Articles de revues : « Spindle position checkpoint »

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Caydasi, Ayse Koca, Bahtiyar Kurtulmus, Maria I. L. Orrico, Astrid Hofmann, Bashar Ibrahim et Gislene Pereira. « Elm1 kinase activates the spindle position checkpoint kinase Kin4 ». Journal of Cell Biology 190, n^o 6 (20 septembre 2010) : 975–89. http://dx.doi.org/10.1083/jcb.201006151.

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Budding yeast asymmetric cell division relies upon the precise coordination of spindle orientation and cell cycle progression. The spindle position checkpoint (SPOC) is a surveillance mechanism that prevents cells with misoriented spindles from exiting mitosis. The cortical kinase Kin4 acts near the top of this network. How Kin4 kinase activity is regulated and maintained in respect to spindle positional cues remains to be established. Here, we show that the bud neck–associated kinase Elm1 participates in Kin4 activation and SPOC signaling by phosphorylating a conserved residue within the activation loop of Kin4. Blocking Elm1 function abolishes Kin4 kinase activity in vivo and eliminates the SPOC response to spindle misalignment. These findings establish a novel function for Elm1 in the coordination of spindle positioning with cell cycle progression via its control of Kin4.

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Caydasi, Ayse K., Bashar Ibrahim et Gislene Pereira. « Monitoring spindle orientation : Spindle position checkpoint in charge ». Cell Division 5, n^o 1 (2010) : 28. http://dx.doi.org/10.1186/1747-1028-5-28.

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Lázaro-Diéguez, Francisco, Iaroslav Ispolatov et Anne Müsch. « Cell shape impacts on the positioning of the mitotic spindle with respect to the substratum ». Molecular Biology of the Cell 26, n^o 7 (avril 2015) : 1286–95. http://dx.doi.org/10.1091/mbc.e14-08-1330.

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All known mechanisms of mitotic spindle orientation rely on astral microtubules. We report that even in the absence of astral microtubules, metaphase spindles in MDCK and HeLa cells are not randomly positioned along their x-z dimension, but preferentially adopt shallow β angles between spindle pole axis and substratum. The nonrandom spindle positioning is due to constraints imposed by the cell cortex in flat cells that drive spindles that are longer and/or wider than the cell's height into a tilted, quasidiagonal x-z position. In rounder cells, which are taller, fewer cortical constraints make the x-z spindle position more random. Reestablishment of astral microtubule–mediated forces align the spindle poles with cortical cues parallel to the substratum in all cells. However, in flat cells, they frequently cause spindle deformations. Similar deformations are apparent when confined spindles rotate from tilted to parallel positions while MDCK cells progress from prometaphase to metaphase. The spindle disruptions cause the engagement of the spindle assembly checkpoint. We propose that cell rounding serves to maintain spindle integrity during its positioning.

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Fraschini, Roberta, Denis Bilotta, Giovanna Lucchini et Simonetta Piatti. « Functional Characterization of Dma1 and Dma2, the Budding Yeast Homologues of Schizosaccharomyces pombe Dma1 and Human Chfr ». Molecular Biology of the Cell 15, n^o 8 (août 2004) : 3796–810. http://dx.doi.org/10.1091/mbc.e04-02-0094.

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Proper transmission of genetic information requires correct assembly and positioning of the mitotic spindle, responsible for driving each set of sister chromatids to the two daughter cells, followed by cytokinesis. In case of altered spindle orientation, the spindle position checkpoint inhibits Tem1-dependent activation of the mitotic exit network (MEN), thus delaying mitotic exit and cytokinesis until errors are corrected. We report a functional analysis of two previously uncharacterized budding yeast proteins, Dma1 and Dma2, 58% identical to each other and homologous to human Chfr and Schizosaccharomyces pombe Dma1, both of which have been previously implicated in mitotic checkpoints. We show that Dma1 and Dma2 are involved in proper spindle positioning, likely regulating septin ring deposition at the bud neck. DMA2 overexpression causes defects in septin ring disassembly at the end of mitosis and in cytokinesis. The latter defects can be rescued by either eliminating the spindle position checkpoint protein Bub2 or overproducing its target, Tem1, both leading to MEN hyperactivation. In addition, dma1Δ dma2Δ cells fail to activate the spindle position checkpoint in response to the lack of dynein, whereas ectopic expression of DMA2 prevents unscheduled mitotic exit of spindle checkpoint mutants treated with microtubule-depolymerizing drugs. Although their primary functions remain to be defined, our data suggest that Dma1 and Dma2 might be required to ensure timely MEN activation in telophase.

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Wang, Mengqiao, et Ruth N. Collins. « A lysine deacetylase Hos3 is targeted to the bud neck and involved in the spindle position checkpoint ». Molecular Biology of the Cell 25, n^o 18 (15 septembre 2014) : 2720–34. http://dx.doi.org/10.1091/mbc.e13-10-0619.

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An increasing number of cellular activities can be regulated by reversible lysine acetylation. Targeting the enzymes responsible for such posttranslational modifications is instrumental in defining their substrates and functions in vivo. Here we show that a Saccharomyces cerevisiae lysine deacetylase, Hos3, is asymmetrically targeted to the daughter side of the bud neck and to the daughter spindle pole body (SPB). The morphogenesis checkpoint member Hsl7 recruits Hos3 to the neck region. Cells with a defect in spindle orientation trigger Hos3 to load onto both SPBs. When associated symmetrically with both SPBs, Hos3 functions as a spindle position checkpoint (SPOC) component to inhibit mitotic exit. Neck localization of Hos3 is essential for its symmetric association with SPBs in cells with misaligned spindles. Our data suggest that Hos3 facilitates cross-talk between the morphogenesis checkpoint and the SPOC as a component of the intricate monitoring of spindle orientation after mitotic entry and before commitment to mitotic exit.

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Nelson, Scott A., et John A. Cooper. « A Novel Pathway that Coordinates Mitotic Exit with Spindle Position ». Molecular Biology of the Cell 18, n^o 9 (septembre 2007) : 3440–50. http://dx.doi.org/10.1091/mbc.e07-03-0242.

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In budding yeast, the spindle position checkpoint (SPC) delays mitotic exit until the mitotic spindle moves into the neck between the mother and bud. This checkpoint works by inhibiting the mitotic exit network (MEN), a signaling cascade initiated and controlled by Tem1, a small GTPase. Tem1 is regulated by a putative guanine exchange factor, Lte1, but the function and regulation of Lte1 remains poorly understood. Here, we identify novel components of the checkpoint that operate upstream of Lte1. We present genetic evidence in agreement with existing biochemical evidence for the molecular mechanism of a pathway that links microtubule-cortex interactions with Lte1 and mitotic exit. Each component of this pathway is required for the spindle position checkpoint to delay mitotic exit until the spindle is positioned correctly.

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Moore, Jeffrey K., Valentin Magidson, Alexey Khodjakov et John A. Cooper. « The Spindle Position Checkpoint Requires Positional Feedback from Cytoplasmic Microtubules ». Current Biology 19, n^o 23 (décembre 2009) : 2026–30. http://dx.doi.org/10.1016/j.cub.2009.10.020.

