Spindle checkpoint silencing requires association of PP1 to both Spc7 and kinesin-8 motors

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.

Some assembly required: Redefining the mitotic checkpoint

The spindle assembly checkpoint (also known as the spindle or mitotic checkpoint) is a surveillance system that ensures fidelity of chromosome segregation. Here we suggest, in light of historical and more recent evidence, that this signaling system monitors kinetochore attachment and spindle assembly by two distinct, but functionally overlapping, pathways.

Taming the Beast: Control of APC/CCdc20-Dependent Destruction

The anaphase-promoting complex/cyclosome (APC/C) is a large multisubunit ubiquitin ligase that triggers the metaphase-to-anaphase transition in the cell cycle by targeting the substrates cyclin B and securin for destruction. APC/C activity toward these two key substrates requires the coactivator Cdc20. To ensure that cells enter mitosis and partition their duplicated genome with high accuracy, APC/C^Cdc20 activity must be tightly controlled. Here, we discuss the mechanisms that regulate APC/C^Cdc20 activity both before and during mitosis. We focus our discussion primarily on the chromosomal pathways that both accelerate and delay APC/C activation by targeting Cdc20 to opposing fates. The findings discussed provide an overview of how cells control the activation of this major cell cycle regulator to ensure both accurate and timely cell division.

Evolving Centromeres and Kinetochores

The genetic material, contained on chromosomes, is often described as the "blueprint for life." During nuclear division, the chromosomes are pulled into each of the two daughter nuclei by the coordination of spindle microtubules, kinetochores, centromeres, and chromatin. These four functional units must link the chromosomes to the microtubules, signal to the cell when the attachment is made so that division can proceed, and withstand the force generated by pulling the chromosomes to either daughter cell. To perform each of these functions, kinetochores are large protein complexes, approximately 5MDa in size, and they contain at least 45 unique proteins. Many of the central components in the kinetochore are well conserved, yielding a common core of proteins forming consistent structures. However, many of the peripheral subcomplexes vary between different taxonomic groups, including changes in primary sequence and gain or loss of whole proteins. It is still unclear how significant these changes are, and answers to this question may provide insights into adaptation to specific lifestyles or progression of disease that involve chromosome instability.

Aurora-B kinase pathway controls the lateral to end-on conversion of kinetochore-microtubule attachments in human cells

Human chromosomes are captured along microtubule walls (lateral attachment) and then tethered to microtubule-ends (end-on attachment) through a multi-step end-on conversion process. Upstream regulators that orchestrate this remarkable change in the plane of kinetochore-microtubule attachment in human cells are not known. By tracking kinetochore movements and using kinetochore markers specific to attachment status, we reveal a spatially defined role for Aurora-B kinase in retarding the end-on conversion process. To understand how Aurora-B activity is counteracted, we compare the roles of two outer-kinetochore bound phosphatases and find that BubR1-associated PP2A, unlike KNL1-associated PP1, plays a significant role in end-on conversion. Finally, we uncover a novel role for Aurora-B regulated Astrin-SKAP complex in ensuring the correct plane of kinetochore-microtubule attachment. Thus, we identify Aurora-B as a key upstream regulator of end-on conversion in human cells and establish a late role for Astrin-SKAP complex in the end-on conversion process.Human chromosomes are captured along microtubule walls and then tethered to microtubule-ends through a multi-step end-on conversion process. Here the authors show that Aurora-B regulates end-on conversion in human cells and establish a late role for Astrin-SKAP complex in the end-on conversion process.

The PP2AB56 phosphatase promotes the association of Cdc20 with APC/C in mitosis

PP2A comprising B56 regulatory subunit isoforms (PP2A^B56) is a serine/threonine phosphatase essential for mitosis. At the kinetochore, PP2A^B56 both stabilizes microtubule binding and promotes silencing of the spindle assembly checkpoint (SAC) through its association with the SAC protein BubR1. Cells depleted of the B56 regulatory subunits of PP2A are delayed in activation of Cdc20-containing APC/C (APC/C^Cdc20), which is an essential step for mitotic exit. It has been hypothesized that this delay arises from increased production of the mitotic checkpoint complex (MCC), an APC/C^Cdc20 inhibitor formed at unattached kinetochores through SAC signaling. In contrast to this prediction, we show that depletion of B56 subunits does not increase the amount or stability of the MCC. Rather, delays in APC/C^Cdc20 activation in B56-depleted cells correlate with impaired Cdc20 binding to APC/C. Stimulation of APC/C^Cdc20 assembly does not require binding between PP2A^B56 and BubR1, and thus this contribution of PP2A^B56 towards mitotic exit is distinct from its functions at kinetochores. PP2A^B56 associates with APC/C constitutively in a BubR1-independent manner. A mitotic phosphorylation site on Cdc20, known to be a substrate of PP2A^B56, modulates APC/C^Cdc20 assembly. These results elucidate the contributions of PP2A^B56 towards completion of mitosis.

