Rozprawy doktorskie na temat „Cryo-EM structures”

In eukaryotic translation initiation, eIF2B, a 295 kDa multisubunit (from α to ε) complex,is the guanine nucleotide exchange factor (GEF) of eIF2, a GTP binding protein, and hasmultiple roles in regulating the level of active eIF2-GTP-Met-tRNAi ternary complexes inthe cytoplasm. Mutations in eIF2B subunits affect global protein synthesis and, in human,are responsible to cause a genetically inherited lethal childhood brain disease calledLeukoencephalopathy with Vanishing White Matter (VWM). Although the genetic aspectseIF2B have been widely studied over decades, detailed structural knowledge only becameavailable in recent years but is still limited. This study aims to gain structural insights intoyeast eIF2B by a range of electron microscopy techniques to improve our understandingtowards its GEF activity with eIF2 and regulatory response. By performing size-exclusion chromatography and multi-angle static light scattering (SECMALS), it was found that eIF2B is a stable dimer of pentamers (~600 kDa). Negativestaining (25.8 Å) and cryo-EM (12.1 Å) eIF2B decamer models that showed 2-foldrotational symmetry were generated by single particle reconstruction. Homology modelingof yeast eIF2B subunits revealed an eIF2B(αβδ)2 hexameric core and two separate arm-likeeIF2Bγε catalytic domains with potential flexibility. To constrain subunit position in thearm structure, Ni-NTA-Nanogold labeling against the multihistidine tag of eIF2Bγ wasperformed. In addition, genetic approaches were applied to eliminate synthesis of eIF2Bα(34 kDa) and eIF2B(βγδε)2 octamer complexes (532 kDa) were purified by SEC-MALSand analysed by negative staining single particle reconstruction. It was speculated thatdeletion of eIF2Bα might have triggered significant conformational rearrangement that ledto high uniformity in the 2D class averages. A hypothetical model was thus proposed forthe octamer where the two arm-like domains clamp together to form a compact structure.

Ma thèse se focalise sur la compréhension d’un phénomène de transcription, appelé backtracking, qui inactive la RNAP et arrête la transcription. La réactivation des complexes RNAP arrêtés et la reprise de la transcription nécessitent un facteur protéique appelé GreB. L’objectif du projet était d’obtenir des informations structurelles sur: i) la façon dont le retour en arrière inactive la RNAP dans E. coli; et ii) comment GreB sauve la RNAP en marche arrière pour continuer la transcription. À l'aide de SP cryo-EM, j’ai capturé quatre instantanés de RNAP dans différents états. Mes résultats montrent que l'ARN n'est plus aligné avec le site actif. De plus, suite à un retour en arrière, la RNAP adopte de nouvelles modifications de conformation permettant la liaison de GreB. En conséquence, le NTD de GreB entre en contact le site actif de la RNAP et donne des résidus acides qui augmentent l'affinité pour un ion magnésium, ce qui est nécessaire pour la catalyse du clivage de l'ARN mal aligné. Ces quatre reconstructions donnent un aperçu du mécanisme catalytique et de la dynamique du clivage et de l'extension de l'ARN
[...]My Ph.D. was focused on the understanding of a transcriptional phenomenon, termed backtracking, which inactivates RNAP and halts transcription. Reactivation of halted RNAP complexes and transcription resumption, requires a protein factor called GreB. The objective of the project was to gain structural information on: i) how backtracking inactivates RNAP inE. coli; and ii) how GreB rescues backtracked RNAP to continue transcription. Using SP cryo- EM, I captured four snapshots of RNAP at different states covering the backtracking and reactivation cycle. My results show that the RNA is no longer aligned with the active center, explaining the transcription halt. Furthermore, as a result of backtracking, RNAP adopts new conformational changes allowing GreB binding. As a consequence, the NTD of GreB contacts RNAP active center and donates acidic residues that increase the affinity towards a magnesium ion, which is required for cleavage catalysis of the misaligned RNA. These four reconstructions give insights on the catalytic mechanism and dynamics of RNA cleavage and extension. [...]

Les kinétoplastides sont un groupe de protozoaires, et qui menace plus de 400 millions de personnes dans le monde entier. Ils possèdent des segments d'expansion d'ARNr (SE) inhabituellement plus larges dans les sous-unités 40S. Ici, nous avons purifié à partir de lysats de cellules de T. cruzi des complexes d'initiation natifs (48S IC) et des sous-unités de 40S natives que nous avons ensuite analysées par cryo-ME. La structure des 48S IC révèle certains des aspects spécifiques de la traduction aux kinétoplastides, tels qu'un réseau d’interaction complexe entre eIF3 et SEs. En outre, notre structure met en évidence le rôle de DDX60 dans l'initiation de la traduction chez les kinétoplastides. La structure d'une sous-unité 40S native révèle l'existence d'un facteur non caractérisé (appelé ηF). Le site de liaison de ηF suggère un rôle dans le contrôle de la traduction. De plus, nous avons rapporté́ la structure d’une nouvelle protéine ribosomale (-r) spécifique des kinétoplastides (KSRP). Notre travail pose les premières bases structurales des aspects spécifiques de l'initiation de la traduction chez les kinétoplastides
Kinetoplastid is a group of flagellated protozoans, which threatens more than 400 million people world-wide. They possess unusual large rRNA expansion segments (ES) in the 40S, such as ES6S, ES7S and ES9S and their location suggests an involvement in the initiation process. Furthermore, all mature mRNAs possess a conserved 5’ spliced-leader. Here, we purified from T. cruzi cell lysates native initiation complexes and native 40S subunits that we then analysed by cryo-EM. The structure of native initiation complexes reveals several kinetoplastid-specific aspects of translation, such as an intricate interaction network between eIF3 and ES6S and ES7S. Furthermore, it reveals the role of DDX60 in translation initiation in kinetoplastids. The structure of native 40S subunits reveals the existence of an uncharacterized factor (termed ηF) bound at platform of the 40S. The binding site of ηF suggests a role in translational control. Moreover, we reported a novel kinetoplastid-specific ribosomal (r-) protein (KSRP) bound to the 40S subunit. Our work represents the first structural characterization of kinetoplastids-specific aspects of translation initiation

The mitochondrial F-ATPases make about 90% of cellular ATP. They are multi-protein assemblies with a membrane extrinsic catalytic domain attached to a membrane embedded sector. They operate by a mechanical rotary mechanism powered by an electro-chemical gradient, generated across the inner mitochondrial membrane by respiration. A detailed molecular description has been provided by X-ray crystallographic studies and "single molecule" observations of the mechanism of the F1 catalytic domain. Details are known also of the architecture of the peripheral stalk of part of the stator and the membrane embedded region of the rotor. However, knowledge of the detailed structure of the rest of the membrane domain, and the detailed mechanism of generation of rotation is lacking. Recently, studies of the intact mitochondrial F-ATPases, determined by cryo-electron microscopy (cryo-em), have provided structural information at intermediate levels of resolution. Whilst these structures have given insights into the mechanism of generation of rotation, the information required for a molecular understanding of this mechanism is still lacking. Moreover, the locations and roles of six supernumerary membrane subunits are unclear. Some of them are likely to be involved in the formation of dimers of the enzyme which line the edges of mitochondrial cristae. Therefore, in this thesis, a procedure is described for the purification of dimers of the bovine and yeast F-ATPases. The structure of the bovine dimer has been determined by cryo-em at a resolution of ca. 6.9 Angstrom. This structure confirms features concerning the trans-membrane spans of the a-, A6L- and b-subunits observed in the monomeric complex. In addition, the single trans-membrane a-helix of the f-subunit has been located, and the subunit appears to mediate dimer formation. The structure of A6L has been extended, and the a-helices of subunits e- and g- have been located. Another novel feature has been assigned to the DAPIT subunit, and may provide links between dimers in forming larger oligomers. Further improvement in the resolution of the structure is hampered by the extreme conformational heterogeneity of the F-ATPase. To this end, the simpler Fo membrane domain has been isolated and characterized initially by electron microscopy in negative stain.

Helical assemblies of proteins are ubiquitous in nature and they perform vital functions in a wide range of organisms. The recent development of direct electron detectors and other imaging techniques in cryo-electron microscopy (cryo-EM) has opened new possibilities in solving helical structures at atomic resolution. Existing software packages for helical processing often require experience in tuning many ad hoc parameters to achieve optimal reconstruction results. REgularised LIkelihood OptimisatioN (RELION), an open-source single-particle analysis package, reduces the need for user expertise by the formulation of an empirical Bayesian framework, and has yielded some of the highest resolution density maps in recent years. Prior information about the helical assemblies can be conveniently incorporated into the statistical framework of RELION and thereby improves the helical reconstructions. This PhD thesis describes the development of a helical processing computation workflow with reduced user intervention in RELION. Chapter 1 introduces the theoretical basis of cryo-EM data acquisition and single-particle data processing, the concepts of helical symmetry, and a previously described method for iterative real-space reconstruction of helical assemblies, to which the RELION implementation bears resemblance. Chapter 2 discusses multiple adaptations to RELION that are necessary for helical processing. Key elements include the imposition and local refinement of helical symmetry, masks on helical segments and references, expressions of angular and translational prior information, manual and automated segment picking as well as initial model generation for helices. Calculations have been performed on four test data sets showing that the developed methods in RELION yield results that are as good as or better than alternative approaches for the tests performed. Chapter 3 describes the same methodology adapted to helical sub-tomogram averaging in RELION. Chapter 4 introduces the local symmetry option developed for special types of filaments with pseudo-helical symmetry. The concept can be extended to general single-particle analysis as well. Chapter 5 describes four helical structures determined in collaboration with other research groups using helical RELION for data processing. Chapter 6 concludes the thesis with a brief summary and future prospects.

Une approche de biologie structurale intégrative a été mise à profit pour l'étude de l’organisation structurale du complexe NuRD. Mon travail s'est focalisé essentiellement sur trois sous-unités du complexe: MBD3, RbAp46 et RbAp48. J'ai mis en place les protocoles de production et de purification de ces différentes sous-unités, et les ai caractérisé biophysiquement par diverses méthodes. Nous avons ensuite entrepris des études de liaisons sur des nucléosomes reconstitués au laboratoire. Pour MBD3, l'optimisation du complexe nous a permis d'obtenir des cristaux diffractant jusqu'à 7 A de résolution. Parallèlement, une reconstruction 3D préliminaire à partir de données de cryo-microscopie électronique a pu être obtenue à 25A de résolution. Pour RbAp46/48, nous avons pu montrer que ces protéines formaient un complexe stable avec le nucléosome, pavant la voie pour leur future étude structurale par cryo-microscopie électronique ou cristallographie aux rayons-X
An integrative structural biology approach has been used to study the structural organization of the NuRD complex.My work focused especially on three subunits of this complex: MBD3, RbAp46 and RbAp48. I set up the preparation of the individual subunits and characterized them by various biophysical methods. We next carried out binding assays with homemade human nucleosomes. For MBD3, optimization of the complex led to crystals diffracting up to 7 Å. In parallel, a preliminary 3-D reconstruction at 25 Å resolution has been solved in cryo-EM. For RbAp46/48, crystal we were able to show that these proteins form stable complexes with the nucleosome, paving the way for future structural analysis by cryo-EM or X-ray crystallography

