Unattached kinetochores catalyze production of an anaphase inhibitor that requires a Mad2 template to prime Cdc20 for BubR1 binding

Genome-wide siRNA screen reveals coupling between mitotic apoptosis and adaptation

The antimitotic anti-cancer drugs, including taxol, perturb spindle dynamics, and induce prolonged, spindle checkpoint-dependent mitotic arrest in cancer cells. These cells then either undergo apoptosis triggered by the intrinsic mitochondrial pathway or exit mitosis without proper cell division in an adaptation pathway. Using a genome-wide small interfering RNA (siRNA) screen in taxol-treated HeLa cells, we systematically identify components of the mitotic apoptosis and adaptation pathways. We show that the Mad2 inhibitor p31(comet) actively promotes mitotic adaptation through cyclin B1 degradation and has a minor separate function in suppressing apoptosis. Conversely, the pro-apoptotic Bcl2 family member, Noxa, is a critical initiator of mitotic cell death. Unexpectedly, the upstream components of the mitochondrial apoptosis pathway and the mitochondrial fission protein Drp1 contribute to mitotic adaption. Our results reveal crosstalk between the apoptosis and adaptation pathways during mitotic arrest.

Thyroid hormone receptor interacting protein 13 (TRIP13) AAA-ATPase is a novel mitotic checkpoint-silencing protein

The mitotic checkpoint (or spindle assembly checkpoint) is a fail-safe mechanism to prevent chromosome missegregation by delaying anaphase onset in the presence of defective kinetochore-microtubule attachment. The target of the checkpoint is the E3 ubiquitin ligase anaphase-promoting complex/cyclosome. Once all chromosomes are properly attached and bioriented at the metaphase plate, the checkpoint needs to be silenced. Previously, we and others have reported that TRIP13 AAA-ATPase binds to the mitotic checkpoint-silencing protein p31(comet). Here we show that endogenous TRIP13 localizes to kinetochores. TRIP13 knockdown delays metaphase-to-anaphase transition. The delay is caused by prolonged presence of the effector for the checkpoint, the mitotic checkpoint complex, and its association and inhibition of the anaphase-promoting complex/cyclosome. These results suggest that TRIP13 is a novel mitotic checkpoint-silencing protein. The ATPase activity of TRIP13 is essential for its checkpoint function, and interference with TRIP13 abolished p31(comet)-mediated mitotic checkpoint silencing. TRIP13 overexpression is a hallmark of cancer cells showing chromosomal instability, particularly in certain breast cancers with poor prognosis. We suggest that premature mitotic checkpoint silencing triggered by TRIP13 overexpression may promote cancer development.

Recruitment of Mad1 to metaphase kinetochores is sufficient to reactivate the mitotic checkpoint

Mad1 recruitment to metaphase kinetochores reactivates the mitotic checkpoint, which requires the C terminus of Mad1 in addition to its Mad2-binding domain.

The mitotic checkpoint monitors kinetochore–microtubule attachment and prevents anaphase until all kinetochores are stably attached. Checkpoint regulation hinges on the dynamic localization of checkpoint proteins to kinetochores. Unattached, checkpoint-active kinetochores accumulate multiple checkpoint proteins, which are depleted from kinetochores upon stable attachment, allowing checkpoint silencing. Because multiple proteins are recruited simultaneously to unattached kinetochores, it is not known what changes at kinetochores are essential for anaphase promoting complex/cyclosome (APC/C) inhibition. Using chemically induced dimerization to manipulate protein localization with temporal control, we show that recruiting the checkpoint protein Mad1 to metaphase kinetochores is sufficient to reactivate the checkpoint without a concomitant increase in kinetochore levels of Mps1 or BubR1. Furthermore, Mad2 binding is necessary but not sufficient for Mad1 to activate the checkpoint; a conserved C-terminal motif is also required. The results of our checkpoint reactivation assay suggest that Mad1, in addition to converting Mad2 to its active conformation, scaffolds formation of a higher-order mitotic checkpoint complex at kinetochores.

Conditional targeting of MAD1 to kinetochores is sufficient to reactivate the spindle assembly checkpoint in metaphase

Fidelity of chromosome segregation is monitored by the spindle assembly checkpoint (SAC). Key components of the SAC include MAD1, MAD2, BUB1, BUB3, BUBR1, and MPS1. These proteins accumulate on kinetochores in early prometaphase but are displaced when chromosomes attach to microtubules and/or biorient on the mitotic spindle. As a result, stable attachment of the final chromosome satisfies the SAC, permitting activation of the anaphase promoting complex/cyclosome (APC/C) and subsequent anaphase onset. SAC satisfaction is reversible, however, as addition of taxol during metaphase stops cyclin B1 degradation by the APC/C. We now show that targeting MAD1 to kinetochores during metaphase is sufficient to reestablish SAC activity after initial silencing. Using rapamycin-induced heterodimerization of FKBP-MAD1 to FRB-MIS12 and live monitoring of cyclin B1 degradation, we show that timed relocalization of MAD1 during metaphase can stop cyclin B1 degradation without affecting chromosome-spindle attachments. APC/C inhibition represented true SAC reactivation, as FKBP-MAD1 required an intact MAD2-interaction motif and MPS1 activity to accomplish this. Our data show that MAD1 kinetochore localization dictates SAC activity and imply that SAC regulatory mechanisms downstream of MAD1 remain functional in metaphase.