Identification of a Sgo2-Dependent but Mad2-Independent Pathway Controlling Anaphase Onset in Fission Yeast

The onset of anaphase is triggered by activation of the anaphase-promoting complex/cyclosome (APC/C) following silencing of the spindle assembly checkpoint (SAC). APC/C triggers ubiquitination of Securin and Cyclin B, which leads to loss of sister chromatid cohesion and inactivation of Cyclin B/Cdk1, respectively. This promotes relocalization of Aurora B kinase and other components of the chromosome passenger complex (CPC) from centromeres to the spindle midzone. In fission yeast, this is mediated by Clp1 phosphatase-dependent interaction of CPC with Klp9/MKLP2 (kinesin-6). When this interaction is disrupted, kinetochores bi-orient normally, but APC/C activation is delayed via a mechanism that requires Sgo2 and some (Bub1, Mph1/Mps1, and Mad3), but not all (Mad1 and Mad2), components of the SAC and the first, but not second, lysine, glutamic acid, glutamine (KEN) box in Mad3. These data indicate that interaction of CPC with Klp9 terminates a Sgo2-dependent, but Mad2-independent, APC/C-inhibitory pathway that is distinct from the canonical SAC.

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.

The mitotic checkpoint complex (MCC): looking back and forth after 15 years

The mitotic checkpoint is a specialized signal transduction pathway that contributes to the fidelity of chromosome segregation. The signaling of the checkpoint originates from defective kinetochore-microtubule interactions and leads to formation of the mitotic checkpoint complex (MCC), a highly potent inhibitor of the Anaphase Promoting Complex/Cyclosome (APC/C)—the E3 ubiquitin ligase essential for anaphase onset. Many important questions concerning the MCC and its interaction with APC/C have been intensively investigated and debated in the past 15 years, such as the exact composition of the MCC, how it is assembled during a cell cycle, how it inhibits APC/C, and how the MCC is disassembled to allow APC/C activation. These efforts have culminated in recently reported structure models for human MCC:APC/C supra-complexes at near-atomic resolution that shed light on multiple aspects of the mitotic checkpoint mechanisms. However, confusing statements regarding the MCC are still scattered in the literature, making it difficult for students and scientists alike to obtain a clear picture of MCC composition, structure, function and dynamics. This review will comb through some of the most popular concepts or misconceptions about the MCC, discuss our current understandings, present a synthesized model on regulation of CDC20 ubiquitination, and suggest a few future endeavors and cautions for next phase of MCC research.

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.

Mitotic phosphatase activity is required for MCC maintenance during the spindle checkpoint

The spindle checkpoint prevents activation of the anaphase-promoting complex (APC/C) until all chromosomes are correctly attached to the mitotic spindle. Early in mitosis, the mitotic checkpoint complex (MCC) inactivates the APC/C by binding the APC/C activating protein CDC20 until the chromosomes are properly aligned and attached to the mitotic spindle, at which point MCC disassembly releases CDC20 to activate the APC/C. Once the APC/C is activated, it targets cyclin B and securin for degradation, and the cell progresses into anaphase. While phosphorylation is known to drive many of the events during the checkpoint, the precise molecular mechanisms regulating spindle checkpoint maintenance and inactivation are still poorly understood. We sought to determine the role of mitotic phosphatases during the spindle checkpoint. To address this question, we treated spindle checkpoint-arrested cells with various phosphatase inhibitors and examined the effect on the MCC and APC/C activation. Using this approach we found that 2 phosphatase inhibitors, calyculin A and okadaic acid (1 μM), caused MCC dissociation and APC/C activation leading to cyclin A and B degradation in spindle checkpoint-arrested cells. Although the cells were able to degrade cyclin B, they did not exit mitosis as evidenced by high levels of Cdk1 substrate phosphorylation and chromosome condensation. Our results provide the first evidence that phosphatases are essential for maintenance of the MCC during operation of the spindle checkpoint.

Phosphorylation of PP1 Regulator Sds22 by PLK1 Ensures Accurate Chromosome Segregation

During cell division, accurate chromosome segregation is tightly regulated by Polo-like kinase 1 (PLK1) and opposing activities of Aurora B kinase and protein phosphatase 1 (PP1). However, the regulatory mechanisms underlying the aforementioned hierarchical signaling cascade during mitotic chromosome segregation have remained elusive. Sds22 is a conserved regulator of PP1 activity, but how it regulates PP1 activity in space and time during mitosis remains elusive. Here we show that Sds22 is a novel and cognate substrate of PLK1 in mitosis, and the phosphorylation of Sds22 by PLK1 elicited an inhibition of PP1-mediated dephosphorylation of Aurora B at threonine 232 (Thr232) in a dose-dependent manner. Overexpression of a phosphomimetic mutant of Sds22 causes a dramatic increase in mitotic delay, whereas overexpression of a non-phosphorylatable mutant of Sds22 results in mitotic arrest. Mechanistically, the phosphorylation of Sds22 by PLK1 strengthens the binding of Sds22 to PP1 and inhibits the dephosphorylation of Thr232 of Aurora B to ensure a robust, error-free metaphase-anaphase transition. These findings delineate a conserved signaling hierarchy that orchestrates dynamic protein phosphorylation and dephosphorylation of critical mitotic regulators during chromosome segregation to guard chromosome stability.

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.

