The kinetochore encodes a mechanical switch to disrupt spindle assembly checkpoint signalling

A conserved site on Ndc80 complex facilitates dynamic recruitment of Mps1 to yeast kinetochores to promote accurate chromosome segregation

Accurate chromosome segregation relies on kinetochores carrying out multiple functions, including establishing and maintaining microtubule attachments, forming precise bi-oriented attachments between sister chromatids, and activating the spindle assembly checkpoint. Central to these processes is the highly conserved Ndc80 complex. This kinetochore subcomplex interacts directly with microtubules but also serves as a critical platform for recruiting kinetochore-associated factors and as a key substrate for error correction kinases. The precise manner in which these kinetochore factors interact and regulate each other's function remains unknown, considerably hindering our understanding of how Ndc80 complex-dependent processes function together to orchestrate accurate chromosome segregation. Here, we aimed to uncover the role of Nuf2's CH domain, a component of the Ndc80 complex, in ensuring these processes. Through extensive mutational analysis, we identified a conserved interaction domain composed of two segments in Nuf2's CH domain that form the binding site for Mps1 within the yeast Ndc80 complex. Interestingly, this site also associates with the Dam1 complex, suggesting Mps1 recruitment may be subject to regulation by competitive binding with other factors. Mutants disrupting this "interaction hub" exhibit defects in spindle assembly checkpoint function and severe chromosome segregation errors. Significantly, specifically restoring Mps1-Ndc80 complex association rescues these defects. Our findings shed light on the intricate regulation of Ndc80 complex-dependent functions and highlight the essential role of Mps1 in kinetochore bi-orientation and accurate chromosome segregation.

Microtubule end-on attachment maturation regulates Mps1 association with its kinetochore receptor

Faithful chromosome segregation requires that sister chromatids establish bi-oriented kinetochore-microtubule attachments. The spindle assembly checkpoint (SAC) prevents premature anaphase onset with incomplete attachments. However, how microtubule attachment and checkpoint signaling are coordinated remains unclear. The conserved kinase Mps1 initiates SAC signaling by localizing transiently to kinetochores in prometaphase and is released upon bi-orientation. Using biochemistry, structure predictions, and cellular assays, we shed light on this dynamic behavior in Saccharomyces cerevisiae. A conserved N-terminal segment of Mps1 binds the neck region of Ndc80:Nuf2, the main microtubule receptor of kinetochores. Mutational disruption of this interface, located at the backside of the paired CH domains and opposite the microtubule-binding site, prevents Mps1 localization, eliminates SAC signaling, and impairs growth. The same interface of Ndc80:Nuf2 binds the microtubule-associated Dam1 complex. We demonstrate that the error correction kinase Ipl1/Aurora B controls the competition between Dam1 and Mps1 for the same binding site. Thus, binding of the Dam1 complex to Ndc80:Nuf2 may release Mps1 from the kinetochore to promote anaphase onset.

An interaction hub on Ndc80 complex facilitates dynamic recruitment of Mps1 to yeast kinetochores to promote accurate chromosome segregation

Accurate chromosome segregation relies on kinetochores carrying out multiple functions, including establishing and maintaining microtubule attachments, forming precise bioriented attachments between sister chromatids, and activating the spindle assembly checkpoint. Central to these processes is the highly conserved Ndc80 complex. This kinetochore subcomplex interacts directly with microtubules, but also serves as a critical platform for recruiting kinetochore-associated factors and as a key substrate for error correction kinases. The precise manner in which these kinetochore factors interact, and regulate each other's function, remains unknown - considerably hindering our understanding of how Ndc80 complex-dependent processes function together to orchestrate accurate chromosome segregation. Here, we aimed to uncover the role of Nuf2's CH domain, a component of the Ndc80 complex, in ensuring accurate chromosome segregation. Through extensive mutational analysis, we identified a conserved "interaction hub" comprising two segments in Nuf2's CH domain, forming the binding site for Mps1 within the yeast Ndc80 complex. Intriguingly, the interaction between Mps1 and the Ndc80 complex seems to be subject to regulation by competitive binding with other factors. Mutants disrupting this interaction hub exhibit defects in spindle assembly checkpoint function and severe chromosome segregation errors. Significantly, specifically restoring Mps1-Ndc80 complex association rescues these defects. Our findings shed light on the intricate regulation of Ndc80 complex-dependent functions and highlight the essential role of Mps1 in kinetochore biorientation and accurate chromosome segregation.