Molecular dynamics simulation on the conformational transition of the mad2 protein from the open to the closed state

The Mad2 protein, with two distinct conformations of open- and closed-states, is a key player in the spindle checkpoint. The closed Mad2 state is more active than the open one. We carried out conventional and targeted molecular dynamics simulations for the two stable Mad2 states and their conformational transition to address the dynamical transition mechanism from the open to the closed state. The intermediate structure in the transition process shows exposure of the β6 strand and an increase of space around the binding sites of β6 strand due to the unfolding of the β7/8 sheet and movement of the β6/4/5 sheet close to the αC helix. Therefore, Mad2 binding to the Cdc20 protein in the spindle checkpoint is made possible. The interconversion between these two states might facilitate the functional activity of the Mad2 protein. Motion correlation analysis revealed the allosteric network between the β1 strand and β7/8 sheet via communication of the β5-αC loop and the β6/4/5 sheet in this transition process.

Phosphorylation regulates the p31Comet-mitotic arrest-deficient 2 (Mad2) interaction to promote spindle assembly checkpoint (SAC) activity

The spindle assembly checkpoint (SAC) ensures the faithful segregation of the genome during mitosis by ensuring that sister chromosomes form bipolar attachments with microtubules of the mitotic spindle. p31(Comet) is an antagonist of the SAC effector Mad2 and promotes silencing of the SAC and mitotic progression. However, p31(Comet) interacts with Mad2 throughout the cell cycle. We show that p31(Comet) binds Mad2 solely in an inhibitory manner. We demonstrate that attenuating the affinity of p31(Comet) for Mad2 by phosphorylation promotes SAC activity in mitosis. Specifically, phosphorylation of Ser-102 weakens p31(Comet)-Mad2 binding and enhances p31(Comet)-mediated bypass of the SAC. Our results provide the first evidence for regulation of p31(Comet) and demonstrate a previously unknown event controlling SAC activity.

Phospho-Bcl-xL(Ser62) influences spindle assembly and chromosome segregation during mitosis

Functional analysis of a series of phosphorylation mutants reveals that Bcl-xL(Ser62Ala) influences cell entry into anaphase and mitotic exit in taxol-exposed cells compared with cells expressing wild-type Bcl-xL or a series of other phosphorylation mutants, an effect that appears to be independent of its anti-apoptotic activity. During normal mitosis progression, Bcl-xL(Ser62) is strongly phosphorylated by PLK1 and MAPK14/SAPKp38α at the prometaphase, metaphase, and the anaphase boundaries, while it is de-phosphorylated at telophase and cytokinesis. Phospho-Bcl-xL(Ser62) localizes in centrosomes with γ-tubulin and in the mitotic cytosol with some spindle-assembly checkpoint signaling components, including PLK1, BubR1, and Mad2. In taxol- and nocodazole-exposed cells, phospho-Bcl-xL(Ser62) also binds to Cdc20- Mad2-, BubR1-, and Bub3-bound complexes, while Bcl-xL(Ser62Ala) does not. Silencing Bcl-xL expression and expressing the phosphorylation mutant Bcl-xL(Ser62Ala) lead to an increased number of cells harboring mitotic spindle defects including multipolar spindle, chromosome lagging and bridging, aneuploidy with micro-, bi-, or multi-nucleated cells, and cells that fail to resolve undergo mitosis within 6 h. Together, the data indicate that during mitosis, Bcl-xL(Ser62) phosphorylation impacts on spindle assembly and chromosome segregation, influencing chromosome stability. Observations of mitotic cells harboring aneuploidy with micro-, bi-, or multi-nucleated cells, and cells that fail to resolve undergo mitosis within 6 h were also made with cells expressing the phosphorylation mutant Bcl-xL(Ser49Ala) and dual mutant Bcl-xL(Ser49/62Ala).

Functional characterization of Anaphase Promoting Complex/Cyclosome (APC/C) E3 ubiquitin ligases in tumorigenesis

The Anaphase Promoting Complex/Cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that primarily governs cell cycle progression. APC/C is composed of at least 14 core subunits and recruits its substrates for ubiquitination via one of the two adaptor proteins, Cdc20 or Cdh1, in M or M/early G1 phase, respectively. Furthermore, recent studies have shed light on crucial functions for APC/C in maintaining genomic integrity, neuronal differentiation, cellular metabolism and tumorigenesis. To gain better insight into the in vivo physiological functions of APC/C in regulating various cellular processes, particularly development and tumorigenesis, a number of mouse models of APC/C core subunits, coactivators or inhibitors have been established and characterized. However, due to their essential role in cell cycle regulation, most of the germline knockout mice targeting the APC/C pathway are embryonic lethal, indicating the need for generating conditional knockout mouse models to assess the role in tumorigenesis for each APC/C signaling component in specific tissues. In this review, we will first provide a brief introduction of the ubiquitin-proteasome system (UPS) and the biochemical activities and cellular functions of the APC/C E3 ligase. We will then focus primarily on characterizing genetic mouse models used to understand the physiological roles of each APC/C signaling component in embryogenesis, cell proliferation, development and carcinogenesis. Finally, we discuss future research directions to further elucidate the physiological contributions of APC/C components during tumorigenesis and validate their potentials as a novel class of anti-cancer targets.

Interplay between the DNA damage proteins MDC1 and ATM in the regulation of the spindle assembly checkpoint

To avoid genomic instability, cells have developed surveillance mechanisms such as the spindle assembly checkpoint (SAC) and the DNA damage response. ATM and MDC1 are central players of the cellular response to DNA double-strand breaks. Here, we identify a new role for these proteins in the regulation of mitotic progression and in SAC activation. MDC1 localizes at mitotic kinetochores following SAC activation in an ATM-dependent manner. ATM phosphorylates histone H2AX at mitotic kinetochores, and this phosphorylation is required for MDC1 localization at kinetochores. ATM and MDC1 are needed for kinetochore localization of the inhibitory mitotic checkpoint complex components, Mad2 and Cdc20, and for the maintenance of the mitotic checkpoint complex integrity. This probably relies on the interaction of MDC1 with the MCC. In this work, we have established that ATM and MDC1 maintain genomic stability not only by controlling the DNA damage response, but also by regulating SAC activation, providing an important link between these two essential biological processes.

