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Tang J, Fu M, Chen X, Zhao Y, Gao L, Cao H, Li X, Zheng SJ, Wang Y. Arrest of Cell Cycle by Avian Reovirus p17 through Its Interaction with Bub3. Viruses 2022; 14:v14112385. [PMID: 36366482 PMCID: PMC9693402 DOI: 10.3390/v14112385] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2022] [Revised: 10/25/2022] [Accepted: 10/25/2022] [Indexed: 01/31/2023] Open
Abstract
Avian reoviruses (ARV) are a group of poultry pathogens that cause runting and stunting syndrome (RSS), a condition otherwise known as "frozen chicken", which are characterized by dramatically delayed growth in broilers. It has been known that p17, a nonstructural protein encoded by ARV, prohibits cellular proliferation by halting the cell cycle at the G2/M phase, the result of which is directly associated with the typical clinical sign of RSS. Nevertheless, the mechanism by which p17 modulates cell-cycle progression remains largely unknown. Here, we screened the interactome of ectopically expressed p17 through a yeast two-hybrid assay and identified Bub3, a cellular mitotic checkpoint protein, as a binding partner of p17. The infection of the Vero cells by ARV downregulated the Bub3 expression, while the knockdown of Bub3 alleviated the p17-modulated cell-cycle arrest during ARV infection. Remarkably, the suppression of Bub3 by RNAi in the Vero cells significantly reduced the viral mRNA and protein abundance, which eventually led to diminished virus replication. Altogether, our findings reveal that ARV p17 impedes host cell proliferation through a Bub3-dependent cell-cycle arrest, which eventually contributes to efficient virus replication. These results also unveil a hitherto unknown therapeutic target for RSS.
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Affiliation(s)
- Junyu Tang
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Mengjiao Fu
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Xiang Chen
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Yimeng Zhao
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Li Gao
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Hong Cao
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Xiaoqi Li
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Shijun J. Zheng
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
| | - Yongqiang Wang
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, China Agricultural University, Beijing 100193, China
- College of Veterinary Medicine, China Agricultural University, Beijing 100193, China
- Correspondence: ; Tel./Fax: +86-(10)-6273-3026
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2
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Deng DJ, Wang X, Yue KY, Wang Y, Jin QW. Analysis of the potential role of fission yeast PP2A in spindle assembly checkpoint inactivation. FASEB J 2022; 36:e22524. [PMID: 36006032 DOI: 10.1096/fj.202101884r] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2021] [Revised: 07/26/2022] [Accepted: 08/17/2022] [Indexed: 11/11/2022]
Abstract
As a surveillance mechanism, the activated spindle assembly checkpoint (SAC) potently inhibits the E3 ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) to ensure accurate chromosome segregation. Although the protein phosphatase 2A (PP2A) has been proposed to be both, directly and indirectly, involved in spindle assembly checkpoint inactivation in mammalian cells, whether it is similarly operating in the fission yeast Schizosaccharomycer pombe has never been demonstrated. Here, we investigated whether fission yeast PP2A is involved in SAC silencing by following the rate of cyclin B (Cdc13) destruction at SPBs during the recovery phase in nda3-KM311 cells released from the inhibition of APC/C by the activated spindle checkpoint. The timing of the SAC inactivation is only slightly delayed when two B56 regulatory subunits (Par1 and Par2) of fission yeast PP2A are absent. Overproduction of individual PP2A subunits either globally in the nda3-KM311 arrest-and-release system or locally in the synthetic spindle checkpoint activation system only slightly suppresses the SAC silencing defects in PP1 deletion (dis2Δ) cells. Our study thus demonstrates that the fission yeast PP2A is not a key regulator actively involved in SAC inactivation.