La pause transcriptionnelle marquée par les ARN polymérases (RNAP) est un mécanisme clé pour réguler l'expression des gènes dans tous les règnes de la vie et est une condition préalable à la terminaison de la transcription. Le facteur de transcription bactérien essentiel NusA stimule à la fois la pause et la terminaison de la transcription, jouant ainsi un rôle central. Ici, je présente des reconstructions par cryo-microscopie électronique (cryo-EM) à une seule particule de NusA lié à des complexes d'élongation en présence et en absence d’ARN en épingle à cheveux dans le canal de sortie de l'ARN. Les structures révèlent quatre interactions entre NusA et RNAP qui suggèrent comment NusA stimule le repliement de l’ARN, la pause et la terminaison de la transcription. Un intermédiaire de translocation asymétrique de l'ARN et de l'ADN convertit le site actif de l'enzyme en un état inactif, fournissant une explication structurelle pour l'inhibition de la catalyse. La comparaison de RNAP à différentes étapes de la mise en pause donne un aperçu de la nature dynamique du processus et du rôle de NusA en tant que facteur de régulation
Transcriptional pausing by RNA polymerases (RNAPs) is a key mechanism to regulate gene expression in all kingdoms of life and is a prerequisite for transcription termination. The essential bacterial transcription factor NusA stimulates both pausing and termination of transcription, thus playing a central role. Here, I present single-particle electron cryo-microscopy (cryo-EM) reconstructions of NusA bound to paused elongation complexes with and without a pause-enhancing hairpin in the RNA exit channel. The structures reveal four interactions between NusA and RNAP that suggest how NusA stimulates RNA folding, pausing, and termination. An asymmetric translocation intermediate of RNA and DNA converts the active site of the enzyme into an inactive state, providing a structural explanation for the inhibition of catalysis. Comparing RNAP at different stages of pausing provides insights on the dynamic nature of the process and the role of NusA as a regulatory factor

Le travail de cette thèse a été consacré à l'analyse structurale par microscopie électronique du « core complexe » de l'appareil photosynthétique bactérien. Ce multicomplexe membranaire est le dernier composant de l'appareil photosynthétique bactérien dont on ne dispose pas de structure atomique. L'enjeu de la détermination de la structure de ce complexe est la compréhension de la transformation de l'énergie lumineuse en séparation de charges et du couplage entre le centre réactionnel et le cytochrome bc1. Pour répondre à ces questions, la démarche a consisté en une analyse structurale comparative de core complexes issus de différentes bactéries du genre Rhodobacter, qui ont été purifées et caractérisés biochimiquement. Ceci a fait appel aux méthodes structurales complémentaires de cristallographie électronique, d'analyse de particules isolées par microscopie électronique et, en collaboration, d'AFM. Celles-ci ont apportés des informations sur les domaines transmembranaires de core complexe reconstitués dans une bicouche lipidique (cryo-microscopie de cristaux 2D), sur les domaines extra-membranaires du core complexe en membrane (AFM) et sur l'ensemble du core complexe en détergent (analyse de particules isolées). Les résultats obtenus ont permis de proposer une organisation des sous-unités au sein du complexe, de dégager les déterminants structuraux impliqués dans la biosynthèse et l'oligomérisation des complexes et d'apporter de nouvelles propositions sur la fonction et le couplages avec les autres composants de l'appareil photosynthétique bactérien.

P-glycoprotein (P-gp or ABCB1) is a membrane-bound active transporter belonging to the ABC protein superfamily. It is responsible for xenobioIc efflux and also contributes to multidrug resistance in diverse diseases including cancer and epilepsy. P-gp has been increasingly recognised as a potential target for future therapeutics. Although the protein has been studied for decades, understanding of the P-gp transport mechanism is still incomplete. Two P-gp orthologues, mouse (m) and human (h), were therefore expressed in yeasts and purified in the presence of the detergent, n-Dodecyl-β-D- Maltoside (DDM). Purified proteins were examined for aggregation and monodispersity via dynamic light scattering (DLS) and their thermal stability was determined by an assay using a thiol-specific dye (CPM). ATPase activity, measured in a detergent environment, showed that the proteins were active with a basal activity of 60 ± 4 and 35 ± 3 nmol/min/mg for mP-gp and hP-gp, respectively. Crystallisation trials were conducted in the presence of nucleotide. In meso crystallisation using commercial monoolein pre- dispensed plates yielded hexagonal crystal-like objects however they failed to diffract X- rays. P-gp samples were also subjected to cryo-EM where mP-gp in the post-hydrolytic (ADP-bound, vanadate-trapped) state provided the highest resolution dataset that led to a reconstruction of 3D density map at the resolution of 7.9 Å which showed an inward- facing conformation. Rigid-body model fitting unveiled densities that were not accounted for by the fitted model illustrating new features such as bound ADP, extended NBD1- TMD2 linker and alternative allocrite-binding sites. Ultimately, the knowledge of P-gp conformation alteration was enhanced and a refined alternating access mechanism of P- gp was proposed based upon information derived from this study.

Les récepteurs Cys-loop sont des canaux ioniques pentamériques activés par un ligand (pLGIC), qui jouent un rôle essentiel dans la neurotransmission rapide. Ils sont la cible de nombreuses familles de médicaments (antiémétiques, anesthésiques généraux, benzodiazépines, médicaments pour arrêter de fumer, etc.) et leurs propriétés physiologiques sont, en conséquent, très étudiées. Lorsque les pLGIC lient des neurotransmetteurs, ils subissent des modifications conformationnelles, d’un état au repos où le pore est fermé vers un état transitoire ouvert. La liaison de ligand peut également provoquer un état conformationnel fermé et désensibilisé. En outre, les propriétés des pLGIC peuvent être influencées par divers composés (lipides, inhibiteurs compétitifs, modulateurs allostériques, ions tels que Ca2 +), ce qui en fait des récepteurs capables d'intégrer différents signaux via des changements de conformation.Dans cette thèse, nous nous concentrons sur des études structurales du récepteur de la sérotonine de type 3 chez la souris (m5-HT3R). La première structure du m5-HT3R, obtenue par cristallographie aux rayons X, représentait une conformation inhibée à pore fermé, stabilisée par des nanobodies {Hassaine: 2014de}. Nous avons cherché à obtenir des structures dans d’autres conformations, afin d’élucider son mécanisme moléculaire de fonctionnement. Pour ce faire, nous avons utilisé à la fois la cristallographie aux rayons X et la cryo-microscopie électronique. Les résultats obtenus sont décrits dans 2 chapitres dédiés de la thèse.Une introduction générale de la famille pLGIC est suivie par une description détaillée de la structure du m5-HT3R. Dans la section résultats, nous présentons le protocole optimisé pour la purification du récepteur, nous expliquons que la mauvaise diffraction est un facteur limitant dans les essais cristallographiques, et nous montrons les freins rencontrés lors de l'utilisation des nanobodies pour la stabilisation conformationnelle du récepteur. Dans les résultats de la microscopie électronique, nous présentons l'optimisation de la préparation de l'échantillon et de la grille qui a finalement permis la collecte de données. Nous décrivons quatre structures différentes représentant des états fonctionnels distincts du m5-HT3R : une conformation inhibée fermée liée au Tropisétron ; un état ouvert et un état pré-actif présumé obtenus en présence de sérotonine ; et enfin un état proche de l’état pré-actif supposé, en présence de sérotonine et du modulateur allostérique TMPPAA. Nous comparons nos données avec les structures du même récepteur obtenues par un autre laboratoire.Notre travail a montré pour la première fois comment l'antagoniste (Tropisétron) et le neurotransmetteur (sérotonine) se lient au m5-HT3R. Nos structures approfondissent également la connaissance du mécanisme d’action du récepteur
Cys-loop receptors are pentameric ligand-gated ion channels (pLGIC), which play a crucial role in rapid neurotransmission. They are the targets of a legion of drugs (antiemetics, general anesthetics, benzodiazepines, smoke cessation drugs, etc.) and their physiological properties are intensively studied. When pLGICs bind neurotransmitters, they undergo conformational changes, from a resting closed-pore state to a transient open-pore state; they can also enter a ligand-bound, closed-pore, desensitized state. Moreover, the gating properties of pLGICs can be influenced by a variety of compounds (e.g. lipids, competitive inhibitors, allosteric modulators, ions such as Ca2+), which makes them flexible receptors capable of integrating different signals into conformational changes.In this thesis we focus on structural studies of the mouse serotonin type 3 receptor (m5-HT3R). The first structure of the m5-HT3R, obtained by X-ray crystallography using stabilizing nanobodies, was a closed-pore inhibited conformation {Hassaine:2014de}. As a follow-up, we aimed to obtain structures of the m5-HT3R in other conformations, in order to elucidate its gating mechanism. For this purpose we used both X-ray crystallography and cryo-electron microscopy and thus the whole thesis follows two story-lines.A general introduction of the pLGIC family is followed by a detailed structural description of the m5-HT3R. In the results section, we present the optimized protocol for the receptor purification, we report that limiting diffraction is a bottleneck in the crystallographic trials and we emphasize limits met using nanobodies for conformational stabilization of the receptor. In the electron microscopy results part we present the optimization of the sample and grid preparation that ultimately permitted data collection. We report four different structures representing distinct functional states of the m5-HT3R: an inhibited tropisetron-bound closed conformation, an open-pore state and a putative pre-active state obtained in the presence of serotonin, and finally a closely-related putative pre-active state in the presence of serotonin and of the allosteric modulator TMPPAA. We compare our data with structures of the same receptor obtained by other laboratory.It was shown for the first time in our work how the antagonist (tropisetron) and the neurotransmitter (serotonin) bind to the full-length m5-HT3R. And our structures deepen the knowledge of the receptor's gating mechanism

Ryanodine receptors (RyR) are intracellular channels that are intricately involved in Ca2+ release. These channels large membrane proteins~2.26MDa in size. In this multi-goal project firstly we successfully studied the gating mechanics of the RyR1 in the presence of Mg2+. We used single particle reconstruction and image processing techniques to obtain the 3D structure of the RyR1 with Mg2+. The 3D structure in the presence of Mg2+ and an ATP analog is the closest representation of human physiological conditions. The open and closed state structures of RyR1 are known. However, the physiologically closed state has not been studied before. Understanding this structure will help in the understanding of protein interactions. Our second goal was the validation of this 3D structure under different levels of noise. Validation under different noise levels analyzed the problem of noise bias is present in the field of cryo-EM and single particle reconstruction in select cases.

X-Linked Retinoschisis (XLRS) is a currently incurable, progressive retinal degeneration that affects approximately 1:20,000 males. Sufferers have a loss of retinal structure and visual acuity, leading to blindness. The condition is caused by mutation of the RS1 gene encoding the retinal-specific protein retinoschisin. Retinoschisin is critical in maintaining the normal, ordered retinal architecture, with deletion in mice models leading to loss of both structure and visual processing, analogous to XLRS sufferers. However, re-introduction of retinoschisin using adeno-associated viral vectors leads to complete rescue in these models. Despite the importance of retinoschisin in maintaining retinal architecture, the mechanism by which it maintains this structure remains unknown. As a result, this study aimed to structurally characterise retinoschisin and XLRS-associated point mutants R141H and H207Q to gain insight into the mechanism of retinoschisin action. To this end, retinoschisin was expressed and purified from HEK 293-EBNA cells and the structure of both monomeric and octameric retinoschisin was investigated using Small-Angle X-Ray Scattering (SAXS) and Cryo-electron microscopy (Cryo-EM). Monomeric retinoschisin was found to adopt an elongated structure that allowed for the tight association of the subunits into a planer propeller structure. However, in solution conditions the octamer also stably self-assembled into a dimer of octamers, for which the structure was solved using cryo-EM. This allowed for construction of a quasi-atomic model, enabling mapping of XLRS-associated point mutations on the complex. Two major classes of mutation were identified, in the intra-octamer and inter-octamer interfaces, suggesting a mechanism of pathology for these mutants. Observation of clustered conservative mutations at the inter-octamer interface suggested the dimer of octamers may be physiologically relevant. Furthermore, comparison of the R141H mutant to the wild-type revealed an additional mutated site in the propeller tips. Here, R141H was suggested to induce a small conformational change and alter an interaction site. Another mutant, H207Q, however, induced a destabilization of the assembled retinoschisin molecule. In conclusion, we purified and structurally characterised human retinoschisin, identifying a new hexadecameric oligomer. The structure of this allowed for identification of distinct classes of mutations on the assembled molecule and a hypothesis of the mechanism of retinoschisin action in the retina.