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Adames, Neil R., Jessica R. Oberle et John A. Cooper. « The Surveillance Mechanism of the Spindle Position Checkpoint in Yeast ». Journal of Cell Biology 153, n^o 1 (2 avril 2001) : 159–68. http://dx.doi.org/10.1083/jcb.153.1.159.

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The spindle position checkpoint in Saccharomyces cerevisiae delays mitotic exit until the spindle has moved into the mother–bud neck, ensuring that each daughter cell inherits a nucleus. The small G protein Tem1p is critical in promoting mitotic exit and is concentrated at the spindle pole destined for the bud. The presumed nucleotide exchange factor for Tem1p, Lte1p, is concentrated in the bud. These findings suggested the hypothesis that movement of the spindle pole through the neck allows Tem1p to interact with Lte1p, promoting GTP loading of Tem1p and mitotic exit. However, we report that deletion of LTE1 had little effect on the timing of mitotic exit. We also examined several mutants in which some cells inappropriately exit mitosis even though the spindle is within the mother. In some of these cells, the spindle pole body did not interact with the bud or the neck before mitotic exit. Thus, some alternative mechanism must exist to coordinate mitotic exit with spindle position. In both wild-type and mutant cells, mitotic exit was preceded by loss of cytoplasmic microtubules from the neck. Thus, the spindle position checkpoint may monitor such interactions.

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Caydasi, Ayse Koca, Maiko Lohel, Gerd Grünert, Peter Dittrich, Gislene Pereira et Bashar Ibrahim. « A dynamical model of the spindle position checkpoint ». Molecular Systems Biology 8, n^o 1 (janvier 2012) : 582. http://dx.doi.org/10.1038/msb.2012.15.

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Moore, Jeffrey K., Prakash Chudalayandi, Richard A. Heil-Chapdelaine et John A. Cooper. « The spindle position checkpoint is coordinated by the Elm1 kinase ». Journal of Cell Biology 191, n^o 3 (1 novembre 2010) : 493–503. http://dx.doi.org/10.1083/jcb.201006092.

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How dividing cells monitor the effective transmission of genomes during mitosis is poorly understood. Budding yeast use a signaling pathway known as the spindle position checkpoint (SPC) to ensure the arrival of one end of the mitotic spindle in the nascent daughter cell. An important question is how SPC activity is coordinated with mother–daughter polarity. We sought to identify factors at the bud neck, the junction between mother and bud, which contribute to checkpoint signaling. In this paper, we show that the protein kinase Elm1 is an obligate regulator of the SPC, and this function requires localization of Elm1 to the bud neck. Furthermore, we show that Elm1 promotes the activity of the checkpoint kinase Kin4. These findings reveal a novel function for Elm1 in the SPC and suggest how checkpoint activity may be linked to cellular organization.

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Yang, Sam S., Elaine Yeh, E. D. Salmon et Kerry Bloom. « Identification of a Mid-anaphase Checkpoint in Budding Yeast ». Journal of Cell Biology 136, n^o 2 (27 janvier 1997) : 345–54. http://dx.doi.org/10.1083/jcb.136.2.345.

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Activation of a facultative, dicentric chromosome provides a unique opportunity to introduce a double strand DNA break into a chromosome at mitosis. Time lapse video enhanced-differential interference contrast analysis of the cellular response upon dicentric activation reveals that the majority of cells initiates anaphase B, characterized by pole–pole separation, and pauses in mid-anaphase for 30–120 min with spindles spanning the neck of the bud before completing spindle elongation and cytokinesis. The length of the spindle at the delay point (3–4 μm) is not dependent on the physical distance between the two centromeres, indicating that the arrest represents surveillance of a dicentric induced aberration. No mid-anaphase delay is observed in the absence of the RAD9 checkpoint gene, which prevents cell cycle progression in the presence of damaged DNA. These observations reveal RAD9- dependent events well past the G2/M boundary and have considerable implications in understanding how chromosome integrity and the position and state of the mitotic spindle are monitored before cytokinesis.

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Bloecher, Andrew, Guglielmo M. Venturi et Kelly Tatchell. « Anaphase spindle position is monitored by the BUB2 checkpoint ». Nature Cell Biology 2, n^o 8 (19 juillet 2000) : 556–58. http://dx.doi.org/10.1038/35019601.

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O'Connell, Christopher B., et Yu-li Wang. « Mammalian Spindle Orientation and Position Respond to Changes in Cell Shape in a Dynein-dependent Fashion ». Molecular Biology of the Cell 11, n^o 5 (mai 2000) : 1765–74. http://dx.doi.org/10.1091/mbc.11.5.1765.

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In animal cells, positioning of the mitotic spindle is crucial for defining the plane of cytokinesis and the size ratio of daughter cells. We have characterized this phenomenon in a rat epithelial cell line using microscopy, micromanipulation, and microinjection. Unmanipulated cells position the mitotic spindle near their geometric center, with the spindle axis lying roughly parallel to the long axis of the cell. Spindles that were initially misoriented underwent directed rotation and caused a delay in anaphase onset. To gain further insight into this process, we gently deformed cells with a blunted glass needle to change the spatial relationship between the cortex and spindle. This manipulation induced spindle movement or rotation in metaphase and/or anaphase, until the spindle reached a proper position relative to the deformed shape. Spindle positioning was inhibited by either treatment with low doses of nocodazole or microinjection of antibodies against dynein, apparently due to the disruption of the organization of dynein and/or astral microtubules. Our results suggest that mitotic cells continuously monitor and maintain the position of the spindle relative to the cortex. This process is likely driven by interactions among astral microtubules, the motor protein dynein, and the cell cortex and may constitute part of a mitotic checkpoint mechanism.

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Ballew, Olivia, et Soni Lacefield. « The DNA damage checkpoint and the spindle position checkpoint : guardians of meiotic commitment ». Current Genetics 65, n^o 5 (26 avril 2019) : 1135–40. http://dx.doi.org/10.1007/s00294-019-00981-z.

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Bertazzi, Daniela Trinca, Bahtiyar Kurtulmus et Gislene Pereira. « The cortical protein Lte1 promotes mitotic exit by inhibiting the spindle position checkpoint kinase Kin4 ». Journal of Cell Biology 193, n^o 6 (13 juin 2011) : 1033–48. http://dx.doi.org/10.1083/jcb.201101056.

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The spindle position checkpoint (SPOC) is an essential surveillance mechanism that allows mitotic exit only when the spindle is correctly oriented along the cell axis. Key SPOC components are the kinase Kin4 and the Bub2–Bfa1 GAP complex that inhibit the mitotic exit–promoting GTPase Tem1. During an unperturbed cell cycle, Kin4 associates with the mother spindle pole body (mSPB), whereas Bub2–Bfa1 is at the daughter SPB (dSPB). When the spindle is mispositioned, Bub2–Bfa1 and Kin4 bind to both SPBs, which enables Kin4 to phosphorylate Bfa1 and thereby block mitotic exit. Here, we show that the daughter cell protein Lte1 physically interacts with Kin4 and inhibits Kin4 kinase activity. Specifically, Lte1 binds to catalytically active Kin4 and promotes Kin4 hyperphosphorylation, which restricts Kin4 binding to the mSPB. This Lte1-mediated exclusion of Kin4 from the dSPB is essential for proper mitotic exit of cells with a correctly aligned spindle. Therefore, Lte1 promotes mitotic exit by inhibiting Kin4 activity at the dSPB.

