Mad2 and Mad3 cooperate to arrest budding yeast in mitosis

The vacuole shapes the nucleus and the ribosomal DNA loop during mitotic delays

Chromosome structuring and condensation is one of the main features of mitosis. Here, Matos-Perdomo et al show how the nuclear envelope reshapes around the vacuole to give rise to the outstanding ribosomal DNA loop in budding yeast.

The ribosomal DNA (rDNA) array of Saccharomyces cerevisiae has served as a model to address chromosome organization. In cells arrested before anaphase (mid-M), the rDNA acquires a highly structured chromosomal organization referred to as the rDNA loop, whose length can double the cell diameter. Previous works established that complexes such as condensin and cohesin are essential to attain this structure. Here, we report that the rDNA loop adopts distinct presentations that arise as spatial adaptations to changes in the nuclear morphology triggered during mid-M arrests. Interestingly, the formation of the rDNA loop results in the appearance of a space under the loop (SUL) which is devoid of nuclear components yet colocalizes with the vacuole. We show that the rDNA-associated nuclear envelope (NE) often reshapes into a ladle to accommodate the vacuole in the SUL, with the nucleus becoming bilobed and doughnut-shaped. Finally, we demonstrate that the formation of the rDNA loop and the SUL require TORC1, membrane synthesis and functional vacuoles, yet is independent of nucleus–vacuole junctions and rDNA-NE tethering.

Distinct requirements for the COMPASS core subunits Set1, Swd1, and Swd3 during meiosis in the budding yeast Saccharomyces cerevisiae

Meiosis-specific chromatin structures, guided by histone modifications, are critical mediators of a meiotic transient transcription program and progression through prophase I. Histone H3K4 can be methylated up to three times by the Set1-containing COMPASS complex and each methylation mark corresponds to a different chromatin conformation. The level of H3K4 modification is directed by the activity of additional COMPASS components. In this study, we characterized the role of the COMPASS subunits during meiosis in Saccharomyces cerevisiae. In vegetative cells, previous studies revealed a role for subunits Swd2, Sdc1, and Bre2 for H3K4me2 while Spp1 supported trimethylation. However, we found that Bre2 and Sdc1 are required for H3K4me3 as yeast prepare to enter meiosis while Spp1 is not. Interestingly, we identified distinct meiotic functions for the core COMPASS complex members that required for all H3K4me, Set1, Swd1, and Swd3. While Set1 and Swd1 are required for progression through early meiosis, Swd3 is critical for late meiosis and spore morphogenesis. Furthermore, the meiotic requirement for Set1 is independent of H3K4 methylation, suggesting the presence of nonhistone substrates. Finally, checkpoint suppression analyses indicate that Set1 and Swd1 are required for both homologous recombination and chromosome segregation. These data suggest that COMPASS has important new roles for meiosis that are independent of its well-characterized functions during mitotic divisions.

Cell-cycle phospho-regulation of the kinetochore

The kinetochore is a mega-dalton protein assembly that forms within centromeric regions of chromosomes and directs their segregation during cell division. Here we review cell cycle-mediated phosphorylation events at the kinetochore, with a focus on the budding yeast Saccharomyces cerevisiae and the insight gained from forced associations of kinases and phosphatases. The point centromeres found in the budding yeast S. cerevisiae are one of the simplest such structures found in eukaryotes. The S. cerevisiae kinetochore comprises a single nucleosome, containing a centromere-specific H3 variant Cse4^CENP-A, bound to a set of kinetochore proteins that connect to a single microtubule. Despite the simplicity of the budding yeast kinetochore, the proteins are mostly homologous with their mammalian counterparts. In some cases, human proteins can complement their yeast orthologs. Like its mammalian equivalent, the regulation of the budding yeast kinetochore is complex: integrating signals from the cell cycle, checkpoints, error correction, and stress pathways. The regulatory signals from these diverse pathways are integrated at the kinetochore by post-translational modifications, notably phosphorylation and dephosphorylation, to control chromosome segregation. Here we highlight the complex interplay between the activity of the different cell-cycle kinases and phosphatases at the kinetochore, emphasizing how much more we have to understand this essential structure.

