Academic literature on the topic 'Viral fusion glycoproteins'

Enveloped viruses encode specialised glycoproteins that mediate fusion of viral and host membranes. Discovery and understanding of the molecular mechanisms of fusion have been achieved through structural analyses of glycoproteins from many different viruses, and yet the fusion mechanisms of some viral genera remain unknown. We have employed systematic genome annotation and AlphaFold modelling to predict the structures of the E1E2 glycoproteins from 60 viral species in the Hepacivirus, Pegivirus, and Pestivirus genera. While the predicted structure of E2 varied widely, E1 exhibited a very consistent fold across genera, despite little or no similarity at the sequence level. Critically, the structure of E1 is unlike any other known viral glycoprotein. This suggests that the Hepaci-, Pegi-, and Pestiviruses may possess a common and novel membrane fusion mechanism. Comparison of E1E2 models from various species reveals recurrent features that are likely to be mechanistically important and sheds light on the evolution of membrane fusion in these viral genera. These findings provide new fundamental understanding of viral membrane fusion and are relevant to structure-guided vaccinology.

Protein interactions play key roles throughout all subcellular compartments. In the present paper, we report the visualization of protein interactions throughout living mammalian cells using two oligomerizing MV (measles virus) transmembrane glycoproteins, the H (haemagglutinin) and the F (fusion) glycoproteins, which mediate MV entry into permissive cells. BiFC (bimolecular fluorescence complementation) has been used to examine the dimerization of these viral glycoproteins. The H glycoprotein is a type II membrane-receptor-binding homodimeric glycoprotein and the F glycoprotein is a type I disulfide-linked membrane glycoprotein which homotrimerizes. Together they co-operate to allow the enveloped virus to enter a cell by fusing the viral and cellular membranes. We generated a pair of chimaeric H glycoproteins linked to complementary fragments of EGFP (enhanced green fluorescent protein) – haptoEGFPs – which, on association, generate fluorescence. Homodimerization of H glycoproteins specifically drives this association, leading to the generation of a fluorescent signal in the ER (endoplasmic reticulum), the Golgi and at the plasma membrane. Similarly, the generation of a pair of corresponding F glycoprotein–haptoEGFP chimaeras also produced a comparable fluorescent signal. Co-expression of H and F glycoprotein chimaeras linked to complementary haptoEGFPs led to the formation of fluorescent fusion complexes at the cell surface which retained their biological activity as evidenced by cell-to-cell fusion.

The entry of enveloped viruses into cells requires the fusion of viral and cellular membranes, driven by conformational changes in viral glycoproteins. Many studies have shown that fusion involves the cooperative action of a large number of these glycoproteins, but the underlying mechanisms are unknown. We used electron microscopy and tomography to study the low pH–induced fusion reaction catalyzed by vesicular stomatitis virus glycoprotein (G). Pre- and post-fusion crystal structures were observed on virions at high and low pH, respectively. Individual fusion events with liposomes were also visualized. Fusion appears to be driven by two successive structural rearrangements of G at different sites on the virion. Fusion is initiated at the flat base of the particle. Glycoproteins located outside the contact zone between virions and liposomes then reorganize into regular arrays. We suggest that the formation of these arrays, which have been shown to be an intrinsic property of the G ectodomain, induces membrane constraints, achieving the fusion reaction.

Viral entry is the first stage in the virus replication cycle and, for enveloped viruses, is mediated by virally encoded glycoproteins. Viral glycoproteins have different receptor affinities and triggering mechanisms. We employed vesicular stomatitis virus (VSV), a BSL-2 enveloped virus that can incorporate non-native glycoproteins, to examine the entry efficiencies of diverse viral glycoproteins. To compare the glycoprotein-mediated entry efficiencies of VSV glycoprotein (G), Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S), Ebola (EBOV) glycoprotein (GP), Lassa (LASV) GP, and Chikungunya (CHIKV) envelope (E) protein, we produced recombinant VSV (rVSV) viruses that produce the five glycoproteins. The rVSV virions encoded a nano luciferase (NLucP) reporter gene fused to a destabilization domain (PEST), which we used in combination with the live-cell substrate EndurazineTM to monitor viral entry kinetics in real time. Our data indicate that rVSV particles with glycoproteins that require more post-internalization priming typically demonstrate delayed entry in comparison to VSV G. In addition to determining the time required for each virus to complete entry, we also used our system to evaluate viral cell surface receptor preferences, monitor fusion, and elucidate endocytosis mechanisms. This system can be rapidly employed to examine diverse viral glycoproteins and their entry requirements.

Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) are prototypical alphaherpesviruses that are characterized by their unique properties to infect trigeminal and dorsal root ganglionic neurons, respectively, and establish life-long latent infections. These viruses initially infect mucosal epithelial tissues and subsequently spread to neurons. They are associated with a significant disease spectrum, including orofacial and ocular infections for HSV-1 and genital and neonatal infections for HSV-2. Viral glycoproteins within the virion envelope bind to specific cellular receptors to mediate virus entry into cells. This is achieved by the fusion of the viral envelope with the plasma membrane. Similarly, viral glycoproteins expressed on cell surfaces mediate cell-to-cell fusion and facilitate virus spread. An interactive complex of viral glycoproteins gB, gD/gH/gL, and gK and other proteins mediate these membrane fusion phenomena with glycoprotein B (gB), the principal membrane fusogen. The requirement for the virion to enter neuronal axons suggests that the heterodimeric protein complex of gK and membrane protein UL20, found only in alphaherpesviruses, constitute a critical determinant for neuronal entry. This hypothesis was substantiated by the observation that a small deletion in the amino terminus of gK prevents entry into neuronal axons while allowing entry into other cells via endocytosis. Cellular receptors and receptor-mediated signaling synergize with the viral membrane fusion machinery to facilitate virus entry and intercellular spread. Unraveling the underlying interactions among viral glycoproteins, envelope proteins, and cellular receptors will provide new innovative approaches for antiviral therapy against herpesviruses and other neurotropic viruses.

