Sequential multisite phospho-regulation of KNL1-BUB3 interfaces at mitotic kinetochores

Spindle checkpoint silencing at kinetochores with submaximal microtubule occupancy

The spindle assembly checkpoint (SAC) ensures proper chromosome segregation by monitoring kinetochore-microtubule interactions. SAC proteins are shed from kinetochores once stable attachments are achieved. Human kinetochores consist of hundreds of SAC protein recruitment modules and bind up to 20 microtubules, raising the question of how the SAC responds to intermediate attachment states. We show that one protein module ('RZZS-MAD1-MAD2') of the SAC is removed from kinetochores at low microtubule occupancy and remains absent at higher occupancies, while another module ('BUB1-BUBR1') is retained at substantial levels irrespective of attachment states. These behaviours reflect different silencing mechanisms: while BUB1 displacement is almost fully dependent on MPS1 inactivation, MAD1 (also known as MAD1L1) displacement is not. Artificially tuning the affinity of kinetochores for microtubules further shows that ∼50% occupancy is sufficient to shed MAD2 and silence the SAC. Kinetochores thus respond as a single unit to shut down SAC signalling at submaximal occupancy states, but retain one SAC module. This may ensure continued SAC silencing on kinetochores with fluctuating occupancy states while maintaining the ability for fast SAC re-activation.

Recent Progress on the Localization of the Spindle Assembly Checkpoint Machinery to Kinetochores

Faithful chromosome segregation during mitosis is crucial for maintaining genome stability. The spindle assembly checkpoint (SAC) is a surveillance mechanism that ensures accurate mitotic progression. Defective SAC signaling leads to premature sister chromatid separation and aneuploid daughter cells. Mechanistically, the SAC couples the kinetochore microtubule attachment status to the cell cycle progression machinery. In the presence of abnormal kinetochore microtubule attachments, the SAC prevents the metaphase-to-anaphase transition through a complex kinase-phosphatase signaling cascade which results in the correct balance of SAC components recruited to the kinetochore. The correct kinetochore localization of SAC proteins is a prerequisite for robust SAC signaling and, hence, accurate chromosome segregation. Here, we review recent progresses on the kinetochore recruitment of core SAC factors.

Efficient mitotic checkpoint signaling depends on integrated activities of Bub1 and the RZZ complex

Kinetochore localized Mad1 is essential for generating a "wait anaphase" signal during mitosis, hereby ensuring accurate chromosome segregation. Inconsistent models for the function and quantitative contribution of the two mammalian Mad1 kinetochore receptors: Bub1 and the Rod-Zw10-Zwilch (RZZ) complex exist. By combining genome editing and RNAi, we achieve penetrant removal of Bub1 and Rod in human cells, which reveals that efficient checkpoint signaling depends on the integrated activities of these proteins. Rod removal reduces the proximity of Bub1 and Mad1, and we can bypass the requirement for Rod by tethering Mad1 to kinetochores or increasing the strength of the Bub1-Mad1 interaction. We find that Bub1 has checkpoint functions independent of Mad1 localization that are supported by low levels of Bub1 suggesting a catalytic function. In conclusion, our results support an integrated model for the Mad1 receptors in which the primary role of RZZ is to localize Mad1 at kinetochores to generate the Mad1-Bub1 complex.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

LUBAC controls chromosome alignment by targeting CENP-E to attached kinetochores

Faithful chromosome segregation requires proper chromosome congression at prometaphase and dynamic maintenance of the aligned chromosomes at metaphase. Chromosome missegregation can result in aneuploidy, birth defects and cancer. The kinetochore-bound KMN network and the kinesin motor CENP-E are critical for kinetochore-microtubule attachment and chromosome stability. The linear ubiquitin chain assembly complex (LUBAC) attaches linear ubiquitin chains to substrates, with well-established roles in immune response. Here, we identify LUBAC as a key player of chromosome alignment during mitosis. LUBAC catalyzes linear ubiquitination of the kinetochore motor CENP-E, which is specifically required for the localization of CENP-E at attached kinetochores, but not unattached ones. KNL1 acts as a receptor of linear ubiquitin chains to anchor CENP-E at attached kinetochores in prometaphase and metaphase. Thus, linear ubiquitination promotes chromosome congression and dynamic chromosome alignment by coupling the dynamic kinetochore microtubule receptor CENP-E to the static one, the KMN network.

During cell division, faithful chromosome segregation requires proper chromosome congression and dynamic maintenance of the aligned chromosomes. Here, the authors find that LUBAC promotes dynamic chromosome congression and alignment by targeting kinetochore motor CENP-E to the KMN network.

Ectopic Activation of the Spindle Assembly Checkpoint Signaling Cascade Reveals Its Biochemical Design

Switch-like activation of the spindle assembly checkpoint (SAC) is critical for accurate chromosome segregation and for cell division in a timely manner. To determine the mechanisms that achieve this, we engineered an ectopic, kinetochore-independent SAC activator: the "eSAC." The eSAC stimulates SAC signaling by artificially dimerizing Mps1 kinase domain and a cytosolic KNL1 phosphodomain, the kinetochore signaling scaffold. By exploiting variable eSAC expression in a cell population, we defined the dependence of the eSAC-induced mitotic delay on eSAC concentration in a cell to reveal the dose-response behavior of the core signaling cascade of the SAC. These quantitative analyses and subsequent mathematical modeling of the dose-response data uncover two crucial properties of the core SAC signaling cascade: (1) a cellular limit on the maximum anaphase-inhibitory signal that the cascade can generate due to the limited supply of SAC proteins and (2) the ability of the KNL1 phosphodomain to produce the anaphase-inhibitory signal synergistically, when it recruits multiple SAC proteins simultaneously. We propose that these properties together achieve inverse, non-linear scaling between the signal output per kinetochore and the number of signaling kinetochores. When the number of kinetochores is low, synergistic signaling by KNL1 enables each kinetochore to produce a disproportionately strong signal output. However, when many kinetochores signal concurrently, they compete for a limited supply of SAC proteins. This frustrates synergistic signaling and lowers their signal output. Thus, the signaling activity of unattached kinetochores will adapt to the changing number of signaling kinetochores to enable the SAC to approximate switch-like behavior.