Le complexe de remodelage des nucléosomes et de désacétylation des histones (NuRD) est l'un des principaux régulateurs épigénétiques du génome. Il contribue à la formation et au maintien de l'hétérochromatine, une structure très dense d'ADN et de protéines réprimant la transcription. La NuRD joue un rôle central dans les processus biologiques pertinents tels que la régulation de la pluripotence ou la tumorigenèse. Malgré cela, sa structure et son mécanisme d'action restent largement inconnus. Cela est dû en grande partie à l'hétérogénéité inhérente de composition et de conformation de NuRD. Dans ce travail de doctorat, nous avons essayé de surmonter ces défis en utilisant une approche multidisciplinaire. Nous avons combiné la purification par affinité de complexes et de sous-unités NuRD endogènes et recombinants, la réticulation, la cryo-microscopie électronique et la spectrométrie de masse. Grâce à cela, nous avons pu identifier un sous-complexe d'histone désacétylase stable à activité indépendante (PMMR) et résoudre sa structure à 16,6 Å. Nous avons également obtenu des structures à faible résolution de plusieurs sous-complexes plus petits. L'étude de ces sous-complexes nous a permis de proposer un processus d'assemblage Holo-NuRD, conduisant à la publication d'un article scientifique.Mots-clés: NuRD, régulation épigénétique, remodelage de la chromatine, histone désacétylase, Cryo-EM
The nucleosome remodeling and histone deacetylase (NuRD) complex is one of the main epigenetic regulators of the genome. It contributes to the formation and maintenance of the heterochromatin, a tightly packed structure of DNA and proteins that represses transcription. NuRD plays a central role in relevant biological processes such as pluripotency regulation or tumorigenesis. Despite that, its structure and action mechanism remain unknown. This is largely due to the inherent compositional and conformational hetesrogeneity of NuRD. In this PhD work we tried to overcome these challenges using a multidisciplinary approach. We combined affinity purification of endogenous and recombinant NuRD complexes and subunits, cross-linking, electron cryo-microscopy and mass spectrometry. Thanks to that we were able to identify a stable histone deacetylase subcomplex with independent activity (PMMR) and solve its structure at 16.6 Å. We also obtained low resolution structures of several smaller subcomplexes. Studying these subcomplexes allowed us to propose a Holo-NuRD assembly pathway, leading to the publication of a scientific paper.Key words: NuRD, Epigenetic regulation, Chromatin remodeling, Histone deacetylase, Cryo-EM

Le ribosome est une machinerie cellulaire importante impliquée dans la synthèse protéique de toute cellule vivante. Par conséquent, le ribosome est l'une des principales cibles des antibiotiques naturels, qui sont capables de tuer les cellules bactériennes en bloquant la synthèse protéique. Toutefois, certaines bactéries sont résistantes à ces antibiotiques en raison de petites modifications au niveau de leurs ribosomes. Entre autres, Staphylococcus aureus (S. aureus) est un agent pathogène responsable de nombreuses infections graves chez l’Homme. Les structures cristallines d'antibiotiques en complexe avec des ribosomes de bactéries non-résistantes, non-pathogènes, Gram négatives ont fourni un aperçu sans précédent des mécanismes d'action de ces antibiotiques. Cependant, aucune structure de ribosome de bactéries pathogènes, hautement résistantes, Gram positives telles que S. aureus n’a encore été identifiée.Dans cette étude, nous présentons la première structure de ribosome de S. aureus à haute résolution (3.9 Å) résolue par cryo-microscopie électronique (cryo-ME). Nous mettons en évidence plusieurs caractéristiques de l'organisation des ribosomes spécifiques des bactéries Gram-positives. Nous décrivons également le protocole de purification et de cristallisation du ribosome de S. aureus pour de futures études de cryo-ME et de cristallographie aux rayons X.Tous les résultats obtenus dans ces travaux, faciliteront la description à l’échelle atomique du ribosome de S. aureus et ses complexes fonctionnels’ ’dans un futur proche. La combinaison des méthodes de cristallographie aux rayons X et de cryo-ME aidera à atteindre cet objectif. Les résultats obtenus serviront de base pour le développement de nouveaux composés contre la bactérie pathogène et extrêmement résistante qu’est S. aureus
The ribosome is a large cellular machinery that performs the protein synthesis in every living cell. Therefore, the ribosome is one of the major targets of naturally produced antibiotics, which can kill bacterial cells by blocking protein synthesis. However, some bacteria are resistant to these antibiotics due to small modifications of their ribosomes. Among them, Staphylococcus aureus (S. aureus) is a severe pathogen that causes numerous infections in humans. The crystal structures of complexes of antibiotics with ribosomes from Gram-negative non-pathogenic non-resistant bacteria have provided unparalleled insight into mechanisms of antibiotics action. However, the structure of the ribosome from Gram-positive pathogenic and highly resistant bacteria such as S. aureus was still unidentified.In this study we present the first high resolution structure of the ribosome from S. aureus solved at 3.9 Å by cryo-electron microscopy (cryo-EM). We demonstrate several features of the ribosome organization which are unique for Gram-positive bacteria. We also describe the protocol of purification and crystallization of S. aureus ribosome for future cryo-EM and X-ray crystallography studies.All the results obtained in this work will help to describe S. aureus ribosome and its functional complexes at the atomic level in the nearest future. The combination of X-ray crystallography and cryo-EM methods will help to achieve this aim. The obtained results will provide a foundation for the development of new compounds against the pathogenic and extremely resistant bacteria S. aureus