Principles and dynamics of spindle assembly checkpoint signalling

The transmission of a complete set of chromosomes to daughter cells during cell division is vital for development and tissue homeostasis. The spindle assembly checkpoint (SAC) ensures correct segregation by informing the cell cycle machinery of potential errors in the interactions of chromosomes with spindle microtubules prior to anaphase. To do so, the SAC monitors microtubule engagement by specialized structures known as kinetochores and integrates local mechanical and chemical cues such that it can signal in a sensitive, responsive and robust manner. In this Review, we discuss how SAC proteins interact to allow production of the mitotic checkpoint complex (MCC) that halts anaphase progression by inhibiting the anaphase-promoting complex/cyclosome (APC/C). We highlight recent advances aimed at understanding the dynamic signalling properties of the SAC and how it interprets various naturally occurring intermediate attachment states. Further, we discuss SAC signalling in the context of the mammalian multisite kinetochore and address the impact of the fibrous corona. We also identify current challenges in understanding how the SAC ensures high-fidelity chromosome segregation.

Nanoscale structural organization and stoichiometry of the budding yeast kinetochore

Proper chromosome segregation is crucial for cell division. In eukaryotes, this is achieved by the kinetochore, an evolutionarily conserved multiprotein complex that physically links the DNA to spindle microtubules and takes an active role in monitoring and correcting erroneous spindle-chromosome attachments. Our mechanistic understanding of these functions and how they ensure an error-free outcome of mitosis is still limited, partly because we lack a complete understanding of the kinetochore structure in the cell. In this study, we use single-molecule localization microscopy to visualize individual kinetochore complexes in situ in budding yeast. For major kinetochore proteins, we measured their abundance and position within the metaphase kinetochore. Based on this comprehensive dataset, we propose a quantitative model of the budding yeast kinetochore. While confirming many aspects of previous reports based on bulk imaging, our results present a unifying nanoscale model of the kinetochore in budding yeast.

ML. The importance of microtubule-dependent tension in accurate chromosome segregation

Accurate chromosome segregation is vital for cell and organismal viability. The mitotic spindle, a bipolar macromolecular machine composed largely of dynamic microtubules, is responsible for chromosome segregation during each cell replication cycle. Prior to anaphase, a bipolar metaphase spindle must be formed in which each pair of chromatids is attached to microtubules from opposite spindle poles. In this bipolar configuration pulling forces from the dynamic microtubules can generate tension across the sister kinetochores. The tension status acts as a signal that can destabilize aberrant kinetochore-microtubule attachments and reinforces correct, bipolar connections. Historically it has been challenging to isolate the specific role of tension in mitotic processes due to the interdependency of attachment and tension status at kinetochores. Recent technical and experimental advances have revealed new insights into how tension functions during mitosis. Here we summarize the evidence that tension serves as a biophysical signal that unifies multiple aspects of kinetochore and centromere function to ensure accurate chromosome segregation.

BubR1 recruitment to the kinetochore via Bub1 enhances spindle assembly checkpoint signaling

During mitosis, unattached kinetochores in a dividing cell activate the spindle assembly checkpoint (SAC) and delay anaphase onset by generating the anaphase-inhibitory mitotic checkpoint complex (MCC). These kinetochores generate the MCC by recruiting its constituent proteins, including BubR1. In principle, BubR1 recruitment to signaling kinetochores should increase its local concentration and promote MCC formation. However, in human cells BubR1 is mainly thought to sensitize the SAC to silencing. Whether BubR1 localization to signaling kinetochores by itself enhances SAC signaling remains unknown. Therefore, we used ectopic SAC activation (eSAC) systems to isolate two molecules that recruit BubR1 to the kinetochore, the checkpoint protein Bub1 and the KI and MELT motifs in the kinetochore protein KNL1, and observed their contribution to eSAC signaling. Our quantitative analyses and mathematical modeling show that Bub1-mediated BubR1 recruitment to the human kinetochore promotes SAC signaling and highlight BubR1's dual role of strengthening the SAC directly and silencing it indirectly.