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Affiliation(s)
- Da-Jie Deng
- State Key Laboratory of Cellular Stress Biology, School of Life Sciences, Faculty of Medicine and Life Sciences, Xiamen University, Xiamen, China
| | - Xi Wang
- State Key Laboratory of Cellular Stress Biology, School of Life Sciences, Faculty of Medicine and Life Sciences, Xiamen University, Xiamen, China
| | - Kai-Ye Yue
- State Key Laboratory of Cellular Stress Biology, School of Life Sciences, Faculty of Medicine and Life Sciences, Xiamen University, Xiamen, China
| | - Yamei Wang
- State Key Laboratory of Cellular Stress Biology, School of Life Sciences, Faculty of Medicine and Life Sciences, Xiamen University, Xiamen, China
| | - Quan-Wen Jin
- State Key Laboratory of Cellular Stress Biology, School of Life Sciences, Faculty of Medicine and Life Sciences, Xiamen University, Xiamen, China
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3
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Zhang H, Deng X, Sun B, Lee Van S, Kang Z, Lin H, Lee YRJ, Liu B. Role of the BUB3 protein in phragmoplast microtubule reorganization during cytokinesis. NATURE PLANTS 2018; 4:485-494. [PMID: 29967519 DOI: 10.1038/s41477-018-0192-z] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/14/2017] [Accepted: 06/04/2018] [Indexed: 05/15/2023]
Abstract
The evolutionarily conserved WD40 protein budding uninhibited by benzimidazole 3 (BUB3) is known for its function in spindle assembly checkpoint control. In the model plant Arabidopsis thaliana, nearly identical BUB3;1 and BUB3;2 proteins decorated the phragmoplast midline through interaction with the microtubule-associated protein MAP65-3 during cytokinesis. BUB3;1 and BUB3;2 interacted with the carboxy-terminal segment of MAP65-3 (but not MAP65-1), which harbours its microtubule-binding domain for its post-mitotic localization. Reciprocally, BUB3;1 and BUB3;2 also regulated MAP65-3 localization in the phragmoplast by enhancing its microtubule association. In the bub3;1 bub3;2 double mutant, MAP65-3 localization was often dissipated away from the phragmoplast midline and abolished upon treatment of low doses of the cytokinesis inhibitory drug caffeine that were tolerated by the control plant. The phragmoplast microtubule array exhibited uncoordinated expansion pattern in the double mutant cells as the phragmoplast edge reached the parental plasma membrane at different times in different areas. Upon caffeine treatment, phragmoplast expansion was halted as if the microtubule array was frozen. As a result, cytokinesis was abolished due to failed cell plate assembly. Our findings have uncovered a novel function of the plant BUB3 in MAP65-3-dependent microtubule reorganization during cytokinesis.
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Affiliation(s)
- Hongchang Zhang
- State Key Laboratory of Crop Stress Biology for Arid Areas and College of Life Sciences, Northwest A&F University, Yangling, Shaanxi, China
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA
| | - Xingguang Deng
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA
- College of Life Sciences, Sichuan University, Chengdu, Sichuan, China
| | - Baojuan Sun
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA
- Vegetable Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou, Guangdong, China
| | - Sonny Lee Van
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA
| | - Zhensheng Kang
- State Key Laboratory of Crop Stress Biology for Arid Areas and College of Plant Protection, Northwest A&F University, Yangling, Shaanxi, China
| | - Honghui Lin
- College of Life Sciences, Sichuan University, Chengdu, Sichuan, China
| | - Yuh-Ru Julie Lee
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA.
| | - Bo Liu
- Department of Plant Biology, College of Biological Sciences, University of California, Davis, CA, USA.
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4
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Niwa O. Determination of the Frequency of Minichromosome Loss to Assess Chromosome Stability in Fission Yeast. Cold Spring Harb Protoc 2018; 2018:pdb.prot091991. [PMID: 27343268 DOI: 10.1101/pdb.prot091991] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Quantitative assessment of chromosome stability in specific genetic backgrounds or under conditions of environmental stress can be addressed by direct cytological examination of chromosome transmission errors (using live or fixed imaging); however, in many cases, this is impractical, particularly when the rate of loss is low. Model chromosomes that allow simple and convenient assessment of chromosome stability are therefore useful. Ch16 is a 530-kb minichromosome constructed by the deletion of large portions of chromosome 3 termini. Ch16 carries the ade6-M216 allele, which interallelically complements the ade6-M210 mutation. Hence, Ade+ is an indication of the presence of Ch16, and Ade- indicates its loss. Ade+ and Ade- are phenotypically discernible as white and red colonies, respectively, on media containing limiting amounts of adenine. When a single cell bearing Ch16 divides on a plate to give rise to two daughter cells, one of which has lost Ch16, it will result in the formation of a half-sectored colony (half of the colony is red and the other half is white). The frequency of half-sectored colonies provides an accurate estimate of mitotic minichromosome loss per cell division. This protocol describes a method to determine half-sectored colony frequency and potential problems associated with the method.