A direct role of Mad1 in the spindle assembly checkpoint beyond Mad2 kinetochore recruitment

The spindle assembly checkpoint (SAC) ensures accurate chromosome segregation by delaying entry into anaphase until all sister chromatids have become bi-oriented. A key component of the SAC is the Mad2 protein, which can adopt either an inactive open (O-Mad2) or active closed (C-Mad2) conformation. The conversion of O-Mad2 into C-Mad2 at unattached kinetochores is thought to be a key step in activating the SAC. The "template model" proposes that this is achieved by the recruitment of soluble O-Mad2 to C-Mad2 bound at kinetochores through its interaction with Mad1. Whether Mad1 has additional roles in the SAC beyond recruitment of C-Mad2 to kinetochores has not yet been addressed. Here, we show that Mad1 is required for mitotic arrest even when C-Mad2 is artificially recruited to kinetochores, indicating that it has indeed an additional function in promoting the checkpoint. The C-terminal globular domain of Mad1 and conserved residues in this region are required for this unexpected function of Mad1.

Stable MCC binding to the APC/C is required for a functional spindle assembly checkpoint

The spindle assembly checkpoint (SAC) delays progression into anaphase until all chromosomes have aligned on the metaphase plate by inhibiting Cdc20, the mitotic co-activator of the APC/C. Mad2 and BubR1 bind and inhibit Cdc20, thereby forming the mitotic checkpoint complex (MCC), which can bind stably to the APC/C. Whether MCC formation per se is sufficient for a functional SAC or MCC association with the APC/C is required remains unclear. Here, we analyze the role of two conserved motifs in Cdc20, IR and C-Box, in binding of the MCC to the APC/C. Mutants in both motifs assemble the MCC normally, but IR motif integrity is particularly important for stable binding to the APC/C. Cells expressing Cdc20 with a mutated IR motif have a compromised SAC, as uninhibited Cdc20 can compete with the MCC for APC/C binding and activate it. We thus show that stable MCC association with the APC/C is critical for a functional SAC.

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.

KNL1: bringing order to the kinetochore

KNL1 is an evolutionarily conserved kinetochore-associated protein essential for accurate chromosome segregation in eukaryotic cells. This large scaffold protein, predicted to be almost entirely unstructured, is involved in diverse mitotic processes including kinetochore assembly, chromosome congression, and mitotic checkpoint signaling. How this kinetochore "hub" coordinates protein-protein interactions spatially and temporally during mitosis to orchestrate these processes is an area of active investigation. Here we summarize the current understanding of KNL1 and discuss possible mechanisms by which this protein actively contributes to multiple aspects of mitotic progression.

APOLLON protein promotes early mitotic CYCLIN A degradation independent of the spindle assembly checkpoint

In the mammalian cell cycle, both CYCLIN A and CYCLIN B are required for entry into mitosis, and their elimination is also essential to complete the process. During mitosis, CYCLIN A and CYCLIN B are ubiquitylated by the anaphase-promoting complex/cyclosome (APC/C) and then subjected to proteasomal degradation. However, CYCLIN A, but not CYCLIN B, begins to be degraded in the prometaphase when APC/C is inactivated by the spindle assembly checkpoint (SAC). Here, we show that APOLLON (also known as BRUCE or BIRC6) plays a role in SAC-independent degradation of CYCLIN A in early mitosis. APPOLON interacts with CYCLIN A that is not associated with cyclin-dependent kinases. APPOLON also interacts with APC/C, and it facilitates CYCLIN A ubiquitylation. In APPOLON-deficient cells, mitotic degradation of CYCLIN A is delayed, and the total, but not the cyclin-dependent kinase-bound, CYCLIN A level was increased. We propose APPOLON to be a novel regulator of mitotic CYCLIN A degradation independent of SAC.

The spindle assembly checkpoint works like a rheostat rather than a toggle switch

The spindle assembly checkpoint (SAC) is essential in mammalian mitosis to ensure the equal segregation of sister chromatids. The SAC generates a mitotic checkpoint complex (MCC) to prevent the anaphase-promoting complex/cyclosome (APC/C) from targeting key mitotic regulators for destruction until all of the chromosomes have attached to the mitotic apparatus. A single unattached kinetochore can delay anaphase for several hours, but how it is able to block the APC/C throughout the cell is not understood. Present concepts of the SAC posit that either it exhibits an all-or-nothing response or there is a minimum threshold sufficient to block the APC/C (ref. 7). Here, we have used gene targeting to measure SAC activity, and find that it does not have an all-or-nothing response. Instead, the strength of the SAC depends on the amount of MAD2 recruited to kinetochores and on the amount of MCC formed. Furthermore, we show that different drugs activate the SAC to different extents, which may be relevant to their efficacy in chemotherapy.

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.