Analysis of the potential role of fission yeast PP2A in spindle assembly checkpoint inactivation

As a surveillance mechanism, the activated spindle assembly checkpoint (SAC) potently inhibits the E3 ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) to ensure accurate chromosome segregation. Although the protein phosphatase 2A (PP2A) has been proposed to be both, directly and indirectly, involved in spindle assembly checkpoint inactivation in mammalian cells, whether it is similarly operating in the fission yeast Schizosaccharomycer pombe has never been demonstrated. Here, we investigated whether fission yeast PP2A is involved in SAC silencing by following the rate of cyclin B (Cdc13) destruction at SPBs during the recovery phase in nda3-KM311 cells released from the inhibition of APC/C by the activated spindle checkpoint. The timing of the SAC inactivation is only slightly delayed when two B56 regulatory subunits (Par1 and Par2) of fission yeast PP2A are absent. Overproduction of individual PP2A subunits either globally in the nda3-KM311 arrest-and-release system or locally in the synthetic spindle checkpoint activation system only slightly suppresses the SAC silencing defects in PP1 deletion (dis2Δ) cells. Our study thus demonstrates that the fission yeast PP2A is not a key regulator actively involved in SAC inactivation.

C9ORF78 partially localizes to centromeres and plays a role in chromosome segregation

C9ORF78 is a poorly characterized protein found in diverse eukaryotes. Previous work indicated overexpression of C9ORF78 in malignant tissues indicating a possible involvement in growth regulatory pathways. Additional studies in fission yeast and humans uncover a potential function in regulating the spliceosome. In studies of GFP-tagged C9ORF78 we observed a dramatic reduction in protein abundance in cells grown to confluence and/or deprived of serum growth factors. Serum stimulation induced synchronous re-expression of the protein in HeLa cells. This effect was also observed with the endogenous protein. Overexpressing either E2F1 or N-Myc resulted in elevated C9ORF78 expression potentially explaining the serum-dependent upregulation of the protein. Immunofluorescence analysis indicates that C9ORF78 localizes to nuclei in interphase but does not appear to concentrate in speckles as would be expected for a splicing protein. Surprisingly, a subpopulation of C9ORF78 co-localizes with ACA, Mad1 and Ndc80 in mitotic cells suggesting that this protein associates with kinetochores or centromeres. Levels of C9ORF78 at the centromere/kinetochore also increased upon activation of the mitotic checkpoint. Furthermore, knocking-down C9ORF78 caused mitotic defects. These studies uncover novel mitotic function and subcellular localization of C9ORF78.

Kre28-Spc105 interaction is essential for Spc105 loading at the kinetochore

Kinetochore (KTs) are macromolecular protein assemblies that attach sister chromatids to spindle microtubules (MTs) and mediate accurate chromosome segregation during mitosis. The outer KT consists of the KMN network, a protein super-complex comprising Knl1 (yeast Spc105), Mis12 (yeast Mtw1), and Ndc80 (yeast Ndc80), which harbours sites for MT binding. Within the KMN network, Spc105 acts as an interaction hub of components involved in spindle assembly checkpoint (SAC) signalling. It is known that Spc105 forms a complex with KT component Kre28. However, where Kre28 physically localizes in the budding yeast KT is not clear. The exact function of Kre28 at the KT is also unknown. Here, we investigate how Spc105 and Kre28 interact and how they are organized within bioriented yeast KTs using genetics and cell biological experiments. Our microscopy data show that Spc105 and Kre28 localize at the KT with a 1 : 1 stoichiometry. We also show that the Kre28-Spc105 interaction is important for Spc105 protein turn-over and essential for their mutual recruitment at the KTs. We created several truncation mutants of kre28 that affect Spc105 loading at the KTs. When over-expressed, these mutants sustain the cell viability, but SAC signalling and KT biorientation are impaired. Therefore, we conclude that Kre28 contributes to chromosome biorientation and high-fidelity segregation at least indirectly by regulating Spc105 localization at the KTs.