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Affiliation(s)
- Osami Niwa
- Kazusa DNA Research Institute, Chiba 292-0818, Japan.,RIKEN, Wako, Saitama 351-0198, Japan
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5
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Generation of a Spindle Checkpoint Arrest from Synthetic Signaling Assemblies. Curr Biol 2016; 27:137-143. [PMID: 28017606 PMCID: PMC5226922 DOI: 10.1016/j.cub.2016.11.014] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2016] [Revised: 10/14/2016] [Accepted: 11/03/2016] [Indexed: 11/24/2022]
Abstract
The spindle checkpoint acts as a mitotic surveillance system, monitoring interactions between kinetochores and spindle microtubules and ensuring high-fidelity chromosome segregation [1, 2, 3]. The checkpoint is activated by unattached kinetochores, and Mps1 kinase phosphorylates KNL1 on conserved MELT motifs to generate a binding site for the Bub3-Bub1 complex [4, 5, 6, 7]. This leads to dynamic kinetochore recruitment of Mad proteins [8, 9], a conformational change in Mad2 [10, 11, 12], and formation of the mitotic checkpoint complex (MCC: Cdc20-Mad3-Mad2 [13, 14, 15]). MCC formation inhibits the anaphase-promoting complex/cyclosome (Cdc20-APC/C), thereby preventing the proteolytic destruction of securin and cyclin and delaying anaphase onset. What happens at kinetochores after Mps1-dependent Bub3-Bub1 recruitment remains mechanistically unclear, and it is not known whether kinetochore proteins other than KNL1 have significant roles to play in checkpoint signaling and MCC generation. Here, we take a reductionist approach, avoiding the complexities of kinetochores, and demonstrate that co-recruitment of KNL1Spc7 and Mps1Mph1 is sufficient to generate a robust checkpoint signal and prolonged mitotic arrest. We demonstrate that a Mad1-Bub1 complex is formed during synthetic checkpoint signaling. Analysis of bub3Δ mutants demonstrates that Bub3 acts to suppress premature checkpoint signaling. This synthetic system will enable detailed, mechanistic dissection of MCC generation and checkpoint silencing. After analyzing several mutants that affect localization of checkpoint complexes, we conclude that spindle checkpoint arrest can be independent of their kinetochore, spindle pole, and nuclear envelope localization. Synthetic signaling scaffolds generate a spindle checkpoint arrest The combination of KNL1Spc7 and Mps1Mph1 kinase generates a robust arrest Kinetochore, spindle, and nuclear envelope enrichment of the scaffold is not required Bub3 acts to inhibit premature checkpoint activation
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6
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Bub3-Bub1 Binding to Spc7/KNL1 Toggles the Spindle Checkpoint Switch by Licensing the Interaction of Bub1 with Mad1-Mad2. Curr Biol 2016; 26:2642-2650. [PMID: 27618268 DOI: 10.1016/j.cub.2016.07.040] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2016] [Revised: 06/22/2016] [Accepted: 07/15/2016] [Indexed: 01/06/2023]
Abstract
The spindle assembly checkpoint (SAC) ensures that sister chromatids do not separate until all chromosomes are attached to spindle microtubules and bi-oriented. Spindle checkpoint proteins, including Mad1, Mad2, Mad3 (BubR1), Bub1, Bub3, and Mph1 (Mps1), are recruited to unattached and/or tensionless kinetochores. SAC activation catalyzes the conversion of soluble Mad2 (O-Mad2) into a form (C-Mad2) that binds Cdc20, BubR1, and Bub3 to form the mitotic checkpoint complex (MCC), a potent inhibitor of the anaphase-promoting complex (APC/C). SAC silencing de-represses Cdc20-APC/C activity allowing poly-ubiquitination of Securin and Cyclin B, leading to the dissolution of sister chromatids and anaphase onset [1]. Understanding how microtubule interaction at kinetochores influences the timing of anaphase requires an understanding of how spindle checkpoint protein interaction with the kinetochore influences spindle checkpoint signaling. We, and others, recently showed that Mph1 (Mps1) phosphorylates multiple conserved MELT motifs in the Spc7 (Spc105/KNL1) protein to recruit Bub1, Bub3, and Mad3 (BubR1) to kinetochores [2-4]. In budding yeast, Mps1 phosphorylation of a central non-catalytic region of Bub1 promotes its association with the Mad1-Mad2 complex, although this association has not yet been detected in other organisms [5]. Here we report that multisite binding of Bub3 to the Spc7 MELT array toggles the spindle checkpoint switch by permitting Mph1 (Mps1)-dependent interaction of Bub1 with Mad1-Mad2.
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7
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Tange Y, Chikashige Y, Takahata S, Kawakami K, Higashi M, Mori C, Kojidani T, Hirano Y, Asakawa H, Murakami Y, Haraguchi T, Hiraoka Y. Inner nuclear membrane protein Lem2 augments heterochromatin formation in response to nutritional conditions. Genes Cells 2016; 21:812-32. [PMID: 27334362 DOI: 10.1111/gtc.12385] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2016] [Accepted: 05/08/2016] [Indexed: 12/29/2022]
Abstract
Inner nuclear membrane proteins interact with chromosomes in the nucleus and are important for chromosome activity. Lem2 and Man1 are conserved members of the LEM-domain nuclear membrane protein family. Mutations of LEM-domain proteins are associated with laminopathy, but their cellular functions remain unclear. Here, we report that Lem2 maintains genome stability in the fission yeast Schizosaccharomyces pombe. S. pombe cells disrupted for the lem2(+) gene (lem2∆) showed slow growth and increased rate of the minichromosome loss. These phenotypes were prominent in the rich culture medium, but not in the minimum medium. Centromeric heterochromatin formation was augmented upon transfer to the rich medium in wild-type cells. This augmentation of heterochromatin formation was impaired in lem2∆ cells. Notably, lem2∆ cells occasionally exhibited spontaneous duplication of genome sequences flanked by the long-terminal repeats of retrotransposons. The resulting duplication of the lnp1(+) gene, which encodes an endoplasmic reticulum membrane protein, suppressed lem2∆ phenotypes, whereas the lem2∆ lnp1∆ double mutant showed a severe growth defect. A combination of mutations in Lem2 and Bqt4, which encodes a nuclear membrane protein that anchors telomeres to the nuclear membrane, caused synthetic lethality. These genetic interactions imply that Lem2 cooperates with the nuclear membrane protein network to regulate genome stability.