Understanding eukaryotic chromosome segregation from a comparative biology perspective

A long-appreciated variation in fundamental cell biological processes between different species is becoming increasingly tractable due to recent breakthroughs in whole-genome analyses and genome editing techniques. However, the bulk of our mechanistic understanding in cell biology continues to come from just a few well-established models. In this Review, I use the highly diverse strategies of chromosome segregation in eukaryotes as an instrument for a more general discussion on phenotypic variation, possible rules underlying its emergence and its utility in understanding conserved functional relationships underlying this process. Such a comparative approach, supported by modern molecular biology tools, might provide a wider, holistic view of biology that is difficult to achieve when concentrating on a single experimental system.

The Ndc80 complex targets Bod1 to human mitotic kinetochores

Regulation of protein phosphatase activity by endogenous protein inhibitors is an important mechanism to control protein phosphorylation in cells. We recently identified Biorientation defective 1 (Bod1) as a small protein inhibitor of protein phosphatase 2A containing the B56 regulatory subunit (PP2A-B56). This phosphatase controls the amount of phosphorylation of several kinetochore proteins and thus the establishment of load-bearing chromosome-spindle attachments in time for accurate separation of sister chromatids in mitosis. Like PP2A-B56, Bod1 directly localizes to mitotic kinetochores and is required for correct segregation of mitotic chromosomes. In this report, we have probed the spatio-temporal regulation of Bod1 during mitotic progression. Kinetochore localization of Bod1 increases from nuclear envelope breakdown until metaphase. Phosphorylation of Bod1 at threonine 95 (T95), which increases Bod1's binding to and inhibition of PP2A-B56, peaks in prometaphase when PP2A-B56 localization to kinetochores is highest. We demonstrate here that kinetochore targeting of Bod1 depends on the outer kinetochore protein Ndc80 and not PP2A-B56. Crucially, Bod1 depletion functionally affects Ndc80 phosphorylation at the N-terminal serine 55 (S55), as well as a number of other phosphorylation sites within the outer kinetochore, including Knl1 at serine 24 and 60 (S24, S60), and threonine T943 and T1155 (T943, T1155). Therefore, Ndc80 recruits a phosphatase inhibitor to kinetochores which directly feeds forward to regulate Ndc80, and Knl1 phosphorylation, including sites that mediate the attachment of microtubules to kinetochores.

Kinase and Phosphatase Cross-Talk at the Kinetochore

Multiple kinases and phosphatases act on the kinetochore to control chromosome segregation: Aurora B, Mps1, Bub1, Plk1, Cdk1, PP1, and PP2A-B56, have all been shown to regulate both kinetochore-microtubule attachments and the spindle assembly checkpoint. Given that so many kinases and phosphatases converge onto two key mitotic processes, it is perhaps not surprising to learn that they are, quite literally, entangled in cross-talk. Inhibition of any one of these enzymes produces secondary effects on all the others, which results in a complicated picture that is very difficult to interpret. This review aims to clarify this picture by first collating the direct effects of each enzyme into one overarching schematic of regulation at the Knl1/Mis12/Ndc80 (KMN) network (a major signaling hub at the outer kinetochore). This schematic will then be used to discuss the implications of the cross-talk that connects these enzymes; both in terms of why it may be needed to produce the right type of kinetochore signals and why it nevertheless complicates our interpretations about which enzymes control what processes. Finally, some general experimental approaches will be discussed that could help to characterize kinetochore signaling by dissociating the direct from indirect effect of kinase or phosphatase inhibition in vivo. Together, this review should provide a framework to help understand how a network of kinases and phosphatases cooperate to regulate two key mitotic processes.

Guiding Mitotic Progression by Crosstalk between Post-translational Modifications

Cell division is tightly regulated to disentangle copied chromosomes in an orderly manner and prevent loss of genome integrity. During mitosis, transcriptional activity is limited and post-translational modifications (PTMs) are responsible for functional protein regulation. Essential mitotic regulators, including polo-like kinase 1 (PLK1) and cyclin-dependent kinases (CDK), as well as the anaphase-promoting complex/cyclosome (APC/C), are members of the enzymatic machinery responsible for protein modification. Interestingly, communication between PTMs ensures the essential tight and timely control during all consecutive phases of mitosis. Here, we present an overview of current concepts and understanding of crosstalk between PTMs regulating mitotic progression.

Mps1 Phosphorylates Its N-Terminal Extension to Relieve Autoinhibition and Activate the Spindle Assembly Checkpoint

Monopolar spindle 1 (Mps1) is a conserved apical kinase in the spindle assembly checkpoint (SAC) that ensures accurate segregation of chromosomes during mitosis. Mps1 undergoes extensive auto- and transphosphorylation, but the regulatory and functional consequences of these modifications remain unclear. Recent findings highlight the importance of intermolecular interactions between the N-terminal extension (NTE) of Mps1 and the Hec1 subunit of the NDC80 complex, which control Mps1 localization at kinetochores and activation of the SAC. Whether the NTE regulates other mitotic functions of Mps1 remains unknown. Here, we report that phosphorylation within the NTE contributes to Mps1 activation through relief of catalytic autoinhibition that is mediated by the NTE itself. Moreover, we find that this regulatory NTE function is independent of its role in Mps1 kinetochore recruitment. We demonstrate that the NTE autoinhibitory mechanism impinges most strongly on Mps1-dependent SAC functions and propose that Mps1 activation likely occurs sequentially through dimerization of a “prone-to-autophosphorylate” Mps1 conformer followed by autophosphorylation of the NTE prior to maximal kinase activation segment trans-autophosphorylation. Our observations underline the importance of autoregulated Mps1 activity in generation and maintenance of a robust SAC in human cells.