Photosynthesis is indisputably the primary biological process to introduce chemical energy and biomass into ecosystems by oxidizing water and reducing carbon dioxide into organic compounds. Photosystem II (PSII) is a unique protein complex, present in thylakoid membranes of all oxygenic photosynthetic organisms, able to catalyze the water-splitting reaction using sunlight as driving force, thus being responsible for the generation of all the molecular oxygen accumulated in the atmosphere for over three billion years. Although its catalytic core has been extremely conserved throughout evolution, from cyanobacteria to higher plants, the necessity of different photosynthetic organisms to cope with ever-changing environmental light conditions led to the emergence of a great variability among its peripheral antenna systems, differentiating in extrinsic phycobilisomes in cyanobacteria and intrinsic light harvesting complexes (LHCII) in green algae and higher plants. LHCII are integral membrane proteins that occur as heterotrimers of Lhcb1-2-3 subunits and monomeric Lhcb4-5-6 polypeptides and associate peripherally with the PSII core in variable numbers, thus forming large supramolecular assemblies called PSII‐LHCII supercomplexes. The minimal functional unit, found in all light conditions, consists of a dimeric PSII core (C2) with two strongly bound LHCII trimers (S2), made of Lhcb1 and Lhcb2, connected by two monomeric Lhcb4 and Lhcb5 subunits, and is called C2S2. In limiting light conditions, the C2S2 can further associate with one or two moderately bound LHCII trimers (M2) which consist of Lhcb1, Lhcb2 and Lhcb3 proteins connected by the monomeric Lhcb6, a peculiar subunit found only in higher plants, originating supercomplexes of type C2S2M1-2. A further supramolecular organization is due to the lateral association of PSII-LHCII supercomplexes within the thylakoid membrane plane, forming PSII-LHCII megacomplexes, or even higher ordered arrays. The LHCII fulfill a dual role by either quenching the excess light energy, often occurring in natural environments, or optimizing its harvesting in ecosystems where there is competition and mutual shading. The rearrangement of the PSII’s modular antenna system through its dynamic interaction with the PSII core, therefore, appears to be a key process in light harvesting regulation. Moreover, plant’s PSII and LHCII are spatially and functionally segregated into piled discs of thylakoid membranes (grana), where they occupy 80% of the surface. Their structural arrangement into PSII-LHCII supercomplexes interacting dynamically with each other appears to be critical in determining the overall membrane architecture and ultimately the efficiency of photosynthesis. Although the overall structure of the basic C2S2 supercomplex in plants has been recently resolved at nearly atomic resolution, there is still a lack of knowledge regarding its structural rearrangement in different light conditions as well as its specific interaction within the membrane plane and between adjacent membranes. During this thesis’ work we have been able to isolate pure PSII-LHCII super- and megacomplexes from pea plants grown in moderate light by mild solubilization of stacked thylakoid membranes. In order to assess their overall functional architecture, the full biochemical characterization of isolated PSII-LHCII supercomplexes, comprehensive of accurate proteomic analyses, was coupled with structural studies. Their structural characterization, performed by transmission electron microscopy (TEM) in cryogenic conditions (cryo-EM) and subsequent single particle analysis, led to a novel 3D structure at about 14 Å resolution of the supercomplexes of type C2S2M. The obtained electron density map revealed that under normal light conditions most of the supercomplexes within the grana are of type C2S2M and occur as paired supercomplexes, whose interactions are mediated by physical connections across the stromal gap of adjacent membranes. The specific overlapping of LHCII trimers facing each other in paired supercomplexes, as already observed in other studies, suggests that this conformation might be representative of their native state within the membranes. The physical connections observed across the stromal gap might be attributable to the mutual interaction between the long N-terminal loops of the monomeric Lhcb4 subunits. These subunits occupy a pivotal position in the 3D map of the paired supercomplexes and are clearly bridged across the stromal gap by electron densities attributable to these loops. In addition, despite the its structural flexibility, the remarkable sequence conservation of this region, even in distant phylogenetic photosynthetic organisms, may suggest its major involvement in structural dynamics. The specific interaction observed in paired supercomplexes seems to be mediated by cations present within the chloroplast in relatively low concentrations as their depletion from buffers used for isolation leads to the dissociation of the paired supercomplexes into single ones. Moreover, this evidence was also strongly supported by the decrease in the PSII excitonic connectivity measured in-vivo. The paired behavior has also been observed in higher oligomerization forms of isolated PSII-LHCII supercomplexes in which two paired supercomplexes laterally interact with each other in the membrane plane, thus forming paired megacomplexes. This novel structure has been obtained by EM and 2D reconstruction of negatively stained particles and, despite its low resolution, reveals how PSII-LHCII supercomplexes may laterally and stromally interact with each other in different ways. The observation of the potential overlapping of LHCII trimers in megacomplexes facing each other, as well as the occurrence of different geometries of interaction between supercomplexes within the membrane plane and between megacomplexes in adjacent membranes, provide intriguing insights on how PSII and LHCII might interact in a very stable manner within the thylakoid membrane and between different discs in the grana. In order to study the PSII-LHCII supercomplex remodeling in the context of ever-changing light environmental conditions, PSII-LHCII supercomplexes have been isolated from pea plants grown at different light intensities: low (LL), moderate (CL) and high light (HL). The accurate profiling and quantitation of the LHCII subunits in the isolated supercomplexes and in the native thylakoids, achieved by using a mass-spectrometry based proteomic approach, was coupled with the evaluation in-vivo of their functional antenna size (ASII). At increasing light intensities, the structural remodeling of the modular PSII’s antenna system led to the reduction of the amount of LHCII M-trimers in the isolated complexes, attested by the decreased level of Lhcb3 and Lhcb6. This specific remodeling does not occur at the same rate in the entire thylakoid membrane. The whole LHCII pool is downregulated only in plants grown in HL, suggesting the occurrence of different acclimation strategies. The remarkable decrease of the ASII observed in HL acclimated plants, when compared to LL plants, can be attributed to the significant increase of the Lhcb4 specific isoform Lhcb4.3, occurring both in isolated supercomplexes and in thylakoid membranes. Unlike isoforms Lhcb4.1-2, the Lhcb4.3 isoform, whose transcription is enhanced upon HL exposure, interestingly has a truncated C-terminus that is located at the binding interface with Lhcb6 within the supercomplex structure. The incorporation of Lhcb4.3 in the PSII-LHCII supercomplex might play a major role in decreasing its functional antenna size by reducing its affinity to bind additional M-trimers, thus regulating its light harvesting efficiency even at moderate light intensities. Conversely, the exposure to HL induces the decrease of the PSII antenna cross-section in isolated supercomplexes and the partial depletion of the whole antenna system of PSII in the thylakoid membranes, thus constitutively preventing damages to the reaction center when light continuously exceeds its energy-processing capacity. These results aim at broadening the current knowledge on how the light harvesting antenna system associated with the PSII core is finely regulated upon plants’ long term acclimation to different light intensities. The flexibility of the PSII’s modular antenna system, accompanied by its finely tuned structural interaction with the core complex, pivotal for the 3D organization of plant thylakoid membranes, certainly played a key role in determining its remarkable evolutionary outcome. Taken together, these results may provide new research directions while certainly broadening the knowledge on how PSII-LHCII assemblies and their supramolecular interaction contribute to maintain the complex architecture of thylakoid membranes and the overall efficiency of photosynthesis in ever changing environmental conditions.
La fotosintesi è indubbiamente il processo biologico principale che introduce energia chimica e biomassa negli ecosistemi ossidando l’acqua e riducendo l'anidride carbonica in composti organici. Il fotosistema II (PSII) è un complesso proteico presente nelle membrane tilacoidali di tutti gli organismi fotosintetici, l’unico in grado di catalizzare la reazione di lisi dell'acqua utilizzando la luce solare come forza motrice e di conseguenza responsabile della generazione di tutto l'ossigeno molecolare presente nell'atmosfera da più di tre miliardi di anni. Nonostante il centro catalitico del PSII sia rimasto fondamentalmente inalterato nel corso dell'evoluzione dai cianobatteri alle piante superiori, la necessità di far fronte alla continua variazione delle condizioni di luce ambientali ha portato all’evoluzione di sistemi di antenne periferiche altamente differenziate, distinte in ficobilisomi estrinseci nei cianobatteri e complessi di membrana intrinseci (LHCII) in alghe verdi e piante superiori. Gli LHCII sono complessi proteici di membrana presenti come etero-trimeri composti dalle subunità Lhcb1-2-3 e subunità monomeriche Lhcb4-5-6 associate perifericamente con il centro catalitico del PSII in numero variabile, formando così associazioni supramolecolari chiamate supercomplessi PSII-LHCII. L'unità funzionale minima, presente in ogni condizione di luce, detta C2S2, è costituita da un PSII centro di reazione dimerico (C2) legato strettamente a due complessi antenna trimerici (S2), composti da Lhcb1 e Lhcb2, mediante due subunità monomeriche Lhcb4 e Lhcb5. In condizioni di luce limitante il C2S2 può ulteriormente associare uno o due complessi antenna trimerici legati moderatamente (M2), costituiti dalle subunità Lhcb1, Lhcb2 e Lhcb3, mediante una peculiare subunità monomerica che si trova solo nelle piante superiori, Lhcb6, generando supercomplessi di tipo C2S2M1-2. I supercomplessi PSII-LHCII possono ulteriormente interagire lateralmente all'interno del piano della membrana tilacoidale formando megacomplessi PSII-LHCII o più estesi arrangiamenti ordinati semicristallini. I complessi antenna LHCII svolgono un duplice ruolo, la dissipazione efficiente dell'energia luminosa, spesso in eccesso negli ambienti naturali, e l’ottimizzazione della sua raccolta negli ambienti in cui vi è concorrenza tra organismi e ombreggiatura reciproca. Il riassetto del sistema di antenne modulari del PSII attraverso la sua interazione dinamica con il centro catalitico sembra quindi essere un processo chiave nella regolazione della raccolta della luce. Inoltre, i PSII e LHCII nelle piante sono spazialmente e funzionalmente segregati in dischi impilati di membrane tilacoidi (grana), dove occupano l'80% della superficie. La loro disposizione strutturale in supercomplessi PSII-LHCII che interagiscono dinamicamente tra loro sembra essere determinante per l'architettura complessiva della membrana tilacoidale e quindi per l'efficienza della fotosintesi. Sebbene la struttura del supercomplesso base C2S2 delle piante sia stata recentemente risolta ad una risoluzione quasi atomica, c'è ancora una lacuna conoscitiva riguardo al ri-arrangiamento strutturale dei PSII-LHCII che avviene in diverse condizioni di luce e alla loro interazione reciproca nel piano della membrana e tra membrane adiacenti dei grana. Durante il lavoro svolto in questa tesi, siamo stati in grado di purificare super- e megacomplessi PSII-LHCII isolati da piante di pisello coltivate in luce moderata mediante la completa solubilizzazione delle membrane tilacoidali. La caratterizzazione biochimica dei supercomplessi PSII-LHCII isolati, complementata da accurate analisi proteomiche, è stata accoppiata con studi strutturali al fine di comprendere la loro architettura funzionale. La caratterizzazione strutturale, eseguita mediante microscopia elettronica a trasmissione (TEM) in condizioni criogeniche (cryo-EM) e successiva analisi d’immagine sulle singole particelle, ha portato ad una nuova struttura tridimensionale (3D) a circa 14 Å di risoluzione del supercomplesso di tipo C2S2M. La mappa di densità elettronica ottenuta ha rivelato che, in condizioni di luce di crescita di intensità moderata, la maggior parte dei supercomplessi è di tipo C2S2M. Essi sono disposti in maniera accoppiata, interagendo mediante collegamenti fisici attraverso l’intervallo stromatico, verosimilmente di membrane adiacenti. La sovrapposizione specifica degli LHCII trimerici, uno di fronte all'altro in supercomplessi accoppiati, come già osservato in altri studi, suggerisce che questa conformazione potrebbe essere rappresentativa del loro stato nativo all'interno delle membrane. I collegamenti fisici osservati nell’intervallo stromatico potrebbero essere attribuibili all'interazione reciproca tra le lunghe porzioni N-terminali di subunità monomeriche Lhcb4 adiacenti. Queste subunità occupano una posizione chiave nella mappa 3D dei supercomplessi accoppiati e le densità elettroniche che attraversano l’intervallo stromatico connettendo i due supercomplessi sono chiaramente attribuibili alle loro porzioni flessibili N-terminali. La sequenza amminoacidica di questa regione, nonostante la sua flessibilità, è sorprendentemente conservata anche in organismi fotosintetici filogeneticamente distanti, il che suggerisce un suo coinvolgimento in dinamiche strutturali fisiologicamente rilevanti per l’apparato fotosintetico. L'interazione specifica osservata nei supercomplessi appaiati sembra essere mediata dai cationi presenti all'interno del cloroplasto in concentrazioni fisiologiche. La loro rimozione dai tamponi utilizzati per l'isolamento, infatti, ne provoca la dissociazione in singoli supercomplessi. Questa evidenza è inoltre sostenuta dalla stima della connettività funzionale misurata in-vivo tramite tecniche di induzione di fluorescenza. Nei supercomplessi appaiati infatti si è evidenziato un potenziale trasferimento di energia maggiore se confrontato con i supercomplessi singolarizzati mediante semplice diluizione dei cationi presenti. L’ appaiamento sul lato stromatico mediato da cationi è stato osservato anche in forme isolate di PSII-LHCII con forme di oligomerizzazione superiore ai supercomplessi, in cui due supercomplessi accoppiati interagiscono lateralmente tra loro nel piano di membrana, formando così megacomplessi appaiati. Questa nuova struttura è stata ottenuta con TEM e ricostruzione bidimensionale a partire da particelle colorate negativamente. Nonostante la bassa risoluzione ottenuta, questa struttura rivela come i supercomplessi PSII-LHCII possono interagire reciprocamente in modi diversi, sia lateralmente che attraverso l’intervallo stromatico. L'osservazione della potenziale sovrapposizione degli LHCII trimerici in megacomplessi accoppiati, così come la presenza di diverse geometrie di interazione tra supercomplessi all'interno del piano di membrana e tra megacomplessi nelle membrane adiacenti, forniscono informazioni interessanti su come PSII e LHCII potrebbero interagire in modo stabile e specifico all'interno della membrana tilacoidale e tra i vari dischi dei grana. Al fine di studiare il rimodellamento dei supercomplessi PSII-LHCII nel contesto di un continuo cambiamento delle condizioni ambientali di luce, sono stati isolati supercomplessi PSII-LHCII da piante di pisello cresciute a diverse intensità di luce: bassa (LL), moderata (CL) e alta (HL). La valutazione in-vivo delle dimensioni dell'antenna funzionale del PSII (ASII) è stata accoppiata con l’identificazione e la quantificazione, mediante analisi proteomiche, delle diverse subunità di LHCII presenti sia nei supercomplessi isolati che nei tilacoidi nativi. All’aumentare dell’intensità di luce di crescita, si evince il rimodellamento strutturale dell’antenna modulare del PSII dovuto alla riduzione della quantità di LHCII trimerici di tipo “M” nei complessi isolati, attestata da una ridotta presenza di Lhcb3 e Lhcb6. Questo rimodellamento specifico non avviene però con le stesse modalità in tutta la membrana tilacoidale. Infatti, la quantità totale di LHCII nei tilacoidi viene significativamente ridotta solo in piante cresciute in HL, suggerendo la presenza di diverse strategie di acclimatazione in grado di ridurre l’antenna funzionale nei tilacoidi. La notevole diminuzione dell’ASII osservata sia nei supercomplessi isolati che nelle membrane tilacoidi di piante cresciute in HL, rispetto alle piante LL, può essere attribuita al significativo incremento di Lhcb4.3, una isoforma di Lhcb4. A differenza delle isoforme Lhcb4.1-2, l'isoforma Lhcb4.3, la cui trascrizione è nota aumentare in seguito all'esposizione ad HL, presenta l’estremità C-terminale troncata. Questa porzione della proteina nella struttura del supercomplesso C2S2M si trova a livello dell’interfaccia di legame con Lhcb6, la subunità monomerica che funge da connettore specifico per l’LHCII trimerico di tipo “M”. L'incorporazione di Lhcb4.3 nel supercomplesso PSII-LHCII sembrerebbe svolgere quindi un ruolo importante nel ridurre le dimensioni dell'antenna funzionale, riducendo l’affinità di legame di antenne aggiuntive (tipo “M”) per ridurre l’efficienza di raccolta della luce già ad intensità moderate. L'esposizione ad HL invece, oltre ad indurre la diminuzione dell'antenna del PSII in supercomplessi isolati, determina anche la riduzione parziale di tutte le antenne del PSII presenti nelle membrane tilacoidi, impedendo quindi danni al centro di reazione quando la luce incidente supera costantemente la sua capacità di utilizzarla efficientemente. Questi risultati contribuiscono ad aumentare le conoscenze su come il sistema di antenne associate al PSII è attivamente regolata a lungo termine modulando l’espressione genica in piante acclimatate a diverse intensità di luce. La flessibilità del sistema modulare di antenne del PSII e la sua interazione strutturale con il centro catalitico, oltre ad essere fondamentale per l’architettura tridimensionale delle membrane tilacoidi delle piante, ha certamente giocato un ruolo chiave nel determinare la loro notevole diversificazione nel corso dell’evoluzione. Nel complesso questi risultati potrebbero fornire nuovi spunti per ampliare la conoscenza di come le associazioni di PSII e LHCII e la loro reciproca interazione contribuiscono a mantenere la complessa architettura delle membrane tilacoidi e quindi l'efficienza complessiva della fotosintesi in condizioni ambientali in continuo mutamento.