Kinesin-8 effects on mitotic microtubule dynamics contribute to spindle function in fission yeast

Kinesin-8 motor proteins destabilize microtubules and increase chromosome loss in mitosis. In fission yeast, aberrant microtubule-driven kinetochore pushing movements, tripolar mitotic spindles, and fluctuations in metaphase spindle length occurred in kinesin-8–deletion mutants. A mathematical model can explain these results.

Kinesin-8 motor proteins destabilize microtubules. Their absence during cell division is associated with disorganized mitotic chromosome movements and chromosome loss. Despite recent work studying effects of kinesin-8s on microtubule dynamics, it remains unclear whether the kinesin-8 mitotic phenotypes are consequences of their effect on microtubule dynamics, their well-established motor activity, or additional, unknown functions. To better understand the role of kinesin-8 proteins in mitosis, we studied the effects of deletion of the fission yeast kinesin-8 proteins Klp5 and Klp6 on chromosome movements and spindle length dynamics. Aberrant microtubule-driven kinetochore pushing movements and tripolar mitotic spindles occurred in cells lacking Klp5 but not Klp6. Kinesin-8–deletion strains showed large fluctuations in metaphase spindle length, suggesting a disruption of spindle length stabilization. Comparison of our results from light microscopy with a mathematical model suggests that kinesin-8–induced effects on microtubule dynamics, kinetochore attachment stability, and sliding force in the spindle can explain the aberrant chromosome movements and spindle length fluctuations seen.

Collaborative Control of Cell Cycle Progression by the RNA Exonuclease Dis3 and Ras Is Conserved Across Species

Dis3 encodes a conserved RNase that degrades or processes all RNA species via an N-terminal PilT N terminus (PIN) domain and C-terminal RNB domain that harbor, respectively, endonuclease activity and 3'-5' exonuclease activity. In Schizosaccharomyces pombe, dis3 mutations cause chromosome missegregation and failure in mitosis, suggesting dis3 promotes cell division. In humans, apparently hypomorphic dis3 mutations are found recurrently in multiple myeloma, suggesting dis3 opposes cell division. Except for the observation that RNAi-mediated depletion of dis3 function drives larval arrest and reduces tissue growth in Drosophila, the role of dis3 has not been rigorously explored in higher eukaryotic systems. Using the Drosophila system and newly generated dis3 null alleles, we find that absence of dis3 activity inhibits cell division. We uncover a conserved CDK1 phosphorylation site that when phosphorylated inhibits Dis3's exonuclease, but not endonuclease, activity. Leveraging this information, we show that Dis3's exonuclease function is required for mitotic cell division: in its absence, cells are delayed in mitosis and exhibit aneuploidy and overcondensed chromosomes. In contrast, we find that modest reduction of dis3 function enhances cell proliferation in the presence of elevated Ras activity, apparently by accelerating cells through G2/M even though each insult by itself delays G2/M. Additionally, we find that dis3 and ras genetically interact in worms and that dis3 can enhance cell proliferation under growth stimulatory conditions in murine B cells. Thus, reduction, but not absence, of dis3 activity can enhance cell proliferation in higher organisms.

The human SKA complex drives the metaphase-anaphase cell cycle transition by recruiting protein phosphatase 1 to kinetochores

The spindle- and kinetochore-associated (Ska) complex is essential for normal anaphase onset in mitosis. The C-terminal domain (CTD) of Ska1 binds microtubules and was proposed to facilitate kinetochore movement on depolymerizing spindle microtubules. Here, we show that Ska complex recruits protein phosphatase 1 (PP1) to kinetochores. This recruitment requires the Ska1 CTD, which binds PP1 in vitro and in human HeLa cells. Ska1 lacking its CTD fused to a PP1-binding peptide or fused directly to PP1 rescues mitotic defects caused by Ska1 depletion. Ska1 fusion to catalytically dead PP1 mutant does not rescue and shows dominant negative effects. Thus, the Ska complex, specifically the Ska1 CTD, recruits PP1 to kinetochores to oppose spindle checkpoint signaling kinases and promote anaphase onset. Microtubule binding by Ska, rather than acting in force production for chromosome movement, may instead serve to promote PP1 recruitment to kinetochores fully attached to spindle microtubules at metaphase.

The selective inhibition of protein phosphatase-1 results in mitotic catastrophe and impaired tumor growth

The serine/threonine protein phosphatase-1 (PP1) complex is a key regulator of the cell cycle. However, the redundancy of PP1 isoforms and the lack of specific inhibitors have hampered studies on the global role of PP1 in cell cycle progression in vertebrates. Here, we show that the overexpression of nuclear inhibitor of PP1 (NIPP1; also known as PPP1R8) in HeLa cells culminated in a prometaphase arrest, associated with severe spindle-formation and chromosome-congression defects. In addition, the spindle assembly checkpoint was activated and checkpoint silencing was hampered. Eventually, most cells either died by apoptosis or formed binucleated cells. The NIPP1-induced mitotic arrest could be explained by the inhibition of PP1 that was titrated away from other mitotic PP1 interactors. Consistent with this notion, the mitotic-arrest phenotype could be rescued by the overexpression of PP1 or the inhibition of the Aurora B kinase, which acts antagonistically to PP1. Finally, we demonstrate that the overexpression of NIPP1 also hampered colony formation and tumor growth in xenograft assays in a PP1-dependent manner. Our data show that the selective inhibition of PP1 can be used to induce cancer cell death through mitotic catastrophe.