All together now: Polo joins the kinase network controlling the spindle assembly checkpoint in Drosophila

Maintenance of genomic stability during eukaryotic cell division relies on the Spindle Assembly Checkpoint (SAC), which has evolved as a surveillance mechanism that monitors kinetochore-microtubule attachment and prevents APC/C-mediated mitotic exit until all chromosomes are properly attached to the mitotic spindle. Reversible protein phosphorylation has long been accredited as a regulatory mechanism of the SAC. Nevertheless, knowledge of how several mitotic kinases act in concert within the signaling pathway to orchestrate SAC function is still emerging. In a recent study, we undertook a comprehensive dissection of the hierarchical framework controlling SAC function in Drosophila cells. We found that Polo lies at the top of the SAC pathway promoting the efficient recruitment of Mps1 to unattached kinetochores. This renders Mps1 fully active to control BubR1 phosphorylation that generates the 3F3/2 phosphoepitope at tensionless kinetochores. We have proposed that Polo is required for SAC function and that the molecular outcome of Mps1-dependent 3F3/2 formation is to promote the association of Cdc20 with BubR1 allowing proper kinetochore recruitment of Cdc20 and efficient assembly of the Mitotic Checkpoint Complex (MCC) required for a sustained SAC response.

Spatial rule-based modeling: a method and its application to the human mitotic kinetochore

A common problem in the analysis of biological systems is the combinatorial explosion that emerges from the complexity of multi-protein assemblies. Conventional formalisms, like differential equations, Boolean networks and Bayesian networks, are unsuitable for dealing with the combinatorial explosion, because they are designed for a restricted state space with fixed dimensionality. To overcome this problem, the rule-based modeling language, BioNetGen, and the spatial extension, SRSim, have been developed. Here, we describe how to apply rule-based modeling to integrate experimental data from different sources into a single spatial simulation model and how to analyze the output of that model. The starting point for this approach can be a combination of molecular interaction data, reaction network data, proximities, binding and diffusion kinetics and molecular geometries at different levels of detail. We describe the technique and then use it to construct a model of the human mitotic inner and outer kinetochore, including the spindle assembly checkpoint signaling pathway. This allows us to demonstrate the utility of the procedure, show how a novel perspective for understanding such complex systems becomes accessible and elaborate on challenges that arise in the formulation, simulation and analysis of spatial rule-based models.

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.

Roles of different pools of the mitotic checkpoint complex and the mechanisms of their disassembly

The mitotic (or spindle assembly) checkpoint system prevents premature separation of sister chromatids in mitosis. When the checkpoint is turned on, the mitotic checkpoint complex (MCC) inhibits the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). MCC is composed of the checkpoint proteins BubR1, Bub3, and Mad2 associated with the APC/C activator Cdc20. The mechanisms of the assembly of MCC when the checkpoint is turned on, and of its disassembly when the checkpoint is inactivated, are not sufficiently understood. Previous reports indicated that APC/C-mediated polyubiquitylation of Cdc20 in MCC is required for the dissociation of APC/C-associated MCC, but not of free MCC. The pool of free MCC is disassembled by an ATP-dependent process stimulated by the Mad2-binding protein p31(comet). It remained unknown whether free MCC is the precursor or the dissociation product of APC/C-bound MCC. By characterizing the mechanisms of the disassembly of APC/C-bound MCC in a purified system, we find that it cannot be the source of free MCC, because it is bound at high affinity and is released only in ubiquitylated or partially disassembled forms. By the use of a cell-free system from Xenopus eggs that reproduces the mitotic checkpoint, we show that MCC can be assembled in the absence of APC/C in a checkpoint-dependent manner. We propose that when the checkpoint is turned on, free MCC is the precursor of APC/C-bound MCC. When the mitotic checkpoint is extinguished, both APC/C-bound and free MCC pools have to be disassembled to release APC/C from inhibition.

Aneuploidy in health, disease, and aging

Aneuploidy, an aberrant number of chromosomes, has been recognized as a feature of human malignancies for over a century, but compelling evidence for causality was largely lacking until mouse models for chromosome number instability were used. These in vivo studies have not only uncovered important new insights into the extremely complex aneuploidy-cancer relationship but also into the molecular mechanisms underlying proper and aberrant chromosome segregation. A series of diverse mouse models for the mitotic checkpoint protein BubR1 has provided evidence for a provocative novel link between aneuploidization and the development of age-related pathologies.

Drosophila Polo regulates the spindle assembly checkpoint through Mps1-dependent BubR1 phosphorylation

Maintenance of genomic stability during eukaryotic cell division relies on the spindle assembly checkpoint (SAC) that prevents mitotic exit until all chromosomes are properly attached to the spindle. Polo is a mitotic kinase proposed to be involved in SAC function, but its role has remained elusive. We demonstrate that Polo and Aurora B functional interdependency comprises a positive feedback loop that promotes Mps1 kinetochore localization and activity. Expression of constitutively active Polo restores normal Mps1 kinetochore levels even after Aurora B inhibition, highlighting a role for Polo in Mps1 recruitment to unattached kinetochores downstream of Aurora B. We also show that Mps1 kinetochore localization is required for BubR1 hyperphosphorylation and formation of the 3F3/2 phosphoepitope. This is essential to allow recruitment of Cdc20 to unattached kinetochores and the assembly of anaphase-promoting complex/cyclosome-inhibitory complexes to levels that ensure long-term SAC activity. We propose a model in which Polo controls Mps1-dependent BubR1 phosphorylation to promote Cdc20 kinetochore recruitment and sustained SAC function.

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.