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Fraschini, Roberta, Marianna Venturetti, Elena Chiroli et Simonetta Piatti. « The spindle position checkpoint : how to deal with spindle misalignment during asymmetric cell division in budding yeast ». Biochemical Society Transactions 36, n^o 3 (21 mai 2008) : 416–20. http://dx.doi.org/10.1042/bst0360416.

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During asymmetric cell division, spindle positioning is critical to ensure the unequal segregation of polarity factors and generate daughter cells with different sizes or fates. In budding yeast the boundary between mother and daughter cell resides at the bud neck, where cytokinesis takes place at the end of the cell cycle. Since budding and bud neck formation occur much earlier than bipolar spindle formation, spindle positioning is a finely regulated process. A surveillance device called the SPOC (spindle position checkpoint) oversees this process and delays mitotic exit and cytokinesis until the spindle is properly oriented along the division axis, thus ensuring genome stability.

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Ballew, Olivia, et Soni Lacefield. « The DNA Damage Checkpoint and the Spindle Position Checkpoint Maintain Meiotic Commitment in Saccharomyces cerevisiae ». Current Biology 29, n^o 3 (février 2019) : 449–60. http://dx.doi.org/10.1016/j.cub.2018.12.043.

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Fraschini, Roberta, Claudio D'Ambrosio, Marianna Venturetti, Giovanna Lucchini et Simonetta Piatti. « Disappearance of the budding yeast Bub2–Bfa1 complex from the mother-bound spindle pole contributes to mitotic exit ». Journal of Cell Biology 172, n^o 3 (30 janvier 2006) : 335–46. http://dx.doi.org/10.1083/jcb.200507162.

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Budding yeast spindle position checkpoint is engaged by misoriented spindles and prevents mitotic exit by inhibiting the G protein Tem1 through the GTPase-activating protein (GAP) Bub2/Bfa1. Bub2 and Bfa1 are found on both duplicated spindle pole bodies until anaphase onset, when they disappear from the mother-bound spindle pole under unperturbed conditions. In contrast, when spindles are misoriented they remain symmetrically localized at both SPBs. Thus, symmetric localization of Bub2/Bfa1 might lead to inhibition of Tem1, which is also present at SPBs. Consistent with this hypothesis, we show that a Bub2 version symmetrically localized on both SPBs throughout the cell cycle prevents mitotic exit in mutant backgrounds that partially impair it. This effect is Bfa1 dependent and can be suppressed by high Tem1 levels. Bub2 removal from the mother-bound SPB requires its GAP activity, which in contrast appears to be dispensable for Tem1 inhibition. Moreover, it correlates with the passage of one spindle pole through the bud neck because it needs septin ring formation and bud neck kinases.

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Lee, Kyunghee, et Kiwon Song. « Functional analysis of the putativeBUB2homologues ofc. elegansin the spindle position checkpoint ». Integrative Biosciences 9, n^o 2 (janvier 2005) : 87–94. http://dx.doi.org/10.1080/17386357.2005.9647256.

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Chan, L. Y., et A. Amon. « The protein phosphatase 2A functions in the spindle position checkpoint by regulating the checkpoint kinase Kin4 ». Genes & ; Development 23, n^o 14 (15 juillet 2009) : 1639–49. http://dx.doi.org/10.1101/gad.1804609.

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Gui, L., et H. Homer. « Spindle assembly checkpoint signalling is uncoupled from chromosomal position in mouse oocytes ». Development 139, n^o 11 (18 avril 2012) : 1941–46. http://dx.doi.org/10.1242/dev.078352.

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Kim, Junwon, Selma Sun Jang et Kiwon Song. « Different Levels of Bfa1/Bub2 GAP Activity Are Required to Prevent Mitotic Exit of Budding Yeast Depending on the Type of Perturbations ». Molecular Biology of the Cell 19, n^o 10 (octobre 2008) : 4328–40. http://dx.doi.org/10.1091/mbc.e08-02-0149.

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In budding yeast, Tem1 is a key regulator of mitotic exit. Bfa1/Bub2 stimulates Tem1 GTPase activity as a GTPase-activating protein (GAP). Lte1 possesses a guanine-nucleotide exchange factor (GEF) domain likely for Tem1. However, recent observations showed that cells may control mitotic exit without either Lte1 or Bfa1/Bub2 GAP activity, obscuring how Tem1 is regulated. Here, we assayed BFA1 mutants with varying GAP activities for Tem1, showing for the first time that Bfa1/Bub2 GAP activity inhibits Tem1 in vivo. A decrease in GAP activity allowed cells to bypass mitotic exit defects. Interestingly, different levels of GAP activity were required to prevent mitotic exit depending on the type of perturbation. Although essential, more Bfa1/Bub2 GAP activity was needed for spindle damage than for DNA damage to fully activate the checkpoint. Conversely, Bfa1/Bub2 GAP activity was insufficient to delay mitotic exit in cells with misoriented spindles. Instead, decreased interaction of Bfa1 with Kin4 was observed in BFA1 mutant cells with a defective spindle position checkpoint. These findings demonstrate that there is a GAP-independent surveillance mechanism of Bfa1/Bub2, which, together with the GTP/GDP switch of Tem1, may be required for the genomic stability of cells with misaligned spindles.

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Merlini, Laura, et Simonetta Piatti. « The mother-bud neck as a signaling platform for the coordination between spindle position and cytokinesis in budding yeast ». Biological Chemistry 392, n^o 8-9 (1 août 2011) : 805–12. http://dx.doi.org/10.1515/bc.2011.090.

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Abstract During asymmetric cell division, spindle positioning is critical for ensuring the unequal inheritance of polarity factors. In budding yeast, the mother-bud neck determines the cleavage plane and a correct nuclear division between mother and daughter cell requires orientation of the mitotic spindle along the mother-bud axis. A surveillance device called the spindle position/orientation checkpoint (SPOC) oversees this process and delays mitotic exit and cytokinesis until the spindle is properly oriented along the division axis, thus ensuring genome stability. Cytoskeletal proteins called septins form a ring at the bud neck that is essential for cytokinesis. Furthermore, septins and septin-associated proteins are implicated in spindle positioning and SPOC. In this review, we discuss the emerging connections between septins and the SPOC and the role of the mother-bud neck as a signaling platform to couple proper chromosome segregation to cytokinesis.

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Schuyler, S. C., et D. Pellman. « Search, capture and signal : games microtubules and centrosomes play ». Journal of Cell Science 114, n^o 2 (15 janvier 2001) : 247–55. http://dx.doi.org/10.1242/jcs.114.2.247.

