Publications
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Mitosis and the spindle checkpoint
Overcoming inhibition in the spindle checkpoint. (2009) Vincent Vanoosthuyse, Kevin G Hardwick. Genes Dev. 23(24):2799-805 review
Abstract: Spindle checkpoint silencing is a critical step during mitosis that initiates chromosome segregation, yet surprisingly little is known about its mechanism. Protein phosphatase I (PP1) was shown recently to be a key player in this process, and in this issue of Genes & Deverlopment, Akiyoshi and colleagues (pp. 2887-2899) identify budding yeast Fin1p as a kinetochore-localized regulator of PP1 activity toward checkpoint targets. Here we review recent mechanistic insights and propose a working model for spindle checkpoint silencing. [PubMed ID: 20008930]
An acetylated form of histone H2A.Z regulates chromosome architecture in Schizosaccharomyces pombe. (2009) Hyun-Soo Kim, Vincent Vanoosthuyse, Jeffrey Fillingham, Assen Roguev, Stephen Watt, Thomas Kislinger, Alex Treyer, Laura Rocco Carpenter, Christopher S Bennett, Andrew Emili, Jack F Greenblatt, Kevin G Hardwick, Nevan J Krogan, Jürg Bähler, Michael-Christopher Keogh. Nat. Struct. Mol. Biol. 16(12):1286-93
Abstract: Histone variant H2A.Z has a conserved role in genome stability, although it remains unclear how this is mediated. Here we demonstrate that the fission yeast Swr1 ATPase inserts H2A.Z (Pht1) into chromatin and Kat5 acetyltransferase (Mst1) acetylates it. Deletion or an unacetylatable mutation of Pht1 leads to genome instability, primarily caused by chromosome entanglement and breakage at anaphase. This leads to the loss of telomere-proximal markers, though telomere protection and repeat length are unaffected by the absence of Pht1. Strikingly, the chromosome entanglement in pht1Delta anaphase cells can be rescued by forcing chromosome condensation before anaphase onset. We show that the condensin complex, required for the maintenance of anaphase chromosome condensation, prematurely dissociates from chromatin in the absence of Pht1. This and other findings suggest an important role for H2A.Z in the architecture of anaphase chromosomes. [PubMed ID: 19915592]
Bub3p facilitates spindle checkpoint silencing in fission yeast. (2009) Vincent Vanoosthuyse, John C Meadows, Sjaak J A van der Sar, Jonathan B A Millar, Kevin G Hardwick. Mol. Biol. Cell 20(24):5096-105
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. [PubMed ID: 19846658]
Getting down to the phosphorylated 'nuts and bolts' of spindle checkpoint signalling. (2010) Judith Zich, Kevin G Hardwick. Trends Biochem. Sci. 35(1):18-27 review new
Abstract: Due to the highly orchestrated stages of mitosis, cells segregate their chromosomes with incredibly high fidelity. One of the principal 'conductors' is the spindle checkpoint, which regulates mitotic progression. Specifically, it delays anaphase onset until all chromosomes are attached in a bi-oriented fashion to spindle microtubules. This delay stems from inhibition of Cdc20, an activator of an E3 ubiquitin ligase known as the anaphase-promoting complex or cyclosome (APC/C). Several recent advances in our mechanistic understanding of this important cell cycle control have been made. Although still poorly understood, signalling roles for checkpoint kinases and their opposing phosphatases continue to be uncovered, and the key substrates gradually identified. [PubMed ID: 19836959]
A novel protein phosphatase 1-dependent spindle checkpoint silencing mechanism. (2009) Vincent Vanoosthuyse, Kevin G Hardwick. Curr. Biol. 19(14):1176-81
Abstract: The spindle checkpoint is a surveillance system acting in mitosis to delay anaphase onset until all chromosomes are properly attached to the mitotic spindle. When the checkpoint is activated, the Mad2 and Mad3 proteins directly bind and inhibit Cdc20, which is an essential activator of an E3 ubiquitin ligase known as the anaphase-promoting complex (APC). When the checkpoint is satisfied, Cdc20-APC is activated and polyubiquitinates securin and cyclin, leading to the dissolution of sister chromatid cohesion and mitotic progression. Several protein kinases play critical roles in spindle checkpoint signaling, but the mechanism (or mechanisms) by which they inhibit mitotic progression remains unclear. Furthermore, it is not known whether their activity needs to be reversed by protein phosphatases before anaphase onset can occur. Here we employ fission yeast to show that Aurora (Ark1) kinase activity is directly required to maintain spindle checkpoint arrest, even in the presence of many unattached kinetochores. Upon Ark1 inhibition, checkpoint complexes are disassembled and cyclin B is rapidly degraded. Importantly, checkpoint silencing and cyclin B degradation require the kinetochore-localized isoform of protein phosphatase 1 (PP1(Dis2)). We propose that PP1(Dis2)-mediated dephosphorylation of checkpoint components forms a novel spindle checkpoint silencing mechanism. [PubMed ID: 19592249]
The spindle checkpoint: assays for the analysis of spindle checkpoint arrest and recovery. (2009) Josefin Fernius, Kevin G Hardwick. Methods Mol. Biol. 545243-58
Abstract: The spindle checkpoint is a surveillance mechanism that ensures the fidelity of chromosome segregation by inhibiting anaphase onset until all chromosomes have established stable bipolar attachments. Here we describe a number of protocols that can be used to assay the ability of budding and fission yeast cells to (1) establish and maintain a spindle checkpoint arrest, and (2) segregate chromosomes efficiently upon recovery from mitotic arrest. We focus on experimental detail of the budding yeast protocols, but also point out important differences between budding and fission yeast assays. [PubMed ID: 19475393]