Making an effective switch at the kinetochore by phosphorylation and dephosphorylation

The kinetochore, the proteinaceous structure on the mitotic centromere, functions as a mechanical latch that hooks onto microtubules to support directional movement of chromosomes. The structure also brings in a number of signaling molecules, such as kinases and phosphatases, which regulate microtubule dynamics and cell cycle progression. Erroneous microtubule attachment is destabilized by Aurora B-mediated phosphorylation of multiple microtubule-binding protein complexes at the kinetochore, such as the KMN network proteins and the Ska/Dam1 complex, while Plk-dependent phosphorylation of BubR1 stabilizes kinetochore-microtubule attachment by recruiting PP2A-B56. Spindle assembly checkpoint (SAC) signaling, which is activated by unattached kinetochores and inhibits the metaphase-to-anaphase transition, depends on kinetochore recruitment of the kinase Bub1 through Mps1-mediated phosphorylation of the kinetochore protein KNL1 (also known as Blinkin in mammals, Spc105 in budding yeast, and Spc7 in fission yeast). Recruitment of protein phosphatase 1 to KNL1 is necessary to silence the SAC upon bioriented microtubule attachment. One of the key unsolved questions in the mitosis field is how a mechanical change at the kinetochore upon microtubule attachment is converted to these and other chemical signals that control microtubule attachment and the SAC. Rapid progress in the field is revealing the existence of an intricate signaling network created right on the kinetochore. Here we review the current understanding of phosphorylation-mediated regulation of kinetochore functions and discuss how this signaling network generates an accurate switch that turns on and off the signaling output in response to kinetochore-microtubule attachment.

Ubiquitin ligases and cell cycle control

The ubiquitin-proteasome system plays a pivotal role in the sequence of events leading to cell division known as the cell cycle. Not only does ubiquitin-mediated proteolysis constitute a critical component of the core oscillator that drives the cell cycle in all eukaryotes, it is also central to the mechanisms that ensure that the integrity of the genome is maintained. These functions are primarily carried out by two families of E3 ubiquitin ligases, the Skp/cullin/F-box-containing and anaphase-promoting complex/cyclosome complexes. However, beyond those functions associated with regulation of central cell cycle events, many peripheral cell cycle-related processes rely on ubiquitylation for signaling, homeostasis, and dynamicity, involving additional types of ubiquitin ligases and regulators. We are only beginning to understand the diversity and complexity of this regulation.

New insights into the role of BubR1 in mitosis and beyond

BubR1 is a critical component of the spindle assembly checkpoint, the surveillance mechanism that helps maintain the high fidelity of mitotic chromosome segregation by preventing cells from initiating anaphase if one or more kinetochores are not attached to the spindle. BubR1 also helps promote the establishment of stable kinetochore-microtubule attachments during prometaphase. In this chapter, we review the structure, functions, and regulation of BubR1 in these "classical roles" at the kinetochore. We discuss its recruitment to kinetochores, its assembly into the inhibitor of anaphase progression, and the importance of its posttranslational modifications. We also consider the evidence for its participation in other roles beyond mitosis, such as the meiosis-specific processes of recombination and prophase arrest of the first meiotic division, the cellular response to DNA damage, and in the regulation of centrosome and basal body function. Finally, studies are presented linking BubR1 dysfunction or misregulation to aging and human disease, particularly cancer.

Bub1 kinase activity drives error correction and mitotic checkpoint control but not tumor suppression

Mice expressing a version of Bub1 that lacks kinase activity have increased chromosome segregation errors and aneuploidy but not increased susceptibility to tumors.

The mitotic checkpoint protein Bub1 is essential for embryogenesis and survival of proliferating cells, and bidirectional deviations from its normal level of expression cause chromosome missegregation, aneuploidy, and cancer predisposition in mice. To provide insight into the physiological significance of this critical mitotic regulator at a modular level, we generated Bub1 mutant mice that lack kinase activity using a knockin gene-targeting approach that preserves normal protein abundance. In this paper, we uncover that Bub1 kinase activity integrates attachment error correction and mitotic checkpoint signaling by controlling the localization and activity of Aurora B kinase through phosphorylation of histone H2A at threonine 121. Strikingly, despite substantial chromosome segregation errors and aneuploidization, mice deficient for Bub1 kinase activity do not exhibit increased susceptibility to spontaneous or carcinogen-induced tumorigenesis. These findings provide a unique example of a modular mitotic activity orchestrating two distinct networks that safeguard against whole chromosome instability and reveal the differential importance of distinct aneuploidy-causing Bub1 defects in tumor suppression.

Kinetochore localized Mad2 and Cdc20 is itself insufficient for triggering the mitotic checkpoint when Mps1 is low in Drosophila melanogaster neuroblasts

The relationships between the kinetochore and checkpoint control remain unresolved. Here, we report the characterization of the in vivo behavior of Cdc20 and Mad2 and the relevant spindle assembly checkpoint (SAC) functions in the neuroblasts of a Drosophila Mps1 weak allele (ald^B4–2). ald^B4–2 third instar larvae brain samples contain only around 16% endogenous Mps1 protein, and the SAC function is abolished. However, this does not lead to rapid anaphase onset and mitotic exit, in contrast to the loss of Mad2 alone in a mad2^EY mutant. The level of GFP-Cdc20 recruitment to the kinetochore is unaffected in ald^B4–2 neuroblasts, while the level of GFP-Mad2 is reduced to just about 20%. Cdc20 and Mad2 display only monophasic exponential kinetics at the kinetochores. The ald^B4–2 heterozygotes expressed approximately 65% of normal Mps1 protein levels, and this is enough to restore the SAC function. The kinetochore recruitment of GFP-Mad2 in response to SAC activation increases by around 80% in heterozygotes, compared with just about 20% in ald^B4–2 mutant. This suggests a correlation between Mps1 levels and Mad2 kinetochore localization and perhaps the existence of a threshold level at which Mps1 is fully functional. The failure to arrest the mitotic progression in ald^B4–2 neuroblasts in response to colchicine treatment suggests that when Mps1 levels are low, approximately 20% of normal GFP-Mad2, alongside normal levels of GFP-Cdc20 kinetochore recruitments, is insufficient for triggering SAC signal propagation.