The Phosphatase PP1 Promotes Mitotic Slippage through Mad3 Dephosphorylation

Accurate chromosome segregation requires bipolar attachment of kinetochores to spindle microtubules. A conserved surveillance mechanism, the spindle assembly checkpoint (SAC), responds to lack of kinetochore-microtubule connections and delays anaphase onset until all chromosomes are bipolarly attached [1]. SAC signaling fires at kinetochores and involves a soluble mitotic checkpoint complex (MCC) that inhibits the anaphase-promoting complex (APC) [2, 3]. The mitotic delay imposed by SAC, however, is not everlasting. If kinetochores fail to establish bipolar connections, cells can escape from the SAC-induced mitotic arrest through a process called mitotic slippage [4]. Mitotic slippage occurs in the presence of SAC signaling at kinetochores [5, 6], but whether and how MCC stability and APC inhibition are actively controlled during slippage is unknown. The PP1 phosphatase has emerged as a key factor in SAC silencing once all kinetochores are bipolarly attached [7, 8]. PP1 turns off SAC signaling through dephosphorylation of the SAC scaffold Knl1/Blinkin at kinetochores [9-11]. Here, we show that, in budding yeast, PP1 is also required for mitotic slippage. However, its involvement in this process is not linked to kinetochores but rather to MCC stability. We identify S268 of Mad3 as a critical target of PP1 in this process. Mad3 S268 dephosphorylation destabilizes the MCC without affecting the initial SAC-induced mitotic arrest. Conversely, it accelerates mitotic slippage and overcomes the slippage defect of PP1 mutants. Thus, slippage is not the mere consequence of incomplete APC inactivation that brings about mitotic exit, as originally proposed, but involves the exertive antagonism between kinases and phosphatases.

Cell-Cycle Regulation of Dynamic Chromosome Association of the Condensin Complex

Eukaryotic cells inherit their genomes in the form of chromosomes, which are formed from the compaction of interphase chromatin by the condensin complex. Condensin is a member of the structural maintenance of chromosomes (SMC) family of ATPases, large ring-shaped protein assemblies that entrap DNA to establish chromosomal interactions. Here, we use the budding yeast Saccharomyces cerevisiae to dissect the role of the condensin ATPase and its relationship with cell-cycle-regulated chromosome binding dynamics. ATP hydrolysis-deficient condensin binds to chromosomes but is defective in chromosome condensation and segregation. By modulating the ATPase, we demonstrate that it controls condensin's dynamic turnover on chromosomes. Mitosis-specific phosphorylation of condensin's Smc4 subunit reduces the turnover rate. However, reducing turnover by itself is insufficient to compact chromosomes. We propose that condensation requires fine-tuned dynamic condensin interactions with more than one DNA. These results enhance our molecular understanding of condensin function during chromosome condensation.

Deficiency of PTEN leads to aberrant chromosome segregation through downregulation of MAD2

Proper spindle formation and accurate chromosome segregation are essential for ensuring mitotic fidelity. Phosphatase and tensin homolog (PTEN) is a multifunctional protein, which is able to maintain the stability of the genome and chromosomes. The present study described an essential role of PTEN in regulating chromosome segregation to prevent gross genomic instability via regulation of mitotic arrest deficient 2 (MAD2). PTEN knockdown induced cell cycle arrest and abnormal chromosome segregation, which manifested as the formation of anaphase bridges, lagging chromosomes and premature chromatid separation. In addition, MAD2 was identified as a potential target of PTEN. Furthermore, the present study revealed that PTEN knockdown resulted in MAD2 degradation via the ubiquitin-proteasomal pathway, while restoration of MAD2 expression partially ameliorated the mitotic defects induced by PTEN loss. The results from the present study proposed a novel mechanism by which PTEN maintains chromosome stability.

Downregulation of Mad2 and BubR1 increase the malignant potential and nocodazole resistance by compromising spindle assembly checkpoint signaling pathway in cervical carcinogenesis

AIM

To explore the involvement of Mad2 and BubR1 in cervical carcinogenesis.

METHODS

The expressions of Mad2 and BubR1 in tissues of high-grade squamous intraepithelial lesions (HSIL), low-grade squamous intraepithelial lesions (LSIL) and chronic cervicitis were analyzed immunohistochemistrily and compared with those of p16^INK4A . PEGFP-Mad2 and pEGFP-BubR1 were transfected into SiHa cells to overexpress Mad2 and BubR1 and Si-RNAs to knockdown. Cell viability was measured by cell counting kit-8 (CCK-8) assay. Migration and invasion capabilities were detected by Transwell. Propidium iodide staining with flow cytometry was used for cell cycle analysis and apoptosis was detected using Annexin V/7-AAD staining after nocodazole treatment.

RESULTS

The expression of Mad2 was significantly lower in HSIL than those in chronic cervicitis and LSIL, however, the expression of BubR1 showed no significant differences. To detect HSIL in cervical lesions, Mad2 had a sensitivity of 88.44% and a specificity of 87.23%, Mad2 was less sensitive and more specific than p16^INK4a . In SiHa cells, knockdown of Mad2 and BubR1 increased cell growth, reinforced invasion capacity and migration potency, inhibited apoptosis and decreased G2-phase distribution after nocodazole treatment. Oppositely, the overexpression strategies made cells show decreased malignant behaviors, raised apoptosis and increased G2-phase distribution.

CONCLUSION

Mad2 negativity was specific to identify HSIL immunohistochemistrily. Downregulation of Mad2 and BubR1 increase the malignant behavior and nocodazole resistance of SiHa cells via causing spindle assembly checkpoint defect. This mechanism may contribute to cervical carcinogenesis and resistance to microtubule-targeting drugs.