The excitatory amino acid transporters (EAATs) play a vital role in the maintenance of glutamatergic neurotransmission within the synapse, involved in fundamental brain functions such as learning and memory. These secondary active transporters enable rapid glutamate reuptake into the surrounding glial cells and neurons by coupling to pre-existing electrochemical gradients of sodium ions, potassium ions and protons. In addition to this conventional transporter function, the EAATs also possess a unique ability to conduct chloride ions in a channel-like process that is thermodynamically uncoupled from substrate transport. Aberrant glutamatergic neurotransmission caused by dysfunction of the EAATs has been associated with excitotoxity-mediated cell death and the pathogenesis of multiple debilitating neurological disorders. In particular, an increased chloride conductance via an EAAT1 mutant has been directly linked to a neurological disease, episodic ataxia type 6. This thesis describes a body of work that investigates the interplay between the elevator mechanism of substrate transport and chloride permeation in the archaeal homolog and sodium-dependent aspartate transporter, GltPh, using X-ray crystallography and single particle cryo-EM. It presents crystal structures of GltPh mutants locked in the outward-facing and inward-facing states, and a novel cryo-EM structure of a GltPh protomer in the chloride conducting state, which fills in the final missing puzzle required to map the complete substrate translocation pathway. This chloride conducting state was further confirmed by a combination of computational and functional analysis, which unveiled two clusters of hydrophobic residues at either side of the membrane that gate the channel. Furthermore, this thesis also explore pathogenic mutations (P206R and L90R in GltPh) associated with episodic ataxia type 6 structurally in an attempt to explain their functional consequences. Together, this thesis enhance our mechanistic understanding of the dual functions conserved in the SLC1A transporter family, opening doors for alternative approaches to treat neurological disorders associated with transporter dysfunctions.

Adeno-associated Virus (AAV) Rep proteins are multifunctional proteins that carry out various DNA transactions required for the life cycle of AAV. The Rep proteins have been found to be important for genome replication, gene regulation, site-specific integration and play an essential role in genome packaging. There are two main groups of Rep proteins: large and small Reps; both groups are SF3 helicase family members. During DNA packaging, studies have shown that the small Rep proteins are critical to produce fully packed particles. Using stopped-flow kinetic analysis, we show a significant difference in helicase activity between the small and large Rep proteins that support the notion that the small Rep proteins are the primary motor to package DNA due to more efficient motor activity. That leaves the large Rep proteins to serve a different role during packaging. In previous studies, we have shown that the large Rep proteins have the ability to change their oligomeric state depending on the nature of the DNA substrate. We can observe double octameric rings with single-stranded DNA (ssDNA) and heptameric complex with double-stranded DNA (dsDNA). To understand Rep protein structural plasticity, we solved a 6.96 Å cryo-EM structure of Rep68*/ssDNA complex illustrating that the formation of Rep octamer rings is dominated by interactions between their N-terminal origin-binding domain (OBD) using the same interface utilized to recognize dsDNA specifically. Our analysis of the structural data suggests that the double octameric ring structure is stabilized by ssDNA that bridges octameric rings together. The structure shows that the helicase domains are highly flexible and that ssDNA is present at the center of the ring. In addition, we have solved a preliminary 12 Å model of Rep68*/dsDNA complex showing a heptameric ring encircling a DNA molecule. Our structural and functional data offer insights to the various Rep-DNA scaffolds that can perform diverse functions during the AAV life cycle.

Le travail retranscrit dans cette thèse porte sur un processus biologique impliqué dans le contrôle qualité de la synthèse protéique bactérienne : la trans-traduction. Ce processus permet de libérer les ribosomes bloqués sur des ARNm défectueux tout en détruisant les peptides et ARNm problématiques impliqués dans le blocage. Il nécessite deux acteurs principaux qui interagissent avec le ribosome: l’ARN transfert-messager (ARNtm) et la protéine SmpB. Dans un premier chapitre, une étude en cryo-microscopie électronique à transmission (cryo-MET) a permis d’obtenir deux structures à l’échelle atomique impliquant le ribosome et différents acteurs de la trans-traduction. La première structure met en évidence l’interaction entre la RNase R, enzyme responsable de la destruction des ARNm défectueux, et le ribosome bactérien. La deuxième structure a mené à la caractérisation des deux premiers états de la trans-traduction à une résolution quasi-atomique. De nouvelles interactions sont notamment observées entre la protéine SmpB et l’hélice H5 de l’ARNtm. Dans un second chapitre, la trans-traduction est exploitée comme cible pour le développement de nouveaux antibiotiques. En effet, cette voie de sauvetage est souvent vitale ou alors indispensable à la virulence bactérienne. Dans l’objectif de découvrir de nouvelles molécules antibiotiques inhibant la trans-traduction, nous avons mis au point un système de détection de la trans-traduction in vitro. Ce système est simple et rapide, basé sur la mesure de la fluorescence d’une GFP tronquée, réassemblée par un ARNtm muté. La validation du système a conduit à la détection de nouveaux composés anti-trans-traduction
This work is focused on a biological process which controls bacterial protein synthesis, trans-translation. This all-in-one process allows the rescuing of ribosomes stalled on defective mRNA, the degradation of the problematic peptides and mRNA. It is driven by two principal actors that interact with the ribosome: transfer-messenger RNA (tmRNA) and Small protein B (SmpB). In a first chapter, by a cryo-electron microscopic (cryo-EM) study, two near-atomic resolution structures, involving the ribosome and various trans-translation actors, were obtained. The first one highlights the interactions between RNase R, an enzyme responsible for mRNA degradation during trans-translation, and the bacterial ribosome. The second one corresponds to the characterization of two early trans-translation states at a near-atomic resolution. New interactions have been observed between SmpB and tmRNA H5 helix. In a second chapter, trans-translation is used as a target for the development of new antibiotic molecules. Indeed, this pathway is often necessary for bacterial survival and pathogenicity. Towards this aim, we designed and set up a new in vitro assay for high-throughput screening assays. This efficient system is based on fluorescence measurements of a GFP reassembled through trans-translation by a mutated tmRNA. This system has been validated and will be used for the discovery of new anti-trans-translation compounds

Electron crystallography of two-dimensional crystals is a structure-determination method well suited to the study of membrane protein structure-function. Two-dimensional crystals consist of ordered arrays of protein within reconstituted lipid bilayers, an arrangement that mimics the natural membrane environment. In this work we describe our recent progress in the use of this method with three different proteins, each providing a window into a separate paradigm in the electron crystallographic pipeline. Specific crystallization conditions for human leukotriene C₄ synthase (LTC₄S) have previously been determined, but our continued refinement of purification and crystallization has identified a number of additional parameters that greatly affect crystal size and quality, and we have developed a protocol to rapidly and reproducibly grow large, non-mosaic crystals of LTC₄S. The human gamma-glutamyl carboxylase (GGCX) has also been crystallized, but is sensitive to cryo-EM sample preparation conditions and we present here the successful reproduction of crystallization and refinement of cryo-EM sample preparation conditions. Lastly, we describe our crystallization screens with the Vibrio cholerae sodium-pumping NADH:ubiquinone reductase complex (Na⁺-NQR), and identify the factors critical to membrane reconstitution of the complex, a necessary first step towards crystallization. We also describe a semi-quantitative crystal screening protocol we have developed that provides quick and accurate method to assess two- dimensional crystallization trials, and discuss some general observations in optimization of membrane protein purification and two-dimensional crystallization for electron crystallography.

Translation is the process by which proteins are synthesized from the instructions in the genetic code. Translation is mediated by the ribosome, a large ribonucleoprotein complex, in concert with messenger RNA (mRNA), transfer RNA (tRNA), and a variety of proteins. The canonical mechanism of translation, introduced in Part I of my thesis, is divided into four distinct phases: initiation, elongation, termination, and recycling. Under unusual circumstances, each phase of translation can also proceed via a number of noncanonical mechanisms, many of which are vitally important for cellular growth or viral infectivity. My thesis describes structural insights into two such noncanonical mechanisms. The aim of the first project, described in Part II, was to structurally characterize a noncanonical mechanism of translational termination in bacteria. In the absence of a stop codon, ribosomes arrest at the 3′ end of an mRNA and are unable to terminate. In bacteria, the primary mechanism for rescuing such nonstop complexes is known as trans-translation. In the absence of a functional trans-translation system, however, the small protein ArfA recognizes the empty mRNA channel and recruits the release factor RF2 to the ribosome, enabling termination to occur. Using single-particle electron cryomicroscopy (cryo-EM), I obtained four high-resolution structures of nonstop complexes that reveal the mechanism of ArfA-mediated ribosome rescue and have wider implications for understanding canonical termination in bacteria. The aim of the second project, described in Part III, was to gain structural insights into a noncanonical mechanism of translational initiation in eukaryotes known as internal ribosome entry. Instead of a 5′ cap, many viruses contain intricately structured, cis-acting internal-ribosome-entry sites (IRESs) within their genomes that direct end-independent initiation. The IRES of hepatitis-C virus (HCV), for example, interacts directly with the mammalian ribosome and functionally replaces many of the canonical initiation factors. However, the mechanism by which the HCV IRES coordinates assembly of an initiation complex and progresses through the initiation phase remains poorly understood. I developed a method for purifying native ribosomal complexes from cell lysate that enabled me to obtain multiple cryo-EM maps of the HCV IRES in complex with the 80S ribosome, including a previously unseen conformation of the IRES induced by rotation of the ribosomal small subunit, and to make progress towards capturing earlier steps in the initiation pathway.