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Accurate distribution of the chromosomes in dividing cells requires coupling of cellular polarity cues with both the orientation of the mitotic spindle and cell cycle progression. Work in budding yeast has demonstrated that cytoplasmic dynein and the kinesin Kip3p define redundant pathways that ensure proper spindle orientation. Furthermore, it has been shown that the Kip3p pathway components Kar9p and Bim1p (Yeb1p) form a complex that provides a molecular link between cortical polarity cues and spindle microtubules. Recently, other studies indicated that the cortical localization of Kar9p depends upon actin cables and Myo2p, a type V myosin. In addition, a BUB2-dependent cell cycle checkpoint has been described that inhibits the mitotic exit network and cytokinesis until proper centrosome position is achieved. Combined, these studies provide molecular insight into how cells link cellular polarity, spindle position and cell cycle progression.

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Vannini, Michael, Victoria R. Mingione, Ashleigh Meyer, Courtney Sniffen, Jenna Whalen et Anupama Seshan. « A Novel Hyperactive Nud1 Mitotic Exit Network Scaffold Causes Spindle Position Checkpoint Bypass in Budding Yeast ». Cells 11, n^o 1 (24 décembre 2021) : 46. http://dx.doi.org/10.3390/cells11010046.

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Mitotic exit is a critical cell cycle transition that requires the careful coordination of nuclear positioning and cyclin B destruction in budding yeast for the maintenance of genome integrity. The mitotic exit network (MEN) is a Ras-like signal transduction pathway that promotes this process during anaphase. A crucial step in MEN activation occurs when the Dbf2-Mob1 protein kinase complex associates with the Nud1 scaffold protein at the yeast spindle pole bodies (SPBs; centrosome equivalents) and thereby becomes activated. This requires prior priming phosphorylation of Nud1 by Cdc15 at SPBs. Cdc15 activation, in turn, requires both the Tem1 GTPase and the Polo kinase Cdc5, but how Cdc15 associates with SPBs is not well understood. We have identified a hyperactive allele of NUD1, nud1-A308T, that recruits Cdc15 to SPBs in all stages of the cell cycle in a CDC5-independent manner. This allele leads to early recruitment of Dbf2-Mob1 during metaphase and requires known Cdc15 phospho-sites on Nud1. The presence of nud1-A308T leads to loss of coupling between nuclear position and mitotic exit in cells with mispositioned spindles. Our findings highlight the importance of scaffold regulation in signaling pathways to prevent improper activation.

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McEwen, Bruce F., Gordon K. T. Chan, Beata Zubrowski, Matthew S. Savoian, Matthew T. Sauer et Tim J. Yen. « CENP-E Is Essential for Reliable Bioriented Spindle Attachment, but Chromosome Alignment Can Be Achieved via Redundant Mechanisms in Mammalian Cells ». Molecular Biology of the Cell 12, n^o 9 (septembre 2001) : 2776–89. http://dx.doi.org/10.1091/mbc.12.9.2776.

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CENP-E is a kinesin-like protein that when depleted from mammalian kinetochores leads to mitotic arrest with a mixture of aligned and unaligned chromosomes. In the present study, we used immunofluorescence, video, and electron microscopy to demonstrate that depletion of CENP-E from kinetochores via antibody microinjection reduces kinetochore microtubule binding by 23% at aligned chromosomes, and severely reduces microtubule binding at unaligned chromosomes. Disruption of CENP-E function also reduces tension across the centromere, increases the incidence of spindle pole fragmentation, and results in monooriented chromosomes approaching abnormally close to the spindle pole. Nevertheless, chromosomes show typical patterns of congression, fast poleward motion, and oscillatory motions. Furthermore, kinetochores of aligned and unaligned chromosomes exhibit normal patterns of checkpoint protein localization. These data are explained by a model in which redundant mechanisms enable kinetochore microtubule binding and checkpoint monitoring in the absence of CENP-E at kinetochores, but where reduced microtubule-binding efficiency, exacerbated by poor positioning at the spindle poles, results in chronically monooriented chromosomes and mitotic arrest. Chromosome position within the spindle appears to be a critical determinant of CENP-E function at kinetochores.

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Caydasi, Ayse Koca, Yagmur Micoogullari, Bahtiyar Kurtulmus, Saravanan Palani et Gislene Pereira. « The 14-3-3 protein Bmh1 functions in the spindle position checkpoint by breaking Bfa1 asymmetry at yeast centrosomes ». Molecular Biology of the Cell 25, n^o 14 (15 juillet 2014) : 2143–51. http://dx.doi.org/10.1091/mbc.e14-04-0890.

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In addition to their well-known role in microtubule organization, centrosomes function as signaling platforms and regulate cell cycle events. An important example of such a function is the spindle position checkpoint (SPOC) of budding yeast. SPOC is a surveillance mechanism that ensures alignment of the mitotic spindle along the cell polarity axis. Upon spindle misalignment, phosphorylation of the SPOC component Bfa1 by Kin4 kinase engages the SPOC by changing the centrosome localization of Bfa1 from asymmetric (one centrosome) to symmetric (both centrosomes). Here we show that, unexpectedly, Kin4 alone is unable to break Bfa1 asymmetry at yeast centrosomes. Instead, phosphorylation of Bfa1 by Kin4 creates a docking site on Bfa1 for the 14-3-3 family protein Bmh1, which in turn weakens Bfa1–centrosome association and promotes symmetric Bfa1 localization. Consistently, BMH1-null cells are SPOC deficient. Our work thus identifies Bmh1 as a new SPOC component and refines the molecular mechanism that breaks Bfa1 centrosome asymmetry upon SPOC activation.

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Nelson, Scott A., Anthony M. Sanson, Hay-Oak Park et John A. Cooper. « A Novel Role for the GTPase-Activating Protein Bud2 in the Spindle Position Checkpoint ». PLoS ONE 7, n^o 4 (25 avril 2012) : e36127. http://dx.doi.org/10.1371/journal.pone.0036127.

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Valerio-Santiago, Mauricio, et Fernando Monje-Casas. « Tem1 localization to the spindle pole bodies is essential for mitotic exit and impairs spindle checkpoint function ». Journal of Cell Biology 192, n^o 4 (14 février 2011) : 599–614. http://dx.doi.org/10.1083/jcb.201007044.

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The mitotic exit network (MEN) is a signaling cascade that triggers inactivation of the mitotic cyclin-dependent kinases and exit from mitosis. The GTPase Tem1 localizes on the spindle pole bodies (SPBs) and initiates MEN signaling. Tem1 activity is inhibited until anaphase by Bfa1-Bub2. These proteins are also part of the spindle position checkpoint (SPOC), a surveillance mechanism that restrains mitotic exit until the spindle is correctly positioned. Here, we show that regulation of Tem1 localization is essential for the proper function of the MEN and the SPOC. We demonstrate that the dynamics of Tem1 loading onto SPBs determine the recruitment of other MEN components to this structure, and reevaluate the interdependence in the localization of Tem1, Bfa1, and Bub2. We also find that removal of Tem1 from the SPBs is critical for the SPOC to impede cell cycle progression. Finally, we demonstrate for the first time that localization of Tem1 to the SPBs is a requirement for mitotic exit.

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Falk, Jill E., Ian W. Campbell, Kelsey Joyce, Jenna Whalen, Anupama Seshan et Angelika Amon. « LTE1 promotes exit from mitosis by multiple mechanisms ». Molecular Biology of the Cell 27, n^o 25 (15 décembre 2016) : 3991–4001. http://dx.doi.org/10.1091/mbc.e16-08-0563.