Many enveloped viruses enter host cells by fusing with acidic endosomes. The fusion activity of multiple viral envelope glycoproteins does not generally affect viral membrane permeability. However, fusion induced by the Lassa virus (LASV) glycoprotein complex (GPc) is always preceded by an increase in viral membrane permeability and the ensuing acidification of the virion interior. Here, systematic investigation of this LASV fusion phenotype using single pseudovirus tracking in live cells reveals that the change in membrane barrier function is associated with the fusogenic conformational reorganization of GPc. We show that a small-molecule fusion inhibitor or mutations that impair viral fusion by interfering with GPc refolding into the post-fusion structure prevent the increase in membrane permeability. We find that the increase in virion membrane permeability occurs early during endosomal maturation and is facilitated by virus-cell contact. This increase is observed using diverse arenavirus glycoproteins, whether presented on lentivirus-based pseudoviruses or arenavirus-like particles, and in multiple different cell types. Collectively, these results suggest that conformational changes in GPc triggered by low pH and cell factor binding are responsible for virion membrane permeabilization and acidification of the virion core prior to fusion. We propose that this viroporin-like activity may augment viral fusion and/or post-fusion steps of infection, including ribonucleoprotein release into the cytoplasm.

ABSTRACT Membrane fusion induced by enveloped viruses proceeds through the actions of viral fusion proteins. Once activated, viral fusion proteins undergo large protein conformational changes to execute membrane fusion. Fusion is thought to proceed through a “hemifusion” intermediate in which the outer membrane leaflets of target and viral membranes mix (lipid mixing) prior to fusion pore formation, enlargement, and completion of fusion. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins—glycoprotein D (gD), glycoprotein B (gB), and a heterodimer of glycoprotein H and L (gH/gL)—to accomplish fusion. gD is primarily thought of as a receptor-binding protein and gB as a fusion protein. The role of gH/gL in fusion has remained enigmatic. Despite experimental evidence that gH/gL may be a fusion protein capable of inducing hemifusion in the absence of gB, the recently solved crystal structure of HSV-2 gH/gL has no structural homology to any known viral fusion protein. We found that in our hands, all HSV entry proteins—gD, gB, and gH/gL—were required to observe lipid mixing in both cell-cell- and virus-cell-based hemifusion assays. To verify that our hemifusion assay was capable of detecting hemifusion, we used glycosylphosphatidylinositol (GPI)-linked hemagglutinin (HA), a variant of the influenza virus fusion protein, HA, known to stall the fusion process before productive fusion pores are formed. Additionally, we found that a mutant carrying an insertion within the short gH cytoplasmic tail, 824L gH, is incapable of executing hemifusion despite normal cell surface expression. Collectively, our findings suggest that HSV gH/gL may not function as a fusion protein and that all HSV entry glycoproteins are required for both hemifusion and fusion. The previously described gH 824L mutation blocks gH/gL function prior to HSV-induced lipid mixing.

ABSTRACT The entry process of the avian sarcoma and leukosis virus (ASLV) family of retroviruses requires first a specific interaction between the viral surface (SU) glycoproteins and a receptor on the cell surface at a neutral pH, triggering conformational changes in the viral SU and transmembrane (TM) glycoproteins, followed by exposure to low pH to complete fusion. The ASLV TM glycoprotein has been proposed to adopt a structure similar to that of the Ebola virus GP2 protein: each contains an internal fusion peptide flanked by cysteine residues predicted to be in a disulfide bond. In a previous study, we concluded that the cysteines flanking the internal fusion peptide in ASLV TM are critical for efficient function of the ASLV viral glycoproteins in mediating entry. In this study, replication-competent ASLV mutant subgroup A [ASLV(A)] variants with these cysteine residues mutated were constructed and genetically selected for improved replication capacity in chicken fibroblasts. Viruses with single cysteine-to-serine mutations reverted to the wild-type sequence. However, viruses with both C9S and C45S (C9,45S) mutations retained both mutations and acquired a second-site mutation that significantly improved the infectivity of the genetically selected virus population. A charged-amino-acid second-site substitution in the TM internal fusion peptide at position 30 is preferred to rescue the C9,45S mutant ASLV(A). ASLV(A) envelope glycoproteins that contain the C9,45S and G30R mutations bind the Tva receptor at wild-type levels and have improved abilities to trigger conformational changes and to form stable TM oligomers compared to those of the C9,45S mutant glycoprotein.

ABSTRACT The envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) function as a homotrimer of gp120/gp41 heterodimers to support virus entry. During the process of virus entry, an individual HIV-1 envelope glycoprotein trimer binds the cellular receptors CD4 and CCR5/CXCR4 and mediates the fusion of the viral and the target cellular membranes. By studying the function of heterotrimers between wild-type and nonfunctional mutant envelope glycoproteins, we found that two wild-type subunits within an envelope glycoprotein trimer are required to support virus entry. Complementation between HIV-1 envelope glycoprotein mutants defective in different functions to allow virus entry was not evident. These results assist our understanding of the mechanisms whereby the HIV-1 envelope glycoproteins mediate virus entry and membrane fusion and guide attempts to inhibit these processes.