The spindle checkpoint functions of Mad3 and Mad2 depend on a Mad3 KEN box-mediated interaction with Cdc20-anaphase-promoting complex (APC/C). (2008) Matylda Sczaniecka, Anna Feoktistova, Karen M May, Jun-Song Chen, Julie Blyth, Kathleen L Gould, Kevin G Hardwick. J. Biol. Chem. 283(34):23039-47
Abstract: Mitotic progression is driven by proteolytic destruction of securin and cyclins. These proteins are labeled for destruction by an ubiquitin-protein isopeptide ligase (E3) known as the anaphase-promoting complex or cyclosome (APC/C). The APC/C requires activators (Cdc20 or Cdh1) to efficiently recognize its substrates, which are specified by destruction (D box) and/or KEN box signals. The spindle assembly checkpoint responds to unattached kinetochores and to kinetochores lacking tension, both of which reflect incomplete biorientation of chromosomes, by delaying the onset of anaphase. It does this by inhibiting Cdc20-APC/C. Certain checkpoint proteins interact directly with Cdc20, but it remains unclear how the checkpoint acts to efficiently inhibit Cdc20-APC/C activity. In the fission yeast, Schizosaccharomyces pombe, we find that the Mad3 and Mad2 spindle checkpoint proteins interact stably with the APC/C in mitosis. Mad3 contains two KEN boxes, conserved from yeast Mad3 to human BubR1, and mutation of either of these abrogates the spindle checkpoint. Strikingly, mutation of the N-terminal KEN box abolishes incorporation of Mad3 into the mitotic checkpoint complex (Mad3-Mad2-Slp1 in S. pombe, where Slp1 is the Cdc20 homolog that we will refer to as Cdc20 hereafter) and stable association of both Mad3 and Mad2 with the APC/C. Our findings demonstrate that this Mad3 KEN box is a critical mediator of Cdc20-APC/C inhibition, without which neither Mad3 nor Mad2 can associate with the APC/C or inhibit anaphase onset. [PubMed ID: 18556659]
The spindle checkpoint: how do cells delay anaphase onset? (2008) Matylda M Sczaniecka, Kevin G Hardwick. SEB Exp Biol Ser 59243-56 review
Abstract: Several models have been suggested above, describing possible modes of spindle checkpoint action: 1. Cdc20 sequestration (by Mad2-Cdc20 and/or MCC). 2. Stable MCC-APC/C association. 3. Cdc20 turnover (in budding yeast). 4. Cdc20-APC/C modification (by Mps1, Bub1, MAPK, Aurora B or BubR1 kinases). Several of these mechanisms could affect APC/C activity by modifying, competing for, and/or blocking the binding site(s) for its substrates. Alternatively, they could reduce the processivity of ubiquitination of substrates, or prevent the release of substrates and thereby reduce substrate turnover. Indeed, the processivity of ubiquitination can determine the order of destruction of APC/C substrates (Rape et al., 2006). Most substrates require multiple APC/C binding events in order to build polyubiquitin chains, and only polyubiquitinated substrates are recognised by the 26S proteasome for destruction. Thus, if the processivity of ubiquitination or the turnover of APC/C substrates were impaired in mitosis, the degradation of securin and cyclin would no longer take place, which would result in mitotic arrest. Our results have highlighted the importance of Mad3 as an anaphase inhibitor, and suggest that it usually acts in concert with Mad2 to efficiently inhibit Cdc20-APC/C. Further experiments are necessary to fully understand their mechanism of action, and this will require a wide range of approaches including dynamic studies of the 'flux' of Mad2 and BubR1 through signalling scaffolds, further structural insights, the identification of important phosphorylation sites on both the checkpoint proteins and Cdc20-APC/C, and an in vitro reconstitution of MCC inhibition of the APC/C. We look forward to seeing the complex regulation of mitotic progression being described over the coming years. [PubMed ID: 18368927]
Bub1 is a fission yeast kinetochore scaffold protein, and is sufficient to recruit other spindle checkpoint proteins to ectopic sites on chromosomes. (2007) Patricia E Rischitor, Karen M May, Kevin G Hardwick. PLoS ONE 2(12):e1342
Abstract: The spindle checkpoint delays anaphase onset until all chromosomes have attached in a bi-polar manner to the mitotic spindle. Mad and Bub proteins are recruited to unattached kinetochores, and generate diffusible anaphase inhibitors. Checkpoint models propose that Mad1 and Bub1 act as stable kinetochore-bound scaffolds, to enhance recruitment of Mad2 and Mad3/BubR1, but this remains untested for Bub1. Here, fission yeast FRAP experiments confirm that Bub1 stably binds kinetochores, and by tethering Bub1 to telomeres we demonstrate that it is sufficient to recruit anaphase inhibitors in a kinase-independent manner. We propose that the major checkpoint role for Bub1 is as a signalling scaffold. [PubMed ID: 18094750]
Bub1 kinase targets Sgo1 to ensure efficient chromosome biorientation in budding yeast mitosis. (2007) Josefin Fernius, Kevin G Hardwick. PLoS Genet. 3(11):e213
Abstract: During cell division all chromosomes must be segregated accurately to each daughter cell. Errors in this process give rise to aneuploidy, which leads to birth defects and is implicated in cancer progression. The spindle checkpoint is a surveillance mechanism that ensures high fidelity of chromosome segregation by inhibiting anaphase until all kinetochores have established bipolar attachments to spindle microtubules. Bub1 kinase is a core component of the spindle checkpoint, and cells lacking Bub1 fail to arrest in response to microtubule drugs and precociously segregate their DNA. The mitotic role(s) of Bub1 kinase activity remain elusive, and it is controversial whether this C-terminal domain of Bub1p is required for spindle checkpoint arrest. Here we make a detailed analysis of budding yeast cells lacking the kinase domain (bub1DeltaK). We show that despite being able to arrest in response to microtubule depolymerisation and kinetochore-microtubule attachment defects, bub1DeltaK cells are sensitive to microtubule drugs. This is because bub1DeltaK cells display significant chromosome mis-segregation upon release from nocodazole arrest. bub1DeltaK cells mislocalise Sgo1p, and we demonstrate that both the Bub1 kinase domain and Sgo1p are required for accurate chromosome biorientation after nocodazole treatment. We propose that Bub1 kinase and Sgo1p act together to ensure efficient biorientation of sister chromatids during mitosis. [PubMed ID: 18081426]