Sharpening the anaphase switch

The segregation of sister chromatids during mitosis is one of the most easily visualized, yet most remarkable, events during the life cycle of a cell. The accuracy of this process is essential to maintain ploidy during cell duplication. Over the past 20 years, substantial progress has been made in identifying components of both the kinetochore and the mitotic spindle that generate the force to move mitotic chromosomes. Additionally, we now have a reasonable, albeit incomplete, understanding of the molecular and biochemical events that are involved in establishing and dissolving sister-chromatid cohesion. However, it is less well-understood how this dissolution of cohesion occurs synchronously on all chromosomes at the onset of anaphase. At the centre of the action is the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that, in association with its activator cell-division cycle protein 20 homologue (Cdc20), is responsible for the destruction of securin. This leads to the activation of separase, a specialized protease that cleaves the kleisin-subunit of the cohesin complex, to relieve cohesion between sister chromatids. APC/C-Cdc20 is also responsible for the destruction of cyclin B and therefore inactivation of the cyclin B-cyclin-dependent kinase 1 (Cdk1). This latter event induces a change in the microtubule dynamics that results in the movement of sister chromatids to spindle poles (anaphase A), spindle elongation (anaphase B) and the onset of cytokinesis. In the present paper, we review the emerging evidence that multiple, spatially and temporally regulated feedback loops ensure anaphase onset is rapid, co-ordinated and irreversible.

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

Regulated recruitment of the kinase-adaptor complex BUB1/BUB3 to kinetochores is crucial for correcting faulty chromosome-spindle attachments and for spindle assembly checkpoint (SAC) signaling. BUB1/BUB3 localizes to kinetochores by binding phosphorylated MELT motifs (MELpT) in the kinetochore scaffold KNL1. Human KNL1 has 19 repeats that contain a MELT-like sequence. The repeats are, however, larger than MELT, and repeat sequences can vary significantly. Using systematic screening, we show that only a limited number of repeats is "active." Repeat activity correlates with the presence of a vertebrate-specific SHT motif C-terminal to the MELT sequence. SHT motifs are phosphorylated by MPS1 in a manner that requires prior phosphorylation of MELT. Phospho-SHT (SHpT) synergizes with MELpT in BUB3/BUB1 binding in vitro and in cells, and human BUB3 mutated in a predicted SHpT-binding surface cannot localize to kinetochores. Our data show sequential multisite regulation of the KNL1-BUB1/BUB3 interaction and provide mechanistic insight into evolution of the KNL1-BUB3 interface.

Regulation of mitotic progression by the spindle assembly checkpoint

Equal segregation of sister chromatids during mitosis requires that pairs of kinetochores establish proper attachment to microtubules emanating from opposite poles of the mitotic spindle. The spindle assembly checkpoint (SAC) protects against errors in segregation by delaying sister separation in response to improper kinetochore–microtubule interactions, and certain checkpoint proteins help to establish proper attachments. Anaphase entry is inhibited by the checkpoint through assembly of the mitotic checkpoint complex (MCC) composed of the 2 checkpoint proteins, Mad2 and BubR1, bound to Cdc20. The outer kinetochore acts as a catalyst for MCC production through the recruitment and proper positioning of checkpoint proteins and recently there has been remarkable progress in understanding how this is achieved. Here, we highlight recent advances in our understanding of kinetochore–checkpoint protein interactions and inhibition of the anaphase promoting complex by the MCC.

Regulatory mechanisms that control mitotic kinesins

During mitosis, the mitotic spindle is assembled to align chromosomes at the spindle equator in metaphase, and to separate the genetic material equally to daughter cells in anaphase. The spindle itself is a macromolecular machine composed of an array of dynamic microtubules and associated proteins that coordinate the diverse events of mitosis. Among the microtubule associated proteins are a plethora of molecular motor proteins that couple the energy of ATP hydrolysis to force production. These motors, including members of the kinesin superfamily, must function at the right time and in the right place to insure the fidelity of mitosis. Misregulation of mitotic motors in disease states, such as cancer, underlies their potential utility as targets for antitumor drug development and highlights the importance of understanding the molecular mechanisms for regulating their function. Here, we focus on recent progress about regulatory mechanisms that control the proper function of mitotic kinesins and highlight new findings that lay the path for future studies.

Analysis of S. pombe SIN protein association to the SPB reveals two genetically separable states of the SIN

The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis, and asymmetric association of SIN proteins with the mitotic spindle pole bodies (SPBs) is important for its regulation. Here, we have used semi-automated image analysis to study SIN proteins in large numbers of wild-type and mutant cells. Our principal conclusions are: first, that the association of Cdc7p with the SPBs in early mitosis is frequently asymmetric, with a bias in favour of the new SPB; second, that the early association of Cdc7p-GFP to the SPB depends on Plo1p but not Spg1p, and is unaffected by mutations that influence its asymmetry in anaphase; third, that Cdc7p asymmetry in anaphase B is delayed by Pom1p and by activation of the spindle assembly checkpoint, and is promoted by Rad24p; and fourth, that the length of the spindle, expressed as a fraction of the length of the cell, at which Cdc7p becomes asymmetric is similar in cells dividing at different sizes. These data reveal that multiple regulatory mechanisms control the SIN in mitosis and lead us to propose a two-state model to describe the SIN.