ABSTRACT Human cytomegalovirus (CMV) infection is dependent on the functions of structural glycoproteins at multiple stages of the viral life cycle. These proteins mediate the initial attachment and fusion events that occur between the viral envelope and a host cell membrane, as well as virion-independent cell-cell spread of the infection. Here we have utilized a cell-based fusion assay to identify the fusogenic glycoproteins of CMV. To deliver the glycoprotein genes to various cell lines, we constructed recombinant retroviruses encoding gB, gH, gL, and gO. Cells expressing individual CMV glycoproteins did not form multinucleated syncytia. Conversely, cells expressing gH/gL showed pronounced syncytium formation, although expression of gH or gL alone had no effect. Anti-gH neutralizing antibodies prevented syncytium formation. Coexpression of gB and/or gO with gH/gL did not yield detectably increased numbers of syncytia. For verification, these results were recapitulated in several cell lines. Additionally, we found that fusion was cell line dependent, as nonimmortalized fibroblast strains did not fuse under any conditions. Thus, the CMV gH/gL complex has inherent fusogenic activity that can be measured in certain cell lines; however, fusion in fibroblast strains may involve a more complex mechanism involving additional viral and/or cellular factors.

Les glycoprotéines virales sont impliquées dans les deux principales étapes d’entrée des virus enveloppés dans leurs cellules hôtes : l’attachement des virus aux récepteurs cellulaires et la fusion des membranes virale et cellulaire. Je me suis d’abord attachée à l’étude structurale de la principale glycoprotéine, E2, de deux hépacivirus : la forme B du virus GB (GBV-B) et le virus de l’hépatite C (HCV). Mes tentatives de cristallisation de l’ectodomaine de la protéine E2 du GBV-B sont restées vaines, mais l’analyse des propriétés de ses fragments a suggéré un rôle de son extrémité C-terminale dans la liaison à son récepteur. En parallèle, j’ai co-cristallisé un peptide synthétique correspondant à la principale boucle de liaison de E2 à son récepteur, avec un fragment d’anticorps dirigé contre cette boucle. Etonnament, le peptide forme une hélice , en nette contradiction avec la conformation étendue adoptée dans un fragment du cœur de E2. Associé à des données biochimiques, cela suggère une flexibilité inattendue de cette région de l’ectodomaine d’E2. Dans un second temps, je me suis intéressée à la glycoprotéine F des baculovirus. J’ai résolu la structure du trimère d’un fragment tryptique de F dans sa conformation post-fusion. Cette structure a validé une prédiction selon laquelle la protéine F était une protéine de fusion de classe I homologue à celle des paramyxovirus. La protéine F des baculovirus est ainsi le premier exemple d’une protéine de fusion de classe I encodée par un virus à ADN. Mes résultats confortent donc l’hypothèse que toutes les protéines F ont un ancêtre commun et suggèrent un lien évolutif intéressant entre les virus à ADN, à ARN et leurs hôtes
Viral glycoproteins are responsible for the two major steps in entry into host cells by enveloped viruses: 1) attachment to cellular receptor/s and 2) fusion of the viral and cellular membranes. My thesis concentrated first on the structural analysis of the major envelope glycoprotein E2 of two hepaciviruses: GB virus B (GBV-B) and hepatitis C virus (HCV). Crystallization of the GBV-B E2 ectodomain remained unsuccessful, but the characterization of truncated versions of E2 suggested an important role of its C-terminal moiety in receptor binding. In parallel, I co-crystallized a synthetic peptide mimicking HCV E2 with an antibody fragment directed against the major receptor-binding loop of E2 that is targeted by broadly neutralizing antibodies. The structure unexpectedly revealed an α-helical peptide conformation, which is in stark contrast to the extended conformation of this region observed in the structure of an E2 core fragment. Together with further biochemical evidence this suggests an unanticipated structural flexibility within this region in the context of the soluble E2 ectodomain. Secondly, I focused on the structural analysis of the baculovirus glycoprotein F. I determined the crystal structure of the post-fusion trimer of a trypsin-truncated F fragment. This structure confirmed previous predictions that baculovirus F protein adopts a class I fusion protein fold and is homologous to the paramyxovirus F protein. Baculovirus F is therefore the first class I fusion protein encoded by a DNA virus. My results support the hypothesis that F proteins may have a common ancestor and imply interesting evolutionary links between DNA and RNA viruses and their hosts

Les glycoprotéines virales sont impliquées dans les deux principales étapes d’entrée des virus enveloppés dans leurs cellules hôtes : l’attachement des virus aux récepteurs cellulaires et la fusion des membranes virale et cellulaire. Je me suis d’abord attachée à l’étude structurale de la principale glycoprotéine, E2, de deux hépacivirus : la forme B du virus GB (GBV-B) et le virus de l’hépatite C (HCV). Mes tentatives de cristallisation de l’ectodomaine de la protéine E2 du GBV-B sont restées vaines, mais l’analyse des propriétés de ses fragments a suggéré un rôle de son extrémité C-terminale dans la liaison à son récepteur. En parallèle, j’ai co-cristallisé un peptide synthétique correspondant à la principale boucle de liaison de E2 à son récepteur, avec un fragment d’anticorps dirigé contre cette boucle. Etonnament, le peptide forme une hélice , en nette contradiction avec la conformation étendue adoptée dans un fragment du cœur de E2. Associé à des données biochimiques, cela suggère une flexibilité inattendue de cette région de l’ectodomaine d’E2. Dans un second temps, je me suis intéressée à la glycoprotéine F des baculovirus. J’ai résolu la structure du trimère d’un fragment tryptique de F dans sa conformation post-fusion. Cette structure a validé une prédiction selon laquelle la protéine F était une protéine de fusion de classe I homologue à celle des paramyxovirus. La protéine F des baculovirus est ainsi le premier exemple d’une protéine de fusion de classe I encodée par un virus à ADN. Mes résultats confortent donc l’hypothèse que toutes les protéines F ont un ancêtre commun et suggèrent un lien évolutif intéressant entre les virus à ADN, à ARN et leurs hôtes
Viral glycoproteins are responsible for the two major steps in entry into host cells by enveloped viruses: 1) attachment to cellular receptor/s and 2) fusion of the viral and cellular membranes. My thesis concentrated first on the structural analysis of the major envelope glycoprotein E2 of two hepaciviruses: GB virus B (GBV-B) and hepatitis C virus (HCV). Crystallization of the GBV-B E2 ectodomain remained unsuccessful, but the characterization of truncated versions of E2 suggested an important role of its C-terminal moiety in receptor binding. In parallel, I co-crystallized a synthetic peptide mimicking HCV E2 with an antibody fragment directed against the major receptor-binding loop of E2 that is targeted by broadly neutralizing antibodies. The structure unexpectedly revealed an α-helical peptide conformation, which is in stark contrast to the extended conformation of this region observed in the structure of an E2 core fragment. Together with further biochemical evidence this suggests an unanticipated structural flexibility within this region in the context of the soluble E2 ectodomain. Secondly, I focused on the structural analysis of the baculovirus glycoprotein F. I determined the crystal structure of the post-fusion trimer of a trypsin-truncated F fragment. This structure confirmed previous predictions that baculovirus F protein adopts a class I fusion protein fold and is homologous to the paramyxovirus F protein. Baculovirus F is therefore the first class I fusion protein encoded by a DNA virus. My results support the hypothesis that F proteins may have a common ancestor and imply interesting evolutionary links between DNA and RNA viruses and their hosts