Aurora B phosphorylates Bub1 to promote spindle assembly checkpoint signaling

Accurate chromosome segregation during cell division requires amphitelic chromosome attachment to the spindle apparatus. It is ensured by the combined activity of the spindle assembly checkpoint (SAC),¹ a signaling mechanism that delays anaphase onset in response to unattached chromosomes, and an error correction mechanism that eliminates syntelic attachments.² The SAC becomes active when Mps1 kinase sequentially phosphorylates the kinetochore protein Spc105/KNL1 and the signaling proteins that Spc105/KNL1 recruits to facilitate the production of the mitotic checkpoint complex (MCC).^3-8 The error correction mechanism is regulated by the Aurora B kinase, but Aurora B also promotes SAC signaling via indirect mechanisms.^9-12 Here we present evidence that Aurora B kinase activity directly promotes MCC production by working downstream of Mps1 in budding yeast and human cells. Using the ectopic SAC activation (eSAC) system, we find that the conditional dimerization of Aurora B in budding yeast and an Aurora B recruitment domain in HeLa cells with either Bub1 or Mad1, but not the phosphodomain of Spc105/KNL1, leads to ectopic MCC production and mitotic arrest.^13-16 Importantly, Bub1 must recruit both Mad1 and Cdc20 for this ectopic signaling activity. These and other data show that Aurora B cooperates with Bub1 to promote MCC production, but only after Mps1 licenses Bub1 recruitment to the kinetochore. This direct involvement of Aurora B in SAC signaling may maintain SAC signaling even after Mps1 activity in the kinetochore is lowered.

Remodeling-defective GPI-anchored proteins on the plasma membrane activate the spindle assembly checkpoint

Newly synthesized glycosylphosphatidylinositol-anchored proteins (GPI-APs) undergo extensive remodeling prior to transport to the plasma membrane. GPI-AP remodeling events serve as quality assurance signatures, and complete remodeling of the anchor functions as a transport warrant. Using a genetic approach in yeast cells, we establish that one remodeling event, the removal of ethanolamine-phosphate from mannose 2 via Ted1p (yPGAP5), is essential for cell viability in the absence of the Golgi-localized putative phosphodiesterase Dcr2p. While GPI-APs in which mannose 2 has not been remodeled in dcr2 ted1-deficient cells can still be delivered to the plasma membrane, their presence elicits a unique stress response. Stress is sensed by Mid2p, a constituent of the cell wall integrity pathway, whereupon signal promulgation culminates in activation of the spindle assembly checkpoint. Our results are consistent with a model in which cellular stress response and chromosome segregation checkpoint pathways are functionally interconnected.

Spindle assembly checkpoint activation and silencing at kinetochores

The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.

Mitotic poleward flux: Finding balance between microtubule dynamics and sliding

Continuous poleward motion of microtubules in metazoan mitotic spindles has been fascinating generations of cell biologists over the last several decades. In human cells, this so-called poleward flux was recently shown to be driven by the coordinated action of four mitotic kinesins. The sliding activities of kinesin-5/EG5 and kinesin-12/KIF15 are sequentially supported by kinesin-7/CENP-E at kinetochores and kinesin-4/KIF4A on chromosome arms, with the individual contributions peaking during prometaphase and metaphase, respectively. Although recent data elucidate the molecular mechanism underlying this cellular phenomenon, the functional roles of microtubule poleward flux during cell division remain largely elusive. Here, we discuss potential contribution of microtubule flux engine to various essential processes at different stages of mitosis.

Chemical tools for dissecting cell division

Components of the cell division machinery typically function at varying cell cycle stages and intracellular locations. To dissect cellular mechanisms during the rapid division process, small-molecule probes act as complementary approaches to genetic manipulations, with advantages of temporal and in some cases spatial control and applicability to multiple model systems. This Review focuses on recent advances in chemical probes and applications to address select questions in cell division. We discuss uses of both enzyme inhibitors and chemical inducers of dimerization, as well as emerging techniques to promote future investigations. Overall, these concepts may open new research directions for applying chemical probes to advance cell biology.

Cell-cycle phospho-regulation of the kinetochore

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

Deprotection of centromeric cohesin at meiosis II requires APC/C activity but not kinetochore tension

Genome haploidization involves sequential loss of cohesin from chromosome arms and centromeres during two meiotic divisions. At centromeres, cohesin's Rec8 subunit is protected from separase cleavage at meiosis I and then deprotected to allow its cleavage at meiosis II. Protection of centromeric cohesin by shugoshin-PP2A seems evolutionarily conserved. However, deprotection has been proposed to rely on spindle forces separating the Rec8 protector from cohesin at metaphase II in mammalian oocytes and on APC/C-dependent destruction of the protector at anaphase II in yeast. Here, we have activated APC/C in the absence of sister kinetochore biorientation at meiosis II in yeast and mouse oocytes, and find that bipolar spindle forces are dispensable for sister centromere separation in both systems. Furthermore, we show that at least in yeast, protection of Rec8 by shugoshin and inhibition of separase by securin are both required for the stability of centromeric cohesin at metaphase II. Our data imply that related mechanisms preserve the integrity of dyad chromosomes during the short metaphase II of yeast and the prolonged metaphase II arrest of mammalian oocytes.