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Affiliation(s)
- Yoshie Tange
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, 565-0871, Japan.,Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan
| | - Yuji Chikashige
- Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan
| | - Shinya Takahata
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Kei Kawakami
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Masato Higashi
- Graduate school of Chemical Sciences and Engineering, Hokkaido University, Sapporo, 060-0810, Japan
| | - Chie Mori
- Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan
| | - Tomoko Kojidani
- Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan.,Laboratory of Electron Microscopy, Faculty of Science, Japan Women's University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo, 112-8681, Japan
| | - Yasuhiro Hirano
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, 565-0871, Japan
| | - Haruhiko Asakawa
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, 565-0871, Japan
| | - Yota Murakami
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Tokuko Haraguchi
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, 565-0871, Japan.,Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan
| | - Yasushi Hiraoka
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, 565-0871, Japan.,Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan
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8
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Prinz F, Puetter V, Holton SJ, Andres D, Stegmann CM, Kwiatkowski D, Prechtl S, Petersen K, Beckmann G, Kreft B, Mumberg D, Fernández-Montalván A. Functional and Structural Characterization of Bub3·BubR1 Interactions Required for Spindle Assembly Checkpoint Signaling in Human Cells. J Biol Chem 2016; 291:11252-67. [PMID: 27030009 DOI: 10.1074/jbc.m115.702142] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2015] [Indexed: 02/02/2023] Open
Abstract
The spindle assembly checkpoint (SAC) is an essential safeguarding mechanism devised to ensure equal chromosome distribution in daughter cells upon mitosis. The proteins Bub3 and BubR1 are key components of the mitotic checkpoint complex, an essential part of the molecular machinery on which the SAC relies. In the present work we have performed a detailed functional and biochemical characterization of the interaction between human Bub3 and BubR1 in cells and in vitro Our results demonstrate that genetic knockdown of Bub3 abrogates the SAC, promotes apoptosis, and inhibits the proliferation of human cancer cells. We also show that the integrity of the human mitotic checkpoint complex depends on the specific recognition between BubR1 and Bub3, for which the BubR1 Gle2 binding sequence motif is essential. This 1:1 binding event is high affinity, enthalpy-driven and with slow dissociation kinetics. The affinity, kinetics, and thermodynamic parameters of the interaction are differentially modulated by small regions in the N and C termini of the Gle2 binding domain sequence, suggesting the existence of "hotspots" for this protein-protein interaction. Furthermore, we show that specific disruption of endogenous BubR1·Bub3 complexes in human cancer cells phenocopies the effects observed in gene targeting experiments. Our work enhances the current understanding of key members of the SAC and paves the road for the pursuit of novel targeted cancer therapies based on SAC inhibition.
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Affiliation(s)
- Florian Prinz
- TRG Oncology, Bayer Pharma AG, Global Drug Discovery, 13353 Berlin, Germany
| | | | | | | | | | | | | | - Kirstin Petersen
- TRG Oncology, Bayer Pharma AG, Global Drug Discovery, 13353 Berlin, Germany
| | | | - Bertolt Kreft
- TRG Oncology, Bayer Pharma AG, Global Drug Discovery, 13353 Berlin, Germany
| | - Dominik Mumberg
- TRG Oncology, Bayer Pharma AG, Global Drug Discovery, 13353 Berlin, Germany
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9
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Zich J, May K, Paraskevopoulos K, Sen O, Syred HM, van der Sar S, Patel H, Moresco JJ, Sarkeshik A, Yates JR, Rappsilber J, Hardwick KG. Mps1Mph1 Kinase Phosphorylates Mad3 to Inhibit Cdc20Slp1-APC/C and Maintain Spindle Checkpoint Arrests. PLoS Genet 2016; 12:e1005834. [PMID: 26882497 PMCID: PMC4755545 DOI: 10.1371/journal.pgen.1005834] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2015] [Accepted: 01/09/2016] [Indexed: 01/01/2023] Open
Abstract
The spindle checkpoint is a mitotic surveillance system which ensures equal segregation of sister chromatids. It delays anaphase onset by inhibiting the action of the E3 ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C). Mad3/BubR1 is a key component of the mitotic checkpoint complex (MCC) which binds and inhibits the APC/C early in mitosis. Mps1Mph1 kinase is critical for checkpoint signalling and MCC-APC/C inhibition, yet few substrates have been identified. Here we identify Mad3 as a substrate of fission yeast Mps1Mph1 kinase. We map and mutate phosphorylation sites in Mad3, producing mutants that are targeted to kinetochores and assembled into MCC, yet display reduced APC/C binding and are unable to maintain checkpoint arrests. We show biochemically that Mad3 phospho-mimics are potent APC/C inhibitors in vitro, demonstrating that Mad3p modification can directly influence Cdc20Slp1-APC/C activity. This genetic dissection of APC/C inhibition