Thesis (Ph.D. in Microbiology) -- University of Colorado Denver, 2008.
Typescript. Includes bibliographical references (leaves 161-183). Free to UCD Anschutz Medical Campus. Online version available via ProQuest Digital Dissertations;

The first step of viral infection requires the binding of the viral attachment protein to cell surface receptors. Following binding, viruses penetrate the cellular membrane to deliver their genome into the host cell. For enveloped viruses, which have a lipid bilayer that surrounds their nucleocapsids, entry into the host cell requires the fusion of viral and cellular membranes. This process is mediated by viral glycoproteins located on the surface of the virus. For many enveloped viruses, such as influenza, Ebola, and human immunodeficiency virus, the fusion protein is responsible for mediating both attachment to cellular receptors and membrane fusion. However, paramyxoviruses are unique among fusion promoting viruses because their receptor binding and fusion activities reside on two separate proteins. This unique distribution of functions necessitates a mechanism by which the two proteins can transmit the juxtaposition of the viral and host cell membranes, mediated by the attachment protein (HN/H), into membrane fusion, mediated by the fusion (F) protein. This mechanism allows for paramyxoviruses to gain entry into and spread between cells, and therefore, is an important aspect of virus infection and disease progression. Despite the conservation of receptor binding activity among members of the Paramyxovirinaesubfamily, for most of these viruses, including Newcastle disease virus (NDV), heterologous HN proteins cannot complement F in the promotion of fusion; both the HN and F proteins must originate from the same virus. This is consistent with the existence of a virus-specific interaction between the two glycoproteins. Thus, one or more domains on the HN and F proteins is thought to mediate a specific interaction between them that is an integral part of the fusion process. Therefore, the primary focus of this thesis is the identification of the site(s) on HN that directly contacts F in the HN-F interaction. The ectodomain of the HN protein consists of a stalk and a terminal globular head. Analysis of the fusion activity of chimeric paramyxovirus HN proteins indicates that the stalk region of HN determines its F protein specificity. The first goal of this research was to address the question of whether the stalk not only determines F-specificity, but does so by directly mediating the interaction with F. To establish a correlation between the amount of fusion and the extent of the HN-F interaction, a specific and quantitative co-immunoprecipitation assay was used that detects the HN-F complex at the cell surface. As an initial probe of the role of the HN stalk in mediating the interaction with F, N-glycans were individually added at several positions in the region. N-glycan addition at positions 69 and 77 in the stalk specifically and completely block both fusion and the HN-F interaction without affecting either HN structure or its other activities. However, though they also prevent fusion, N-glycans added at other positions in the stalk also modulate activities that reside in the globular head of HN. This correlates with an alteration of the tetrameric structure of the protein as indicated by sucrose gradient sedimentation analyses. These additional N-glycans likely indirectly affect fusion, perhaps by interfering with changes in the conformation of HN that link receptor binding to the fusion activation of F. To address the issue of whether N-glycan addition at any position in HN would abolish fusion, an N-glycan was added in another region at the base of the globular head of HN (residues 124-152), which was previously predicted by a peptide-based analysis to mediate the interaction with F. HN carrying this additional N-glycan exhibits significant fusion promoting activity, arguing against this site being part of the F-interactive domain in HN. These data support the idea that the F-interactive site on HN is defined by the stalk region of the protein. Site-directed mutagenesis was used to begin to explore the role of individual residues in the stalk in the interaction with F. The characteristics of the F-interactive domain in the stalk of HN are that it is a conserved motif with enough sequence heterogeneity to account for the specificity of the interaction. One such region that meets these requirements is the intervening region (IR) (residues 89-95); a non-helical domain situated between two conserved heptad repeats. Several amino acid substitutions for a completely conserved proline residue in this region impair not only fusion and the HN-F interaction, but also decrease neuraminidase activity in the globular domain and alter the structure of the protein, suggesting that the substitutions indirectly affect the HN-F interaction. Substitutions for L94 also interfere with fusion, but have no significant effect on any other HN function or its structure. Amino acid substitutions at two other positions in the IR (A89 and L90) also modulate only fusion. In all cases, diminished fusion correlates with a decreased ability of the mutated HN protein to interact with F at the cell surface. These findings indicate that the IR is critical to the role of HN in the promotion of fusion and are consistent with its direct involvement in the interaction with the homologous F protein. These are the first point mutations in the HN protein for which a correlation has been demonstrated between the extent of the HN-F interaction and the amount of fusion. This argues strongly that the co-IP assay is an accurate reflection of the HN-F interaction. The second goal of this research was to address the HN-F interaction from the perspective of the F protein by investigating the relationship between receptor binding, the HN-F interaction, and fusion using a highly fusogenic form of the F protein. It has previously been shown that an L289A substitution in NDV F eliminates the requirement for HN in the promotion of fusion and enhances HN-dependent fusion above wild-type (wt) levels. Here, it was shown that the HN-independent fusion exhibited by L289A-F in Cos-7 cells cannot be duplicated in BHK cells. However, when L289A-F is co-expressed with wt HN, enhanced fusion above wt levels is observed in BHK cells. Additionally, when L289A-F is co-expressed with IR-mutated HN proteins previously shown to promote low levels of fusion with wt F, a 2.5-fold increase in fusion was observed. However, similar to wt F, an interaction between L289A-F and the IR-mutated HN proteins was not detected. These results imply that the attachment function of HN, as well as the conformational change in L289A-F, are necessary for the enhanced level of fusion exhibited by HN proteins co-expressed with L289A-F. Indeed, two MAbs detected a conformational difference between L289A-F and the wt F protein. These findings support the idea that the L289A substitution converts F to a form that is less dependent on an interaction with HN for conversion to the fusion-active form. The last goal of this research was to address the cellular site of the HN-F interaction, still a controversial issue based on conflicting data from studies of different paramyxoviruses, using various approaches. This is a particular point of interest, as it speaks to the mechanism by which the HN-F interaction regulates fusion. Thus, NDV HN and F were successfully retained intracellularly with a multiple arginine or KK motif, respectively. The results of Endoglycosidase H resistance and F cleavage studies indicate that the mutated proteins, HN-ER and F-ER, are retained in a compartment prior to the medial-Golgi apparatus and that they are unable to interact with a high enough affinity to co-retain or even cause reduced transport of their wt partner glycoproteins. This is consistent with the HN-F interaction occurring at the cell surface, possibly triggered by receptor binding. In conclusion, this thesis presents evidence to argue that the IR in the stalk of the NDV HN protein directly mediates the interaction with the F protein that is necessary for fusion. Overall, the data presented in this thesis extend the current knowledge of the mechanism by which the paramyxovirus attachment protein can trigger the F protein to initiate membrane fusion. A clear understanding of this process has the potential to identify new anti-viral strategies, such as small molecule inhibitors, aimed at controlling paramyxovirus infection by interfering with early steps in the virus infection cycle.