Ipl1p-dependent phosphorylation of Mad3p is required for the spindle checkpoint response to lack of tension at kinetochores. (2007) Emma M J King, Najma Rachidi, Nick Morrice, Kevin G Hardwick, Michael J R Stark. Genes Dev. 21(10):1163-8
Abstract: The spindle checkpoint delays anaphase onset until all chromosomes are correctly attached to microtubules. Ipl1 protein kinase (Aurora B) is required to correct inappropriate kinetochore-microtubule attachments and for the response to lack of tension between sister kinetochores. Here we identify residues in the checkpoint protein Mad3p that are phosphorylated by Ipl1p. When phosphorylation of Mad3p at two sites is prevented, the cell's response to reduced kinetochore tension is dramatically curtailed. Our data provide strong evidence for a distinct checkpoint pathway responding to lack of sister kinetochore tension, in which Ipl1p-dependent phosphorylation of Mad3p is a key step. [PubMed ID: 17504936]
Mad3 KEN boxes mediate both Cdc20 and Mad3 turnover, and are critical for the spindle checkpoint. (2007) Emma M J King, Sjaak J A van der Sar, Kevin G Hardwick. PLoS ONE 2(4):e342
Abstract: Mitotic progression is controlled by proteolytic destruction of securin and cyclin. The mitotic E3 ubiquitin ligase, known as the anaphase promoting complex or cyclosome (APC/C), in partnership with its activators Cdc20p and Cdh1p, targets these proteins for degradation. In the presence of defective kinetochore-microtubule interactions, APC/C(Cdc20) is inhibited by the spindle checkpoint, thereby delaying anaphase onset and providing more time for spindle assembly. Cdc20p interacts directly with Mad2p, and its levels are subject to careful regulation, but the precise mode(s) of APC/C( Cdc20) inhibition remain unclear. The mitotic checkpoint complex (MCC, consisting of Mad3p, Mad2p, Bub3p and Cdc20p in budding yeast) is a potent APC/C inhibitor. Here we focus on Mad3p and how it acts, in concert with Mad2p, to efficiently inhibit Cdc20p. We identify and analyse the function of two motifs in Mad3p, KEN30 and KEN296, which are conserved from yeast Mad3p to human BubR1. These KEN amino acid sequences resemble 'degron' signals that confer interaction with APC/C activators and target proteins for degradation. We show that both Mad3p KEN boxes are necessary for spindle checkpoint function. Mutation of KEN30 abolished MCC formation and stabilised Cdc20p in mitosis. In addition, mutation of Mad3-KEN30, APC/C subunits, or Cdh1p, stabilised Mad3p in G1, indicating that the N-terminal KEN box could be a Mad3p degron. To determine the significance of Mad3p turnover, we analysed the consequences of MAD3 overexpression and found that four-fold overproduction of Mad3p led to chromosome bi-orientation defects and significant chromosome loss during recovery from anti-microtubule drug induced checkpoint arrest. In conclusion, Mad3p KEN30 mediates interactions that regulate the proteolytic turnover of Cdc20p and Mad3p, and the levels of both of these proteins are critical for spindle checkpoint signaling and high fidelity chromosome segregation. [PubMed ID: 17406666]
Novel sfi1 alleles uncover additional functions for Sfi1p in bipolar spindle assembly and function. (2007) Victoria E Anderson, John Prudden, Simon Prochnik, Thomas H Giddings, Kevin G Hardwick. Mol. Biol. Cell 18(6):2047-56
Abstract: A variety of spindle and kinetochore defects have been shown to induce a mitotic delay through activation of the spindle checkpoint. With the aim of identifying novel mitotic defects we carried out a mad1 synthetic lethal screen in budding yeast. In this screen, four novel alleles of sfi1 were isolated. SFI1 is an essential gene, previously identified through its interaction with centrin/CDC31 and shown to be required for spindle pole body (SPB) duplication. The new mutations were all found in the C-terminal domain of Sfi1p, which has no known function, but it is well conserved among budding yeasts. Analysis of the novel sfi1 mutants, through a combination of light and electron microscopy, revealed duplicated SPBs <0.3 microm apart. Importantly, these SPBs have completed duplication, but they are not separated, suggesting a possible defect in splitting of the bridge. We discuss possible roles for Sfi1p in this step in bipolar spindle assembly. [PubMed ID: 17392514]
Shugoshin 2 regulates localization of the chromosomal passenger proteins in fission yeast mitosis. (2007) Vincent Vanoosthuyse, Sergey Prykhozhij, Kevin G Hardwick. Mol. Biol. Cell 18(5):1657-69
Abstract: Fission yeast has two members of the Shugoshin family, Sgo1 and Sgo2. Although Sgo1 has clearly been established as a protector of centromere cohesion in meiosis I, the roles of Sgo2 remain elusive. Here we show that Sgo2 is required to ensure proper chromosome biorientation upon recovery from a prolonged spindle checkpoint arrest. Consistent with this, Sgo2 is essential for maintaining the Passenger proteins on centromeres upon checkpoint activation. Interestingly, lack of Sgo2 has a more penetrant effect on the localization of Survivin than on the two other Passenger proteins INCENP and Aurora B, and the Survivin-INCENP complex but not the INCENP-Aurora B complex is destabilized in the absence of Sgo2. Finally we show that the conserved C-terminus of Sgo2 is crucial to maintain Sgo2 and Passenger proteins localization on centromeres upon prolonged checkpoint activation. Taken together, our results demonstrate that Sgo2 is important for chromosome biorientation and that it controls docking of the Passenger proteins on chromosomes in early mitotic cells. [PubMed ID: 17301288]