Mps1 promotes poleward chromosome movements in meiotic prometaphase

In prophase of meiosis I, homologous chromosomes pair and become connected by cross-overs. Chiasmata, the connections formed by cross-overs, enable the chromosome pair, called a bivalent, to attach as a single unit to the spindle. When the meiotic spindle forms in prometaphase, most bivalents are associated with one spindle pole and then go through a series of oscillations on the spindle, attaching to and detaching from microtubules until the partners of the bivalent become bioriented—attached to microtubules from opposite sides of the spindle. The conserved kinase, Mps1, is essential for the bivalents to be pulled by microtubules across the spindle in prometaphase. Here we show that MPS1 is needed for efficient triggering of the migration of microtubule-attached kinetochores toward the poles and promotes microtubule depolymerization. Our data support the model Mps1 acts at the kinetochore to coordinate the successful attachment of a microtubule and the triggering of microtubule depolymerization to then move the chromosome.

Meiotic regulation of the Ndc80 complex composition and function

This review describes the current models for how the subunit abundance of the Ndc80 complex, a key kinetochore component, is regulated in budding yeast and metazoan meiosis. The past decades of kinetochore research have established the Ndc80 complex to be a key microtubule interactor and a central hub for regulating chromosome segregation. Recent studies further demonstrate that Ndc80 is the limiting kinetochore subunit that dictates the timing of kinetochore activation in budding yeast meiosis. Here, we discuss the molecular circuits that regulate Ndc80 protein synthesis and degradation in budding yeast meiosis and compare the findings with those from metazoans. We envision the regulatory principles discovered in budding yeast to be conserved in metazoans, thereby providing guidance into future investigations on kinetochore regulation in human health and disease.

Structural basis of Stu2 recruitment to yeast kinetochores

Chromosome segregation during cell division requires engagement of kinetochores of sister chromatids with microtubules emanating from opposite poles. As the corresponding microtubules shorten, these 'bioriented' sister kinetochores experience tension-dependent stabilization of microtubule attachments. The yeast XMAP215 family member and microtubule polymerase, Stu2, associates with kinetochores and contributes to tension-dependent stabilization in vitro. We show here that a C-terminal segment of Stu2 binds the four-way junction of the Ndc80 complex (Ndc80c) and that residues conserved both in yeast Stu2 orthologs and in their metazoan counterparts make specific contacts with Ndc80 and Spc24. Mutations that perturb this interaction prevent association of Stu2 with kinetochores, impair cell viability, produce biorientation defects, and delay cell cycle progression. Ectopic tethering of the mutant Stu2 species to the Ndc80c junction restores wild-type function in vivo. These findings show that the role of Stu2 in tension-sensing depends on its association with kinetochores by binding with Ndc80c.

Kinetochore phosphatases suppress autonomous Polo-like kinase 1 activity to control the mitotic checkpoint

Local phosphatase regulation is needed at kinetochores to silence the mitotic checkpoint (a.k.a. spindle assembly checkpoint [SAC]). A key event in this regard is the dephosphorylation of MELT repeats on KNL1, which removes SAC proteins from the kinetochore, including the BUB complex. We show here that PP1 and PP2A-B56 phosphatases are primarily required to remove Polo-like kinase 1 (PLK1) from the BUB complex, which can otherwise maintain MELT phosphorylation in an autocatalytic manner. This appears to be their principal role in the SAC because both phosphatases become redundant if PLK1 is inhibited or BUB-PLK1 interaction is prevented. Surprisingly, MELT dephosphorylation can occur normally under these conditions even when the levels or activities of PP1 and PP2A are strongly inhibited at kinetochores. Therefore, these data imply that kinetochore phosphatase regulation is critical for the SAC, but primarily to restrain and extinguish autonomous PLK1 activity. This is likely a conserved feature of the metazoan SAC, since the relevant PLK1 and PP2A-B56 binding motifs have coevolved in the same region on MADBUB homologues.