Measles (MV) virions, like those of other enveloped viruses, enter cells by fusing their lipid membranes with those of the target host cells. Additionally, infected tissues often possess giant multinucleate cells, known as syncytia, which are formed by fusion of infected cells with uninfected neighbors. Expression of both the MV attachment (H) and fusion (F) proteins is required for membrane fusion. MV H mediates receptor binding in order to bring the two membranes into close proximity prior to F activation and is thought to trigger F activation through a specific interaction between the two proteins. Although measles H and F are efficiently transported to the cell surface when expressed independently, evidence has been reported in support of an intracellular interaction between the two proteins that can be detected using an ER co-retention approach. However, it was not determined if the putative co-retention was specific to the two measles glycoproteins, as is their ability to complement each other for efficient fusion promotion. Thus, in this thesis, the formation of an intracellular complex between MV H and F was re-examined. Consistent with the formation of an intracellular complex, cell surface expression and receptor binding of untagged wt MV H is slightly reduced by co-expression of an excess of ER-tagged MV F compared to co-expression with wt F. However, the reduction in surface expression is non-specific in that it can also be induced with heterologous proteins of NDV, which lack significant homology with those of MV. Although this approach did not detect a specific intracellular interaction between MV H and F, it cannot be ruled out that there is a weak association of the proteins that is undetectable by this method. This led to the use of an alternative approach to investigate the cellular site(s) of interaction between the measles H and F proteins. Consistent with a cell surface interaction between MV H and F, the combination of surface biotinylation and co-immunoprecipitation detects formation of a virus-specific H-F complex. Approximately, 21% of the total amount of MV H at the cell surface can be captured with MV F using an antibody against the latter protein. Two complementary approaches were used to address the relationship between this cell surface interaction and receptor recognition by MV H. First, the proteins were co-immunoprecipitated from the surface of Chinese hamster ovary (CHO) cells, which do not express either MV receptor, CD46 or CD150. Similar levels of MV H can be co-immunoprecipitated with F from the surfaces of parental CHO cells and stably transfected cells that express, human CD46 (CHO-CD46), indicating that binding to CD46 is not the trigger for the H-F interaction. Second, MV H proteins, carrying mutations that dramatically reduce CD46 binding, were shown to co-immunoprecipitate efficiently with F from the surface of HeLa cells. Significantly, these results indicate that MV H and F interact in the absence of, and thus prior to, receptor binding. This is in direct contrast to the NDV HN-F cell surface interaction, which is thought to be triggered by receptor binding. Identification of the domains of the para myxovirus attachment and fusion proteins that mediate membrane fusion activities is an essential part of understanding the mechanism of fusion. As a result of the H-F interaction prior to receptor binding, MV H attachment to its cellular receptor must result in conformational changes that trigger activation of the F protein. Site-directed mutagenesis analyses of two regions of MV H indicate that a HR domain in the stalk of the attachment protein is essential to the ability of H to activate F. However, either it is not the only region of H that interacts with F or it is indirectly involved in F activation because mutations in the HR do not disrupt MV H-F complex formation at the cell surface. Additionally, the functional interaction between MV H and F may be mediated, at least in part, by Loop 1 of the amino terminus of the C-rich region of the fusion protein. However, the exact role of this region of the F protein in fusion promotion remains to be determined. Importantly, the cell surface interaction between MV H and F proteins appears to be mediated by more that one region of each protein. In contrast to NDV, in no case has a definitive link between any single amino acid difference in MV H or F and an inability to form the cell surface H-F complex been established. In conclusion, the data presented in this dissertation support a model of measles membrane fusion in which the Hand F proteins form a complex prior to receptor recognition. This complex may hold F in its meta-stable pre-fusion state until binding of H to receptors at the cell surface triggers dissociation of the complex, releasing F to assume its fusogenic form. Importantly, these data also indicate that, although paramyxoviruses may all use the same general process. for promotion of membrane fusion, the mechanism may vary in multiple aspects. A more complete understanding of the means by which measles promotes membrane fusion may direct the development of specific strategies aimed at interfering with the early stages of infection.