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Mps1 autophosphorylation at the NTE promotes activity independent of localization

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NTE phosphorylation relieves an NTE-dependent autoinhibition

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Mps1 autophosphorylation at its NTE is essential for the SAC, but not congression

A fine balancing act: A delicate kinase-phosphatase equilibrium that protects against chromosomal instability and cancer

Cancer cells rewire signalling networks to acquire specific hallmarks needed for their proliferation, survival, and dissemination throughout the body. Although this is often associated with the constitutive activation or inactivation of protein phosphorylation networks, there are other contexts when the dysregulation must be much milder. For example, chromosomal instability is a widespread cancer hallmark that relies on subtle defects in chromosome replication and/or division, such that these processes remain functional, but nevertheless error-prone. In this article, we will discuss how perturbations to the delicate kinase-phosphatase balance could lie at the heart of this type of dysregulation. In particular, we will explain how the two principle mechanisms that safeguard the chromosome segregation process rely on an equilibrium between at least two kinases and two phosphatases to function correctly. This balance is set during mitosis by a central complex that has also been implicated in chromosomal instability - the BUB1/BUBR1/BUB3 complex - and we will put forward a hypothesis that could link these two findings. This could be relevant for cancer treatment because most tumours have evolved by pushing the boundaries of chromosomal instability to the limit. If this involves subtle changes to the kinase-phosphatase equilibrium, then it may be possible to exacerbate these defects and tip tumour cells over the edge, whilst still maintaining the viability of healthy cells.

Unattached kinetochores drive their own capturing by sequestering a CLASP

Kinetochores that are not attached to microtubules prevent chromosome missegregation via the spindle assembly checkpoint. We show that they also promote their own capturing. Similar to what governs the localization of spindle assembly checkpoint proteins, the phosphorylation of Spc105 by Mps1 allows unattached kinetochores to sequester Stu1 in cooperation with Slk19. The withdrawal of Stu1, a CLASP essential for spindle integrity, from microtubules and attached kinetochores disrupts the organization of the spindle and thus allows the enhanced formation of dynamic random microtubules that span the nucleus and are ideal to capture unattached kinetochores. The enhanced formation of nuclear random microtubules does not occur if Stu1 sequestering to unattached kinetochores fails and the spindle remains uncompromised. Consequently, these cells exhibit a severely decreased capturing efficiency. After the capturing event, Stu1 is relocated to the capturing microtubule and prevents precocious microtubule depolymerization as long as kinetochores are laterally or incompletely end-on attached.

Kinetochores (KT) that are not attached to microtubules prevent chromosome missegregation via the spindle assembly checkpoint. Here the authors show that Mps1 localizes Stu1 at unattached KTs together with Slk19, causing a reorganization of the nuclear MT network that favors the capturing of unattached KT.

The Ins and Outs of Aurora B Inner Centromere Localization

Error-free chromosome segregation is essential for the maintenance of genomic integrity during cell division. Aurora B, the enzymatic subunit of the Chromosomal Passenger Complex (CPC), plays a crucial role in this process. In early mitosis Aurora B localizes predominantly to the inner centromere, a specialized region of chromatin that lies at the crossroads between the inter-kinetochore and inter-sister chromatid axes. Two evolutionarily conserved histone kinases, Haspin and Bub1, control the positioning of the CPC at the inner centromere and this location is thought to be crucial for the CPC to function. However, recent studies sketch a subtler picture, in which not all functions of the CPC require strict confinement to the inner centromere. In this review we discuss the molecular pathways that direct Aurora B to the inner centromere and deliberate if and why this specific localization is important for Aurora B function.

BubR1 Promotes Bub3-Dependent APC/C Inhibition during Spindle Assembly Checkpoint Signaling

The spindle assembly checkpoint (SAC) prevents premature sister chromatid separation during mitosis. Phosphorylation of unattached kinetochores by the Mps1 kinase promotes recruitment of SAC machinery that catalyzes assembly of the SAC effector mitotic checkpoint complex (MCC). The SAC protein Bub3 is a phospho-amino acid adaptor that forms structurally related stable complexes with functionally distinct paralogs named Bub1 and BubR1. A short motif (“loop”) of Bub1, but not the equivalent loop of BubR1, enhances binding of Bub3 to kinetochore phospho-targets. Here, we asked whether the BubR1 loop directs Bub3 to different phospho-targets. The BubR1 loop is essential for SAC function and cannot be removed or replaced with the Bub1 loop. BubR1 loop mutants bind Bub3 and are normally incorporated in MCC in vitro but have reduced ability to inhibit the MCC target anaphase-promoting complex (APC/C), suggesting that BubR1:Bub3 recognition and inhibition of APC/C requires phosphorylation. Thus, small sequence differences in Bub1 and BubR1 direct Bub3 to different phosphorylated targets in the SAC signaling cascade.

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The molecular basis of kinetochore recruitment of Bub1 and BubR1 is dissected

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Bub1 and BubR1 modulate the ability of Bub3 to recognize phosphorylated targets

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A newly identified BubR1 motif targets Bub3 to the anaphase-promoting complex

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The newly identified motif of BubR1 is required for checkpoint signaling

Playing polo during mitosis: PLK1 takes the lead

Polo-like kinase 1 (PLK1), the prototypical member of the polo-like family of serine/threonine kinases, is a pivotal regulator of mitosis and cytokinesis in eukaryotes. Many layers of regulation have evolved to target PLK1 to different subcellular structures and to its various mitotic substrates in line with its numerous functions during mitosis. Collective work is starting to illuminate an important set of substrates for PLK1: the mitotic kinases that together ensure the fidelity of the cell division process. Amongst these, recent developments argue that PLK1 regulates the activity of the histone kinases Aurora B and Haspin to define centromere identity, of MPS1 to initiate spindle checkpoint signaling, and of BUB1 and its pseudokinase paralog BUBR1 to coordinate spindle checkpoint activation and inactivation. Here, we review the recent work describing the regulation of these kinases by PLK1. We highlight common themes throughout and argue that a major mitotic function of PLK1 is as a master regulator of these key kinases.