demonstrates that Mps1Mph1 kinase-dependent modifications of Mad3 and Mad2 act in a concerted manner to maintain spindle checkpoint arrests. When cells divide they need to ensure that a complete copy of their genetic material is transmitted to both daughter cells. Cells have evolved many controls to ensure that every division is carried out with very high fidelity. The spindle checkpoint is one such control, which acts as a surveillance system during mitosis. Defects in this checkpoint control lead to unequal segregation of DNA/chromosomes, termed aneuploidy, which is responsible for human birth defects and is very common in tumour cells. The molecular components of the spindle checkpoint, identified initially through yeast genetics, include several protein kinases. Surprisingly few of their substrates have been identified. Here we identify the checkpoint protein Mad3 as an important substrate of the Mps1Mph1 kinase. We show that Mps1Mph1-dependent modification of Mad3 and Mad2 acts to delay cell division in situations where the genetic material would not be equally inherited by daughter cells. This delay enables the cell to correct any problems within the division machinery and thus avoid aneuploidy.
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Affiliation(s)
- Judith Zich
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Karen May
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Konstantinos Paraskevopoulos
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Onur Sen
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Heather M. Syred
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Sjaak van der Sar
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
| | - Hitesh Patel
- Edinburgh Cancer Research UK Centre, Institute of Genetics and Molecular Medicine, University of Edinburgh, Edinburgh, United Kingdom
| | - James J. Moresco
- Scripps Research Institute, La Jolla, California, United States of America
| | - Ali Sarkeshik
- Scripps Research Institute, La Jolla, California, United States of America
| | - John R. Yates
- Scripps Research Institute, La Jolla, California, United States of America
| | - Juri Rappsilber
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
- Department of Bioanalytics, Institute of Biotechnology, Technische Universitat Berlin, Berlin, Germany
| | - Kevin G. Hardwick
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh, United Kingdom
- * E-mail:
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10
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Tang NH, Toda T. Alp7/TACC recruits kinesin-8-PP1 to the Ndc80 kinetochore protein for timely mitotic progression and chromosome movement. J Cell Sci 2014; 128:354-63. [PMID: 25472718 PMCID: PMC4294777 DOI: 10.1242/jcs.160036] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Upon establishment of proper kinetochore–microtubule attachment, the spindle assembly checkpoint (SAC) must be silenced to allow onset of anaphase, which is when sister chromatids segregate equally to two daughter cells. However, how proper kinetochore–microtubule attachment leads to timely anaphase onset remains elusive. Furthermore, the molecular mechanisms of chromosome movement during anaphase A remain unclear. In this study, we show that the fission yeast Alp7/TACC protein recruits a protein complex consisting of the kinesin-8 (Klp5–Klp6) and protein phosphatase 1 (PP1) to the kinetochore upon kinetochore–microtubule attachment. Accumulation of this complex at the kinetochore, on the one hand, facilitates SAC inactivation through PP1, and, on the other hand, accelerates polewards chromosome movement driven by the Klp5–Klp6 motor. We identified an alp7 mutant that had specific defects in binding to the Klp5–Klp6–PP1 complex but with normal localisation to the microtubule and kinetochore. Consistent with our proposition, this mutant shows delayed anaphase onset and decelerated chromosome movement during anaphase A. We propose that the recruitment of kinesin-8–PP1 to the kinetochore through Alp7/TACC interaction plays a crucial role in regulation of timely mitotic progression and chromosome movement during anaphase A.
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Affiliation(s)
- Ngang Heok Tang
- Laboratory of Cell Regulation, Cancer Research UK, London Research Institute, Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK
| | - Takashi Toda
- Laboratory of Cell Regulation, Cancer Research UK, London Research Institute, Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3PX, UK
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11
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Abstract
During mitosis and meiosis, the spindle assembly checkpoint acts to maintain genome stability by delaying cell division until accurate chromosome segregation can be guaranteed. Accuracy requires that chromosomes become correctly attached to the microtubule spindle apparatus via their kinetochores. When not correctly attached to the spindle, kinetochores activate the spindle assembly checkpoint network, which in turn blocks cell cycle progression. Once all kinetochores become stably attached to the spindle, the checkpoint is inactivated, which alleviates the cell cycle block and thus allows chromosome segregation and cell division to proceed. Here we review recent progress in our understanding of how the checkpoint signal is generated, how it blocks cell cycle progression and how it is extinguished.