The spindle checkpoint. (2006) Karen M May, Kevin G Hardwick. J. Cell. Sci. 119(Pt 20):4139-42 review
Abstract: not available. [PubMed ID: 17038540]
Control of Shugoshin function during fission-yeast meiosis. (2005) Sabine Vaur, Fabien Cubizolles, Guillaume Plane, Sylvie Genier, Peter K Rabitsch, Juraj Gregan, Kim Nasmyth, Vincent Vanoosthuyse, Kevin G Hardwick, Jean-Paul Javerzat. Curr. Biol. 15(24):2263-70
Abstract: Meiosis consists of a single round of DNA replication followed by two consecutive nuclear divisions. During the first division (MI), sister kinetochores must orient toward the same pole to favor reductional segregation. Correct chromosome segregation during the second division (MII) requires the retention of centromeric cohesion until anaphase II. The spindle checkpoint protein Bub1 is essential for both processes in fission yeast . When bub1 is deleted, the Shugoshin protein Sgo1 is not recruited to centromeres, cohesin Rec8 does not persist at centromeres, and sister-chromatid cohesion is lost by the end of MI. Deletion of bub1 also affects kinetochore orientation because sister centromeres can move to opposite spindle poles in approximately 30% of MI divisions. We show here that these two functions are separable within the Bub1 protein. The N terminus of Bub1 is necessary and sufficient for Sgo1 targeting to centromeres and the protection of cohesion, whereas the C-terminal kinase domain acts together with Sgo2, the second fission-yeast Shugoshin protein, to promote sister-kinetochore co-orientation during MI. Additional analyses suggest that the protection of centromeric cohesion does not operate when sister kinetochores attach to opposite spindle poles during MI. Sgo1-mediated protection of centromere cohesion might therefore be regulated by the mode of kinetochore attachment. [PubMed ID: 16360688]
Bub1 and the multilayered inhibition of Cdc20-APC/C in mitosis. (2005) Vincent Vanoosthuyse, Kevin G Hardwick. Trends Cell Biol. 15(5):231-3 review
Abstract: To ensure the accuracy of chromosome segregation in mitosis, the spindle checkpoint blocks the activity of the anaphase-promoting complex APC/C until all chromosomes are properly bi-orientated on the metaphase spindle. How the checkpoint machinery actually inhibits the APC/C is still unclear. A new paper by Tang and coworkers helps further our understanding of this complex and fundamental process. [PubMed ID: 15866025]
Checkpoint signalling: Mad2 conformers and signal propagation. (2005) Kevin G Hardwick. Curr. Biol. 15(4):R122-4 review
Abstract: The spindle checkpoint protein Mad2 has a tendency to form multimers and adopts at least two structural conformations. New work highlights the importance of the Mad2-Mad2 interaction, and suggests how spindle checkpoint signals are propagated away from kinetochores. [PubMed ID: 15723780]
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. (2005) Sheila Kadura, Xiangwei He, Vincent Vanoosthuyse, Kevin G Hardwick, Shelley Sazer. Mol. Biol. Cell 16(1):385-95
Abstract: 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. [PubMed ID: 15525673]
Kinetochore targeting of fission yeast Mad and Bub proteins is essential for spindle checkpoint function but not for all chromosome segregation roles of Bub1p. (2004) Vincent Vanoosthuyse, Rebekka Valsdottir, Jean-Paul Javerzat, Kevin G Hardwick. Mol. Cell. Biol. 24(22):9786-801
Abstract: Several lines of evidence suggest that kinetochores are organizing centers for the spindle checkpoint response and the synthesis of a "wait anaphase" signal in cases of incomplete or improper kinetochore-microtubule attachment. Here we characterize Schizosaccharomyces pombe Bub3p and study the recruitment of spindle checkpoint components to kinetochores. We demonstrate by chromatin immunoprecipitation that they all interact with the central domain of centromeres, consistent with their role in monitoring kinetochore-microtubule interactions. Bub1p and Bub3p are dependent upon one another, but independent of the Mad proteins, for their kinetochore localization. We demonstrate a clear role for the highly conserved N-terminal domain of Bub1p in the robust targeting of Bub1p, Bub3p, and Mad3p to kinetochores and show that this is crucial for an efficient checkpoint response. Surprisingly, neither this domain nor kinetochore localization is required for other functions of Bub1p in chromosome segregation. [PubMed ID: 15509783]
The complexity of Bub1 regulation--phosphorylation, phosphorylation, phosphorylation. (2003 Mar-Apr) Vincent Vanoosthuyse, Kevin G Hardwick. Cell Cycle 2(2):118-9
Abstract: not available. [PubMed ID: 12695661]
Analysis of Bub3 spindle checkpoint function in Xenopus egg extracts. (2003) Leigh Campbell, Kevin G Hardwick. J. Cell. Sci. 116(Pt 4):617-28
Abstract: The spindle checkpoint delays the onset of anaphase if there are any defects in the interactions between spindle microtubules and kinetochores. This checkpoint has been reconstituted in vitro in Xenopus egg extracts, and here we use antibodies to Xenopus Bub3 (XBub3) to show that this protein is required for both the activation and the maintenance of a spindle checkpoint arrest in egg extracts. We detect two forms of XBub3 in egg extracts and find both to be complexed with the XBub1 and XBubR1 kinases. Only one form of XBub3 is apparent in Xenopus tissue culture (XTC) cells, and localisation studies reveal that, unlike the Mad proteins, which are concentrated at the nuclear periphery, XBub3 is diffusely localised throughout the nucleus during interphase. During early prophase it is recruited to kinetochores, where it remains until chromosomes align at the metaphase plate. We discuss the mechanism by which our alpha-XBub3 antibodies interfere with the checkpoint and possible roles for XBub3 in the spindle checkpoint pathway. [PubMed ID: 12538762]