Understanding inhibitor resistance in Mps1 kinase through novel biophysical assays and structures

Monopolar spindle 1 (Mps1/TTK) is a protein kinase essential in mitotic checkpoint signaling, preventing anaphase until all chromosomes are properly attached to spindle microtubules. Mps1 has emerged as a potential target for cancer therapy, and a variety of compounds have been developed to inhibit its kinase activity. Mutations in the catalytic domain of Mps1 that give rise to inhibitor resistance, but retain catalytic activity and do not display cross-resistance to other Mps1 inhibitors, have been described. Here we characterize the interactions of two such mutants, Mps1 C604Y and C604W, which raise resistance to two closely related compounds, NMS-P715 and its derivative Cpd-5, but not to the well characterized Mps1 inhibitor, reversine. We show that estimates of the IC₅₀ (employing a novel specific and efficient assay that utilizes a fluorescently labeled substrate) and the binding affinity (K_D ) indicate that, in both mutants, Cpd-5 should be better tolerated than the closely related NMS-P715. To gain further insight, we determined the crystal structure of the Mps1 kinase mutants bound to Cpd-5 and NMS-P715 and compared the binding modes of Cpd-5, NMS-P715, and reversine. The difference in steric hindrance between Tyr/Trp⁶⁰⁴ and the trifluoromethoxy moiety of NMS-P715, the methoxy moiety of Cpd-5, and complete absence of such a group in reversine, account for differences we observe in vitro Our analysis enforces the notion that inhibitors targeting Mps1 drug-resistant mutations can emerge as a feasible intervention strategy based on existing scaffolds, if the clinical need arises.

Bub1 positions Mad1 close to KNL1 MELT repeats to promote checkpoint signalling

Proper segregation of chromosomes depends on a functional spindle assembly checkpoint (SAC) and requires kinetochore localization of the Bub1 and Mad1/Mad2 checkpoint proteins. Several aspects of Mad1/Mad2 kinetochore recruitment in human cells are unclear and in particular the underlying direct interactions. Here we show that conserved domain 1 (CD1) in human Bub1 binds directly to Mad1 and a phosphorylation site exists in CD1 that stimulates Mad1 binding and SAC signalling. Importantly, fusion of minimal kinetochore-targeting Bub1 fragments to Mad1 bypasses the need for CD1, revealing that the main function of Bub1 is to position Mad1 close to KNL1 MELT repeats. Furthermore, we identify residues in Mad1 that are critical for Mad1 functionality, but not Bub1 binding, arguing for a direct role of Mad1 in the checkpoint. This work dissects functionally relevant molecular interactions required for spindle assembly checkpoint signalling at kinetochores in human cells.

The spindle assembly checkpoint ensures correct chromosome segregation and relies on kinetochore localization of the Bub1 and Mad1/Mad2 checkpoint proteins. Here the authors show that main function of Bub1 is to position Mad1 close to KNL1 MELT repeats in human cells.

Protein Phosphatase 1 inactivates Mps1 to ensure efficient Spindle Assembly Checkpoint silencing

Faithfull genome partitioning during cell division relies on the Spindle Assembly Checkpoint (SAC), a conserved signaling pathway that delays anaphase onset until all chromosomes are attached to spindle microtubules. Mps1 kinase is an upstream SAC regulator that promotes the assembly of an anaphase inhibitor through a sequential multi-target phosphorylation cascade. Thus, the SAC is highly responsive to Mps1, whose activity peaks in early mitosis as a result of its T-loop autophosphorylation. However, the mechanism controlling Mps1 inactivation once kinetochores attach to microtubules and the SAC is satisfied remains unknown. Here we show in vitro and in Drosophila that Protein Phosphatase 1 (PP1) inactivates Mps1 by dephosphorylating its T-loop. PP1-mediated dephosphorylation of Mps1 occurs at kinetochores and in the cytosol, and inactivation of both pools of Mps1 during metaphase is essential to ensure prompt and efficient SAC silencing. Overall, our findings uncover a mechanism of SAC inactivation required for timely mitotic exit.

DOI:http://dx.doi.org/10.7554/eLife.25366.001

Dual recognition of chromatin and microtubules by INCENP is important for mitotic progression

The chromosomal passenger complex (CPC), composed of inner centromere protein (INCENP), Survivin, Borealin, and the kinase Aurora B, contributes to the activation of the mitotic checkpoint. The regulation of CPC function remains unclear. Here, we reveal that in addition to Survivin and Borealin, the single α-helix (SAH) domain of INCENP supports CPC localization to chromatin and the mitotic checkpoint. The INCENP SAH domain also mediates INCENP's microtubule binding, which is negatively regulated by Cyclin-dependent kinase-mediated phosphorylation of segments flanking the SAH domain. The microtubule-binding capacity of the SAH domain is important for mitotic arrest in conditions of suppressed microtubule dynamics, and the duration of mitotic arrest dictates the probability, but not the timing, of cell death. Although independent targeting of INCENP to microtubules or the kinetochore/centromere promotes the mitotic checkpoint, it is insufficient for a robust mitotic arrest. Altogether, our results demonstrate that dual recognition of chromatin and microtubules by CPC is important for checkpoint maintenance and determination of cell fate in mitosis.

Molecular Mechanisms of Spindle Assembly Checkpoint Activation and Silencing

In eukaryotic cell division, the Spindle Assembly Checkpoint (SAC) plays a key regulatory role by monitoring the status of chromosome-microtubule attachments and allowing chromosome segregation only after all chromosomes are properly attached to spindle microtubules. While the identities of SAC components have been known, in some cases, for over two decades, the molecular mechanisms of the SAC have remained mostly mysterious until very recently. In the past few years, advances in biochemical reconstitution, structural biology, and bioinformatics have fueled an explosion in the molecular understanding of the SAC. This chapter seeks to synthesize these recent advances and place them in a biological context, in order to explain the mechanisms of SAC activation and silencing at a molecular level.