Orientadores: Fernando José Von Zuben, Rodrigo Villares Portugal
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação
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Resumo: Análise de Partículas Isoladas é uma técnica que permite o estudo da estrutura tridimensional de proteínas e outros complexos macromoleculares de interesse biológico. Seus dados primários consistem em imagens de microscopia eletrônica de transmissão de múltiplas cópias da molécula em orientações aleatórias. Tais imagens são bastante ruidosas devido à baixa dose de elétrons utilizada. Reconstruções 3D podem ser obtidas combinando-se muitas imagens de partículas em orientações similares e estimando seus ângulos relativos. Entretanto, estados conformacionais heterogêneos frequentemente coexistem na amostra, porque os complexos moleculares podem ser flexíveis e também interagir com outras partículas. Heterogeneidade representa um desafio na reconstrução de modelos 3D confiáveis e degrada a resolução dos mesmos. Entre os algoritmos mais populares usados para classificação estrutural estão o agrupamento por k-médias, agrupamento hierárquico, mapas autoorganizáveis e estimadores de máxima verossimilhança. Tais abordagens estão geralmente entrelaçadas à reconstrução dos modelos 3D. No entanto, trabalhos recentes indicam ser possível inferir informações a respeito da estrutura das moléculas diretamente do conjunto de projeções 2D. Dentre estas descobertas, está a relação entre a variabilidade estrutural e manifolds em um espaço de atributos multidimensional. Esta dissertação investiga se um comitê de algoritmos de não-supervisionados é capaz de separar tais "manifolds conformacionais". Métodos de "consenso" tendem a fornecer classificação mais precisa e podem alcançar performance satisfatória em uma ampla gama de conjuntos de dados, se comparados a algoritmos individuais. Nós investigamos o comportamento de seis algoritmos de agrupamento, tanto individualmente quanto combinados em comitês, para a tarefa de classificação de heterogeneidade conformacional. A abordagem proposta foi testada em conjuntos sintéticos e reais contendo misturas de imagens de projeção da proteína Mm-cpn nos estados "aberto" e "fechado". Demonstra-se que comitês de agrupadores podem fornecer informações úteis na validação de particionamentos estruturais independetemente de algoritmos de reconstrução 3D
Abstract: Single Particle Analysis is a technique that allows the study of the three-dimensional structure of proteins and other macromolecular assemblies of biological interest. Its primary data consists of transmission electron microscopy images from multiple copies of the molecule in random orientations. Such images are very noisy due to the low electron dose employed. Reconstruction of the macromolecule can be obtained by averaging many images of particles in similar orientations and estimating their relative angles. However, heterogeneous conformational states often co-exist in the sample, because the molecular complexes can be flexible and may also interact with other particles. Heterogeneity poses a challenge to the reconstruction of reliable 3D models and degrades their resolution. Among the most popular algorithms used for structural classification are k-means clustering, hierarchical clustering, self-organizing maps and maximum-likelihood estimators. Such approaches are usually interlaced with the reconstructions of the 3D models. Nevertheless, recent works indicate that it is possible to infer information about the structure of the molecules directly from the dataset of 2D projections. Among these findings is the relationship between structural variability and manifolds in a multidimensional feature space. This dissertation investigates whether an ensemble of unsupervised classification algorithms is able to separate these "conformational manifolds". Ensemble or "consensus" methods tend to provide more accurate classification and may achieve satisfactory performance across a wide range of datasets, when compared with individual algorithms. We investigate the behavior of six clustering algorithms both individually and combined in ensembles for the task of structural heterogeneity classification. The approach was tested on synthetic and real datasets containing a mixture of images from the Mm-cpn chaperonin in the "open" and "closed" states. It is shown that cluster ensembles can provide useful information in validating the structural partitionings independently of 3D reconstruction methods
Mestrado
Engenharia de Computação
Mestre em Engenharia Elétrica

Le travail retranscrit dans cette thèse regroupe l'étude de différents processus biologiques impliqués dans la synthèse protéique bactérienne. Dans un premier chapitre, les origines de la synthèse protéique au temps du monde ARN sont traitées en guise d'introduction. Ce travail théorique se poursuit par la présentation d'une structure à haute résolution du facteur d'élongation G (EF-G) en complexe avec le ribosome par cryo-microscopie électronique à transmission (cryo-MET). Grâce aux avancées techniques de la cryo-MET, nous avons observé pour la première fois EF-G lié au ribosome en l'absence de tout inhibiteur. Cet état particulièr d'EF-G permet de visualiser une flexibilité de son doamine III. Cette étude permet aussi de rationaliser le fonctionnement de l'antibiotique acide fusidique. Nous nous sommes ensuite intéressés aux voies de sauvetage de la synthèse protéique et plus particulièrement de la trans-traduction. Ce mécanisme fascinant permet le recyclage des ribosomes bloqués sur un ARN messager défectueux. Cette voie de sauvetage est généralement vitale ou alors indispensable pour la virulence bactérienne. Nous avons réalisé une étude structurale préliminaire de la dégradation de l'ARNm défectueux durant ce processus. Après une revue traitant du sujet, nous présentons une étude de la trans-traduction comme cible pour le développement de nouveaux antibiotiques. Pour cela, nous avons mis au point un système rapporteur avec contrôle interne de l'activité trans-traductionnelle bactérienne. Après avoir mis au point ce système et validé son utilisation, nous l'avons exploité en testant des molécules ciblant la trans-traduction
The current PhD work brings together various studies linked to bacterial protein synthesis. The first chapter is about the origins of protein synthesis at the time of the RNA world. This theoretical work continues with the presentation of a high-resolution structure of the elongation factor G (EF-G) in complex with the ribosome by cryo-electron transmission microscopy (cryo-TEM). We describe for the first time EF-G bound to the ribosome in the absence of any inhibitor. This particular structure of EF-G displays a yet unseen positioning of its third domain, which becomes very flexible. This study helps to understand the way the antibiotic fusidic acid blocks translation. The work then switches to a study of trans-translation, the main rescuing system of stalled ribosomes in bacteria. Trans-translation is generally vital or at least necessary for bacterial virulence. We conducted a preliminary structural study on the way faulty mRNAs are degraded during this process. This is why we present a study of trans-translation as a target for the development of new antibiotics. For this we developed and validated a reporter system for trans-translation, which is used to screen molecules targeting trans-translation

Viruses are entities which are made of a few genes and are reliant on obligate parasitism to propagate. Due to the obligate connection to their hosts, virus evolution is constrained to the type of host. Viruses however do transmit to evolutionary distinct hosts; in these cases, the phylogenetic relationship of the hosts usually are close. In some instances, RNA-viruses have made host jumps between evolutionary distant hosts, such as the host jump from invertebrates to vertebrates, and fungi to arthropod. Partitiviruses are double stranded RNA viruses which mainly infect fungi and plants. The defining characteristic of these double stranded RNA viruses are the double layered capsids which are formed by a single open reading frame (ORF). The capsid proteins form icosahedral virus particles which are in the magnitude of 30-40 nm. Metagenomic studies have discovered partitiviruses originating from an insect in the Odanata family, a finding which contradicts the fungal host specificity of partitiviruses. The finding of the Hubei.PLV 11 thus implies the existence of a partitiviruses containing structural elements in their capsids which could be involved in the infection of arthropods. Thus, this virus could be used as a model for a structural comparison with its fungi infecting relatives with hopes to identify common viral structural factors necessary for the infection of arthropods. For this purpose, the Hubei.PLV ORF was cloned and then transfected into insect Spodoptera frugiperda (Sf-9) cells using a baculovirus expression system, “bac-to-bac” expression system. The FLAG-tagged capsid proteins were expressed by the Sf-9 cells to be approximately 60 kDa. After ultra-centrifugation in a sucrose gradient, some spontaneous assembly into the expected ~40 nm icosahedral virus-like particles were observed using low resolution scanning electron microscopy. The observed particles were also confirmed by a dynamic light scattering experiment (DLS) and a higher resolution cryo-EM microscope. Thus, the bac-to-bac expression system can be used to produce VLPs from this genus of viruses, and this metagenomically derived virus genome. However, for future success in defining a high-resolution model of this virus, it is recommended that the Sf-9 culture volume is sufficiently high for enough particle production which is necessary for a high-resolution map. The other virus, the Southern pygmy squid Flavilike virus (SpSFV) has been suggested to be the oldest relative of the land based flaviviruses. The SpSFV was found to be the most divergent of the flaviviruses, and to infect invertebrates. Solving for the structure of the SpSFV and comparing it to vertebrate infecting flaviviruses could therefore lead to the identification of factors necessary for the adaptation to vertebrates and thus the humoral immunity by flaviviruses. The soluble E-protein was expressed using the bac-to-bac expression system. The protein was indicated to be multiglycosylated and approximately 50 kDa which is in line with other strains in the genus. Affinity chromatography did not elute this protein, likely due to the His-tag not being spatially available. Cation exchange could elute some protein, but not much from the small ~30 mL culture. To conclude, VLP assembly was confirmed by the Hubei.PLV, thus, solving for the structure is a distinct possibility when a larger Sf-9 culture is used to produce the VLPs. For the SpSFV soluble E-protein, the protein is secreted into the supernatant of the Sf-9 cultures, making purification a possibility. For this, a large Sf-9 culture can be used to produce this protein and then purify it with a cat-ion exchange chromatography.

Accuracy of 3-D structures of proteins is crucial, both in terms of its agreement with the experimental data used to determine the structure and stereochemistry, especially when they are utilized for applications like drug design. Hence, stereochemical validation of 3-D structures is of fundamental importance. In this thesis, we have performed stereochemical studies on various systems, starting from peptide structures to high-resolution protein structures and large multi-protein assemblies. In peptide structures, we noted that subtle deviations in backbone bond lengths and angles from ideal bond geometry significantly alters the allowed Ramachandran (ϕ,ψ) space, which is visualized using bond geometry-specific steric maps. We utilized these bond geometry-specific steric maps for stereochemical validation of residues in protein structures where we distinguish between genuine (ϕ,ψ) angle outliers and outliers due to modeling errors. This was done by analyzing if the observed (ϕ,ψ) value of a residue occurs in disallowed or allowed regions of their own bond geometry-specific steric map. We showed that these maps are significantly affected through variations in backbone bond geometry during atomic vibrations. Thus, disallowed (ϕ,ψ) regions at one timepoint can become allowed at another timepoint. We also suggested how high energy barriers in the (ϕ,ψ) space are potentially crossed during conformational transitions. Our analysis of the relationship between conformational strain in protein structures due to unfavorable (ϕ,ψ) angles and flexibility in local regions of proteins showed that they are only weakly related. This is likely due to the variable nature of allowed (ϕ,ψ) space itself, where adjustments to bond lengths and angles could lower the supposedly high-energy associated with an unfavorable (ϕ,ψ) conformation. With the emergence of an increasing number of cryoEM structures, we assessed their stereochemical quality and highlighted areas requiring improvement. We also showed that global resolution of cryo-EM structures is not a robust indicator of their quality. Our comparison of atomic packing in cryo-EM and crystal structures revealed that cryo-EM structures are less tightly packed than crystal structures, which is likely due to the nature of samples used in structure determination. We also suggest that the level of atomic packing seen in cryo-EM structures resembles the native state better. Overall, in this thesis, our studies on stereochemistry of peptides and proteins have generated a new framework for Ramachandran angle validation. We have also explored the implications of these studies on the flexibility and dynamics of proteins. Stereochemical studies on cryo-EM structures, which are predominantly multi-protein assemblies, have highlighted the red flags in these structures that their users should be aware of.