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In budding yeast, alignment of the anaphase spindle along the mother–bud axis is crucial for maintaining genome integrity. If the anaphase spindle becomes misaligned in the mother cell compartment, cells arrest in anaphase because the mitotic exit network (MEN), an essential Ras-like GTPase signaling cascade, is inhibited by the spindle position checkpoint (SPoC). Distinct localization patterns of MEN and SPoC components mediate MEN inhibition. Most components of the MEN localize to spindle pole bodies. If the spindle becomes mispositioned in the mother cell compartment, cells arrest in anaphase due to inhibition of the MEN by the mother cell–restricted SPoC kinase Kin4. Here we show that a bud-localized activating signal is necessary for full MEN activation. We identify Lte1 as this signal and show that Lte1 activates the MEN in at least two ways. It inhibits small amounts of Kin4 that are present in the bud via its central domain. An additional MEN-activating function of Lte1 is mediated by its N- and C-terminal GEF domains, which, we propose, directly activate the MEN GTPase Tem1. We conclude that control of the MEN by spindle position is exerted by both negative and positive regulatory elements that control the pathway’s GTPase activity.

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Li, Yan, et Kiwon Song. « The N-Terminal Domain of Bfa1 Coordinates Mitotic Exit Independent of GAP Activity in Saccharomyces cerevisiae ». Cells 11, n^o 14 (12 juillet 2022) : 2179. http://dx.doi.org/10.3390/cells11142179.

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The spindle position checkpoint (SPOC) of budding yeast delays mitotic exit in response to misaligned spindles to ensure cell survival and the maintenance of genomic stability. The GTPase-activating protein (GAP) complex Bfa1–Bub2, a key SPOC component, inhibits the GTPase Tem1 to induce mitotic arrest in response to DNA and spindle damage, as well as spindle misorientation. However, previous results strongly suggest that Bfa1 exerts a GAP-independent function in blocking mitotic exit in response to misaligned spindles. Thus, the molecular mechanism by which Bfa1 controls mitotic exit in response to misaligned spindles remains unclear. Here, we observed that overexpression of the N-terminal domain of Bfa1 (Bfa1-D16), which lacks GAP activity and cannot localize to the spindle pole body (SPB), induced cell cycle arrest along with hyper-elongation of astral microtubules (aMTs) as Bfa1 overexpression in Δbub2. We found that Δbub2 cells overexpressing Bfa1 or Bfa1-D16 inhibited activation of Mob1, which is responsible for mitotic exit. In anaphase-arrested cells, Bfa1-D16 overexpression inhibited Tem1 binding to the SPB as well as Bfa1 overexpression. Additionally, endogenous levels of Bfa1-D16 showed minor SPOC activity that was not regulated by Kin4. These results suggested that Bfa1-D16 may block mitotic exit through inhibiting Tem1 activity outside of SPBs. Alternatively, Bfa1-D16 dispersed out of SPBs may block Tem1 binding to SPBs by physically interacting with Tem1 as previously reported. Moreover, we observed hyper-elongated aMTs in tem1-3, cdc15-2, and dbf2-2 mutants that induce anaphase arrest and cannot undergo mitotic exit at restrictive temperatures, suggesting that aMT dynamics are closely related to the regulation of mitotic exit. Altogether, these observations suggest that Bfa1 can control the SPOC independent of its GAP activity and SPB localization.

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Falk, J. E., L. Y. Chan et A. Amon. « Lte1 promotes mitotic exit by controlling the localization of the spindle position checkpoint kinase Kin4 ». Proceedings of the National Academy of Sciences 108, n^o 31 (27 juin 2011) : 12584–90. http://dx.doi.org/10.1073/pnas.1107784108.

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West, Robert R., Terra Malmstrom et J. Richard McIntosh. « Kinesinsklp5+ andklp6+ are required for normal chromosome movement in mitosis ». Journal of Cell Science 115, n^o 5 (1 mars 2002) : 931–40. http://dx.doi.org/10.1242/jcs.115.5.931.

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Proper mitotic chromosome segregation requires dynamic interactions between spindle microtubules and kinetochores. Here we demonstrate that two related fission yeast kinesins, klp5+ and klp6+, are required for normal chromosome segregation in mitosis. Null mutants frequently lack a normal metaphase chromosome alignment. Chromosome pairs move back and forth along the spindle for an extended period prior to sister chromatid separation, a phenotype reminiscent of the loss of CENP-E in metazoans. Ultimately, sister chromatids segregate, regardless of chromosome position along the spindle, and viable daughter cells are usually produced. The initiation of anaphase B is sometimes delayed, but the rate of spindle elongation is similar to wildtype. Despite a delay, anaphase B often begins before anaphase A is completed. The klp5Δ and klp6Δ null mutants are synthetically lethal with a deletion of the spindle assembly checkpoint gene, bub1+, several mutants in components of the anaphase promoting complex, and a cold sensitive allele of the kinetochore and microtubule-binding protein, Dis1p. Klp5p-GFP and Klp6p-GFP localize to kinetochores from prophase to the onset of anaphase A, but relocalize to the spindle midzone during anaphase B. These data indicate that Klp5p and Klp6p are kinetochore kinesins required for normal chromosome movement in prometaphase.

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Kadura, Sheila, Xiangwei He, Vincent Vanoosthuyse, Kevin G. Hardwick et Shelley Sazer. « The A78V Mutation in the Mad3-like Domain of Schizosaccharomyces pombe Bub1p Perturbs Nuclear Accumulation and Kinetochore Targeting of Bub1p, Bub3p, and Mad3p and Spindle Assembly Checkpoint Function ». Molecular Biology of the Cell 16, n^o 1 (janvier 2005) : 385–95. http://dx.doi.org/10.1091/mbc.e04-07-0558.

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During mitosis, the spindle assembly checkpoint (SAC) responds to faulty attachments between kinetochores and the mitotic spindle by imposing a metaphase arrest until the defect is corrected, thereby preventing chromosome missegregation. A genetic screen to isolate SAC mutants in fission yeast yielded point mutations in three fission yeast SAC genes: mad1, bub3, and bub1. The bub1-A78V mutant is of particular interest because it produces a wild-type amount of protein that is mutated in the conserved but uncharacterized Mad3-like region of Bub1p. Characterization of mutant cells demonstrates that the alanine at position 78 in the Mad3-like domain of Bub1p is required for: 1) cell cycle arrest induced by SAC activation; 2) kinetochore accumulation of Bub1p in checkpoint-activated cells; 3) recruitment of Bub3p and Mad3p, but not Mad1p, to kinetochores in checkpoint-activated cells; and 4) nuclear accumulation of Bub1p, Bub3p, and Mad3p, but not Mad1p, in cycling cells. Increased targeting of Bub1p-A78V to the nucleus by an exogenous nuclear localization signal does not significantly increase kinetochore localization or SAC function, but GFP fused to the isolated Bub1p Mad 3-like accumulates in the nucleus. These data indicate that Bub1p-A78V is defective in both nuclear accumulation and kinetochore targeting and that a threshold level of nuclear Bub1p is necessary for the nuclear accumulation of Bub3p and Mad3p.