Alp7/TACC recruits kinesin-8-PP1 to the Ndc80 kinetochore protein for timely mitotic progression and chromosome movement

Upon establishment of proper kinetochore–microtubule attachment, the spindle assembly checkpoint (SAC) must be silenced to allow onset of anaphase, which is when sister chromatids segregate equally to two daughter cells. However, how proper kinetochore–microtubule attachment leads to timely anaphase onset remains elusive. Furthermore, the molecular mechanisms of chromosome movement during anaphase A remain unclear. In this study, we show that the fission yeast Alp7/TACC protein recruits a protein complex consisting of the kinesin-8 (Klp5–Klp6) and protein phosphatase 1 (PP1) to the kinetochore upon kinetochore–microtubule attachment. Accumulation of this complex at the kinetochore, on the one hand, facilitates SAC inactivation through PP1, and, on the other hand, accelerates polewards chromosome movement driven by the Klp5–Klp6 motor. We identified an alp7 mutant that had specific defects in binding to the Klp5–Klp6–PP1 complex but with normal localisation to the microtubule and kinetochore. Consistent with our proposition, this mutant shows delayed anaphase onset and decelerated chromosome movement during anaphase A. We propose that the recruitment of kinesin-8–PP1 to the kinetochore through Alp7/TACC interaction plays a crucial role in regulation of timely mitotic progression and chromosome movement during anaphase A.

Negative feedback at kinetochores underlies a responsive spindle checkpoint signal

Kinetochores are specialised multi-protein complexes that play a crucial role in maintaining genome stability 1. They bridge attachments between chromosomes and microtubules during mitosis and they activate the spindle assembly checkpoint (SAC) to arrest division until all chromosomes are attached 2. Kinetochores are able to efficiently integrate these two processes because they can rapidly respond to changes in microtubule occupancy by switching localised SAC signalling ON or OFF 2–4. We show that this responsiveness arises because the SAC primes kinetochore phosphatases to induce negative feedback and silence its own signal. Active SAC signalling recruits PP2A-B56 to kinetochores where it antagonises Aurora B to promote PP1 recruitment. PP1 in turn silences the SAC and delocalises PP2A-B56. Preventing or bypassing key regulatory steps demonstrates that this spatiotemporal control of phosphatase feedback underlies rapid signal switching at the kinetochore by; 1) allowing the SAC to quickly transition to the ON state in the absence of antagonising phosphatase activity, and 2) ensuring phosphatases are then primed to rapidly switch the SAC signal OFF when kinetochore kinase activities are diminished by force-producing microtubule attachments.

Signalling dynamics in the spindle checkpoint response

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

Inhibitor-3 ensures bipolar mitotic spindle attachment by limiting association of SDS22 with kinetochore-bound protein phosphatase-1

Faithful chromosome segregation during mitosis is tightly regulated by opposing activities of Aurora B kinase and protein phosphatase-1 (PP1). PP1 function at kinetochores has been linked to SDS22, but the exact localization of SDS22 and how it affects PP1 are controversial. Here, we confirm that SDS22 is required for PP1 activity, but show that SDS22 does not normally localize to kinetochores. Instead, SDS22 is kept in solution by formation of a ternary complex with PP1 and inhibitor-3 (I3). Depletion of I3 does not affect the amount of PP1 at kinetochores but causes quantitative association of SDS22 with PP1 on KNL1 at the kinetochore. Such accumulation of SDS22 at kinetochores interferes with PP1 activity and inhibits Aurora B threonine-232 dephosphorylation, which leads to increased Aurora B activity in metaphase and persistence in anaphase accompanied with segregation defects. We propose a model in which I3 regulates an SDS22-mediated PP1 activation step in solution that precedes SDS22 dissociation and transfer of PP1 to kinetochores, and which is required for PP1 to efficiently antagonize Aurora B.

The human mitotic kinesin KIF18A binds protein phosphatase 1 (PP1) through a highly conserved docking motif

Protein phosphatase 1 (PP1), a serine/threonine protein phosphatase, controls diverse key cellular events. PP1 catalytic subunits form complexes with a variety of interacting proteins that control its ability to dephosphorylate substrates. Here we show that the human mitotic kinesin-8, KIF18A, directly interacts with PP1γ through a conserved RVxF motif. Our phylogenetic analyses of the kinesins further uncovered the broad conservation of this interaction potential within the otherwise highly diverse motor-protein superfamily. This suggests an ancestral origin of PP1 recruitment to KIF18A and a strategic role in human mitotic cells.

PP2A-B56 opposes Mps1 phosphorylation of Knl1 and thereby promotes spindle assembly checkpoint silencing

The kinetochore surveillance phosphatase PP2A-B56 dephosphorylates Knl1 to silence the spindle assembly checkpoint after all chromosomes have been correctly attached to microtubules.