Evolution and function of the mitotic checkpoint

The mitotic checkpoint evolved to prevent cell division when chromosomes have not established connections with the chromosome segregation machinery. Many of the fundamental molecular principles that underlie the checkpoint, its spatiotemporal activation, and its timely inactivation have been uncovered. Most of these are conserved in eukaryotes, but important differences between species exist. Here we review current concepts of mitotic checkpoint activation and silencing. Guided by studies in model organisms and our phylogenomics analysis of checkpoint constituents and their functional domains and motifs, we highlight ancient and taxa-specific aspects of the core checkpoint modules in the context of mitotic checkpoint function.

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.

In vivo FRET imaging revealed a regulatory role of RanGTP in kinetochore-microtubule attachments via Aurora B kinase

Under the fluctuating circumstances provided by the innate dynamics of microtubules and opposing tensions resulted from microtubule-associated motors, it is vital to ensure stable kinetochore-microtubule attachments for accurate segregation. However, a comprehensive understanding of how this regulation is mechanistically achieved remains elusive. Using our newly designed live cell FRET time-lapse imaging, we found that post-metaphase RanGTP is crucial in the maintenance of stable kinetochore-microtubule attachments by regulating Aurora B kinase via the NES-bearing Mst1. More importantly, our study demonstrates that by ensuring stable alignment of metaphase chromosomes prior to segregation, RanGTP is indispensible in governing the genomic integrity and the fidelity of cell cycle progression. Our findings suggest an additional role of RanGTP beyond its known function in mitotic spindle assembly during the prometaphase-metaphase transition.

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.

Reconstituting the kinetochore–microtubule interface: what, why, and how

The kinetochore is the proteinaceous complex that governs the movement of duplicated chromosomes by interacting with spindle microtubules during mitosis and meiosis. Faithful chromosome segregation requires that kinetochores form robust load-bearing attachments to the tips of dynamic spindle microtubules, correct microtubule attachment errors, and delay the onset of anaphase until all chromosomes have made proper attachments. To understand how this macromolecular machine operates to segregate duplicated chromosomes with exquisite accuracy, it is critical to reconstitute and study kinetochore–microtubule interactions in vitro using defined components. Here, we review the current status of reconstitution as well as recent progress in understanding the microtubule-binding functions of kinetochores in vivo.

Reprogramming to pluripotency can conceal somatic cell chromosomal instability

The discovery that somatic cells are reprogrammable to pluripotency by ectopic expression of a small subset of transcription factors has created great potential for the development of broadly applicable stem-cell-based therapies. One of the concerns regarding the safe use of induced pluripotent stem cells (iPSCs) in therapeutic applications is loss of genomic integrity, a hallmark of various human conditions and diseases, including cancer. Structural chromosome defects such as short telomeres and double-strand breaks are known to limit reprogramming of somatic cells into iPSCs, but whether defects that cause whole-chromosome instability (W-CIN) preclude reprogramming is unknown. Here we demonstrate, using aneuploidy-prone mouse embryonic fibroblasts (MEFs) in which chromosome missegregation is driven by BubR1 or RanBP2 insufficiency, that W-CIN is not a barrier to reprogramming. Unexpectedly, the two W-CIN defects had contrasting effects on iPSC genomic integrity, with BubR1 hypomorphic MEFs almost exclusively yielding aneuploid iPSC clones and RanBP2 hypomorphic MEFs karyotypically normal iPSC clones. Moreover, BubR1-insufficient iPSC clones were karyotypically unstable, whereas RanBP2-insufficient iPSC clones were rather stable. These findings suggest that aneuploid cells can be selected for or against during reprogramming depending on the W-CIN gene defect and present the novel concept that somatic cell W-CIN can be concealed in the pluripotent state. Thus, karyotypic analysis of somatic cells of origin in addition to iPSC lines is necessary for safe application of reprogramming technology.

iPSC technology has the potential to revolutionize stem-cell based regenerative medicine and would also allow for the production of patient-specific cells for disease modeling and drug discovery. One of the primary safety concerns of iPSCs is genetic instability, which is associated with cancer and various other diseases and includes abnormalities in both chromosomal structure and number. Whereas certain structural chromosome changes have been shown to preclude somatic cell reprogramming, the effect of whole-chromosome reshuffling on this process is completely unknown. Here we show that BubR1 and RanBP2 hypomorphic MEF lines, which are highly prone to erroneous chromosome segregation due to mitotic checkpoint and DNA decatenation failure, respectively, reprogram to pluripotency with normal efficiency. However, while RanBP2 hypomorphic MEFs yielded karyotypically normal iPSC clones with generally low chromosomal instability rates, BubR1 hypomorphic MEFs almost exclusively yielded aneuploid iPSC clones with high instability rates. These data provide important new insights into the genomic integrity requirements during somatic cell reprogramming, and they establish that the safe application of iPSC technology requires screening of both iPSCs and the iPSC-founder cells for chromosome number instability.