Le virus de la stomatite vésiculaire (VSV) est un virus enveloppé appartenant au genre Vésiculovirus et à la famille des Rhabdoviridae. Son unique glycoprotéine G reconnait un récepteur à la surface de la cellule hôte puis, après endocytose du virion, déclenche la fusion membranaire grâce à une transition structurale induite à faible pH depuis la forme pré-fusion de G vers sa forme post-fusion. Par ailleurs, G est la cible des anticorps neutralisant le virus. Les structures cristallographiques des formes pré- et post-fusion de l'ectodomaine soluble de G (i.e. sans sa partie transmembranaire) ont été déterminées par radiocristallographie. Ces structures ont installé G comme étant le prototype des glycoprotéines de fusion de classe III. L'organisation de l'extrémité carboxyterminale de l'ectodomaine et du domaine transmembranaire de G, qui jouent un rôle important durant le processus de fusion, n'est cependant pas connue. Nous avons donc réalisé une étude par cryo-microscopie électronique sur la glycoprotéine complète, purifiée à partir de particules virales, seule ou en complexe avec un anticorps monoclonal. Cette étude nous a permis de compléter les structures de l'ectodomaine dans ses conformations pré et post-fusion. Elle suggère que les domaines transmembranaires sont mobiles au sein de la membrane. Par ailleurs, nous avons résolu deux structures de G en complexe avec un FAb dérivé d'un anticorps neutralisant, reconnaissant à la fois les formes pré- et post-fusion de plusieurs souches de Vésiculovirus. Cette première structure d'un complexe entre G et un anticorps nous a permis de caractériser finement l'épitope, d'identifier les résidus de G clefs dans l'interaction et de proposer un mécanisme de neutralisation. Ce travail augmente de façon significative nos connaissances sur la structure de G qui est la glycoprotéine la plus utilisée en biotechnologie pour délivrer un cargo et en thérapie génique pour le pseudotypage de vecteurs lentiviraux. Nous avons également débuté une étude visant à caractériser les glycoprotéines des Lyssavirus, genre appartenant également à la famille des Rhabdoviridae, et dont le virus de la rage est le prototype. Nous avons produit et purifié les ectodomaines de plusieurs Lyssavirus, et nous avons pu obtenir une structure cristallographique de l'ectodomaine du virus Ikoma (IKOV G) qui correspondrait à un intermédiaire monomérique tardif. Afin de poursuivre le travail de caractérisation de cette structure, plusieurs approches sont en cours. Nous avons notamment réalisé une sélection par phage display de ligands alphaReps dirigés contre IKOV G. Les alphareps sont des protéines artificielles constituées par des répétitions hélicoïdales. 6 des 11 alphareps sélectionnées sont capables de lier IKOV G. La caractérisation des complexes IKOV G est en cours. Nous envisageons i) d'utiliser ces alphareps pour piéger des conformations différentes de G que celle obtenues pour en obtenir la structure par cristallographie ou cryo-EM ii) tester l'activité antivirale des apharep sélectionnées
Vesicular stomatitis virus (VSV), an enveloped virus, is the prototype species of the genus Vesiculovirus within the family Rhabdoviridae. Its G glycoprotein is responsible for receptor recognition, on the host cell surface, that triggers clathrin-mediated endocytosis of VSV. Then, within the acidic environment of the endosome, VSV G undergoes a fusogenic conformational change from the pre-fusion form of G to its post-fusion form, leading the fusion of both membranes. G is also the target of virus-neutralizing antibodies. Both structures of the pre- and post-fusion forms of the soluble ectodomain of G (i.e. without its transmembrane part) were determined by radiocrystallography. These structures established G as the prototype of class III fusion glycoproteins. However, the organization of the carboxyterminal part of the ectodomain and the transmembrane domain of G, which play an important role during the fusion process, remains unknown. Therefore, we carried out a cryo-electron microscopy study on the complete glycoprotein, directly purified from viral particles, alone or in complex with a monoclonal antibody. This study led to complete the structures of the ectodomain in its pre- and post-fusion conformations. It also revealed that the transmembrane domains are mobile within the membrane. We have also solved two structures of G in complex with a FAb derived from a neutralizing antibody, recognizing both pre- and post-fusion forms of G from several strains of Vesiculovirus. Based on these first structures of a complex between G and an antibody, we could characterize the epitope, identify the key G residues in the interaction and propose a neutralization mechanism. This work significantly increases our knowledge of the structure of G, which is the most widely used glycoprotein in biotechnology for cargo delivery and in gene therapy (by lentivirus pseudotyping).We also initiated a study aimed at characterizing the glycoproteins of Lyssaviruses, a genus also part of the Rhabdoviridae family, and for which rabies virus is the prototype. We produced and purified the ectodomains of several Lyssaviruses, and we were able to obtain a crystallographic structure of the ectodomain of Ikoma virus (IKOV G), which corresponds to a late monomeric intermediate. Several approaches are underway to further characterize this structure. We also carried out a phage display selection of alphaReps directed against IKOV G. Alphareps are artificial proteins binders consisting of helical repeats. 6 out of 11 alphareps are able to bind IKOVG. Complexes of IKOV G with alphareps are currently being characterized. We plan to i) use these tools as crystallization helpers to trap different conformations of G in crystallography or cryo-EM ii) evaluate the potential antiviral activity of these alphareps