The spindle checkpoint: structural insights into dynamic signalling. (2002) Andrea Musacchio, Kevin G Hardwick. Nat. Rev. Mol. Cell Biol. 3(10):731-41 review
Abstract: Chromosome segregation is a complex and astonishingly accurate process whose inner working is beginning to be understood at the molecular level. The spindle checkpoint plays a key role in ensuring the fidelity of this process. It monitors the interactions between chromosomes and microtubules, and delays mitotic progression to allow extra time to correct defects. Here, we review and integrate findings on the dynamics of checkpoint proteins at kinetochores with structural information about signalling complexes. [PubMed ID: 12360190]
Distinct chromosome segregation roles for spindle checkpoint proteins. (2002) Cheryl D Warren, D Michelle Brady, Raymond C Johnston, Joseph S Hanna, Kevin G Hardwick, Forrest A Spencer. Mol. Biol. Cell 13(9):3029-41
Abstract: The spindle checkpoint plays a central role in the fidelity of chromosome transmission by ensuring that anaphase is initiated only after kinetochore-microtubule associations of all sister chromatid pairs are complete. In this study, we find that known spindle checkpoint proteins do not contribute equally to chromosome segregation fidelity in Saccharomyces cerevisiae. Loss of Bub1 or Bub3 protein elicits the largest effect. Analysis of Bub1p reveals the presence of two molecular functions. An N-terminal 608-amino acid (nonkinase) portion of the protein supports robust checkpoint activity, and, as expected, contributes to chromosome segregation. A C-terminal kinase-encoding segment independently contributes to chromosome segregation through an unknown mechanism. Both molecular functions depend on association with Bub3p. A 156-amino acid fragment of Bub1p functions in Bub3p binding and in kinetochore localization by one-hybrid assay. An adjacent segment is required for Mad1p binding, detected by deletion analysis and coimmunoprecipitation. Finally, overexpression of wild-type BUB1 or MAD3 genes leads to chromosome instability. Analysis of this activity indicates that the Bub3p-binding domain of Bub1p contributes to this phenotype through disruption of checkpoint activity as well as through introduction of kinetochore or spindle damage. [PubMed ID: 12221113]
The awesome power of multiple model systems: interpreting the complex nature of spindle checkpoint signaling. (2002) David N Millband, Leigh Campbell, Kevin G Hardwick. Trends Cell Biol. 12(5):205-9 review
Abstract: The spindle checkpoint coordinates the cell biology of mitosis with cell-cycle progression. It ensures that sister-chromatid separation only takes place when all kinetochores have formed stable bipolar microtubule attachments. Here, we discuss recent advances in our understanding of what activates this checkpoint pathway, the molecular nature of the checkpoint signal and its mode of transmission, and how the checkpoint might be inactivated. [PubMed ID: 12062159]
Fission yeast Mad3p is required for Mad2p to inhibit the anaphase-promoting complex and localizes to kinetochores in a Bub1p-, Bub3p-, and Mph1p-dependent manner. (2002) David N Millband, Kevin G Hardwick. Mol. Cell. Biol. 22(8):2728-42
Abstract: The spindle checkpoint delays the metaphase-to-anaphase transition in response to spindle and kinetochore defects. Genetic screens in budding yeast identified the Mad and Bub proteins as key components of this conserved regulatory pathway. Here we present the fission yeast homologue of Mad3p. Cells devoid of mad3(+) are unable to arrest their cell cycle in the presence of microtubule defects. Mad3p coimmunoprecipitates Bub3p, Mad2p, and the spindle checkpoint effector Slp1/Cdc20p. We demonstrate that Mad3p function is required for the overexpression of Mad2p to result in a metaphase arrest. Mad1p, Bub1p, and Bub3p are not required for this arrest. Thus, Mad3p appears to have a crucial role in transducing the inhibitory "wait anaphase" signal to the anaphase-promoting complex (APC). Mad3-green fluorescent protein (GFP) is recruited to unattached kinetochores early in mitosis and accumulates there upon prolonged checkpoint activation. For the first time, we have systematically studied the dependency of Mad3/BubR1 protein recruitment to kinetochores. We find Mad3-GFP kinetochore localization to be dependent upon Bub1p, Bub3p, and the Mph1p kinase, but not upon Mad1p or Mad2p. We discuss the implications of these findings in the context of our current understanding of spindle checkpoint function. [PubMed ID: 11909965]
Cdc28 activates exit from mitosis in budding yeast. (2000) A D Rudner, K G Hardwick, A W Murray. J. Cell Biol. 149(7):1361-76
Abstract: The activity of the cyclin-dependent kinase 1 (Cdk1), Cdc28, inhibits the transition from anaphase to G1 in budding yeast. CDC28-T18V, Y19F (CDC28-VF), a mutant that lacks inhibitory phosphorylation sites, delays the exit from mitosis and is hypersensitive to perturbations that arrest cells in mitosis. Surprisingly, this behavior is not due to a lack of inhibitory phosphorylation or increased kinase activity, but reflects reduced activity of the anaphase-promoting complex (APC), a defect shared with other mutants that lower Cdc28/Clb activity in mitosis. CDC28-VF has reduced Cdc20- dependent APC activity in mitosis, but normal Hct1- dependent APC activity in the G1 phase of the cell cycle. The defect in Cdc20-dependent APC activity in CDC28-VF correlates with reduced association of Cdc20 with the APC. The defects of CDC28-VF suggest that Cdc28 activity is required to induce the metaphase to anaphase transition and initiate the transition from anaphase to G1 in budding yeast. [PubMed ID: 10871278]