The spindle assembly checkpoint (SAC) monitors correct attachment of chromosomes to microtubules, an important safeguard mechanism ensuring faithful chromosome segregation in eukaryotic cells. How the SAC signal is turned off once all the chromosomes have successfully attached to the spindle remains an unresolved question. Mps1 phosphorylation of Knl1 results in recruitment of the SAC proteins Bub1, Bub3, and BubR1 to the kinetochore and production of the wait-anaphase signal. SAC silencing is therefore expected to involve a phosphatase opposing Mps1. Here we demonstrate in vivo and in vitro that BubR1-associated PP2A-B56 is a key phosphatase for the removal of the Mps1-mediated Knl1 phosphorylations necessary for Bub1/BubR1 recruitment in mammalian cells. SAC silencing is thus promoted by a negative feedback loop involving the Mps1-dependent recruitment of a phosphatase opposing Mps1. Our findings extend the previously reported role for BubR1-associated PP2A-B56 in opposing Aurora B and suggest that BubR1-bound PP2A-B56 integrates kinetochore surveillance and silencing of the SAC.

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

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

Pre-anaphase chromosome oscillations are regulated by the antagonistic activities of Cdk1 and PP1 on Kif18A

Upon congression at the spindle equator, vertebrate chromosomes display oscillatory movements which typically decline as cells progress towards anaphase. Kinesin-8 Kif18A has been identified as a suppressor of chromosome movements, but how its activity is temporally regulated to dampen chromosome oscillations before anaphase onset remained mysterious. Here, we identify a regulatory network composed of cyclin-dependent kinase-1 (Cdk1) and protein phosphatase-1 (PP1) that antagonistically regulate Kif18A. Cdk1-mediated inhibitory phosphorylation of Kif18A promotes chromosome oscillations in early metaphase. PP1 induces metaphase plate thinning by directly dephosphorylating Kif18A. Chromosome attachment induces Cdk1 inactivation and kinetochore recruitment of PP1α/γ. Thus, we propose that chromosome biorientation mediates the alignment of chromosomes at the metaphase plate by tipping the balance in favour of dephosphorylated Kif18A capable of suppressing the oscillatory movements of chromosomes. Notably, interfering with chromosome oscillations severely impairs the fidelity of sister chromatid segregation demonstrating the importance of timely controlled chromosome dynamics for the maintenance of genome integrity.

The dynamic protein Knl1 - a kinetochore rendezvous

Knl1 (also known as CASC5, UniProt Q8NG31) is an evolutionarily conserved scaffolding protein that is required for proper kinetochore assembly, spindle assembly checkpoint (SAC) function and chromosome congression. A number of recent reports have confirmed the prominence of Knl1 in these processes and provided molecular details and structural features that dictate Knl1 functions in higher organisms. Knl1 recruits SAC components to the kinetochore and is the substrate of certain protein kinases and phosphatases, the interplay of which ensures the exquisite regulation of the aforementioned processes. In this Commentary, we discuss the overall domain organization of Knl1 and the roles of this protein as a versatile docking platform. We present emerging roles of the protein interaction motifs present in Knl1, including the RVSF, SILK, MELT and KI motifs, and their role in the recruitment and regulation of the SAC proteins Bub1, BubR1, Bub3 and Aurora B. Finally, we explore how the regions of low structural complexity that characterize Knl1 are implicated in the cooperative interactions that mediate binding partner recognition and scaffolding activity by Knl1.

CPF-associated phosphatase activity opposes condensin-mediated chromosome condensation

Functional links connecting gene transcription and condensin-mediated chromosome condensation have been established in species ranging from prokaryotes to vertebrates. However, the exact nature of these links remains misunderstood. Here we show in fission yeast that the 3′ end RNA processing factor Swd2.2, a component of the Cleavage and Polyadenylation Factor (CPF), is a negative regulator of condensin-mediated chromosome condensation. Lack of Swd2.2 does not affect the assembly of the CPF but reduces its association with chromatin. This causes only limited, context-dependent effects on gene expression and transcription termination. However, CPF-associated Swd2.2 is required for the association of Protein Phosphatase 1 PP1^Dis2 with chromatin, through an interaction with Ppn1, a protein that we identify as the fission yeast homologue of vertebrate PNUTS. We demonstrate that Swd2.2, Ppn1 and PP1^Dis2 form an independent module within the CPF, which provides an essential function in the absence of the CPF-associated Ssu72 phosphatase. We show that Ppn1 and Ssu72, like Swd2.2, are also negative regulators of condensin-mediated chromosome condensation. We conclude that Swd2.2 opposes condensin-mediated chromosome condensation by facilitating the function of the two CPF-associated phosphatases PP1 and Ssu72.

Failure to properly condense chromosomes prior to their segregation in mitosis can lead to genome instability. The evolutionary-conserved condensin complex is key to the condensation process but the molecular mechanisms underlying its localization pattern on chromosomes remain unclear. Previous observations showed that the localization of condensin is intimately linked to regions of high transcription, although, somewhat paradoxically, its association with chromatin is disrupted by a processive polymerase activity. Here we identify several RNA processing factors as negative regulators of condensin in fission yeast. Two of these factors associate with PP1 phosphatase as an independent entity within the Cleavage and Polyadenylation Factor (CPF), a complex key for 3′ end RNA processing. Lack of this module induces only minor and context-dependent effects on gene expression. Our data suggest that this module helps maintaining the proper level of phosphatase activity within the CPF and thereby opposes the function of condensin in mitotic chromosome condensation.