Electron cryotomography analysis of Dam1C/DASH at the kinetochore-spindle interface in situ

In dividing cells, depolymerizing spindle microtubules move chromosomes by pulling at their kinetochores. While kinetochore subcomplexes have been studied extensively in vitro, little is known about their in vivo structure and interactions with microtubules or their response to spindle damage. Here we combine electron cryotomography of serial cryosections with genetic and pharmacological perturbation to study the yeast chromosome segregation machinery in vivo. Each kinetochore microtubule has one (rarely, two) Dam1C/DASH outer kinetochore assemblies. Dam1C/DASH contacts the microtubule walls and does so with its flexible "bridges"; there are no contacts with the protofilaments' curved tips. In metaphase, ∼40% of the Dam1C/DASH assemblies are complete rings; the rest are partial rings. Ring completeness and binding position along the microtubule are sensitive to kinetochore attachment and tension, respectively. Our study and those of others support a model in which each kinetochore must undergo cycles of conformational change to couple microtubule depolymerization to chromosome movement.

Assays for the spindle assembly checkpoint in cell culture

The spindle assembly checkpoint (SAC) is crucial to maintain genomic stability since it prevents premature separation of sister chromatids in mitosis and ensures the fidelity of chromosome segregation. The SAC arrests cells in mitosis and is not satisfied until all kinetochores are stably attached to the mitotic spindle. Improperly attached kinetochores activate the SAC and catalyze the formation of the mitotic checkpoint complex (MCC), containing Mad2, Cdc20, BubR1, and Bub3 proteins. The MCC binds and thereby inhibits the APC/C E3 ubiquitin ligase until the last kinetochore has attached to microtubules. Once the SAC is satisfied, the APC/C promptly activates and targets cyclin B1 and securin for degradation, thus allowing sister chromatids to separate and the cell to exit mitosis. Our understanding of SAC signaling has increased thanks to the development of new genetic, biochemical, molecular, and structural biology techniques. Here, we describe how live-cell imaging microscopy in combination with gene-targeting strategies and biochemical assays can be exploited to investigate the intrinsic properties of the SAC in mammalian cultured cells.

Rewiring the Budding Yeast Proteome using Synthetic Physical Interactions

Artificially tethering two proteins or protein fragments together is a powerful method to query molecular mechanisms. However, this approach typically relies upon a prior understanding of which two proteins, when fused, are most likely to provide a specific function and is therefore not readily amenable to large-scale screening. Here, we describe the Synthetic Physical Interaction (SPI) method to create proteome-wide forced protein associations in the budding yeast Saccharomyces cerevisiae. This method allows thousands of protein-protein associations to be screened for those that affect either normal growth or sensitivity to drugs or specific conditions. The method is amenable to proteins, protein domains, or any genetically encoded peptide sequence.

Molecular Regulation of the Spindle Assembly Checkpoint by Kinases and Phosphatases

The spindle assembly checkpoint (SAC) is a surveillance mechanism contributing to the preservation of genomic stability by monitoring the microtubule attachment to, and/or the tension status of, each kinetochore during mitosis. The SAC halts metaphase to anaphase transition in the presence of unattached and/or untensed kinetochore(s) by releasing the mitotic checkpoint complex (MCC) from these improperly-oriented kinetochores to inhibit the anaphase-promoting complex/cyclosome (APC/C). The reversible phosphorylation of a variety of substrates at the kinetochore by antagonistic kinases and phosphatases is one major signaling mechanism for promptly turning on or turning off the SAC. In such a complex network, some kinases act at the apex of the SAC cascade by either generating (monopolar spindle 1, MPS1/TTK and likely polo-like kinase 1, PLK1), or contributing to generate (Aurora kinase B) kinetochore phospho-docking sites for the hierarchical recruitment of the SAC proteins. Aurora kinase B, MPS1 and budding uninhibited by benzimidazoles 1 (BUB1) also promote sister chromatid biorientation by modulating kinetochore microtubule stability. Moreover, MPS1, BUB1, and PLK1 seem to play key roles in APC/C inhibition by mechanisms dependent and/or independent on MCC assembly. The protein phosphatase 1 and 2A (PP1 and PP2A) are recruited to kinetochores to oppose kinase activity. These phosphatases reverse the phosphorylation of kinetochore targets promoting the microtubule attachment stabilization, sister kinetochore biorientation and SAC silencing. The kinase-phosphatase network is crucial as it renders the SAC a dynamic, graded-signaling, high responsive, and robust process thereby ensuring timely anaphase onset and preventing the generation of proneoplastic aneuploidy.