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Affiliation(s)
- Pablo Lara-Gonzalez
- Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester M13 9PT, UK
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12
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MPS1/Mph1 phosphorylates the kinetochore protein KNL1/Spc7 to recruit SAC components. Nat Cell Biol 2012; 14:746-52. [DOI: 10.1038/ncb2515] [Citation(s) in RCA: 260] [Impact Index Per Article: 21.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2012] [Accepted: 05/08/2012] [Indexed: 12/13/2022]
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13
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Shepperd LA, Meadows JC, Sochaj AM, Lancaster TC, Zou J, Buttrick GJ, Rappsilber J, Hardwick KG, Millar JB. Phosphodependent recruitment of Bub1 and Bub3 to Spc7/KNL1 by Mph1 kinase maintains the spindle checkpoint. Curr Biol 2012; 22:891-9. [PMID: 22521786 PMCID: PMC3780767 DOI: 10.1016/j.cub.2012.03.051] [Citation(s) in RCA: 228] [Impact Index Per Article: 19.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2012] [Revised: 02/22/2012] [Accepted: 03/12/2012] [Indexed: 11/21/2022]
Abstract
The spindle assembly checkpoint (SAC) is the major surveillance system that ensures that sister chromatids do not separate until all chromosomes are correctly bioriented during mitosis. Components of the checkpoint include Mad1, Mad2, Mad3 (BubR1), Bub3, and the kinases Bub1, Mph1 (Mps1), and Aurora B. Checkpoint proteins are recruited to kinetochores when individual kinetochores are not bound to spindle microtubules or not under tension. Kinetochore association of Mad2 causes it to undergo a conformational change, which promotes its association to Mad3 and Cdc20 to form the mitotic checkpoint complex (MCC). The MCC inhibits the anaphase-promoting complex/cyclosome (APC/C) until the checkpoint is satisfied. SAC silencing derepresses Cdc20-APC/C activity. This triggers the polyubiquitination of securin and cyclin, which promotes the dissolution of sister chromatid cohesion and mitotic progression. We, and others, recently showed that association of PP1 to the Spc7/Spc105/KNL1 family of kinetochore proteins is necessary to stabilize microtubule-kinetochore attachments and silence the SAC. We now report that phosphorylation of the conserved MELT motifs in Spc7 by Mph1 (Mps1) recruits Bub1 and Bub3 to the kinetochore and that this is required to maintain the SAC signal.
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Affiliation(s)
- Lindsey A. Shepperd
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - John C. Meadows
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Alicja M. Sochaj
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Theresa C. Lancaster
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Juan Zou
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Graham J. Buttrick
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Juri Rappsilber
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Kevin G. Hardwick
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Jonathan B.A. Millar
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
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14
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Zich J, Sochaj A, Syred H, Milne L, Cook A, Ohkura H, Rappsilber J, Hardwick K. Kinase activity of fission yeast Mph1 is required for Mad2 and Mad3 to stably bind the anaphase promoting complex. Curr Biol 2012; 22:296-301. [PMID: 22281223 PMCID: PMC3315010 DOI: 10.1016/j.cub.2011.12.049] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2011] [Revised: 11/14/2011] [Accepted: 12/16/2011] [Indexed: 01/19/2023]
Abstract
Defects in chromosome segregation result in aneuploidy, which can lead to disease or cell death [1, 2]. The spindle checkpoint delays anaphase onset until all chromosomes are attached to spindle microtubules in a bipolar fashion [3, 4]. Mad2 is a key checkpoint component that undergoes conformational activation, catalyzed by a Mad1-Mad2 template enriched at unattached kinetochores [5]. Mad2 and Mad3 (BubR1) then bind and inhibit Cdc20 to form the mitotic checkpoint complex (MCC), which binds and inhibits the anaphase promoting complex (APC/C). Checkpoint kinases (Aurora, Bub1, and Mps1) are critical for checkpoint signaling, yet they have poorly defined roles and few substrates have been identified [6–8]. Here we demonstrate that a kinase-dead allele of the fission yeast MPS1 homolog (Mph1) is checkpoint defective and that levels of APC/C-associated Mad2 and Mad3 are dramatically reduced in this mutant. Thus, MCC binding to fission yeast APC/C is dependent on Mph1 kinase activity. We map and mutate several phosphorylation sites in Mad2, producing mutants that display reduced Cdc20-APC/C binding and an inability to maintain checkpoint arrest. We conclude that Mph1 kinase regulates the association of Mad2 with its binding partners and thereby mitotic arrest.