Complex formation between Mad1p, Bub1p and Bub3p is crucial for spindle checkpoint function. (2000) D M Brady, K G Hardwick. Curr. Biol. 10(11):675-8
Abstract: The spindle checkpoint delays the metaphase to anaphase transition in response to defects in kinetochore-microtubule interactions in the mitotic apparatus (see [1] [2] [3] [4] for reviews). The Mad and Bub proteins were identified as key components of the spindle checkpoint through budding yeast genetics [5] [6] and are highly conserved [3]. Most of the spindle checkpoint proteins have been localised to kinetochores, yet almost nothing is known about the molecular events which take place there. Mad1p forms a tight complex with Mad2p [7], and has been shown to recruit Mad2p to kinetochores [8]. Similarly, Bub3p binds to Bub1p [9] and may target it to kinetochores [10]. Here, we show that budding yeast Mad1p has a regulated association with Bub1p and Bub3p during a normal cell cycle and that this complex is found at significantly higher levels once the spindle checkpoint is activated. We find that formation of this complex requires Mad2p and Mps1p but not Mad3p or Bub2p. In addition, we identify a conserved motif within Mad1p that is essential for Mad1p-Bub1p-Bub3p complex formation. Mutation of this motif abolishes checkpoint function, indicating that formation of the Mad1p-Bub1p-Bub3p complex is a crucial step in the spindle checkpoint mechanism. [PubMed ID: 10837255]
MAD3 encodes a novel component of the spindle checkpoint which interacts with Bub3p, Cdc20p, and Mad2p. (2000) K G Hardwick, R C Johnston, D L Smith, A W Murray. J. Cell Biol. 148(5):871-82
Abstract: We show that MAD3 encodes a novel 58-kD nuclear protein which is not essential for viability, but is an integral component of the spindle checkpoint in budding yeast. Sequence analysis reveals two regions of Mad3p that are 46 and 47% identical to sequences in the NH(2)-terminal region of the budding yeast Bub1 protein kinase. Bub1p is known to bind Bub3p (Roberts et al. 1994) and we use two-hybrid assays and coimmunoprecipitation experiments to show that Mad3p can also bind to Bub3p. In addition, we find that Mad3p interacts with Mad2p and the cell cycle regulator Cdc20p. We show that the two regions of homology between Mad3p and Bub1p are crucial for these interactions and identify loss of function mutations within each domain of Mad3p. We discuss roles for Mad3p and its interactions with other spindle checkpoint proteins and with Cdc20p, the target of the checkpoint. [PubMed ID: 10704439]
The spindle checkpoint of budding yeast depends on a tight complex between the Mad1 and Mad2 proteins. (1999) R H Chen, D M Brady, D Smith, A W Murray, K G Hardwick. Mol. Biol. Cell 10(8):2607-18
Abstract: The spindle checkpoint arrests the cell cycle at metaphase in the presence of defects in the mitotic spindle or in the attachment of chromosomes to the spindle. When spindle assembly is disrupted, the budding yeast mad and bub mutants fail to arrest and rapidly lose viability. We have cloned the MAD2 gene, which encodes a protein of 196 amino acids that remains at a constant level during the cell cycle. Gel filtration and co-immunoprecipitation analyses reveal that Mad2p tightly associates with another spindle checkpoint component, Mad1p. This association is independent of cell cycle stage and the presence or absence of other known checkpoint proteins. In addition, Mad2p binds to all of the different phosphorylated isoforms of Mad1p that can be resolved on SDS-PAGE. Deletion and mutational analysis of both proteins indicate that association of Mad2p with Mad1p is critical for checkpoint function and for hyperphosphorylation of Mad1p. [PubMed ID: 10436016]
Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae. (1999) K G Hardwick, R Li, C Mistrot, R H Chen, P Dann, A Rudner, A W Murray. Genetics 152(2):509-18
Abstract: The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint. [PubMed ID: 10353895]
Fission yeast bub1 is a mitotic centromere protein essential for the spindle checkpoint and the preservation of correct ploidy through mitosis. (1998) P Bernard, K Hardwick, J P Javerzat. J. Cell Biol. 143(7):1775-87
Abstract: The spindle checkpoint ensures proper chromosome segregation by delaying anaphase until all chromosomes are correctly attached to the mitotic spindle. We investigated the role of the fission yeast bub1 gene in spindle checkpoint function and in unperturbed mitoses. We find that bub1(+) is essential for the fission yeast spindle checkpoint response to spindle damage and to defects in centromere function. Activation of the checkpoint results in the recruitment of Bub1 to centromeres and a delay in the completion of mitosis. We show that Bub1 also has a crucial role in normal, unperturbed mitoses. Loss of bub1 function causes chromosomes to lag on the anaphase spindle and an increased frequency of chromosome loss. Such genomic instability is even more dramatic in Deltabub1 diploids, leading to massive chromosome missegregation events and loss of the diploid state, demonstrating that bub1(+ )function is essential to maintain correct ploidy through mitosis. As in larger eukaryotes, Bub1 is recruited to kinetochores during the early stages of mitosis. However, unlike its vertebrate counterpart, a pool of Bub1 remains centromere-associated at metaphase and even until telophase. We discuss the possibility of a role for the Bub1 kinase after the metaphase-anaphase transition. [PubMed ID: 9864354]