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Vukušić, Kruno, et Iva M. Tolić. « Polar Chromosomes—Challenges of a Risky Path ». Cells 11, n^o 9 (3 mai 2022) : 1531. http://dx.doi.org/10.3390/cells11091531.

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The process of chromosome congression and alignment is at the core of mitotic fidelity. In this review, we discuss distinct spatial routes that the chromosomes take to align during prometaphase, which are characterized by distinct biomolecular requirements. Peripheral polar chromosomes are an intriguing case as their alignment depends on the activity of kinetochore motors, polar ejection forces, and a transition from lateral to end-on attachments to microtubules, all of which can result in the delayed alignment of these chromosomes. Due to their undesirable position close to and often behind the spindle pole, these chromosomes may be particularly prone to the formation of erroneous kinetochore-microtubule interactions, such as merotelic attachments. To prevent such errors, the cell employs intricate mechanisms to preposition the spindle poles with respect to chromosomes, ensure the formation of end-on attachments in restricted spindle regions, repair faulty attachments by error correction mechanisms, and delay segregation by the spindle assembly checkpoint. Despite this protective machinery, there are several ways in which polar chromosomes can fail in alignment, mis-segregate, and lead to aneuploidy. In agreement with this, polar chromosomes are present in certain tumors and may even be involved in the process of tumorigenesis.

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Shannon, Katie B., Julie C. Canman, C. Ben Moree, Jennifer S. Tirnauer et E. D. Salmon. « Taxol-stabilized Microtubules Can Position the Cytokinetic Furrow in Mammalian Cells ». Molecular Biology of the Cell 16, n^o 9 (septembre 2005) : 4423–36. http://dx.doi.org/10.1091/mbc.e04-11-0974.

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How microtubules act to position the plane of cell division during cytokinesis is a topic of much debate. Recently, we showed that a subpopulation of stable microtubules extends past chromosomes and interacts with the cell cortex at the site of furrowing, suggesting that these stabilized microtubules may stimulate contractility. To test the hypothesis that stable microtubules can position furrows, we used taxol to rapidly suppress microtubule dynamics during various stages of mitosis in PtK1 cells. Cells with stabilized prometaphase or metaphase microtubule arrays were able to initiate furrowing when induced into anaphase by inhibition of the spindle checkpoint. In these cells, few microtubules contacted the cortex. Furrows formed later than usual, were often aberrant, and did not progress to completion. Images showed that furrowing correlated with the presence of one or a few stable spindle microtubule plus ends at the cortex. Actin, myosin II, and anillin were all concentrated in these furrows, demonstrating that components of the contractile ring can be localized by stable microtubules. Inner centromere protein (INCENP) was not found in these ingressions, confirming that INCENP is dispensable for furrow positioning. Taxol-stabilization of the numerous microtubule-cortex interactions after anaphase onset delayed furrow initiation but did not perturb furrow positioning. We conclude that taxol-stabilized microtubules can act to position the furrow and that loss of microtubule dynamics delays the timing of furrow onset and prevents completion. We discuss our findings relative to models for cleavage stimulation.

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Geymonat, Marco, Adonis Spanos, Geoffroy de Bettignies et Steven G. Sedgwick. « Lte1 contributes to Bfa1 localization rather than stimulating nucleotide exchange by Tem1 ». Journal of Cell Biology 187, n^o 4 (16 novembre 2009) : 497–511. http://dx.doi.org/10.1083/jcb.200905114.

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Lte1 is a mitotic regulator long envisaged as a guanosine nucleotide exchange factor (GEF) for Tem1, the small guanosine triphosphatase governing activity of the Saccharomyces cerevisiae mitotic exit network. We demonstrate that this model requires reevaluation. No GEF activity was detectable in vitro, and mutational analysis of Lte1’s putative GEF domain indicated that Lte1 activity relies on interaction with Ras for localization at the bud cortex rather than providing nucleotide exchange. Instead, we found that Lte1 can determine the subcellular localization of Bfa1 at spindle pole bodies (SPBs). Under conditions in which Lte1 is essential, Lte1 promoted the loss of Bfa1 from the maternal SPB. Moreover, in cells with a misaligned spindle, mislocalization of Lte1 in the mother cell promoted loss of Bfa1 from one SPB and allowed bypass of the spindle position checkpoint. We observed that lte1 mutants display aberrant localization of the polarity cap, which is the organizer of the actin cytoskeleton. We propose that Lte1’s role in cell polarization underlies its contribution to mitotic regulation.

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Nicklas, R. B., et P. Arana. « Evolution and the meaning of metaphase ». Journal of Cell Science 102, n^o 4 (1 août 1992) : 681–90. http://dx.doi.org/10.1242/jcs.102.4.681.

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We used an evolutionary test to ask whether the congression of chromosomes to the spindle equator is important in itself or just a mitotic happenstance. If congression matters, then it might evolve if absent initially. Previous workers established that newly made trivalents, meiotic units of three chromosomes, generally do not congress to the spindle equator. Instead, these young trivalents lie close to the pole to which two of the three chromosomes are oriented. We studied ancient sex-chromosome trivalents that arose hundreds of thousands to several million years ago in several species of praying mantids and one grasshopper. All these old trivalents lie near the spindle equator at metaphase; some of them congress as precisely to the equator as the ordinary chromosomes in the same cells. We conclude that congression evolved independently two or three times in the materials studied. Therefore, the metaphase position of chromosomes midway between the poles appears to matter, but why? In the praying mantids, the evident answer is that metaphase is a quality-control checkpoint. Sometimes the three chromosomes are not associated in a trivalent but rather are present as a bivalent plus an unpaired chromosome, which lies near one pole. Earlier workers showed that such cells are blocked in metaphase and eventually degenerate; this prevents the formation of sperm with abnormal combinations of sex chromosomes. We suggest that the quality-control system would have trouble distinguishing an unpaired chromosome from an uncongressed, newly arisen trivalent, both of which would lie near a spindle pole. If so, the confused quality-control system would block anaphase imprudently, causing a loss of cells that would have produced normal sperm. Hence, we conclude that the congression of the trivalent to the equator probably evolved along with the metaphase quality-control checkpoint. The mechanism of congression in old trivalents is uncertain, but probably involves an interesting force-sensitive regulation of the motors associated with particular chromosomes. We also examined the congression of two newly made quadrivalents when they orient with three kinetochores to one pole and one to the other. As others have described, one of these quadrivalents does not congress, while the other quadrivalent comes closer than expected to the spindle equator. Such variation in the extent of congression may provide materials on which natural selection can act, leading to the evolution of congression. The trivalents of praying mantids are attractive materials for further studies of the mechanism of congression and of the idea that metaphase is a checkpoint for progression through the cell cycle.

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Merlini, Laura, Roberta Fraschini, Barbara Boettcher, Yves Barral, Giovanna Lucchini et Simonetta Piatti. « Budding Yeast Dma Proteins Control Septin Dynamics and the Spindle Position Checkpoint by Promoting the Recruitment of the Elm1 Kinase to the Bud Neck ». PLoS Genetics 8, n^o 4 (26 avril 2012) : e1002670. http://dx.doi.org/10.1371/journal.pgen.1002670.