C-terminal region of Mad2 plays an important role during mitotic spindle checkpoint in fission yeast Schizosaccharomyces pombe

The mitotic arrest deficiency 2 (Mad2) protein is an essential component of the spindle assembly checkpoint that interacts with Cdc20/Slp1 and inhibit its ability to activate anaphase promoting complex/cyclosome (APC/C). In bladder cancer cell line the C-terminal residue of the mad2 gene has been found to be deleted. In this study we tried to understand the role of the C-terminal region of mad2 on the spindle checkpoint function. To envisage the role of C-terminal region of Mad2, we truncated 25 residues of Mad2 C-terminal region in fission yeast S.pombe and characterized its effect on spindle assembly checkpoint function. The cells containing C-terminal truncation of Mad2 exhibit sensitivity towards microtubule destabilizing agent suggesting perturbation of spindle assembly checkpoint. Further, the C-terminal truncation of Mad2 exhibit reduced viability in the nda3-KM311 mutant background at non-permissive temperature. Truncation in mad2 gene also affects its foci forming ability at unattached kinetochore suggesting that the mad2-∆CT mutant is unable to maintain spindle checkpoint activation. However, in response to the defective microtubule, only brief delay of mitotic progression was observed in Mad2 C-terminal truncation mutant. In addition we have shown that the deletion of two β strands of Mad2 protein abolishes its ability to interact with APC activator protein Slp1/Cdc20. We purpose that the truncation of two β strands (β7 and β8) of Mad2 destabilize the safety belt and affect the Cdc20-Mad2 interaction leading to defects in the spindle checkpoint activation.

Synthetic Physical Interactions Map Kinetochore-Checkpoint Activation Regions

The spindle assembly checkpoint (SAC) is a key mechanism to regulate the timing of mitosis and ensure that chromosomes are correctly segregated to daughter cells. The recruitment of the Mad1 and Mad2 proteins to the kinetochore is normally necessary for SAC activation. This recruitment is coordinated by the SAC kinase Mps1, which phosphorylates residues at the kinetochore to facilitate binding of Bub1, Bub3, Mad1, and Mad2. There is evidence that the essential function of Mps1 is to direct recruitment of Mad1/2. To test this model, we have systematically recruited Mad1, Mad2, and Mps1 to most proteins in the yeast kinetochore, and find that, while Mps1 is sufficient for checkpoint activation, recruitment of either Mad1 or Mad2 is not. These data indicate an important role for Mps1 phosphorylation in SAC activation, beyond the direct recruitment of Mad1 and Mad2.

The Molecular Biology of Spindle Assembly Checkpoint Signaling Dynamics

The spindle assembly checkpoint is a safeguard mechanism that coordinates cell-cycle progression during mitosis with the state of chromosome attachment to the mitotic spindle. The checkpoint prevents mitotic cells from exiting mitosis in the presence of unattached or improperly attached chromosomes, thus avoiding whole-chromosome gains or losses and their detrimental effects on cell physiology. Here, I review a considerable body of recent progress in the elucidation of the molecular mechanisms underlying checkpoint signaling, and identify a number of unresolved questions.

The C-terminal domains of human neurofibromin and its budding yeast homologs Ira1 and Ira2 regulate the metaphase to anaphase transition

The human tumor suppressor neurofibromin contains a cysteine and serine-rich domain/Ras-GTPase activating protein domain (CSRD/RasGAP) and a C-terminal domain (CTD). Domain studies of neurofibromin suggest it has other functions in addition to being a RasGAP, but the mechanisms underlying its tumor suppressor activity are not well understood. The budding yeast Saccharomyces cerevisiae is a good model system for studying neurofibromin function because it possesses Ira1 and Ira2, which are homologous to human neurofibromin in both sequence and function. We found that overexpression of CTD or a neurofibromin CTD-homologous domain (CHD) of Ira1/2 in budding yeast delayed degradation of the securin protein Pds1, whereas overexpression of CSRD/RasGAP did not affect Pds1 degradation. We also found that when CTD or CHD was overexpressed, the number of cells in metaphase was higher than in the control. These results demonstrate that CTD and CHD function in the metaphase to anaphase transition. In addition, Δira1Δira2 cells bypassed mitotic arrest in response to spindle damage, indicating that Ira1 and Ira2 may be involved in the spindle assembly checkpoint (SAC). However, Δira1Δira2Δmad2 cells are more sensitive to spindle damage than Δmad2 or Δira1Δira2 cells are, suggesting that Ira1/2 and Mad2 function in different pathways. Overexpression of CTD but not CSRD/RasGAP partially rescued the hypersensitivity of Δira1Δira2Δmad2 cells to microtubule-destabilizing drugs, indicating a role for CTD in the SAC pathway. Taken together, independently of RasGAP activity, the C-terminal domains of neurofibromin, Ira1, and Ira2 regulate the metaphase to anaphase transition in a Mad2-independent fashion.