Tension-dependent removal of pericentromeric shugoshin is an indicator of sister chromosome biorientation

During mitosis and meiosis, sister chromatid cohesion resists the pulling forces of microtubules, enabling the generation of tension at kinetochores upon chromosome biorientation. How tension is read to signal the bioriented state remains unclear. Shugoshins form a pericentromeric platform that integrates multiple functions to ensure proper chromosome biorientation. Here we show that budding yeast shugoshin Sgo1 dissociates from the pericentromere reversibly in response to tension. The antagonistic activities of the kinetochore-associated Bub1 kinase and the Sgo1-bound phosphatase protein phosphatase 2A (PP2A)-Rts1 underlie a tension-dependent circuitry that enables Sgo1 removal upon sister kinetochore biorientation. Sgo1 dissociation from the pericentromere triggers dissociation of condensin and Aurora B from the centromere, thereby stabilizing the bioriented state. Conversely, forcing sister kinetochores to be under tension during meiosis I leads to premature Sgo1 removal and precocious loss of pericentromeric cohesion. Overall, we show that the pivotal role of shugoshin is to build a platform at the pericentromere that attracts activities that respond to the absence of tension between sister kinetochores. Disassembly of this platform in response to intersister kinetochore tension signals the bioriented state. Therefore, tension sensing by shugoshin is a central mechanism by which the bioriented state is read.

Kinetochore composition and its function: lessons from yeasts

Proper chromosome segregation during cell division is essential for proliferation, and this is facilitated by kinetochores, large protein complexes assembled on the centromeric region of the chromosomes. Although the sequences of centromeric DNA differ totally among organisms, many components of the kinetochores assembled on centromeres are very well conserved among eukaryotes. To define the identity of centromeres, centromere protein A (CENP-A), which is homologous to canonical histone H3, acts as a landmark for kinetochore assembly. Kinetochores mediate spindle–microtubule attachment and control the movement of chromosomes during mitosis and meiosis. To conduct faithful chromosome segregation, kinetochore assembly and microtubule attachment are elaborately regulated. Here we review the current understanding of the composition, assembly, functions and regulation of kinetochores revealed mainly through studies on fission and budding yeasts. Moreover, because recent cumulative evidence suggests the importance of the regulation of the orientation of kinetochore–microtubule attachment, which differs distinctly between mitosis and meiosis, we focus especially on the molecular mechanisms underlying this regulation.

Mechanisms promoting escape from mitotic stress-induced tumor cell death

Non-small cell lung cancer (NSCLC) is notorious for its paltry responses to first-line therapeutic regimens. In contrast to acquired chemoresistance, little is known about the molecular underpinnings of the intrinsic resistance of chemo-naïve NSCLC. Here we report that intrinsic resistance to paclitaxel in NSCLC occurs at a cell-autonomous level because of the uncoupling of mitotic defects from apoptosis. To identify components that permit escape from mitotic stress-induced death, we used a genome-wide RNAi-based strategy, which combines a high-throughput toxicity screen with a live-cell imaging platform to measure mitotic fate. This strategy revealed that prolonging mitotic arrest with a small molecule inhibitor of the APC/cyclosome could sensitize otherwise paclitaxel-resistant NSCLC. We also defined novel roles for CASC1 and TRIM69 in supporting resistance to spindle poisons. CASC1, which is frequently co-amplified with KRAS in lung tumors, is essential for microtubule polymerization and satisfaction of the spindle assembly checkpoint. TRIM69, which associates with spindle poles and promotes centrosomal clustering, is essential for formation of a bipolar spindle. Notably, RNAi-mediated attenuation of CASC1 or TRIM69 was sufficient to inhibit tumor growth in vivo. On the basis of our results, we hypothesize that tumor evolution selects for a permissive mitotic checkpoint, which may promote survival despite chromosome segregation errors. Attacking this adaptation may restore the apoptotic consequences of mitotic damage to permit the therapeutic eradication of drug-resistant cancer cells.

KNL1 facilitates phosphorylation of outer kinetochore proteins by promoting Aurora B kinase activity

KNL1 is essential for efficient kinetochore–microtubule attachment dynamics during mitosis, in part through promotion of Bub1-mediated targeting of Aurora B to the kinetochore.

Aurora B kinase phosphorylates kinetochore proteins during early mitosis, increasing kinetochore–microtubule (MT) turnover and preventing premature stabilization of kinetochore–MT attachments. Phosphorylation of kinetochore proteins during late mitosis is low, promoting attachment stabilization, which is required for anaphase onset. The kinetochore protein KNL1 recruits Aurora B–counteracting phosphatases and the Aurora B–targeting factor Bub1, yet the consequences of KNL1 depletion on Aurora B phospho-regulation remain unknown. Here, we demonstrate that the KNL1 N terminus is essential for Aurora B activity at kinetochores. This region of KNL1 is also required for Bub1 kinase activity at kinetochores, suggesting that KNL1 promotes Aurora B activity through Bub1-mediated Aurora B targeting. However, ectopic targeting of Aurora B to kinetochores does not fully rescue Aurora B activity in KNL1-depleted cells, suggesting KNL1 influences Aurora B activity through an additional pathway. Our findings establish KNL1 as a requirement for Aurora B activity at kinetochores and for wild-type kinetochore–MT attachment dynamics.