Budding yeast Cdc20: a target of the spindle checkpoint. (1998) L H Hwang, L F Lau, D L Smith, C A Mistrot, K G Hardwick, E S Hwang, A Amon, A W Murray. Science 279(5353):1041-4
Abstract: The spindle checkpoint regulates the cell division cycle by keeping cells with defective spindles from leaving mitosis. In the two-hybrid system, three proteins that are components of the checkpoint, Mad1, Mad2, and Mad3, were shown to interact with Cdc20, a protein required for exit from mitosis. Mad2 and Mad3 coprecipitated with Cdc20 at all stages of the cell cycle. The binding of Mad2 depended on Mad1 and that of Mad3 on Mad1 and Mad2. Overexpression of Cdc20 allowed cells with a depolymerized spindle or damaged DNA to leave mitosis but did not overcome the arrest caused by unreplicated DNA. Mutants in Cdc20 that were resistant to the spindle checkpoint no longer bound Mad proteins, suggesting that Cdc20 is the target of the spindle checkpoint. [PubMed ID: 9461437]
The spindle checkpoint. (1998) K G Hardwick. Trends Genet. 14(1):1-4 review
Abstract: not available. [PubMed ID: 9448456]
Activation of the budding yeast spindle assembly checkpoint without mitotic spindle disruption. (1996) K G Hardwick, E Weiss, F C Luca, M Winey, A W Murray. Science 273(5277):953-6
Abstract: The spindle assembly checkpoint keeps cells with defective spindles from initiating chromosome segregation. The protein kinase Mps1 phosphorylates the yeast protein Mad1p when this checkpoint is activated, and the overexpression of Mps1p induces modification of Mad1p and arrests wild-type yeast cells in mitosis with morphologically normal spindles. Spindle assembly checkpoint mutants overexpressing Mps1p pass through mitosis without delay and can produce viable progeny, which demonstrates that the arrest of wild-type cells results from inappropriate activation of the checkpoint in cells whose spindle is fully functional. Ectopic activation of cell-cycle checkpoints might be used to exploit the differences in checkpoint status between normal and tumor cells and thus improve the selectivity of chemotherapy. [PubMed ID: 8688079]
Mad1p, a phosphoprotein component of the spindle assembly checkpoint in budding yeast. (1995) K G Hardwick, A W Murray. J. Cell Biol. 131(3):709-20
Abstract: The spindle assembly checkpoint prevents cells from initiating anaphase until the spindle has been fully assembled. We previously isolated mitotic arrest deficient (mad) mutants that inactivate this checkpoint and thus increase the sensitivity of cells to benomyl, a drug that interferes with mitotic spindle assembly by depolymerizing microtubules. We have cloned the MAD1 gene and show that when it is disrupted yeast cells have the same phenotype as the previously isolated mad1 mutants: they fail to delay the metaphase to anaphase transition in response to microtubule depolymerization. MAD1 is predicted to encode a 90-kD coiled-coil protein. Anti-Mad1p antibodies give a novel punctate nuclear staining pattern and cell fractionation reveals that the bulk of Mad1p is soluble. Mad1p becomes hyperphosphorylated when wild-type cells are arrested in mitosis by benomyl treatment, or by placing a cold sensitive tubulin mutant at the restrictive temperature. This modification does not occur in G1-arrested cells treated with benomyl or in cells arrested in mitosis by defects in the mitotic cyclin proteolysis machinery, suggesting that Mad1p hyperphosphorylation is a step in the activation of the spindle assembly checkpoint. Analysis of Mad1p phosphorylation in other spindle assembly checkpoint mutants reveals that this response to microtubule-disrupting agents is defective in some (mad2, bub1, and bub3) but not all (mad3, bub2) mutant strains. We discuss the possible functions of Mad1p at this cell cycle checkpoint. [PubMed ID: 7593191]
Protein sorting
SED6 is identical to ERG6, and encodes a putative methyltransferase required for ergosterol synthesis. (1994) K G Hardwick, H R Pelham. Yeast 10(2):265-9
Abstract: Luminal endoplasmic reticulum (ER) proteins carry a sorting signal that allows them to be retrieved from the Golgi apparatus by a specific receptor. In yeast, this receptor is encoded by the ERD2 gene. Although retrieval of ER proteins does not appear to be an essential process, cells lacking ERD2 do not grow. Several multicopy suppressors of this growth defect have been isolated. The sequence of one of these, SED6, is presented here. Its product contains motifs characteristic of methyltransferases, and it is identical to ERG6, the presumed structural gene for S-adenosylmethionine:delta 24-sterol-C-methyltransferase. The gene is located adjacent to PDR4, near the centromere of chromosome XIII. [PubMed ID: 8203167]
SED5 encodes a 39-kD integral membrane protein required for vesicular transport between the ER and the Golgi complex. (1992) K G Hardwick, H R Pelham. J. Cell Biol. 119(3):513-21
Abstract: The ERD2 gene, which encodes the yeast HDEL (His-Asp-Glu-Leu) receptor, is essential for growth (Semenza, J. C., K. G. Hardwick, N. Dean, and H. R. B. Pelham. 1990. Cell. 61:1349-1357; Lewis, M. J., D. J. Sweet, and H. R. B. Pelham. 1990. Cell. 61:1359-1363). SED5, when present in multiple copies, enables cells to grow in the absence of Erd2p. Sequence analysis of SED5 reveals no significant homology with ERD2 or other known genes. We have raised antibodies to Sed5p which specifically recognize a 39-kD integral membrane protein. A stretch of hydrophobic residues at the COOH terminus is predicted to hold Sed5p on the cytoplasmic face of intracellular membranes. Cells that are depleted of Sed5p are unable to transport carboxypeptidase Y to the Golgi complex, and stop growing after a dramatic accumulation of ER membranes and vesicles. We conclude that the SED5 gene is essential for growth and that Sed5p is required for ER to Golgi transport. When Sed5p is overexpressed the efficiency of ER to Golgi transport is reduced, vesicles accumulate, and cellular morphology is perturbed. Immunofluorescence studies reveal that the bulk of Sed5p is not found on ER membranes but on punctate structures throughout the cytoplasm, the number of which increases upon SED5 overexpression. We suggest that Sed5p has an essential role in vesicular transport between ER and Golgi compartments and that it may itself cycle between these organelles. [PubMed ID: 1400588]