MAD2 and CDC20 are upregulated in high-grade squamous intraepithelial lesions and squamous cell carcinomas of the uterine cervix

Abnormal expression of the spindle assembly checkpoint proteins causes tumor cell aneuploidy, which has been reported in various malignancies. The expression of mitotic arrest deficient 2 (MAD2) and cell-division cycle 20 homolog (CDC20), the key spindle assembly checkpoint proteins, has not been studied in cervical carcinogenesis. In this study, we compared the expression of MAD2 and CDC20 in 332 cases, including normal cervical tissues, low-grade squamous intraepithelial lesions, high-grade squamous intraepithelial lesions (HSILs), and invasive squamous cell carcinomas (SCCs). Both MAD2 and CDC20 were overexpressed in the nuclei or cytoplasm of dysplastic and malignant tumor cells. The frequency of MAD2 overexpression was markedly increased from undetectable (0/100) in normal cervical tissues and 2% (1/50) in low-grade squamous intraepithelial lesions to 67.1% (53/79) in HSILs and 52.4% (54/103) in SCCs. Similarly, CDC20 was overexpressed in 49.4% (39/79) of HSILs and 22.3% (23/103) of SCCs, whereas CDC20 was not detectable (0/100) in normal cervical tissues and overexpressed only in 8.0% (4/50) of low-grade squamous intraepithelial lesions. In SCC cases, MAD2 overexpression correlated with a patient age of less than 60 yr (P=0.043), nonkeratinizing histologic type (P=0.018), and a lesser degree of stromal invasion (P=0.026). In conclusion, MAD2 and CDC20 overexpression was increased in HSILs and SCCs, suggesting their involvement in the initiation of cervical cancers. Controlling CDC20 and MAD2 expression may be a therapeutic strategy for cervical cancer.

The mitotic checkpoint complex binds a second CDC20 to inhibit active APC/C

The spindle assembly checkpoint (SAC) maintains genomic stability by delaying chromosome segregation until the last chromosome has attached to the mitotic spindle. The SAC prevents the anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase from recognizing cyclin B and securin by catalysing the incorporation of the APC/C co-activator, CDC20, into a complex called the mitotic checkpoint complex (MCC). The SAC works through unattached kinetochores generating a diffusible 'wait anaphase' signal that inhibits the APC/C in the cytoplasm, but the nature of this signal remains a key unsolved problem. Moreover, the SAC and the APC/C are highly responsive to each other: the APC/C quickly targets cyclin B and securin once all the chromosomes attach in metaphase, but is rapidly inhibited should kinetochore attachment be perturbed. How this is achieved is also unknown. Here, we show that the MCC can inhibit a second CDC20 that has already bound and activated the APC/C. We show how the MCC inhibits active APC/C and that this is essential for the SAC. Moreover, this mechanism can prevent anaphase in the absence of kinetochore signalling. Thus, we propose that the diffusible 'wait anaphase' signal could be the MCC itself, and explain how reactivating the SAC can rapidly inhibit active APC/C.

Insights into the anaphase-promoting complex: a molecular machine that regulates mitosis

The anaphase-promoting complex/cyclosome (APC/C) is a large multimeric complex that functions as a RING domain E3 ubiquitin ligase to regulate ordered transitions through the cell cycle. It does so by controlling the ubiquitin-mediated proteolysis of cell cycle proteins, notably cyclins and securins, whose degradation triggers sister chromatid disjunction and mitotic exit. Regulation of APC/C activity and modulation of its substrate specificity are subject to intricate cell cycle checkpoints and control mechanisms involving the switching of substrate-specifying cofactors, association of regulatory protein complexes and post-translational modifications. This review discusses the recent progress towards understanding the overall architecture of the APC/C, the molecular basis for degron recognition and ubiquitin chain synthesis, and how these activities are regulated.

Signalling dynamics in the spindle checkpoint response

The spindle checkpoint ensures proper chromosome segregation during cell division. Unravelling checkpoint signalling has been a long-standing challenge owing to the complexity of the structures and forces that regulate chromosome segregation. New reports have now substantially advanced our understanding of checkpoint signalling mechanisms at the kinetochore, the structure that connects microtubules and chromatin. In contrast to the traditional view of a binary checkpoint response - either completely on or off - new findings indicate that the checkpoint response strength is variable. This revised perspective provides insight into how checkpoint bypass can lead to aneuploidy and informs strategies to exploit these errors for cancer treatments.

The dynamics of signal amplification by macromolecular assemblies for the control of chromosome segregation

The control of chromosome segregation relies on the spindle assembly checkpoint (SAC), a complex regulatory system that ensures the high fidelity of chromosome segregation in higher organisms by delaying the onset of anaphase until each chromosome is properly bi-oriented on the mitotic spindle. Central to this process is the establishment of multiple yet specific protein-protein interactions in a narrow time-space window. Here we discuss the highly dynamic nature of multi-protein complexes that control chromosome segregation in which an intricate network of weak but cooperative interactions modulate signal amplification to ensure a proper SAC response. We also discuss the current structural understanding of the communication between the SAC and the kinetochore; how transient interactions can regulate the assembly and disassembly of the SAC as well as the challenges and opportunities for the definition and the manipulation of the flow of information in SAC signaling.