Macromolecular assemblies such as the ribosome, spliceosome, polymerases are imperative for cellular functions. The current understanding of these important machineries and many other assemblies at the molecular level is poor. The lack of structural data for many macromolecular assemblies further causes a bottleneck in understanding the cellular processes and the various disease manifestations. Hence, it is essential to characterize the structures and molecular architectures of these macromolecular assemblies. Though the number of 3-D structures for individual proteins structures or domains in the Protein Data Bank (PDB) is growing, the number of structures deposited for macromolecular assemblies is relatively poor. Hence, apart from the use of experimental techniques for characterizing macromolecular assembly structures, the use of computational techniques would help in supplementing the growth of macromolecular assembly structures. This thesis deals with the use of integrative approaches where computational methods are combined with experimental data to model and understand the mechanistic features of macromolecular assemblies with a special focus on a sub-complex of the spliceosome machinery. Chapter 1 of this thesis provides an introduction to protein-protein interactions and macromolecular assemblies. Further, the modelling of macromolecular assemblies using integrative methods are discussed, with a subsequent introduction to the spliceosome machinery. In chapter 2, modelling studies were performed on the proteins involved in the general amino acid control mechanism, which is triggered in yeast under amino acid starvation conditions. The proteins involved in the study were Gcn1, a ribosome binding protein and the RWD-domain containing proteins Gcn2, Yih1, Gir2 and Mtc5. From laboratory experiments it is known that in order for Gcn2 activation, an eIF2α kinase, its RWD-domain has to bind to Gcn1 and the residue Arg-2259 is important for this interaction. As the 3-D structure for the Gcn1 region containing Arg-2259 is not currently available, its 3-D structure was inferred using fold recognition and comparative modelling techniques. Further, in order to understand the Gcn2 RWD domain-Gcn1 molecular interaction, a complex structure was inferred by using a restrained protein-protein docking procedure. As the proteins, Yih1 and Gir2 are known to bind to Gcn1 using their RWD-domains, first the structures of the RWD-domain containing proteins including Mtc5 were inferred using a Gcn2 RWD domain NMR structure. Additionally, the Gcn1-Gcn2 complex was used to build a set of complexes to explain the binding of other RWD domain containing proteins Yih1, Gir2 and Mtc5. The important molecular interactions were obtained on analysing the interacting residues in these complexes. Thus, the Gcn1-Gcn2 interaction at the molecular level has been proposed for the first time. Future experiments guided by the protein-protein complex models and the proposed set of mutations should provide an understanding about the critical molecular interactions involved in the general amino acid control mechanism. Chapter 3 describes an integrative approach that was used to decipher a pseudo-atomic model of the closed form of human SF3b complex. SF3b is a multi-protein complex containing seven components – p14, SF3b49, SF3b155, SF3b145, SF3b130, SF3b14b and SF3b10. It recognizes the branch point adenosine in the pre-mRNA as part of U2 snRNP or U11/U12 di-snRNP in the spliceosome. Although, the cryo-EM map for human SF3b complex has been available for more than a decade, the structure and relative spatial arrangement of all components in the complex are not yet known. The integrative modelling approach used here involved utilizing structural data in the form of available X-ray and NMR structures, fold recognition and comparative modelling as well as currently available experimental datasets, along with the available cryo-EM density map to provide a model with high structural coverage. Hence, the molecular architecture of closed form human SF3b complex was derived that can now provide insights into the functioning of SF3b in splicing. This might also help the future high resolution structure determination efforts of the entire human spliceosome machinery In chapter 4, the molecular architecture of the closed form of SF3b complex obtained from the use of integrative modelling approach (Chapter 3) is extensively discussed. The structure-function relationships for some of the SF3b components based on the pseudo-atomic model has also been provided. In addition, the extreme flexibility associated with some of the SF3b components based on dynamics analysis has also been examined. Further, using an existing U11/U12 di-snRNP cryo-EM map and the closed form SF3b complex pseudo-atomic model, an open form of the SF3b complex was modelled and the component structures were fit into it. Hence, it was found that the transition between closed and open forms is primarily caused by a flap containing the HEAT repeat protein, SF3b155. This Protein is also known to harbour cancer causing mutations and has the potential to affect the Closed to open transition as well as SF3b complex structure and stability. Thus, this provides a framework for the future understanding of the closed to open transition in SF3b functioning within the spliceosome. Chapter 5 builds upon the integrative modelling approach (Chapter 3) that proposed the molecular architecture of the closed form of human SF3b complex and an open form of SF3b that was derived due to a flap opening of the closed form and which might help in accommodating RNA and other trans-acting factors within the U11/U12 di-snRNP (Chapter 4). In the current chapter, the SF3b open form and its interaction with the RNA elements is studied. The 5' end of U12 snRNA and its interaction with pre-mRNA in branch point duplex was modelled guided by the open form of SF3b that provided the necessary structural constraints and the RNA model is topologically consistent with the existing biochemical data. Further, utilizing the SF3b opens form-RNA model and the existing experimental knowledge, an extensive discussion has been provided on how the architecture of SF3b acts as a scaffold for U12 snRNA: pre-mRNA branch point duplex formation as well as its potential implications for branch point adenosine recognition fidelity. Moreover, the reasons for SF3b to be defined as a “fuzzy” complex - a complex with highly flexible folded regions along with intrinsically disordered regions is also discussed. Hence, the current work adds to the excellent developments made previously and deepens the understanding of the structure-function relationship of the human SF3b complex in the context of the spliceosome machinery. In chapter 6, a methodology has been proposed for the use of evolutionary conservation of protein-protein interfacial residues in multiple protein cryo-EM density based fitting of the protein components in the low-resolution density maps of multi-protein assemblies. First, the methodology was tested on a dataset of simulated density maps generated at four different resolutions -10, 15, 20 and 25 Å. On utilizing the evolutionary conservation scores obtained from multiple sequence alignments to score the fitted complexes, it was found that there was a decrease in the conservation scores when compared to that of the crystal structures, which were used to generate the simulated density maps. Further, the assessment of the multiple protein density fitting technique to align the actual protein-protein interface residues correctly using a performance metric called F-measure showed there was a decrease in performance as the resolutions became poorer. Hence, based on evolutionary conservations scores as well as F-measure the decrease in conservation scores or performance was found to be mainly due to the errors associated with the fitting process. Subsequently, a refinement methodology was designed involving the use of conservation scores, which improved the accuracy of the fitted models and the same, was observed in an experimental cryo-EM density test case of RyR1-FKBP12 complex. Hence, the conservation information acts as an effective filter to distinguish the incorrectly fitted structures and improves the accuracy of the fitting of the protein structures in the density maps. Thus, one can incorporate the conserved surface residues information in the current density fitting tools to reduce ambiguity and improve the accuracy of the macromolecular assembly structures determined using cryo-EM. In the concluding chapter 7, the learnings on the structural and mechanistic features of protein assemblies obtained from the use of computational techniques and integration of experimental datasets is discussed. In chapter 2, the modelling of a binary macromolecular complex such as the Gcn1-Gcn2 complex was performed using computational structure prediction strategies to understand the molecular basis of its interaction. Due to the potential inaccuracies which can exist in computational modelling, the chapters 3 to 5 dealt with the use of integrative approaches, primarily guided by the cryo-EM map, in order to decipher the molecular architecture of the human SF3b complex in the closed and open forms as well as its contribution for branch point adenosine recognition. Based on the extensive experience gained in modelling of assemblies using cryo-EM data in the previous chapters, a new method has been proposed on the use of evolutionary conservation information to improve the accuracy of cryo-EM density based fitting. Hence, these studies have provided strategies for modelling macromolecular assemblies as well as a deeper understanding of its mechanistic features.

Macromolecular assemblies such as the ribosome, spliceosome, polymerases are imperative for cellular functions. The current understanding of these important machineries and many other assemblies at the molecular level is poor. The lack of structural data for many macromolecular assemblies further causes a bottleneck in understanding the cellular processes and the various disease manifestations. Hence, it is essential to characterize the structures and molecular architectures of these macromolecular assemblies. Though the number of 3-D structures for individual proteins structures or domains in the Protein Data Bank (PDB) is growing, the number of structures deposited for macromolecular assemblies is relatively poor. Hence, apart from the use of experimental techniques for characterizing macromolecular assembly structures, the use of computational techniques would help in supplementing the growth of macromolecular assembly structures. This thesis deals with the use of integrative approaches where computational methods are combined with experimental data to model and understand the mechanistic features of macromolecular assemblies with a special focus on a sub-complex of the spliceosome machinery. Chapter 1 of this thesis provides an introduction to protein-protein interactions and macromolecular assemblies. Further, the modelling of macromolecular assemblies using integrative methods are discussed, with a subsequent introduction to the spliceosome machinery. In chapter 2, modelling studies were performed on the proteins involved in the general amino acid control mechanism, which is triggered in yeast under amino acid starvation conditions. The proteins involved in the study were Gcn1, a ribosome binding protein and the RWD-domain containing proteins Gcn2, Yih1, Gir2 and Mtc5. From laboratory experiments it is known that in order for Gcn2 activation, an eIF2α kinase, its RWD-domain has to bind to Gcn1 and the residue Arg-2259 is important for this interaction. As the 3-D structure for the Gcn1 region containing Arg-2259 is not currently available, its 3-D structure was inferred using fold recognition and comparative modelling techniques. Further, in order to understand the Gcn2 RWD domain-Gcn1 molecular interaction, a complex structure was inferred by using a restrained protein-protein docking procedure. As the proteins, Yih1 and Gir2 are known to bind to Gcn1 using their RWD-domains, first the structures of the RWD-domain containing proteins including Mtc5 were inferred using a Gcn2 RWD domain NMR structure. Additionally, the Gcn1-Gcn2 complex was used to build a set of complexes to explain the binding of other RWD domain containing proteins Yih1, Gir2 and Mtc5. The important molecular interactions were obtained on analysing the interacting residues in these complexes. Thus, the Gcn1-Gcn2 interaction at the molecular level has been proposed for the first time. Future experiments guided by the protein-protein complex models and the proposed set of mutations should provide an understanding about the critical molecular interactions involved in the general amino acid control mechanism. Chapter 3 describes an integrative approach that was used to decipher a pseudo-atomic model of the closed form of human SF3b complex. SF3b is a multi-protein complex containing seven components – p14, SF3b49, SF3b155, SF3b145, SF3b130, SF3b14b and SF3b10. It recognizes the branch point adenosine in the pre-mRNA as part of U2 snRNP or U11/U12 di-snRNP in the spliceosome. Although, the cryo-EM map for human SF3b complex has been available for more than a decade, the structure and relative spatial arrangement of all components in the complex are not yet known. The integrative modelling approach used here involved utilizing structural data in the form of available X-ray and NMR structures, fold recognition and comparative modelling as well as currently available experimental datasets, along with the available cryo-EM density map to provide a model with high structural coverage. Hence, the molecular architecture of closed form human SF3b complex was derived that can now provide insights into the functioning of SF3b in splicing. This might also help the future high resolution structure determination efforts of the entire human spliceosome machinery In chapter 4, the molecular architecture of the closed form of SF3b complex obtained from the use of integrative modelling approach (Chapter 3) is extensively discussed. The structure-function relationships for some of the SF3b components based on the pseudo-atomic model has also been provided. In addition, the extreme flexibility associated with some of the SF3b components based on dynamics analysis has also been examined. Further, using an existing U11/U12 di-snRNP cryo-EM map and the closed form SF3b complex pseudo-atomic model, an open form of the SF3b complex was modelled and the component structures were fit into it. Hence, it was found that the transition between closed and open forms is primarily caused by a flap containing the HEAT repeat protein, SF3b155. This Protein is also known to harbour cancer causing mutations and has the potential to affect the Closed to open transition as well as SF3b complex structure and stability. Thus, this provides a framework for the future understanding of the closed to open transition in SF3b functioning within the spliceosome. Chapter 5 builds upon the integrative modelling approach (Chapter 3) that proposed the molecular architecture of the closed form of human SF3b complex and an open form of SF3b that was derived due to a flap opening of the closed form and which might help in accommodating RNA and other trans-acting factors within the U11/U12 di-snRNP (Chapter 4). In the current chapter, the SF3b open form and its interaction with the RNA elements is studied. The 5' end of U12 snRNA and its interaction with pre-mRNA in branch point duplex was modelled guided by the open form of SF3b that provided the necessary structural constraints and the RNA model is topologically consistent with the existing biochemical data. Further, utilizing the SF3b opens form-RNA model and the existing experimental knowledge, an extensive discussion has been provided on how the architecture of SF3b acts as a scaffold for U12 snRNA: pre-mRNA branch point duplex formation as well as its potential implications for branch point adenosine recognition fidelity. Moreover, the reasons for SF3b to be defined as a “fuzzy” complex - a complex with highly flexible folded regions along with intrinsically disordered regions is also discussed. Hence, the current work adds to the excellent developments made previously and deepens the understanding of the structure-function relationship of the human SF3b complex in the context of the spliceosome machinery. In chapter 6, a methodology has been proposed for the use of evolutionary conservation of protein-protein interfacial residues in multiple protein cryo-EM density based fitting of the protein components in the low-resolution density maps of multi-protein assemblies. First, the methodology was tested on a dataset of simulated density maps generated at four different resolutions -10, 15, 20 and 25 Å. On utilizing the evolutionary conservation scores obtained from multiple sequence alignments to score the fitted complexes, it was found that there was a decrease in the conservation scores when compared to that of the crystal structures, which were used to generate the simulated density maps. Further, the assessment of the multiple protein density fitting technique to align the actual protein-protein interface residues correctly using a performance metric called F-measure showed there was a decrease in performance as the resolutions became poorer. Hence, based on evolutionary conservations scores as well as F-measure the decrease in conservation scores or performance was found to be mainly due to the errors associated with the fitting process. Subsequently, a refinement methodology was designed involving the use of conservation scores, which improved the accuracy of the fitted models and the same, was observed in an experimental cryo-EM density test case of RyR1-FKBP12 complex. Hence, the conservation information acts as an effective filter to distinguish the incorrectly fitted structures and improves the accuracy of the fitting of the protein structures in the density maps. Thus, one can incorporate the conserved surface residues information in the current density fitting tools to reduce ambiguity and improve the accuracy of the macromolecular assembly structures determined using cryo-EM. In the concluding chapter 7, the learnings on the structural and mechanistic features of protein assemblies obtained from the use of computational techniques and integration of experimental datasets is discussed. In chapter 2, the modelling of a binary macromolecular complex such as the Gcn1-Gcn2 complex was performed using computational structure prediction strategies to understand the molecular basis of its interaction. Due to the potential inaccuracies which can exist in computational modelling, the chapters 3 to 5 dealt with the use of integrative approaches, primarily guided by the cryo-EM map, in order to decipher the molecular architecture of the human SF3b complex in the closed and open forms as well as its contribution for branch point adenosine recognition. Based on the extensive experience gained in modelling of assemblies using cryo-EM data in the previous chapters, a new method has been proposed on the use of evolutionary conservation information to improve the accuracy of cryo-EM density based fitting. Hence, these studies have provided strategies for modelling macromolecular assemblies as well as a deeper understanding of its mechanistic features.