Genes that allow yeast cells to grow in the absence of the HDEL receptor. (1992) K G Hardwick, J C Boothroyd, A D Rudner, H R Pelham. EMBO J. 11(11):4187-95
Abstract: The ERD2 gene of Saccharomyces cerevisiae encodes the HDEL receptor that sorts ER proteins; it is essential for growth. In the absence of Erd2p the Golgi apparatus is both functionally and morphologically perturbed. Here we describe the isolation of four SED genes (suppressors of the erd2-deletion) which, when present in multiple copies, allow cells to grow in the absence of ERD2. The suppressed strains secrete the ER protein BiP and their internal membranes show a variety of morphological abnormalities. Sequence analysis indicates that all these SED genes encode membrane proteins: SED1 encodes a probable cell surface glycoprotein; SED2 is identical to SEC12, a gene required for the formation of ER-derived transport vesicles; SED4 encodes a protein whose cytoplasmic domain is 45% identical to that of Sec12p; SED3 is DPM1, the structural gene for dolichol-P-mannose synthase. We suggest that the absence of ERD2 causes an imbalance between membrane flow into and out of the Golgi apparatus, and that the SED gene products can compensate for this either by slowing transport from the ER or by stimulating vesicle budding from Golgi membranes. [PubMed ID: 1327759]
ERD2, a yeast gene required for the receptor-mediated retrieval of luminal ER proteins from the secretory pathway. (1990) J C Semenza, K G Hardwick, N Dean, H R Pelham. Cell 61(7):1349-57
Abstract: Resident proteins of the ER lumen carry a specific tetrapeptide signal (KDEL or HDEL) that prevents their secretion. We have previously described the isolation of yeast mutants that fail to retain such resident proteins within the cell. Here we describe ERD2, a gene required for retention. It encodes a 26 kd integral membrane protein whose abundance determines the efficiency and capacity of the retention system. Reduced expression of ERD2 leads to secretion of proteins bearing the HDEL signal, whereas overexpression of ERD2 improves retention both in wild-type cells and in other mutants. These results are consistent with other evidence that ERD2 encodes the HDEL receptor (see accompanying paper). The gene is also required, perhaps indirectly, for normal protein transport through the Golgi, and hence for growth. We discuss possible roles for ERD2 in the secretory pathway. [PubMed ID: 2194670]
ERS1 a seven transmembrane domain protein from Saccharomyces cerevisiae. (1990) K G Hardwick, H R Pelham. Nucleic Acids Res. 18(8):2177
Abstract: not available. [PubMed ID: 2186379]
ERD1, a yeast gene required for the retention of luminal endoplasmic reticulum proteins, affects glycoprotein processing in the Golgi apparatus. (1990) K G Hardwick, M J Lewis, J Semenza, N Dean, H R Pelham. EMBO J. 9(3):623-30
Abstract: We have previously shown that the C-terminal sequence HDEL acts as a retention signal for luminal endoplasmic reticulum (ER) proteins in Saccharomyces cerevisiae, and that it is possible to isolate mutants that fail to retain an invertase fusion protein bearing this signal. Analysis of many such mutants defines two genes, ERD1 and ERD2. Cells lacking the ERD1 gene secrete the endogenous ER protein, BiP. Under normal growth conditions, the rate of secretion is equivalent to the rate at which wild-type cells secrete a modified form of BiP that lacks the HDEL signal altogether. Thus, erd1 cells show a profound disruption of the retention system. The mutant cells have no gross abnormality of their intracellular membrane system, but show defects in the Golgi-dependent modification of glycoproteins. We suggest that sorting of luminal ER proteins normally occurs in the Golgi, and that the function of ERD1 is required for the correct interaction of an HDEL receptor with its ligands. The sequence of ERD1 predicts a membrane protein with several transmembrane domains, a conclusion supported by analysis of ERD1-SUC2 fusion proteins. [PubMed ID: 2178921]
Sorting of soluble ER proteins in yeast. (1988) H R Pelham, K G Hardwick, M J Lewis. EMBO J. 7(6):1757-62
Abstract: In animal cells, luminal endoplasmic reticulum (ER) proteins are prevented from being secreted by a sorting system that recognizes the C-terminal sequence KDEL. We show that yeast has a similar sorting system, but it recognizes HDEL, rather than KDEL: derivatives of the enzyme invertase that bear the HDEL signal fail to be secreted. An invertase fusion protein that is retained in the cells is partially modified by outer-chain mannosyl transferases, which reside in the Golgi element. This supports the view, based on studies in animal cells, that ER targeting is achieved by continuous retrieval of proteins from the Golgi. We have used an invertase fusion gene to screen for mutants that are defective in this sorting system. Over 60 mutants were obtained; eight of these are alleles of a single gene, erd1. The mutant strains grow normally at 30 degrees C, but instead of retaining the fusion protein in the cells, they secrete it. [PubMed ID: 3049074]