Bimodal activation of BubR1 by Bub3 sustains mitotic checkpoint signaling

The mitotic checkpoint (also known as the spindle assembly checkpoint) prevents premature anaphase onset through generation of an inhibitor of the E3 ubiquitin ligase APC/C, whose ubiquitination of cyclin B and securin targets them for degradation. Combining in vitro reconstitution and cell-based assays, we now identify dual mechanisms through which Bub3 promotes mitotic checkpoint signaling. Bub3 enhances signaling at unattached kinetochores not only by facilitating binding of BubR1 but also by enhancing Cdc20 recruitment to kinetochores mediated by BubR1's internal Cdc20 binding site. Downstream of kinetochore-produced complexes, Bub3 promotes binding of BubR1's conserved, amino terminal Cdc20 binding domain to a site in Cdc20 that becomes exposed by initial Mad2 binding. This latter Bub3-stimulated event generates the final mitotic checkpoint complex of Bub3-BubR1-Cdc20 that selectively inhibits ubiquitination of securin and cyclin B by APC/C(Cdc20). Thus, Bub3 promotes two distinct BubR1-Cdc20 interactions, involving each of the two Cdc20 binding sites of BubR1 and acting at unattached kinetochores or cytoplasmically, respectively, to facilitate production of the mitotic checkpoint inhibitor.

WT1 interacts with MAD2 and regulates mitotic checkpoint function

Tumour suppressors safeguard the fidelity of the mitotic checkpoint by transcriptional regulation of genes that encode components of the mitotic checkpoint complex (MCC). Here we report a new role for the tumour suppressor and transcription factor, WT1, in the mitotic checkpoint. We show that WT1 regulates the MCC by directly interacting with the spindle assembly checkpoint protein, MAD2. WT1 colocalizes with MAD2 during mitosis and preferentially binds to the functionally active, closed-conformer, C-MAD2. Furthermore, WT1 associates with the MCC containing MAD2, BUBR1 and CDC20, resulting in prolonged inhibition of the anaphase-promoting complex/cyclosome (APC/C) and delayed degradation of its substrates SECURIN and CYCLIN B1. Strikingly, RNA interference-mediated depletion of WT1 leads to enhanced turnover of SECURIN, decreased lag time to anaphase and defects in chromosome segregation. Our findings identify WT1 as a regulator of the mitotic checkpoint and chromosomal stability.

Joined at the hip: kinetochores, microtubules, and spindle assembly checkpoint signaling

Error-free chromosome segregation relies on stable connections between kinetochores and spindle microtubules. The spindle assembly checkpoint (SAC) monitors such connections and relays their absence to the cell cycle machinery to delay cell division. The molecular network at kinetochores that is responsible for microtubule binding is integrated with the core components of the SAC signaling system. Molecular-mechanistic understanding of how the SAC is coupled to the kinetochore-microtubule interface has advanced significantly in recent years. The latest insights not only provide a striking view of the dynamics and regulation of SAC signaling events at the outer kinetochore but also create a framework for understanding how that signaling may be terminated when kinetochores and microtubules connect.

Chromatin compaction by condensin I, intra-kinetochore stretch and tension, and anaphase onset, in collective spindle assembly checkpoint interaction

The control mechanism in mitosis and meiosis by which cells decide to inhibit or allow segregation, the so-called spindle assembly checkpoint (SAC), increases the fidelity of chromosome segregation. It acts like a clockwork mechanism which measures time in units of stable attachments of microtubules (MTs) to kinetochores (the order parameter). Stable MT-kinetochore attachments mediate poleward forces and 'unstable' attachments, acting alone or together with motor proteins on kinetochores via chromosomes, antipoleward forces. Stable and unstable attachments could be separated, and the non-equilibrium integrated MT mediated force acting on stably attached kinetochores was derived in a collective interaction (Matsson 2009 J. Phys.: Condens. Matter 21 502101), in which kinetochores were treated as rigid protein complexes. As forces and tension in that model became equally distributed in all bioriented sister chromatid (SC) pairs, segregation was inhibited without need of a 'wait-anaphase' signal. In this generalization, the kinetochore is divided into an inner chromatin proximal complex and an outer MT proximal complex, and the integrated MT mediated force is divided into an integrated poleward and an integrated antipoleward force. The model also describes the collective interaction of condensin I with chromatin, which together with the MT mediated dynamics yields the putative in vivo tension in kinetochores and centromeric and pericentromeric chromatin, as a non-linear function of the order parameter. Supported by the compaction force and an increased stiffness in chromatin towards the end of metaphase, the two opposing integrated MT mediated poleward forces, together with metaphase oscillations, induce a swift and synchronized anaphase onset by first increasing the intra-kinetochore stretch. This increase lowers the SAC energy threshold, making a cleavage by separase of all cohesin tethering SC pairs in anaphase energetically possible, thereby reducing the risk for aneuploidy and cancer. It is also shown how this risk might increase in condensin I depleted cells. Moreover, a solution is provided to the fundamental statistical physics problem with a system containing an increasing number of particles (molecular complexes) that become strongly correlated in space.