Generation of a Spindle Checkpoint Arrest from Synthetic Signaling Assemblies

The spindle checkpoint acts as a mitotic surveillance system, monitoring interactions between kinetochores and spindle microtubules and ensuring high-fidelity chromosome segregation [1, 2, 3]. The checkpoint is activated by unattached kinetochores, and Mps1 kinase phosphorylates KNL1 on conserved MELT motifs to generate a binding site for the Bub3-Bub1 complex [4, 5, 6, 7]. This leads to dynamic kinetochore recruitment of Mad proteins [8, 9], a conformational change in Mad2 [10, 11, 12], and formation of the mitotic checkpoint complex (MCC: Cdc20-Mad3-Mad2 [13, 14, 15]). MCC formation inhibits the anaphase-promoting complex/cyclosome (Cdc20-APC/C), thereby preventing the proteolytic destruction of securin and cyclin and delaying anaphase onset. What happens at kinetochores after Mps1-dependent Bub3-Bub1 recruitment remains mechanistically unclear, and it is not known whether kinetochore proteins other than KNL1 have significant roles to play in checkpoint signaling and MCC generation. Here, we take a reductionist approach, avoiding the complexities of kinetochores, and demonstrate that co-recruitment of KNL1^Spc7 and Mps1^Mph1 is sufficient to generate a robust checkpoint signal and prolonged mitotic arrest. We demonstrate that a Mad1-Bub1 complex is formed during synthetic checkpoint signaling. Analysis of bub3Δ mutants demonstrates that Bub3 acts to suppress premature checkpoint signaling. This synthetic system will enable detailed, mechanistic dissection of MCC generation and checkpoint silencing. After analyzing several mutants that affect localization of checkpoint complexes, we conclude that spindle checkpoint arrest can be independent of their kinetochore, spindle pole, and nuclear envelope localization.

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Synthetic signaling scaffolds generate a spindle checkpoint arrest

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The combination of KNL1^Spc7 and Mps1^Mph1 kinase generates a robust arrest

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Kinetochore, spindle, and nuclear envelope enrichment of the scaffold is not required

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Bub3 acts to inhibit premature checkpoint activation

A Cell Biological Perspective on Past, Present and Future Investigations of the Spindle Assembly Checkpoint

The spindle assembly checkpoint (SAC) is a quality control mechanism that ensures accurate chromosome segregation during cell division. It consists of a mechanochemical signal transduction mechanism that senses the attachment of chromosomes to the spindle, and a signaling cascade that inhibits cell division if one or more chromosomes are not attached. Extensive investigations of both these component systems of the SAC have synthesized a comprehensive understanding of the underlying molecular mechanisms. This review recounts the milestone results that elucidated the SAC, compiles a simple model of the complex molecular machinery underlying the SAC, and highlights poorly understood facets of the biochemical design and cell biological operation of the SAC that will drive research forward in the near future.

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.

Cryo-EM of Mitotic Checkpoint Complex-Bound APC/C Reveals Reciprocal and Conformational Regulation of Ubiquitin Ligation

The mitotic checkpoint complex (MCC) coordinates proper chromosome biorientation on the spindle with ubiquitination activities of CDC20-activated anaphase-promoting complex/cyclosome (APC/C(CDC20)). APC/C(CDC20) and two E2s, UBE2C and UBE2S, catalyze ubiquitination through distinct architectures for linking ubiquitin (UB) to substrates and elongating polyUB chains, respectively. MCC, which contains a second molecule of CDC20, blocks APC/C(CDC20)-UBE2C-dependent ubiquitination of Securin and Cyclins, while differentially determining or inhibiting CDC20 ubiquitination to regulate spindle surveillance, checkpoint activation, and checkpoint termination. Here electron microscopy reveals conformational variation of APC/C(CDC20)-MCC underlying this multifaceted regulation. MCC binds APC/C-bound CDC20 to inhibit substrate access. However, rotation about the CDC20-MCC assembly and conformational variability of APC/C modulate UBE2C-catalyzed ubiquitination of MCC's CDC20 molecule. Access of UBE2C is limiting for subsequent polyubiquitination by UBE2S. We propose that conformational dynamics of APC/C(CDC20)-MCC modulate E2 activation and determine distinctive ubiquitination activities as part of a response mechanism ensuring accurate sister chromatid segregation.

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.

Two functionally distinct kinetochore pools of BubR1 ensure accurate chromosome segregation

The BubR1/Bub3 complex is an important regulator of chromosome segregation as it facilitates proper kinetochore–microtubule interactions and is also an essential component of the spindle assembly checkpoint (SAC). Whether BubR1/Bub3 localization to kinetochores in human cells stimulates SAC signalling or only contributes to kinetochore–microtubule interactions is debated. Here we show that two distinct pools of BubR1/Bub3 exist at kinetochores and we uncouple these with defined BubR1/Bub3 mutants to address their function. The major kinetochore pool of BubR1/Bub3 is dependent on direct Bub1/Bub3 binding and is required for chromosome alignment but not for the SAC. A distinct pool of BubR1/Bub3 localizes by directly binding to phosphorylated MELT repeats on the outer kinetochore protein KNL1. When we prevent the direct binding of BubR1/Bub3 to KNL1 the checkpoint is weakened because BubR1/Bub3 is not incorporated into checkpoint complexes efficiently. In conclusion, kinetochore localization supports both known functions of BubR1/Bub3.

The BubR1/Bub3 complex regulates chromosome segregation to enable proper kinetochore-microtubule interactions and is also required for the spindle assembly checkpoint. Here the authors show that two distinct pools of BubR1/Bub3 exist at kinetochores to support both known functions of BubR1/Bub3.