The APC/C subunit Mnd2/Apc15 promotes Cdc20 autoubiquitination and spindle assembly checkpoint inactivation

The fidelity of chromosome segregation depends on the spindle assembly checkpoint (SAC). In the presence of unattached kinetochores, anaphase is delayed when three SAC components (Mad2, Mad3/BubR1, and Bub3) inhibit Cdc20, the activating subunit of the anaphase-promoting complex (APC/C). We analyzed the role of Cdc20 autoubiquitination in the SAC of budding yeast. Reconstitution with purified components revealed that a Mad3-Bub3 complex synergizes with Mad2 to lock Cdc20 on the APC/C and stimulate Cdc20 autoubiquitination, while inhibiting ubiquitination of substrates. SAC-dependent Cdc20 autoubiquitination required the Mnd2/Apc15 subunit of the APC/C. General inhibition of Cdc20 ubiquitination in vivo resulted in high Cdc20 levels and a failure to establish a SAC arrest, suggesting that SAC establishment depends on low Cdc20 levels. Specific inhibition of SAC-dependent ubiquitination, by deletion of Mnd2, allowed establishment of a SAC arrest but delayed release from the arrest, suggesting that Cdc20 ubiquitination is also required for SAC inactivation.

CENP-E--dependent BubR1 autophosphorylation enhances chromosome alignment and the mitotic checkpoint

How the state of spindle microtubule capture at the kinetochore is translated into mitotic checkpoint signaling remains largely unknown. In this paper, we demonstrate that the kinetochore-associated mitotic kinase BubR1 phosphorylates itself in human cells and that this autophosphorylation is dependent on its binding partner, the kinetochore motor CENP-E. This CENP-E-dependent BubR1 autophosphorylation at unattached kinetochores is important for a full-strength mitotic checkpoint to prevent single chromosome loss. Replacing endogenous BubR1 with a nonphosphorylatable BubR1 mutant, as well as depletion of CENP-E, the BubR1 kinase activator, results in metaphase chromosome misalignment and a decrease of Aurora B-mediated Ndc80 phosphorylation at kinetochores. Furthermore, expressing a phosphomimetic BubR1 mutant substantially reduces the incidence of polar chromosomes in CENP-E-depleted cells. Thus, the state of CENP-E-dependent BubR1 autophosphorylation in response to spindle microtubule capture by CENP-E is important for kinetochore function in achieving accurate chromosome segregation.

Up-regulation of the mitotic checkpoint component Mad1 causes chromosomal instability and resistance to microtubule poisons

The mitotic checkpoint is the major cell cycle checkpoint acting during mitosis to prevent aneuploidy and chromosomal instability, which are hallmarks of tumor cells. Reduced expression of the mitotic checkpoint component Mad1 causes aneuploidy and promotes tumors in mice [Iwanaga Y, et al. (2007) Cancer Res 67:160-166]. However, the prevalence and consequences of Mad1 overexpression are currently unclear. Here we show that Mad1 is frequently overexpressed in human cancers and that Mad1 up-regulation is a marker of poor prognosis. Overexpression of Mad1 causes aneuploidy and chromosomal instability through weakening mitotic checkpoint signaling caused by mislocalization of the Mad1 binding partner Mad2. Cells overexpressing Mad1 are resistant to microtubule poisons, including currently used chemotherapeutic agents. These results suggest that levels of Mad1 must be tightly regulated to prevent aneuploidy and transformation and that Mad1 up-regulation may promote tumors and cause resistance to current therapies.

BRCA2 fine-tunes the spindle assembly checkpoint through reinforcement of BubR1 acetylation

Germline mutations that inactivate BRCA2 promote early-onset cancer with chromosome instability. Here, we report that BRCA2 regulates the spindle assembly checkpoint (SAC). Previously, we reported that BubR1 acetylation is essential for SAC activity. In this study we show that BRCA2 recruits the PCAF acetyltransferase and aids in BubR1 acetylation during mitosis. In the absence of BRCA2, BubR1 acetylation is abolished, and the level of BubR1 decreases during mitosis. Similarly, Brca2-deficient mouse embryonic fibroblasts exhibited weak SAC activity. Transgenic mice that were engineered to have interruptions in the BRCA2-BubR1 association exhibited marked decrease of BubR1 acetylation, weakened SAC activity, and aneuploidy. These transgenic mice developed spontaneous tumors at 40% penetrance. Moreover, immunohistochemical analyses of human breast cancer specimens suggested that BRCA2 mutation and BubR1 status is closely linked. Our results provide an explanation for how mutation of BRCA2 can lead to chromosome instability without apparent mutations in SAC components.

Timing of anaphase-promoting complex activation in mouse oocytes is predicted by microtubule-kinetochore attachment but not by bivalent alignment or tension

Homologous chromosome segregation errors during meiosis I are common and generate aneuploid embryos. Here, we provide a reason for this susceptibility to mis-segregation by live cell imaging of mouse oocytes. Our results show that stable kinetochore-microtubule attachments form in mid-prometaphase, 3-4 hours before anaphase. This coincided with the loss of Mad2 from kinetochores and with the start of anaphase-promoting complex/cyclosome (APC/C)-mediated cyclin B1 destruction. Therefore, the spindle assembly checkpoint (SAC) ceased to inhibit the APC/C from mid-prometaphase. This timing did not coincide with bivalent congression in one-third of all oocytes examined. Non-aligned bivalents were weakly positive for Mad2, under less tension than congressed bivalents and, by live-cell imaging, appeared to be in the process of establishing correct bi-orientation. The time from when the APC/C became active until anaphase onset was affected by the rate of loss of CDK1 activity, rather than by these non-aligned bivalents, which occasionally persisted until anaphase, resulting in homolog non-disjunction. We conclude that, in oocytes, a few erroneous attachments of bivalent kinetochores to microtubules do not generate a sufficient SAC 'wait anaphase' signal to inhibit the APC/C.