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Affiliation(s)
- Judith Zich
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Alicja M. Sochaj
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Heather M. Syred
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Laura Milne
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Atlanta G. Cook
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Hiro Ohkura
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Juri Rappsilber
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
| | - Kevin G. Hardwick
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JR, UK
- Corresponding author
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15
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Centromere-tethered Mps1 pombe homolog (Mph1) kinase is a sufficient marker for recruitment of the spindle checkpoint protein Bub1, but not Mad1. Proc Natl Acad Sci U S A 2011; 109:209-14. [PMID: 22184248 DOI: 10.1073/pnas.1114647109] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The spindle checkpoint delays the onset of anaphase until all of the chromosomes properly achieve bipolar attachment to the spindle. It has been shown that unattached kinetochores are the site that emits a signal for activation of the checkpoint. Although the components of the checkpoint such as Bub1, Mad1 and Mad2 selectively accumulate at unattached kinetochores, the answer to how they recognize unattached kinetochores has remained elusive. Mps1 pombe homolog (Mph1) kinase has been shown to function upstream of most of the components of the checkpoint and thus it is thought to recognize unattached kinetochores by itself and recruit other components. In this study we have expressed a fusion protein of Mph1 and Ndc80 (a kinetochore protein of the outer plate) and shown that the fusion protein arrests cell cycle progression in a spindle-checkpoint\x{2013}dependent manner in fission yeast. When expression of Mad2 is turned off, the cells grow normally with Mph1 constitutively localized at centromeres/kinetochores. Under this condition, Bub1 can be found with Mph1 throughout the cell cycle, indicating that localization of Mph1 at centromeres/kinetochores is sufficient to recruit Bub1. In contrast, Mad1 is found to transiently localize at kinetochores, which are presumably unattached to the spindle, but soon it dissociates from kinetochores. We propose that Mph1 is a sufficient marker for recruitment of Bub1. Mad1, in contrast, requires an additional condition/component for stable association with kinetochores.
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16
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Meadows JC, Shepperd LA, Vanoosthuyse V, Lancaster TC, Sochaj AM, Buttrick GJ, Hardwick KG, Millar JB. Spindle checkpoint silencing requires association of PP1 to both Spc7 and kinesin-8 motors. Dev Cell 2011; 20:739-50. [PMID: 21664573 PMCID: PMC3792844 DOI: 10.1016/j.devcel.2011.05.008] [Citation(s) in RCA: 131] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2011] [Revised: 04/28/2011] [Accepted: 05/09/2011] [Indexed: 02/06/2023]
Abstract
The spindle checkpoint is the prime cell-cycle control mechanism that ensures sister chromatids are bioriented before anaphase takes place. Aurora B kinase, the catalytic subunit of the chromosome passenger complex, both destabilizes kinetochore attachments that do not generate tension and simultaneously maintains the spindle checkpoint signal. However, it is unclear how the checkpoint is silenced following chromosome biorientation. We demonstrate that association of type 1 phosphatase (PP1(Dis2)) with both the N terminus of Spc7 and the nonmotor domains of the Klp5-Klp6 (kinesin-8) complex is necessary to counteract Aurora B kinase to efficiently silence the spindle checkpoint. The role of Klp5 and Klp6 in checkpoint silencing is specific to this class of kinesin and independent of their motor activities. These data demonstrate that at least two distinct pools of PP1, one kinetochore associated and the other motor associated, are needed to silence the spindle checkpoint.
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Affiliation(s)
- John C. Meadows
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Lindsey A. Shepperd
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Vincent Vanoosthuyse
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
- Laboratoire de Biologie Moléculaire de la Cellule, Ecole Normale Supérieure de Lyon, Site Jacques Monod, Lyon, France
| | - Theresa C. Lancaster
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Alicja M. Sochaj
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Graham J. Buttrick
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
| | - Kevin G. Hardwick
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
| | - Jonathan B.A. Millar
- Division of Biomedical Cell Biology, Warwick Medical School, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
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17
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Vanoosthuyse V, Meadows JC, van der Sar SJA, Millar JBA, Hardwick KG. Bub3p facilitates spindle checkpoint silencing in fission yeast. Mol Biol Cell 2010; 20:5096-105. [PMID: 19846658 DOI: 10.1091/mbc.e09-09-0762] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Although critical for spindle checkpoint signaling, the role kinetochores play in anaphase promoting complex (APC) inhibition remains unclear. Here we show that spindle checkpoint proteins are severely depleted from unattached kinetochores in fission yeast cells lacking Bub3p. Surprisingly, a robust mitotic arrest is maintained in the majority of bub3 Delta cells, yet they die, suggesting that Bub3p is essential for successful checkpoint recovery. During recovery, two defects are observed: (1) cells mis-segregate chromosomes and (2) anaphase onset is significantly delayed. We show that Bub3p is required to activate the APC upon inhibition of Aurora kinase activity in checkpoint-arrested cells, suggesting that Bub3p is required for efficient checkpoint silencing downstream of Aurora kinase. Together, these results suggest that spindle checkpoint signals can be amplified in the nucleoplasm, yet kinetochore localization of spindle checkpoint components is required for proper recovery from a spindle checkpoint-dependent arrest.