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Cook, Diana, Sarah Long, John Stanton, Patrick Cusick, Colleen Lawrimore, Elaine Yeh, Sarah Grant et Kerry Bloom. « Behavior of dicentric chromosomes in budding yeast ». PLOS Genetics 17, n^o 3 (18 mars 2021) : e1009442. http://dx.doi.org/10.1371/journal.pgen.1009442.

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DNA double-strand breaks arisein vivowhen a dicentric chromosome (two centromeres on one chromosome) goes through mitosis with the two centromeres attached to opposite spindle pole bodies. Repair of the DSBs generates phenotypic diversity due to the range of monocentric derivative chromosomes that arise. To explore whether DSBs may be differentially repaired as a function of their spatial position in the chromosome, we have examined the structure of monocentric derivative chromosomes from cells containing a suite of dicentric chromosomes in which the distance between the two centromeres ranges from 6.5 kb to 57.7 kb. Two major classes of repair products, homology-based (homologous recombination (HR) and single-strand annealing (SSA)) and end-joining (non-homologous (NHEJ) and micro-homology mediated (MMEJ)) were identified. The distribution of repair products varies as a function of distance between the two centromeres. Genetic dependencies on double strand break repair (Rad52), DNA ligase (Lif1), and S phase checkpoint (Mrc1) are indicative of distinct repair pathway choices for DNA breaks in the pericentromeric chromatin versus the arms.

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Lew, Daniel J., et Daniel J. Burke. « The Spindle Assembly and Spindle Position Checkpoints ». Annual Review of Genetics 37, n^o 1 (décembre 2003) : 251–82. http://dx.doi.org/10.1146/annurev.genet.37.042203.120656.

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Chang, F., A. Woollard et P. Nurse. « Isolation and characterization of fission yeast mutants defective in the assembly and placement of the contractile actin ring ». Journal of Cell Science 109, n^o 1 (1 janvier 1996) : 131–42. http://dx.doi.org/10.1242/jcs.109.1.131.

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Fission yeast cells divide by medial cleavage using an actin-based contractile ring. We have conducted a genetic screen for temperature-sensitive mutants defective in the assembly and placement of this actin ring. Six genes necessary for actin ring formation and one gene necessary for placement of the actin ring have now been identified. The genes can be further organized into different phenotypic groups, suggesting that the gene products may have different functions in actin ring formation. Mutants of cdc3 and cdc8, which encode profilin and tropomyosin respectively, display disorganized actin patches in all cells. cdc12 and cdc15 mutants display disorganized actin patches during mitosis, but normal interphase actin patterns. cdc4 and rng2 mutants display disorganized actin cables during mitosis, but normal interphase actin patterns. In mid1 mutants, the actin ring and septum are positioned at random locations and angles on the cell surface, although the nucleus is positioned normally, indicating that the mid1 gene product is required to couple the division site to the position of the nucleus. mid1 mutant cells may reveal a new cell cycle checkpoint in telophase that coordinates cell division and the proper distribution of nuclei. The actin ring forms medially in a beta-tubulin mutant, showing that actin ring formation and placement are not dependent on the mitotic spindle.

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Garvanska, Dimitriya H., et Jakob Nilsson. « Specificity determinants of phosphoprotein phosphatases controlling kinetochore functions ». Essays in Biochemistry 64, n^o 2 (5 juin 2020) : 325–36. http://dx.doi.org/10.1042/ebc20190065.

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Abstract Kinetochores are instrumental for accurate chromosome segregation by binding to microtubules in order to move chromosomes and by delaying anaphase onset through the spindle assembly checkpoint (SAC). Dynamic phosphorylation of kinetochore components is key to control these activities and is tightly regulated by temporal and spatial recruitment of kinases and phosphoprotein phosphatases (PPPs). Here we focus on PP1, PP2A-B56 and PP2A-B55, three PPPs that are important regulators of mitosis. Despite the fact that these PPPs share a very similar active site, they target unique ser/thr phosphorylation sites to control kinetochore function. Specificity is in part achieved by PPPs binding to short linear motifs (SLiMs) that guide their substrate specificity. SLiMs bind to conserved pockets on PPPs and are degenerate in nature, giving rise to a range of binding affinities. These SLiMs control the assembly of numerous substrate specifying complexes and their position and binding strength allow PPPs to target specific phosphorylation sites. In addition, the activity of PPPs is regulated by mitotic kinases and inhibitors, either directly at the activity level or through affecting PPP–SLiM interactions. Here, we discuss recent progress in understanding the regulation of PPP specificity and activity and how this controls kinetochore biology.

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Howell, Rowan S. M., Cinzia Klemm, Peter H. Thorpe et Attila Csikász-Nagy. « Unifying the mechanism of mitotic exit control in a spatiotemporal logical model ». PLOS Biology 18, n^o 11 (12 novembre 2020) : e3000917. http://dx.doi.org/10.1371/journal.pbio.3000917.

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The transition from mitosis into the first gap phase of the cell cycle in budding yeast is controlled by the Mitotic Exit Network (MEN). The network interprets spatiotemporal cues about the progression of mitosis and ensures that release of Cdc14 phosphatase occurs only after completion of key mitotic events. The MEN has been studied intensively; however, a unified understanding of how localisation and protein activity function together as a system is lacking. In this paper, we present a compartmental, logical model of the MEN that is capable of representing spatial aspects of regulation in parallel to control of enzymatic activity. We show that our model is capable of correctly predicting the phenotype of the majority of mutants we tested, including mutants that cause proteins to mislocalise. We use a continuous time implementation of the model to demonstrate that Cdc14 Early Anaphase Release (FEAR) ensures robust timing of anaphase, and we verify our findings in living cells. Furthermore, we show that our model can represent measured cell–cell variation in Spindle Position Checkpoint (SPoC) mutants. This work suggests a general approach to incorporate spatial effects into logical models. We anticipate that the model itself will be an important resource to experimental researchers, providing a rigorous platform to test hypotheses about regulation of mitotic exit.

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Varma, Dileep, Xiaohu Wan, Dhanya Cheerambathur, Reto Gassmann, Aussie Suzuki, Josh Lawrimore, Arshad Desai et E. D. Salmon. « Spindle assembly checkpoint proteins are positioned close to core microtubule attachment sites at kinetochores ». Journal of Cell Biology 202, n^o 5 (26 août 2013) : 735–46. http://dx.doi.org/10.1083/jcb.201304197.