Structure of human Mad1 C-terminal domain reveals its involvement in kinetochore targeting

The spindle checkpoint prevents aneuploidy by delaying anaphase onset until all sister chromatids achieve proper microtubule attachment. The kinetochore-bound checkpoint protein complex Mad1-Mad2 promotes the conformational activation of Mad2 and serves as a catalytic engine of checkpoint signaling. How Mad1 is targeted to kinetochores is not understood. Here, we report the crystal structure of the conserved C-terminal domain (CTD) of human Mad1. Mad1 CTD forms a homodimer and, unexpectedly, has a fold similar to those of the kinetochore-binding domains of Spc25 and Csm1. Nonoverlapping Mad1 fragments retain detectable kinetochore targeting. Deletion of the CTD diminishes, does not abolish, Mad1 kinetochore localization. Mutagenesis studies further map the functional interface of Mad1 CTD in kinetochore targeting and implicate Bub1 as its receptor. Our results indicate that CTD is a part of an extensive kinetochore-binding interface of Mad1, and rationalize graded kinetochore targeting of Mad1 during checkpoint signaling.

The MPS1 family of protein kinases

MPS1 protein kinases are found widely, but not ubiquitously, in eukaryotes. This family of potentially dual-specific protein kinases is among several that regulate a number of steps of mitosis. The most widely conserved MPS1 kinase functions involve activities at the kinetochore in both the chromosome attachment and the spindle checkpoint. MPS1 kinases also function at centrosomes. Beyond mitosis, MPS1 kinases have been implicated in development, cytokinesis, and several different signaling pathways. Family members are identified by virtue of a conserved C-terminal kinase domain, though the N-terminal domain is quite divergent. The kinase domain of the human enzyme has been crystallized, revealing an unusual ATP-binding pocket. The activity, level, and subcellular localization of Mps1 family members are tightly regulated during cell-cycle progression. The mitotic functions of Mps1 kinases and their overexpression in some tumors have prompted the identification of Mps1 inhibitors and their active development as anticancer drugs.

Structure of the mitotic checkpoint complex

In mitosis, the spindle assembly checkpoint (SAC) ensures genome stability by delaying chromosome segregation until all sister chromatids have achieved bipolar attachment to the mitotic spindle. The SAC is imposed by the mitotic checkpoint complex (MCC), whose assembly is catalysed by unattached chromosomes and which binds and inhibits the anaphase-promoting complex/cyclosome (APC/C), the E3 ubiquitin ligase that initiates chromosome segregation. Here, using the crystal structure of Schizosaccharomyces pombe MCC (a complex of mitotic spindle assembly checkpoint proteins Mad2, Mad3 and APC/C co-activator protein Cdc20), we reveal the molecular basis of MCC-mediated APC/C inhibition and the regulation of MCC assembly. The MCC inhibits the APC/C by obstructing degron recognition sites on Cdc20 (the substrate recruitment subunit of the APC/C) and displacing Cdc20 to disrupt formation of a bipartite D-box receptor with the APC/C subunit Apc10. Mad2, in the closed conformation (C-Mad2), stabilizes the complex by optimally positioning the Mad3 KEN-box degron to bind Cdc20. Mad3 and p31(comet) (also known as MAD2L1-binding protein) compete for the same C-Mad2 interface, which explains how p31(comet) disrupts MCC assembly to antagonize the SAC. This study shows how APC/C inhibition is coupled to degron recognition by co-activators.

Tex14, a Plk1-regulated protein, is required for kinetochore-microtubule attachment and regulation of the spindle assembly checkpoint

Proper assembly of kinetochores (KTs) during mitosis is required for bipolar attachment of spindle microtubules (MTs) and the accumulation of spindle assembly checkpoint (SAC) components. Here we show that testis-expressed protein 14 (Tex14), which has been implicated in midbody function, is recruited to KTs by Plk1 in a Cdk1-dependent manner during early mitosis. Exclusion of Tex14 from kinetochores results in an inability to efficiently localize outer KT components, impaired KT-MT attachment, chromosome congression defects, and whole-chromosome instability. In addition, we demonstrate that phosphorylation of Tex14 by Plk1 during metaphase promotes APC(Cdc20)-mediated Tex14 degradation. Inhibition of this phosphorylation event causes retention of Tex14 at KTs and results in delayed metaphase-to-anaphase transition and chromosome segregation defects. Our findings identify Tex14 as an important mediator of KT structure and function and the fidelity of chromosome separation.

Spindle assembly checkpoint: the third decade

The spindle assembly checkpoint controls cell cycle progression during mitosis, synchronizing it with the attachment of chromosomes to spindle microtubules. After the discovery of the mitotic arrest deficient (MAD) and budding uninhibited by benzymidazole (BUB) genes as crucial checkpoint components in 1991, the second decade of checkpoint studies (2001–2010) witnessed crucial advances in the elucidation of the mechanism through which the checkpoint effector, the mitotic checkpoint complex, targets the anaphase-promoting complex (APC/C) to prevent progression into anaphase. Concomitantly, the discovery that the Ndc80 complex and other components of the microtubule-binding interface of kinetochores are essential for the checkpoint response finally asserted that kinetochores are crucial for the checkpoint response. Nevertheless, the relationship between kinetochores and checkpoint control remains poorly understood. Crucial advances in this area in the third decade of checkpoint studies (2011–2020) are likely to be brought about by the characterization of the mechanism of kinetochore recruitment, activation and inactivation of checkpoint proteins, which remains elusive for the majority of checkpoint components. Here, we take a molecular view on the main challenges hampering this task.