RNAi silencing of the SoxE gene suppresses cell proliferation in silkworm BmN4 cells

The transcription factor SoxE is mainly expressed in the gonad and involved in the regulation of gonad development and sex determination in animals. Here, we used the silkworm ovary-derived BmN4-SID1 cell line to survey the roles of the silkworm SoxE protein (BmSoxE) and predict its candidate binding targets. RNAi-mediated silencing of BmSoxE expression suppressed cell proliferation in BmN4-SID1 cells. A further cell cycle analysis revealed that this inhibition of cell proliferation was largely due to cell cycle arrest in G1 phase when BmSoxE expression was blocked in BmN4-SID1 cells. Genome-wide microarray expression analyses demonstrated that the expression levels of a set of genes were significantly altered following BmSoxE RNAi. More than half of these genes contained conserved binding sites for HMG box domain of the Sox proteins and were predicted to be candidate binding targets for BmSoxE. Importantly, some of the candidate targets may be associated with the effect of BmSoxE on cell proliferation. Several candidate target genes showed gonad-specific expression in silkworm larvae. Taken together, these data demonstrate that BmSoxE is required for cell proliferation in silkworm BmN4-SID1 cells and provide valuable information for further investigations of the molecular control exerted by the BmSoxE protein over cell proliferation and gonad development in the silkworm.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

Chromosome segregation in budding yeast: sister chromatid cohesion and related mechanisms

Studies on budding yeast have exposed the highly conserved mechanisms by which duplicated chromosomes are evenly distributed to daughter cells at the metaphase-anaphase transition. The establishment of proteinaceous bridges between sister chromatids, a function provided by a ring-shaped complex known as cohesin, is central to accurate segregation. It is the destruction of this cohesin that triggers the segregation of chromosomes following their proper attachment to microtubules. Since it is irreversible, this process must be tightly controlled and driven to completion. Furthermore, during meiosis, modifications must be put in place to allow the segregation of maternal and paternal chromosomes in the first division for gamete formation. Here, I review the pioneering work from budding yeast that has led to a molecular understanding of the establishment and destruction of cohesion.

Monopolar spindle 1 (MPS1) kinase promotes production of closed MAD2 (C-MAD2) conformer and assembly of the mitotic checkpoint complex

MPS1 kinase is an essential component of the spindle assembly checkpoint (SAC), but its functioning mechanisms are not fully understood. We have shown recently that direct interaction between BUBR1 and MAD2 is critical for assembly and function of the human mitotic checkpoint complex (MCC), the SAC effector. Here we report that inhibition of MPS1 kinase activity by reversine disrupts BUBR1-MAD2 as well as CDC20-MAD2 interactions, causing premature activation of the anaphase-promoting complex/cyclosome. The effect of MPS1 inhibition is likely due to reduction of closed MAD2 (C-MAD2), as expressing a MAD2 mutant (MAD2(L13A)) that is locked in the C conformation rescued the checkpoint defects. In the presence of reversine, exogenous C-MAD2 does not localize to unattached kinetochores but is still incorporated into the MCC. Contrary to a previous report, we found that sustained MPS1 activity is required for maintaining both the MAD1·C-MAD2 complex and open MAD2 (O-MAD2) at unattached kinetochores to facilitate C-MAD2 production. Additionally, mitotic phosphorylation of BUBR1 is also affected by MPS1 inhibition but seems dispensable for MCC assembly. Our results support the notion that MPS1 kinase promotes C-MAD2 production and subsequent MCC assembly to activate the SAC.

Mad2 "opens" Cdc20 for BubR1 binding

In this issue of Molecular Cell, Han et al. (2013) demonstrate that Mad2 induces a conformational change in Cdc20 that permits BubR1 binding, thereby producing the physiologically relevant APC/C(Cdc20) inhibitor.

Catalytic assembly of the mitotic checkpoint inhibitor BubR1-Cdc20 by a Mad2-induced functional switch in Cdc20

The mitotic checkpoint acts to maintain chromosome content by generation of a diffusible anaphase inhibitor. Unattached kinetochores catalyze a conformational shift in Mad2, converting an inactive open form into a closed form that can capture Cdc20, the mitotic activator of the APC/C ubiquitin ligase. Mad2 binding is now shown to promote a functional switch in Cdc20, exposing a previously inaccessible site for binding to BubR1's conserved Mad3 homology domain. BubR1, but not Mad2, binding to APC/C(Cdc20) is demonstrated to inhibit ubiquitination of cyclin B. Closed Mad2 is further shown to catalytically amplify production of BubR1-Cdc20 without necessarily being part of the complex. Thus, the mitotic checkpoint is produced by a cascade of two catalytic steps: an initial step acting at unattached kinetochores to produce a diffusible Mad2-Cdc20 intermediate and a diffusible step in which that intermediate amplifies production of BubR1-Cdc20, the inhibitor of cyclin B ubiquitination, by APC/C(Cdc20).