Role and regulation of kinesin-8 motors through the cell cycle

Members of the kinesin-8 motor family play a central role in controlling microtubule length throughout the eukaryotic cell cycle. Inactivation of kinesin-8 causes defects in cell polarity during interphase and astral and mitotic spindle length, metaphase chromosome alignment, timing of anaphase onset and accuracy of chromosome segregation. Although the biophysical mechanism by which kinesin-8 molecules influence microtubule dynamics has been studied extensively in a variety of species, a consensus view has yet to emerge. One reason for this might be that some members of the kinesin-8 family can associate to other microtubule-associated proteins, cell cycle regulatory proteins and other kinesin family members. In this review we consider how cell cycle specific modification and its association to other regulatory proteins may modulate the function of kinesin-8 to enable it to function as a master regulator of microtubule dynamics.

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

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

Modular assembly of RWD domains on the Mis12 complex underlies outer kinetochore organization

Faithful chromosome segregation is mandatory for cell and organismal viability. Kinetochores, large protein assemblies embedded in centromeric chromatin, establish a mechanical link between chromosomes and spindle microtubules. The KMN network, a conserved 10-subunit kinetochore complex, harbors the microtubule-binding interface. RWD domains in the KMN subunits Spc24 and Spc25 mediate kinetochore targeting of the microtubule-binding subunits by interacting with the Mis12 complex, a KMN subcomplex that tethers directly onto the underlying chromatin layer. Here, we show that Knl1, a KMN subunit involved in mitotic checkpoint signaling, also contains RWD domains that bind the Mis12 complex and that mediate kinetochore targeting of Knl1. By reporting the first 3D electron microscopy structure of the KMN network, we provide a comprehensive framework to interpret how interactions of RWD-containing proteins with the Mis12 complex shape KMN network topology. Our observations unveil a regular pattern in the construction of the outer kinetochore.

Linked in: formation and regulation of microtubule attachments during chromosome segregation

Accurate segregation of the replicated genome during cell division depends on dynamic attachments formed between chromosomes and the microtubule polymers of the spindle. Here we review recent advances in mechanistic analysis of microtubule attachment formation and regulation.

The signaling network that silences the spindle assembly checkpoint upon the establishment of chromosome bipolar attachment

Improper kinetochore attachments activate the spindle assembly checkpoint (SAC) to prevent anaphase onset, but it is poorly understood how this checkpoint is silenced to allow anaphase onset. Chromosome bipolar attachment applies tension on sister kinetochores, and the lack of tension delays anaphase onset. In budding yeast, the delay induced by tension defects depends on the intact SAC as well as increase in ploidy (Ipl1)/Aurora kinase and a centromere-associated protein ShuGOshin (Sgo1). Here we provide evidence indicating that Ipl1-dependent phosphorylation of the kinetochore protein Duo1 and Mps1 interacting (Dam1) prevents SAC silencing when tension is absent. The nonphosphorylatable dam1 mutant cells, as well as sgo1 mutant cells, are competent in SAC activation but unable to prevent SAC silencing in response to tension defects. We further found that phosphomimetic dam1 mutants exhibited delayed anaphase onset mainly due to the failure in SAC silencing, but destabilized kinetochore attachment likely plays a minor role in this delay. Because the tension resulting from bipolar attachment triggers the dephosphorylation of Dam1 by protein phosphatase 1, this dephosphorylation likely coordinates SAC silencing with chromosome bipolar attachment. Therefore, Sgo1, Ipl1 kinase, Dam1, and protein phosphatase 1 comprise the SAC silencing network that ensures the correct timing for anaphase onset.

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.

Interplay between mitotic kinesins and the Aurora kinase–PP1 (protein phosphatase 1) axis

Correct transmission of genetic information from mother to daughter cells is necessary for development and survival. Accurate segregation is achieved by bipolar attachment of sister kinetochores in each chromatid pair to spindle microtubules emanating from opposite spindle poles, a process known as chromosome bi-orientation. Achieving this requires dynamic interplay between kinetochore proteins, kinesin motor proteins and cell cycle regulators. Chromosome bi-orientation is monitored by a surveillance mechanism known as the SAC (spindle assembly checkpoint). The Aurora B kinase, which is bound to the inner centromere during early mitosis, plays a central role in both chromosome bi-orientation and the spindle checkpoint. The application of tension across centromeres establishes a spatial gradient of high phosphorylation activity at the inner centromere and low phosphorylation activity at the outer kinetochore. This gradient is further refined by the association of PP1 (protein phosphatase 1) to the outer kinetochore, which stabilizes kinetochore–microtubule interactions and silences the spindle checkpoint by dephosphorylating Aurora B kinase targets when chromosome bi-orientation is achieved. In the present review, I discuss emerging evidence that bidirectional cross-talk between mitotic kinesins and the Aurora kinase–PP1 axis is crucial for co-ordinating chromosome bi-orientation and spindle checkpoint signalling in eukaryotes.

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.