Les structures pré- et post-fusion de G VSV ont été élucidées par radiocristallographie,permettent d'identifier certains résidus qui pourraient jouer le rôle d'interrupteurs moléculaires sensibles au pH depuis la forme post- vers la forme pré-fusion. Ces acides aminés acides ont donc été remplacés par de simples(D268L,N,V;D274N;E276Q;D393N;D395N: et doubles mutants(D274N-D395N,E276-393N). L'expression,repliement,transport à la membrane cellulaire ont été analysés par immunofluorescence et FACS. Des expériences de fusion cellule-cellule ont montré que la plupart de ces mutants sont affectés dans leur propriétés de fusion. En particulier,les mutants en position 268 ont complètement perdu leur activité de fusion. Les mutants D268V/L sont piégés dans leur conformation post-fusion alors que la mutation D268N entraîne la création d'un site de glycosylation,qui rend le mutant incapable de former le trimère post-fusion. Tous ces mutants,sauf E276Q,ont des capacités réduites à pseudotyper un virus dépourvu du gène de la G. De façon inattendue,la stabilisation du trimère post-fusion empêche l'incorporation de G dans les particules virales. Nous avons utilisé la génétique inverse pour produire des particules virales recombinantes. La plupart des mutants on immédiatement réverté vers la séquence de type sauvage révélant l'importance de celle-ci. Ces données démontrent que les résidus identifiés précédemment sont bien des régulateurs du changement de conformation de G et que D268 est k résidu majeur dans cet rôle. Elles suggèrent aussi que la conformation de G régule son incorporation dans les particule naissantes et/ou la libération de la nucléocapside virale dans le cytoplasme après fusion
The structure of the pre- and post-fusion of VSV G states have been elucidated by X-ray crystallography,allowing the identification of some amino acid residues which could play the role of pH-dependent molecular switches,triggering the conformational change from the post- toward the pre-fusion state. To confirm this,these acidic amino acids were mutated,creating single(D268L,N,V;D274N;E276Q;D393N;D395N) and double mutants (D274N-D395N,E276Q-D393N). Their expression,folding and transport to the tell membrane were analyzed by immunofluorescence and FACS. A cell-cell fusion assay showed that most of these mutants are affected in their membrane fusion properties. Particularly,mutants D268L/V/N have completely lost their fusion activity. We demonstrated that D268LN are trapped in their post-fusion state whereas the mutation D268N,which results in the creation of an efficient glycosylation, is unable to form the post fusion trimer. All these mutants, except mutant E276Q, do not or very inefficiently pseudotype a virus lacking the glycoprotein gene. Surprisingly,the stabilization of the post fusion trimer precludes G incorporation into the viral particles. We used a reverse genetic approach to produce recombinant viral particles. Most of the mutants immediately reverted toward the wild type sequence indicating the importance of the wild type sequence in this region. These data demonstrate that the residues identified are indeed regulators of G conformational change and that D268 is the major pH sensitive switch. It also suggests that the conformation of G regulates its incorporation in nascent particles and/or the release of viral nucleocapsid into the cytoplasm alter fusion

Le virus de Chikungunya (CHIKV) est un alphavirus émergent, transmis par les moustiques, qui a provoqué des épidémies de maladies débilitantes chez homme pendant les dernières cinq années. L'invasion de CHIKV dans les cellules sensibles est médiée par deux glycoprotéines virales, E1 et E2, qui portent respectivement les boucles de fusion à la membrane et les déterminants antigéniques principaux, et forment une couche protéinique icosaédrique à la surface du virion. La glycoprotéine E2, provenant du clivage par la furine du précurseur p62 (en E3 et E2), est responsable de la liaison au récepteur, tandis que E1 est impliqué dans la fusion membranaire. Dans le cadre d'un effort multidisciplinaire pour comprendre la biologie de CHIKV, nous avons déterminé les structures cristallines de l'hétérodimère précurseur immature (p62-E1; 2. 17 A de résolution) et du complexe mature (E3-E2-E1; 2. 6 A de résolution). Les structures atomiques nous ont permis de faire la synthèse d'une multitude de données génétiques, biochimiques, immunologiques et de microscopie électronique accumulées pendant plusieurs années sur les arbovirus en général. Cette analyse donne une image détaillée de l'architecture fonctionnelle de la couche de surface (25 MDa) des alphavirus. Les structures des complexes matures et immatures de CHIKV a aussi permis de décrire les causes et les mécanismes du changement de conformation des protéines d'enveloppe du virion lors du passage de celui-ci dans l'endosome à pH acide et précédant la fusion membranaire
Chikungunya is an emerging mosquito-bome alphavirus that has caused widespread outbreaks of debilitating human disease in the past five years. CHIKV invasion of susceptible cells is mediated by two viral glycoproteins, E1 and E2, which carry the main antigenic determinants and form an icosahedral shell at the virion surface. Glycoprotein E2, derived from furin cleavage of the p62 precursor to E3 and E2 is responsible for receptor binding and is the major viral antigen. The E1 protein is responsible for inducing the fusion of viral and cellular membranes in the target cell endosome which is required for release of the viral nucleocapsid into the cytoplasm to initiale infection of a cell. While the structure of E1 has been determined, the structure of E2"has remained elusive over the years. This thesis reports the atomic structures of the mature (E3/E2/E1) and immature (P62/E1) envelope glycoprotein complexes from Chikungunya virus determined by X-ray crystallography using a recombinant protein construct. This construct contained the covalently linked ectodomains of p62 and E1. Diffracting crystals of the purified complexes were obtained at neutral pH when the linker joining the ectodomains was cleaved. The glycoprotein structures were fit into reconstructions of the alphavirus virion obtained from cryo-electron microscopy (cryoEM). This analysis resulted in an inferred atomic model of the entire 25MDa surface of the highly conserved alphavirus virion and allowed for the synthesis of a wealth of genetic, biochemical, immunological and electron microscopy data accumulated over the years on alphaviruses in general

Abstract The envelope glycoproteins of human immunodeficiency viruses (HIV types 1 and 2) play a central role in the transmission of acquired immunodeficiency syndrome (AIDS) (1). The viral envelope mediates attachment of the virus to the CD4 receptor present on target cells (2-4) and the spread of virus by cellto-cell fusion (5, 6). In the infected cells, the viral envelope of HIV-1 is synthesized as a precursor glycoprotein (gp160) which is subsequently cleaved into an external surface protein gp120 and a transmembrane protein gp41, upon maturation in the Golgi (7-9). After cleavage, the two glycoproteins (held together by non-covalent interactions) are transported to the cell surface.