Aurora B prevents premature removal of spindle assembly checkpoint proteins from the kinetochore: A key role for Aurora B in mitosis

Accurate chromosome segregation is dependent on the spindle assembly checkpoint (SAC). In current models, the key direct role of Aurora B in the SAC has been suggested to be to promote rapid kinetochore localisation of MPS1, allowing MPS1 to generate the checkpoint signal. However, Aurora B is also thought to play an indirect role in the SAC through the destabilisation of kinetochore-microtubule (KT-MT) attachments. Here, we demonstrate that Aurora B activity is not required for the kinetochore recruitment of the majority of SAC proteins. More importantly, we show that the primary role of Aurora B in the SAC is to prevent the premature removal of SAC proteins from the kinetochore, which is strictly dependent on KT-MT interactions. Moreover, in the presence of KT-MT interactions, Aurora B inhibition silences a persistent SAC induced by tethering MPS1 to the kinetochore. This explains the highly synergistic interaction between Aurora B and MPS1 inhibitors to override the SAC, which is lost when cells are pre-arrested in nocodazole. Furthermore, we show that Aurora B and MPS1 inhibitors synergistically kill a panel of breast and colon cancer cell lines, including cells that are otherwise insensitive to Aurora B inhibitors alone. These data demonstrate that the major role of Aurora B in SAC is to prevent the removal of SAC proteins from tensionless kinetochores, thus inhibiting premature SAC silencing, and highlights a therapeutic strategy through combination of Aurora B and MPS1 inhibitors.

Dual mechanisms regulate the recruitment of spindle assembly checkpoint proteins to the budding yeast kinetochore

Quantitative knowledge of the recruitment of spindle assembly checkpoint (SAC) proteins by the kinetochore is essential to understanding the mechanisms that regulate protein recruitment and hence the strength of the SAC. Here this recruitment is quantified, and novel mechanisms are identified that strongly modulate SAC protein recruitment by the kinetochore.

Recruitment of spindle assembly checkpoint (SAC) proteins by an unattached kinetochore leads to SAC activation. This recruitment is licensed by the Mps1 kinase, which phosphorylates the kinetochore protein Spc105 at one or more of its six MELT repeats. Spc105 then recruits the Bub3-Bub1 and Mad1-Mad2 complexes, which produce the inhibitory signal that arrests cell division. The strength of this signal depends, in part, on the number of Bub3-Bub1 and Mad1-Mad2 molecules that Spc105 recruits. Therefore regulation of this recruitment will influence SAC signaling. To understand this regulation, we established the physiological binding curves that describe the binding of Bub3-Bub1 and Mad1-Mad2 to the budding yeast kinetochore. We find that the binding of both follows the mass action law. Mps1 likely phosphorylates all six MELT repeats of Spc105. However, two mechanisms prevent Spc105 from recruiting six Bub3-Bub1 molecules: low Bub1 abundance and hindrance in the binding of more than one Bub3-Bub1 molecule to the same Spc105. Surprisingly, the kinetochore recruits two Mad1-Mad2 heterotetramers for every Bub3-Bub1 molecule. Finally, at least three MELT repeats per Spc105 are needed for accurate chromosome segregation. These data reveal that kinetochore-intrinsic and -extrinsic mechanisms influence the physiological operation of SAC signaling, potentially to maximize chromosome segregation accuracy.

Bub1 targeting to centromeres is sufficient for Sgo1 recruitment in the absence of kinetochores

Centromeric chromatin containing the histone H3 variant centromere protein A (CENP-A) directs kinetochore assembly through a hierarchical binding of CENPs, starting with CENP-C and CENP-T. Centromeres are also the chromosomal regions where cohesion, mediated by cohesin, is most prominently maintained in mitosis. While most cohesin dissociates from chromosome arms in prophase, Shugoshin 1 (Sgo1) prevents this process at centromeres. Centromeric localization of Sgo1 depends on histone H2A phosphorylation by the kinase Bub1, but whether additional interactions with kinetochore components are required for Sgo1 recruitment is unclear. Using the Xenopus egg cell-free system, we here show that both CENP-C and CENP-T can independently drive centromeric accumulation of Sgo1 through recruitment of Bub1 to the KNL1, MIS12, NDC80 (KMN) network. The spindle assembly checkpoint (SAC) kinase Mps1 is also required for this pathway even in the absence of checkpoint signaling. Sgo1 recruitment is abolished in chromosomes lacking kinetochore components other than CENP-A. However, forced targeting of Bub1 to centromeres is sufficient to restore Sgo1 localization under this condition.

Attachment issues: kinetochore transformations and spindle checkpoint silencing

Cell division culminates in the segregation of duplicated chromosomes in opposite directions prior to cellular fission. This process is guarded by the spindle assembly checkpoint (SAC), which prevents the anaphase of cell division until stable connections between spindle microtubules and the kinetochores of all chromosomes are established. The anaphase inhibitor is generated at unattached kinetochores and inhibitor production is prevented when microtubules are captured. Understanding the molecular changes in the kinetochore that are evoked by microtubule attachments is crucial for understanding the mechanisms of SAC signaling and silencing. Here, we highlight the most recent findings on these events, pinpoint some remaining mysteries, and argue for incorporating holistic views of kinetochore dynamics in order to understand SAC silencing.

The kinetochore interaction network (KIN) of ascomycetes

Chromosome segregation relies on coordinated activity of a large assembly of proteins, the kinetochore interaction network (KIN). How conserved the underlying mechanisms driving the epigenetic phenomenon of centromere and kinetochore assembly and maintenance are remains unclear, even though various eukaryotic models have been studied. More than 50 different proteins, many in multiple copies, comprise the KIN or are associated with fungal centromeres and kinetochores. Proteins isolated from immune sera recognized centromeric regions on chromosomes and thus were named centromere proteins (CENPs). CENP-A, sometimes called centromere-specific H3 (CenH3), is incorporated into nucleosomes within or near centromeres. The constitutive centromere-associated network (CCAN) assembles on this specialized chromatin, likely based on specific interactions with and requiring presence of CENP-C. The outer kinetochore comprises the Knl1-Mis12-Ndc80 (KMN) protein complexes that connect CCAN to spindles, accomplished by binding and stabilizing microtubules (MTs) and in the process generating load-bearing assemblies for chromatid segregation. In most fungi the Dam1/DASH complex connects the KMN complexes to MTs. Fungi present a rich resource to investigate mechanistic commonalities but also differences in kinetochore architecture. While ascomycetes have sets of CCAN and KMN proteins that are conserved with those of budding yeast or metazoans, searching other major branches of the fungal kingdom revealed that CCAN proteins are poorly conserved at the primary sequence level. Several conserved binding motifs or domains within KMN complexes have been described recently, and these features of ascomycete KIN proteins are shared with most metazoan proteins. In addition, several ascomycete-specific domains have been identified here.