Panta rhei: the APC/C at steady state

The anaphase-promoting complex or cyclosome (APC/C) is a conserved, multisubunit E3 ubiquitin (Ub) ligase that is active both in dividing and in postmitotic cells. Its contributions to life are especially well studied in the domain of cell division, in which the APC/C lies at the epicenter of a regulatory network that controls the directionality and timing of cell cycle events. Biochemical and structural work is shedding light on the overall organization of APC/C subunits and on the mechanism of substrate recognition and Ub chain initiation and extension as well as on the molecular mechanisms of a checkpoint that seizes control of APC/C activity during mitosis. Here, we review how these recent advancements are modifying our understanding of the APC/C.

The spindle assembly checkpoint

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.

Tracking spindle checkpoint signals from kinetochores to APC/C

Accurate chromosome segregation during mitosis is critical for maintaining genomic stability. The kinetochore--a large protein assembly on centromeric chromatin--functions as the docking site for spindle microtubules and a signaling hub for the spindle checkpoint. At metaphase, spindle microtubules from opposing spindle poles capture each pair of sister kinetochores, exert pulling forces, and create tension across sister kinetochores. The spindle checkpoint detects improper kinetochore-microtubule attachments and translates these defects into biochemical activities that inhibit the anaphase-promoting complex or cyclosome (APC/C) throughout the cell to delay anaphase onset. A deficient spindle checkpoint leads to premature sister-chromatid separation and aneuploidy. Here, we review recent progress on the generation, propagation, transmission, and silencing of the spindle checkpoint signals from kinetochores to APC/C.

The Mad1-Mad2 balancing act--a damaged spindle checkpoint in chromosome instability and cancer

Cancer cells are commonly aneuploid. The spindle checkpoint ensures accurate chromosome segregation by controlling cell cycle progression in response to aberrant microtubule-kinetochore attachment. Damage to the checkpoint, which is a partial loss or gain of checkpoint function, leads to aneuploidy during tumorigenesis. One form of damage is a change in levels of the checkpoint proteins mitotic arrest deficient 1 and 2 (Mad1 and Mad2), or in the Mad1:Mad2 ratio. Changes in Mad1 and Mad2 levels occur in human cancers, where their expression is regulated by the tumor suppressors p53 and retinoblastoma 1 (RB1). By employing a standard assay, namely the addition of a mitotic poison at mitotic entry, it has been shown that checkpoint function is normal in many cancer cells. However, in several experimental systems, it has been observed that this standard assay does not always reveal checkpoint aberrations induced by changes in Mad1 or Mad2, where excess Mad1 relative to Mad2 can lead to premature anaphase entry, and excess Mad2 can lead to a delay in entering anaphase. This Commentary highlights how changes in the levels of Mad1 and Mad2 result in a damaged spindle checkpoint, and explores how these changes cause chromosome instability that can lead to aneuploidy during tumorigenesis.

Mad2 and the APC/C compete for the same site on Cdc20 to ensure proper chromosome segregation

The spindle assembly checkpoint (SAC) is essential to ensure proper chromosome segregation and thereby maintain genomic stability. The SAC monitors chromosome attachment, and any unattached chromosomes generate a "wait anaphase" signal that blocks chromosome segregation. The target of the SAC is Cdc20, which activates the anaphase-promoting complex/cyclosome (APC/C) that triggers anaphase and mitotic exit by ubiquitylating securin and cyclin B1. The inhibitory complex formed by the SAC has recently been shown to inhibit Cdc20 by acting as a pseudosubstrate inhibitor, but in this paper, we show that Mad2 also inhibits Cdc20 by binding directly to a site required to bind the APC/C. Mad2 and the APC/C competed for Cdc20 in vitro, and a Cdc20 mutant that does not bind stably to Mad2 abrogated the SAC in vivo. Thus, we provide insights into how Cdc20 binds the APC/C and uncover a second mechanism by which the SAC inhibits the APC/C.

Don't make me mad, Bub!

The history of Bub1, a spindle checkpoint component, reveals a spectacular case of parallel evolution. In this issue of Developmental Cell, Suijkerbuijk et al. (2012) provide evidence that Bub1 has duplicated and diverged many times during eukaryotic evolution, dividing the functions of its ancestor between the two duplicated copies.

Mitosis: short-circuiting spindle checkpoint signaling

The spindle checkpoint forms an intricate signaling circuit to sense unattached kinetochores, to inhibit the anaphase-promoting complex/cyclosome (APC/C), and to delay anaphase onset. Using clever genetic experiments in the budding yeast, Lau and Murray define the endpoint of checkpoint signaling and provide key mechanistic insights into checkpoint inhibition of APC/C.