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Spindle assembly checkpoint proteins have been thought to reside in the peripheral corona region of the kinetochore, distal to microtubule attachment sites at the outer plate. However, recent biochemical evidence indicates that checkpoint proteins are closely linked to the core kinetochore microtubule attachment site comprised of the Knl1–Mis12–Ndc80 (KMN) complexes/KMN network. In this paper, we show that the Knl1–Zwint1 complex is required to recruit the Rod–Zwilch–Zw10 (RZZ) and Mad1–Mad2 complexes to the outer kinetochore. Consistent with this, nanometer-scale mapping indicates that RZZ, Mad1–Mad2, and the C terminus of the dynein recruitment factor Spindly are closely juxtaposed with the KMN network in metaphase cells when their dissociation is blocked and the checkpoint is active. In contrast, the N terminus of Spindly is ∼75 nm outside the calponin homology domain of the Ndc80 complex. These results reveal how checkpoint proteins are integrated within the substructure of the kinetochore and will aid in understanding the coordination of microtubule attachment and checkpoint signaling during chromosome segregation.

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Zivadinovic, Radomir, Aleksandra Petric, Goran Lilic, Vekoslav Lilic et Biljana Djordjevic. « Persistent human papillomavirus infection in the etiology of cervical carcinoma : The role of immunological, genetic, viral and cellular factors ». Srpski arhiv za celokupno lekarstvo 142, n^o 5-6 (2014) : 378–83. http://dx.doi.org/10.2298/sarh1406378z.

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The aim of this paper was to present the role of human papillomavirus (HPV) in cervical carcinogenesis from several aspects. By explaining the HPV virus lifecycle and structure, its effect on cervical cell cycle and subversion of immune response can be better understood. Early E region of the viral genome encodes proteins that are directly involved in carcinogenesis. The E6 protein binds to p53 protein (product of tumor-suppressor gene) blocking and degrading it, which in turn prevents cell cycle arrest and apoptosis induction. E6 is also capable of telomerase activation, which leads to cell immortalization; it also reacts with host proto-oncogene c-jun, responsible for transcription, shortens G1 phase and speeds up the transition from G1 to S phase of the cells infected by HPV. E7 forms bonds with retinoblastoma protein (product of tumor-suppressor gene) and inactivates it. It can inactivate cyclin inhibitors p21, p27, and abrogate the mitotic spindle checkpoint with the loss of protective effect of pRB and p53. The immune system cannot initiate early immunological reaction since the virus is non-lytic, while the concentration of viral proteins - antigens is low and has a basal intracellular position. Presentation through Langerhans cells (LC) is weak, because the number of these cells is low due to the effect of HPV. E7 HPV reduces the expression of E-cadherin, which is responsible for LC adhesion to HPVtransformed keratinocytes. Based on these considerations, it may be concluded that the process of cervical carcinogenesis includes viral, genetic, cellular, molecular-biological, endocrine, exocrine and immunological factors.

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Maekawa, Hiromi, Claire Priest, Johannes Lechner, Gislene Pereira et Elmar Schiebel. « The yeast centrosome translates the positional information of the anaphase spindle into a cell cycle signal ». Journal of Cell Biology 179, n^o 3 (29 octobre 2007) : 423–36. http://dx.doi.org/10.1083/jcb.200705197.

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The spindle orientation checkpoint (SPOC) of budding yeast delays mitotic exit when cytoplasmic microtubules (MTs) are defective, causing the spindle to become misaligned. Delay is achieved by maintaining the activity of the Bfa1–Bub2 guanosine triphosphatase–activating protein complex, an inhibitor of mitotic exit. In this study, we show that the spindle pole body (SPB) component Spc72, a transforming acidic coiled coil–like molecule that interacts with the γ-tubulin complex, recruits Kin4 kinase to both SPBs when cytoplasmic MTs are defective. This allows Kin4 to phosphorylate the SPB-associated Bfa1, rendering it resistant to inactivation by Cdc5 polo kinase. Consistently, forced targeting of Kin4 to both SPBs delays mitotic exit even when the anaphase spindle is correctly aligned. Moreover, we present evidence that Spc72 has an additional function in SPOC regulation that is independent of the recruitment of Kin4. Thus, Spc72 provides a missing link between cytoplasmic MT function and components of the SPOC.

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Lara-Gonzalez, Pablo, Taekyung Kim, Karen Oegema, Kevin Corbett et Arshad Desai. « A tripartite mechanism catalyzes Mad2-Cdc20 assembly at unattached kinetochores ». Science 371, n^o 6524 (31 décembre 2020) : 64–67. http://dx.doi.org/10.1126/science.abc1424.

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During cell division, kinetochores couple chromosomes to spindle microtubules. To protect against chromosome gain or loss, kinetochores lacking microtubule attachment locally catalyze association of the checkpoint proteins Cdc20 and Mad2, which is the key event in the formation of a diffusible checkpoint complex that prevents mitotic exit. We elucidated the mechanism of kinetochore-catalyzed Mad2-Cdc20 assembly with a probe that specifically monitors this assembly reaction at kinetochores in living cells. We found that catalysis occurs through a tripartite mechanism that includes localized delivery of Mad2 and Cdc20 substrates and two phosphorylation-dependent interactions that geometrically constrain their positions and prime Cdc20 for interaction with Mad2. These results reveal how unattached kinetochores create a signal that ensures genome integrity during cell division.

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Tan, Chia Huei, Ivana Gasic, Sabina P. Huber-Reggi, Damian Dudka, Marin Barisic, Helder Maiato et Patrick Meraldi. « The equatorial position of the metaphase plate ensures symmetric cell divisions ». eLife 4 (18 juillet 2015). http://dx.doi.org/10.7554/elife.05124.

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Chromosome alignment in the middle of the bipolar spindle is a hallmark of metazoan cell divisions. When we offset the metaphase plate position by creating an asymmetric centriole distribution on each pole, we find that metaphase plates relocate to the middle of the spindle before anaphase. The spindle assembly checkpoint enables this centering mechanism by providing cells enough time to correct metaphase plate position. The checkpoint responds to unstable kinetochore–microtubule attachments resulting from an imbalance in microtubule stability between the two half-spindles in cells with an asymmetric centriole distribution. Inactivation of the checkpoint prior to metaphase plate centering leads to asymmetric cell divisions and daughter cells of unequal size; in contrast, if the checkpoint is inactivated after the metaphase plate has centered its position, symmetric cell divisions ensue. This indicates that the equatorial position of the metaphase plate is essential for symmetric cell divisions.

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Metchat, Aïcha, Manuel Eguren, Julius M. Hossain, Antonio Z. Politi, Sébastien Huet et Jan Ellenberg. « An actin-dependent spindle position checkpoint ensures the asymmetric division in mouse oocytes ». Nature Communications 6, n^o 1 (15 juillet 2015). http://dx.doi.org/10.1038/ncomms8784.

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Abstract Faithful chromosome segregation, during meiosis, is of critical importance to prevent aneuploidy in the resulting embryo. In mammalian oocytes, the segregation of homologous chromosomes takes place with the spindle located at the cell’s periphery. The spindle is often assembled close to the centre of the cell, which necessitates the actin network for spindle transport to the cell cortex. In this study, we investigate how the segregation of chromosomes is coordinated with the positioning of the metaphase I spindle. We develop different assays to perturb the spindle’s position and to delay its relocation to the cell periphery. We find that anaphase is delayed until the spindle is positioned in close proximity with the oocyte cortex. We further show that the metaphase arrest is dependent on a functional actin network, in addition to the spindle assembly checkpoint. Our work provides the first evidence for the existence of a functional spindle position checkpoint.

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