Mps1Mph1 Kinase Phosphorylates Mad3 to Inhibit Cdc20Slp1-APC/C and Maintain Spindle Checkpoint Arrests

The spindle checkpoint is a mitotic surveillance system which ensures equal segregation of sister chromatids. It delays anaphase onset by inhibiting the action of the E3 ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C). Mad3/BubR1 is a key component of the mitotic checkpoint complex (MCC) which binds and inhibits the APC/C early in mitosis. Mps1^Mph1 kinase is critical for checkpoint signalling and MCC-APC/C inhibition, yet few substrates have been identified. Here we identify Mad3 as a substrate of fission yeast Mps1^Mph1 kinase. We map and mutate phosphorylation sites in Mad3, producing mutants that are targeted to kinetochores and assembled into MCC, yet display reduced APC/C binding and are unable to maintain checkpoint arrests. We show biochemically that Mad3 phospho-mimics are potent APC/C inhibitors in vitro, demonstrating that Mad3p modification can directly influence Cdc20^Slp1-APC/C activity. This genetic dissection of APC/C inhibition demonstrates that Mps1^Mph1 kinase-dependent modifications of Mad3 and Mad2 act in a concerted manner to maintain spindle checkpoint arrests.

When cells divide they need to ensure that a complete copy of their genetic material is transmitted to both daughter cells. Cells have evolved many controls to ensure that every division is carried out with very high fidelity. The spindle checkpoint is one such control, which acts as a surveillance system during mitosis. Defects in this checkpoint control lead to unequal segregation of DNA/chromosomes, termed aneuploidy, which is responsible for human birth defects and is very common in tumour cells.

The molecular components of the spindle checkpoint, identified initially through yeast genetics, include several protein kinases. Surprisingly few of their substrates have been identified. Here we identify the checkpoint protein Mad3 as an important substrate of the Mps1^Mph1 kinase. We show that Mps1^Mph1-dependent modification of Mad3 and Mad2 acts to delay cell division in situations where the genetic material would not be equally inherited by daughter cells. This delay enables the cell to correct any problems within the division machinery and thus avoid aneuploidy.

A Centromere-Signaling Network Underlies the Coordination among Mitotic Events

There is increasing evidence that regulators of the spindle checkpoint, kinetochore-microtubule attachments, and sister chromatid cohesion are part of an interconnected mitotic regulatory circuit with two positive feedback loops and the chromosome passenger complex (CPC) at its center. If true, this conceptual breakthrough needs to be integrated into models of mitosis. In this review, we describe this circuit and point out how the double feedback loops could provide insights into the self-organization of some mitotic processes and the autonomy of every chromosome on the mitotic spindle. We also provide working models for how mitotic events may be coordinated by this circuit.

The Aurora B Kinase in Chromosome Bi-Orientation and Spindle Checkpoint Signaling

Aurora B, a member of the Aurora family of serine/threonine protein kinases, is a key player in chromosome segregation. As part of a macromolecular complex known as the chromosome passenger complex, Aurora B concentrates early during mitosis in the proximity of centromeres and kinetochores, the sites of attachment of chromosomes to spindle microtubules. There, it contributes to a number of processes that impart fidelity to cell division, including kinetochore stabilization, kinetochore–microtubule attachment, and the regulation of a surveillance mechanism named the spindle assembly checkpoint. In the regulation of these processes, Aurora B is the fulcrum of a remarkably complex network of interactions that feed back on its localization and activation state. In this review, we discuss the multiple roles of Aurora B during mitosis, focusing in particular on its role at centromeres and kinetochores. Many details of the network of interactions at these locations remain poorly understood, and we focus here on several crucial outstanding questions.

Bub1 autophosphorylation feeds back to regulate kinetochore docking and promote localized substrate phosphorylation

During mitosis, Bub1 kinase phosphorylates histone H2A-T120 to promote centromere sister chromatid cohesion through recruitment of shugoshin (Sgo) proteins. The regulation and dynamics of H2A-T120 phosphorylation are poorly understood. Using quantitative phosphoproteomics we show that Bub1 is autophosphorylated at numerous sites. We confirm mitosis-specific autophosphorylation of a several residues and show that Bub1 activation is primed in interphase but fully achieved only in mitosis. Mutation of a single autophosphorylation site T589 alters kinetochore turnover of Bub1 and results in uniform H2A-T120 phosphorylation and Sgo recruitment along chromosome arms. Consequently, improper sister chromatid resolution and chromosome segregation errors are observed. Kinetochore tethering of Bub1-T589A refocuses H2A-T120 phosphorylation and Sgo1 to centromeres. Recruitment of the Bub1-Bub3-BubR1 axis to kinetochores has recently been extensively studied. Our data provide novel insight into the regulation and kinetochore residency of Bub1 and indicate that its localization is dynamic and tightly controlled through feedback autophosphorylation.