Vacuolar-type ATPases (V-ATPases) are ubiquitous membrane-bound protein complexes present in the endo-membrane system of all eukaryotic cells. In eukaryotic cells, the reversible dissociation of the V1 and Vo regions is an essential mechanism for regulating V-ATPase activity. Therefore, knowledge of the structure of the dissociated V1-ATPase is necessary for understanding the regulation of V-ATPase activity. In this thesis, I showed that by introducing a 3xFLAG tag at the C terminus of different V1-ATPase subunits, highly purified V1-ATPase complex could be isolated. Electron cryomicroscopy (cryo-EM) was used for initial analysis of the intact V1-ATPase. In addition to the intact complex, partial V1-ATPase subcomplexes with different subunit compositions were isolated from yeast cells in late log phase. All of the isolated subcomplexes were found to contain the major V1-ATPase subunits A and B, but differed in the peripheral stalk subunit composition.

Flaviviruses are enveloped, positive-strand RNA viruses that are spread by mosquitoes and ticks and can cause serious disease in humans. Flavivirus virions undergo extensive structural changes during their life cycle, including during maturation and fusion. Flaviviruses are initially assembled at the endoplasmic reticulum in a non-infectious, immature state, and then traffic to the trans-Golgi network, where a pH drop triggers a structural rearrangement of glycoproteins prM and E on the virus surface from 60 trimers to 90 dimers. A host protease, furin, then cleaves prM which makes the transition irreversible. Upon exiting the host cell, pr disassociates from the virus and the infectious, mature virus is able to enter a new cell.

In Chapter 1, an overview of flaviviruses is presented, including a brief history of their discovery and interaction with humans, followed by what is known about their life cycle and the maturation process. The structure of a mature flavivirus is then described, including the symmetrical arrangement of glycoproteins on the virion surface, the lipid membrane, and the nucleocapsid core, followed by an introduction of the structural proteins that assemble into the virion. The structure of the immature flavivirus is then described. The chapter concludes with a description of the dynamics and heterogeneity observed for flaviviruses.

The conformational rearrangements that occur during flavivirus maturation remain unclear. The structures of immature and mature flaviviruses determined with cryo-electron microscopy (cryo-EM) demonstrated that flaviviruses are icosahedral particles with 180 copies of glycoproteins on their surface. Icosahedral viruses typically have a quasi-equivalent arrangement of glycoproteins, but flaviviruses lack quasi-equivalence and instead the three subunits within an asymmetric unit occupy different chemical environments. Although the subunits are the same proteins, the unique environment of each subunit can be exploited for tracking subunits during conformational rearrangements. For example, the unique labeling of a subunit can be used to identify it in the immature and mature virion.

In Chapter 2, the maturation process was studied by developing tools to differentially label protein subunits and trap potential intermediates of maturation. The tools included heavy-atom compounds and antibody Fabs, which were used to probe Kunjin virus (KUNV), an Australian subtype of West Nile virus (WNV). One heavy-atom compound, potassium tetranitroplatinate(II), was found to derivatize immature KUNV, likely at sites on both E and prM. Higher-resolution studies will be required to determine if the compound differentially labeled the three subunits. The other tool developed was the E16 Fab. E16 Fab, originally isolated from a mouse immunized with WNV E and found to bind to two out of three subunits on mature WNV, was used to differentially label subunits in immature KUNV. Based on poor epitope accessibility on immature KUNV, E16 Fab was hypothesized to trap an intermediate state of maturation. In the cryo-EM reconstruction of E16 Fab bound to immature KUNV it was found that the virion had localized distorted density and apparent non-uniform binding of the E16 Fab. Based on this result it was proposed that flaviviruses had imperfect icosahedral symmetry.

The structural asymmetry of immature and mature flaviviruses was investigated in Chapter 3. Icosahedral symmetry has always been imposed during cryo-EM reconstructions of flaviviruses, as it led to stable convergence of orientations. When reconstructions of immature KUNV and ZIKV were performed without imposing symmetry, the reconstructions showed that the flaviviruses had an eccentric nucleocapsid core, which was positioned closer to the membrane at one pole. At the opposite pole, the glycoprotein and inner leaflet densities were weak and distorted. Furthermore, there were protrusions from the core that contacted the transmembrane helices of the glycoproteins. In the asymmetric reconstruction of mature KUNV, the core was positioned concentric with the glycoprotein shell, in contrast to the immature virion, indicating that maturation alters the interactions between the core and the glycoproteins. The asymmetric reconstructions suggested that there is variable contact between the core and glycoproteins during assembly, which may be due to membrane curvature restrictions in the budding process.

In Chapter 4, extracellular vesicles (EVs) that were released during dengue virus (DENV) infection were characterized by mass spectrometry. EVs may play a significant role in flavivirus infection, as they have been shown to transport both viral proteins and infectious RNA. EVs likely represent alternative modes of virus transmission and aid in immune evasion. However, previous studies on EVs are controversial because EVs are potential contaminated during assays by co-purifying virions and other particulates. The identification of EV biomarkers would greatly reduce contamination because biomarkers would enable isolation of pure EVs by affinity purification. Therefore, a strategy was developed to isolate EVs and profile them with proteomics. The four proteins cystatin-A, filamin B, fibrinogen beta chain, and endothelin converting enzyme 1 were found to be statistically enriched in the DENV sample and represent potential EV biomarkers.

Legionella pneumophila (Lp) is a gram-negative, intracellular parasite. These bacteria evade the host response with the help of a Type IVb secretion system (T4bSS), composed of Defective in organelle trafficking (Dot) and Intracellular multiplication (Icm) proteins. This secretion system delivers over 300 effectors into the host, and a large number of these molecules are dependent on IcmS and IcmW. These effectors are essential for the bacterium’s survival in the host. This work and previous studies have shown that IcmS and IcmW interact to stabilize each other and the C-terminal "tail" of DotL (residues 655-783), a coupling protein in the T4bSS, binds to the IcmSW complex to further increase its stability. All three components are α-helical, making the complex amenable to structural studies by X-ray crystallography and cryo-electron microscopy. Three maps of the IcmSW-DotL-tail complex (~42 kDa) were generated from cryo-EM images recorded with a Volta phase plate and K2 Summit direct electron detector at 500-1000 nm under-defocus. The final maps were processed with RELION-2 and resolved to 5.5-6.5 Å resolution using 57k, 60k and 80k particles, respectively. Concurrent with work in this thesis, a crystal structure of IcmSW-DotL(656-783) was solved by Dr. Byung-Ha Oh’s group at KAIST. This structure was used as a comparative model for our cryo-EM 3D reconstructions which were determined to evaluate size limits imposed on single particle methods with current technology and to provide snapshots of the complex in solution. Comparisons between the crystal structure and cryo-EM maps show that the overall structure is similar in solution, but there is significant flexibility within each subunit with a repositioning of some α-helices and surface loops. Flexibility in the absence of a central subunit (LvgA), and a low number of good particles may have limited the final resolution. Although the current maps were determined at α-helical resolution, this work provides a road map for solving near atomic structures at or near the size of IcmSW-DotL-tail. This structural technology will provide a means to probe the solution structure and function of biological machines in a large range of sizes and conformations.

PdxR is a bacterial transcription factor belonging to the MocR family, which is involved in the regulation of the de novo biosynthesis of pyridoxal 5’-phosphate (PLP). PdxR has been extensively studied at the functional level and its target regulon has been characterized. However, a structural characterization of this transcription factor, and of MocRs in general, is missing. In fact, to date no structure of MocRs in complex with DNA is available, and a mechanism of their regulatory activity has been hypothesised on the basis of functional and computational data. In this thesis I report the structure of PdxR from Bacillus clausii in complex with its target DNA sequence of 48-bp, determined by cryo-electron microscopy (cryo-EM). Our data revealed that in PdxR the DNA-binding winged helix-turn-helix domain (wHTH) and the aspartate aminotransferase (AAT)-like domain are arranged in a domain-swap homodimeric assembly and are connected by a long flexible linker. Single particle analysis allowed us to isolate two different conformational states of the PdxR-DNA complex, an “open” and a “closed” one, differently bound to the target oligonucleotide, and to identify residues on the protein and bases on the DNA involved in sequence recognition and complex formation. Moreover, we analysed the role of both the DNA sequence and conformation in the specific recognition by PdxR, evaluating the complex stability using rationally designed DNA sequences. Our results provide relevant information on the structure and the dynamics of PdxR-DNA complex and represent a fundamental step to clarify the mechanism governing the DNA-binding mode and the transcriptional regulation of MocRs transcription factors.