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Affiliation(s)
- Vincent Vanoosthuyse
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, University of Edinburgh, EH9 3JR, United Kingdom.
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18
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Windecker H, Langegger M, Heinrich S, Hauf S. Bub1 and Bub3 promote the conversion from monopolar to bipolar chromosome attachment independently of shugoshin. EMBO Rep 2009; 10:1022-8. [PMID: 19680287 PMCID: PMC2728212 DOI: 10.1038/embor.2009.183] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2009] [Revised: 07/09/2009] [Accepted: 07/10/2009] [Indexed: 11/09/2022] Open
Abstract
The eukaryotic spindle assembly checkpoint (SAC) delays anaphase in the presence of chromosome attachment errors. Bub3 has been reported to be required for SAC activity in all eukaryotes examined so far. We find that Bub3, unlike its binding partner Bub1, is not essential for the SAC in fission yeast. As Bub3 is needed for the efficient kinetochore localization of Bub1, and of Mad1, Mad2 and Mad3, this implies that most SAC proteins do not need to be enriched at the kinetochores for the SAC to function. We find that Bub3 is also dispensable for shugoshin localization to the centromeres, which is the second known function of Bub1. Instead, Bub3, together with Bub1, has a specific function in promoting the conversion from chromosome mono-orientation to bi-orientation.
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Affiliation(s)
- Hanna Windecker
- Friedrich Miescher Laboratory of the Max Planck Society, Spemannstrasse 39, 72076 Tuebingen, Germany
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19
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Klebig C, Korinth D, Meraldi P. Bub1 regulates chromosome segregation in a kinetochore-independent manner. ACTA ACUST UNITED AC 2009; 185:841-58. [PMID: 19487456 PMCID: PMC2711590 DOI: 10.1083/jcb.200902128] [Citation(s) in RCA: 159] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
The kinetochore-bound protein kinase Bub1 performs two crucial functions during mitosis: it is essential for spindle checkpoint signaling and for correct chromosome alignment. Interestingly, Bub1 mutations are found in cancer tissues and cancer cell lines. Using an isogenic RNA interference complementation system in transformed HeLa cells and untransformed RPE1 cells, we investigate the effect of structural Bub1 mutants on chromosome segregation. We demonstrate that Bub1 regulates mitosis through the same mechanisms in both cell lines, suggesting a common regulatory network. Surprisingly, Bub1 can regulate chromosome segregation in a kinetochore-independent manner, albeit at lower efficiency. Its kinase activity is crucial for chromosome alignment but plays only a minor role in spindle checkpoint signaling. We also identify a novel conserved motif within Bub1 (amino acids 458–476) that is essential for spindle checkpoint signaling but does not regulate chromosome alignment, and we show that several cancer-related Bub1 mutants impair chromosome segregation, suggesting a possible link to tumorigenesis.
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Affiliation(s)
- Christiane Klebig
- Institute of Biochemistry, Eidgenössische Technische Hochschule (ETH) Zurich, 8093 Zurich, Switzerland
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20
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Current awareness on yeast. Yeast 2009. [DOI: 10.1002/yea.1567] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
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21
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Meadows JC, Millar J. Latrunculin A delays anaphase onset in fission yeast by disrupting an Ase1-independent pathway controlling mitotic spindle stability. Mol Biol Cell 2008; 19:3713-23. [PMID: 18562692 DOI: 10.1091/mbc.e08-02-0164] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
It has been proposed previously that latrunculin A, an inhibitor of actin polymerization, delays the onset of anaphase by causing spindle misorientation in fission yeast. However, we show that Delta mto1 cells, which are defective in nucleation of cytoplasmic microtubules, have profoundly misoriented spindles but are not delayed in the timing of sister chromatid separation, providing compelling evidence that fission yeast does not possess a spindle orientation checkpoint. Instead, we show that latrunculin A delays anaphase onset by disrupting interpolar microtubule stability. This effect is abolished in a latrunculin A-insensitive actin mutant and exacerbated in cells lacking Ase1, which cross-links antiparallel interpolar microtubules at the spindle midzone both before and after anaphase. These data indicate that both Ase1 and an intact actin cytoskeleton are required for preanaphase spindle stability. Finally, we show that loss of Ase1 activates a checkpoint that requires only the Mad3, Bub1, and Mph1, but not Mad1, Mad2, or Bub3 checkpoint proteins.
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Affiliation(s)
- John C Meadows
- Division of Yeast Genetics, National Institute for Medical Research, London NW7 1AA, United Kingdom
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