Widespread Recurrent Patterns of Rapid Repeat Evolution in the Kinetochore Scaffold KNL1

The outer kinetochore protein scaffold KNL1 is essential for error-free chromosome segregation during mitosis and meiosis. A critical feature of KNL1 is an array of repeats containing MELT-like motifs. When phosphorylated, these motifs form docking sites for the BUB1–BUB3 dimer that regulates chromosome biorientation and the spindle assembly checkpoint. KNL1 homologs are strikingly different in both the amount and sequence of repeats they harbor. We used sensitive repeat discovery and evolutionary reconstruction to show that the KNL1 repeat arrays have undergone extensive, often species-specific array reorganization through iterative cycles of higher order multiplication in conjunction with rapid sequence diversification. The number of repeats per array ranges from none in flowering plants up to approximately 35–40 in drosophilids. Remarkably, closely related drosophilid species have independently expanded specific repeats, indicating near complete array replacement after only approximately 25–40 Myr of evolution. We further show that repeat sequences were altered by the parallel emergence/loss of various short linear motifs, including phosphosites, which supplement the MELT-like motif, signifying modular repeat evolution. These observations point to widespread recurrent episodes of concerted KNL1 repeat evolution in all eukaryotic supergroups. We discuss our findings in the light of the conserved function of KNL1 repeats in localizing the BUB1–BUB3 dimer and its role in chromosome segregation.

Dynamic localization of Mps1 kinase to kinetochores is essential for accurate spindle microtubule attachment

The spindle assembly checkpoint (SAC) is a conserved signaling pathway that monitors faithful chromosome segregation during mitosis. As a core component of SAC, the evolutionarily conserved kinase monopolar spindle 1 (Mps1) has been implicated in regulating chromosome alignment, but the underlying molecular mechanism remains unclear. Our molecular delineation of Mps1 activity in SAC led to discovery of a previously unidentified structural determinant underlying Mps1 function at the kinetochores. Here, we show that Mps1 contains an internal region for kinetochore localization (IRK) adjacent to the tetratricopeptide repeat domain. Importantly, the IRK region determines the kinetochore localization of inactive Mps1, and an accumulation of inactive Mps1 perturbs accurate chromosome alignment and mitotic progression. Mechanistically, the IRK region binds to the nuclear division cycle 80 complex (Ndc80C), and accumulation of inactive Mps1 at the kinetochores prevents a dynamic interaction between Ndc80C and spindle microtubules (MTs), resulting in an aberrant kinetochore attachment. Thus, our results present a previously undefined mechanism by which Mps1 functions in chromosome alignment by orchestrating Ndc80C-MT interactions and highlight the importance of the precise spatiotemporal regulation of Mps1 kinase activity and kinetochore localization in accurate mitotic progression.

Plk1 and Mps1 Cooperatively Regulate the Spindle Assembly Checkpoint in Human Cells

Equal mitotic chromosome segregation is critical for genome integrity and is monitored by the spindle assembly checkpoint (SAC). We have previously shown that the consensus phosphorylation motif of the essential SAC kinase Monopolar spindle 1 (Mps1) is very similar to that of Polo-like kinase 1 (Plk1). This prompted us to ask whether human Plk1 cooperates with Mps1 in SAC signaling. Here, we demonstrate that Plk1 promotes checkpoint signaling at kinetochores through the phosphorylation of at least two Mps1 substrates, including KNL-1 and Mps1 itself. As a result, Plk1 activity enhances Mps1 catalytic activity as well as the recruitment of the SAC components Mad1:C-Mad2 and Bub3:BubR1 to kinetochores. We conclude that Plk1 strengthens the robustness of SAC establishment at the onset of mitosis and supports SAC maintenance during prolonged mitotic arrest.

Natural Loss of Mps1 Kinase in Nematodes Uncovers a Role for Polo-like Kinase 1 in Spindle Checkpoint Initiation

The spindle checkpoint safeguards against chromosome loss during cell division by preventing anaphase onset until all chromosomes are attached to spindle microtubules. Checkpoint signal is generated at kinetochores, the primary attachment site on chromosomes for spindle microtubules. Mps1 kinase initiates checkpoint signaling by phosphorylating the kinetochore-localized scaffold protein Knl1 to create phospho-docking sites for Bub1/Bub3. Mps1 is widely conserved but is surprisingly absent in many nematode species. Here, we show that PLK-1, which targets a substrate motif similar to that of Mps1, functionally substitutes for Mps1 in C. elegans by phosphorylating KNL-1 to direct BUB-1/BUB-3 kinetochore recruitment. This finding led us to re-examine checkpoint initiation in human cells, where we found that Plk1 co-inhibition significantly reduced Knl1 phosphorylation and Bub1 kinetochore recruitment relative to Mps1 inhibition alone. Thus, the finding that PLK-1 functionally substitutes for Mps1 in checkpoint initiation in C. elegans uncovered a role for Plk1 in species that have Mps1.

The (phospho) needle in the (MELT) Haystack

The spindle assembly checkpoint promotes chromosome bi-orientation and halts mitotic progression in the presence of improper kinetochore-microtubule attachments. Knl1, a kinetochore protein, acts as a scaffold for SAC signaling. A new study unveils remarkable complexity in the interplay of Knl1 phosphorylation and SAC function (Vleugel et al., 2015).

Dissecting the roles of human BUB1 in the spindle assembly checkpoint

Mitotic chromosome segregation is initiated by the anaphase promoting complex/cyclosome (APC/C) and its co-activator CDC20. APC/CCDC20 is inhibited by the spindle assembly checkpoint (SAC) when chromosomes have not attached to spindle microtubules. Unattached kinetochores catalyze the formation of a diffusible APC/CCDC20 inhibitor that is composed of BUBR1, BUB3, MAD2 and a second molecule of CDC20. Kinetochore recruitment of these proteins as well as SAC activation rely on the mitotic kinase BUB1, but the molecular mechanism by which BUB1 accomplishes this in human cells is unknown. We show that BUBR1 and BUB3 kinetochore recruitment by BUB1 is dispensable for SAC activation. Unlike its yeast and nematode orthologs, human BUB1 does not associate stably with the MAD2 activator MAD1 and, although required for accelerating loading of MAD1 onto kinetochores, is dispensable for its steady-state levels there. Instead, we identify a 50 amino acid segment harboring the recently reported ABBA motif close to a KEN box as critical for BUB1's role in SAC signaling. The presence of this segment correlates with SAC activity and efficient binding of CDC20 but not MAD1 to kinetochores.