Newly identified beta-coronavirus i.e. the 2019 novel coronavirus is associated with a contagious transmittable respiratory disease called COVID-19. This disease has been declared as a “pandemic” by the World Health Organization (WHO). The entry of coronavirus in the human respiratory epithelial cells depends upon the interaction between host cell receptor ACE2 and viral S-glycoprotein. However, this type of molecular recognition in between cell surface receptors and envelope glycoproteins are mediated by specific glycan epitopes and attribute to viral entry through membrane fusion. Glycans are essential biomolecules made by all living organisms, have roles in serving structure, energy storage, and system regulatory purposes. The glycan shield plays a crucial role in concealing the surface S protein from molecular recognition. The immunomodulatory properties of Glycan-binding proteins (GBPs) like Lectins, build them as an attractive candidates for vaccine adjuvant. Investigations involving the complement system activation by the lectin pathway in COVID-19 and diseases are in need of the hour. The innate immune response involving complement system could have varied biological effects against an array of microbial infections. The advances in glycoprotein style methods especially immunomodulatory action of some lectins are necessary to boost the effectiveness of treatment of COVID-19 and other pandemics.

Abstract HIV-l, HIV-2, and SIV belong to the group of primate lentiviruses which use the CD4 molecule as their main port of entry. Binding of HIV virions to CD4 results in conformational changes in the virus envelope glycoproteins, gpl20 and gp41. These structural alterations are likely to be essential for virion and cell membranes to fuse. Many studies indicate that a second, still undefined, cell surface component is needed in addition to CD4, for fusion. HIV-1 is able to bind to other cell surface molecules, including the glycolipid galactocereboside but these mediate viral entry very inefficiently at best. Some strains of HIV-2 have been isolated which can fuse and infect cells independently of CD4 but the alternative receptor used by this virus has not been identified yet. CD4-independent entry of HIV-1 in vitro has also been reported to occur via Fc receptors (of antibody coated virions) or the complement receptors CR2 and CR3 (of virions coated with the C3b/C3d fragments of the complement component C3). However, it is unknown whether these routes of entry are of any significance in vivo.

Abstract The complex replication cycle of HIV, offers several potential sites for intervention with antiretroviral drugs. H (shown in Fig. 17- 1 The host becomes infected by exposure to blood or body fluids containing HIV. The virus then attaches to target cells through binding of the viral surface glycoprotein gp120 to CD4 molecules located on the membranes of certain T lymphocytes and macrophages/monocytes, and the virus is in ternalized by fusion with the host cell membrane. After internalization, the HIV virion is uncoated, releasing viral genomic RNA into the host cell.

Abstract The life-cycle of human immunodeficiency virus (HIV) begins with the binding of the viral envelope glycoprotein (gpl20) to the CD4 receptor of the host cells. Fusion between the viral envelope and the cell membrane follows, whereupon the viral nucleocapsid gains entry into the cell. After entry, uncoating of the nucleocapsid takes place. Once it has been released (at least partially) from its core (group-specific antigen (gag)) proteins, the genomic RNA is converted into double-stranded (proviral) DNA by reverse transcriptase (RT). The pro viral DNA then migrates to the nucleus where it is integrated into the host genome by integrase.

SARS-CoV-2 belongs to the family coronviradae and the disease caused by this virus is known as COVID-19. Viral entry into the cell is favored by spike glycoprotein, which interacts with Angiotensin-converting-enzyme-2 (ACE-2). Moreover, proteins such as Transmembrane Protease Serine-2 (TMPRSS-2), are responsible for viral fusion with cellular epithelium. Traditional drug discovery methods and their development process are time-consuming as well as expensive. Thus, there is a need for a method that can overcome such drawbacks. Drug repurposing is an approach in which we can use an existing drug that is already being used for another disease. The repurposing of drugs is also known as repositioning. It is the process that identifies new therapeutic use for existing or available drugs. Hydroxychloroquine inhibits ACE-2 glycosylation virus entry to the host body; arbidol prevents fusion of viral lipid shell with cell membrane hence restricting contact and penetration of virus. Drug repurposing could be a successful strategy for the treatment of sporadic, neglected diseases, difficult-to-treat diseases, and the current pandemic situation, i.e., COVID-19. However, there is no denying the fact that there are several limitations to this approach.

Toll-Like Receptor-4 (TLR4) senses life-threatening Ebola virus Glycoprotein (GP) and produces pro-inflammatory cytokines, resulting in lethal Ebola virus infections. GP2-subunit of Ebola promotes viral entry via membrane fusion. The present study models, optimizes and demonstrates the 3D monomer of the responsible human protein. The essential residue (studied from wet-laboratory research) was observed to be functionally conserved from multiple-sequence alignment. Thus, after performing point-mutation, the mutant protein was satisfactorily re-modelled; keeping its functionality preserved. Comparable residual participation in GP2 and each of the proteins was examined, individually. Stability of the proteins and protein-GP2 complexes on mutation; were discerned via energy calculations, solvent-accessibility area and conformational switching, with supportive statistical significances. Therefore, this probe paves a pathway to examine the weaker interaction of the stable mutated human protein with Ebola GP2 protein, thereby defending the Ebola viral entry.