Microtubule Attachment and Centromeric Tension Shape the Protein Architecture of the Human Kinetochore

The nanoscale protein architecture of the kinetochore plays an integral role in specifying the mechanisms underlying its functions in chromosome segregation. However, defining this architecture in human cells remains challenging because of the large size and compositional complexity of the kinetochore. Here, we use Förster resonance energy transfer to reveal the architecture of individual kinetochore-microtubule attachments in human cells. We find that the microtubule-binding domains of the Ndc80 complex cluster at the microtubule plus end. This clustering occurs only after microtubule attachment, and it increases proportionally with centromeric tension. Surprisingly, Ndc80 complex clustering is independent of the organization and number of its centromeric receptors. Moreover, this clustering is similar in yeast and human kinetochores despite significant differences in their centromeric organizations. These and other data suggest that the microtubule-binding interface of the human kinetochore behaves like a flexible "lawn" despite being nucleated by repeating biochemical subunits.

Structural view of the yeast Dam1 complex, a ring-shaped molecular coupler for the dynamic microtubule end

In a dividing eukaryotic cell, proper chromosome segregation requires the dynamic yet persistent attachment of kinetochores to spindle microtubules. In the budding yeast Saccharomyces cerevisiae, this function is especially crucial because each kinetochore is attached to a single microtubule; consequently, loss of attachment could lead to unrecoverable chromosome loss. The highly specialized heterodecameric Dam1 protein complex achieves this coupling by assembling into a microtubule-encircling ring that glides near the end of the dynamic microtubule to mediate chromosome motion. In recent years, we have learned a great deal about the structural properties of the Dam1 heterodecamer, its mechanism of self-assembly into rings, and its tethering to the kinetochore by the elongated Ndc80 complex. The most remarkable progress has resulted from defining the fine structures of helical bundles within Dam1 heterodecamer. In this review, we critically analyze structural observations collected by diverse approaches with the goal of obtaining a unified view of Dam1 ring architecture. A considerable consistency between different studies supports a coherent model of the circular core of the Dam1 ring. However, there are persistent uncertainties about the composition of ring protrusions and flexible extensions, as well as their roles in mediating ring core assembly and interactions with the Ndc80 complex and microtubule.

Polo kinase recruitment via the constitutive centromere-associated network at the kinetochore elevates centromeric RNA

The kinetochore, a multi-protein complex assembled on centromeres, is essential to segregate chromosomes during cell division. Deficiencies in kinetochore function can lead to chromosomal instability and aneuploidy-a hallmark of cancer cells. Kinetochore function is controlled by recruitment of regulatory proteins, many of which have been documented, however their function often remains uncharacterized and many are yet to be identified. To identify candidates of kinetochore regulation we used a proteome-wide protein association strategy in budding yeast and detected many proteins that are involved in post-translational modifications such as kinases, phosphatases and histone modifiers. We focused on the Polo-like kinase, Cdc5, and interrogated which cellular components were sensitive to constitutive Cdc5 localization. The kinetochore is particularly sensitive to constitutive Cdc5 kinase activity. Targeting Cdc5 to different kinetochore subcomplexes produced diverse phenotypes, consistent with multiple distinct functions at the kinetochore. We show that targeting Cdc5 to the inner kinetochore, the constitutive centromere-associated network (CCAN), increases the levels of centromeric RNA via an SPT4 dependent mechanism.

Chronic cadmium exposure causes oocyte meiotic arrest by disrupting spindle assembly checkpoint and maturation promoting factor

Cadmium (Cd) is a bioaccumulative heavy metal element with potential toxicity on the female reproductive system, but the exact molecular mechanisms have not yet been clearly defined. In this study, female mice were exposed to 0.5 mg/kg/day of CdCl₂ for 60 consecutive days. We found that chronic Cd exposure significantly decreased the fecundity of female mice by affecting oocyte meiotic progression as indicated by disrupted spindle assembly, chromosome alignment and kinetochore-microtubule attachments, consequently resulting in aneuploid oocytes. Further studies showed that the periodic fluctuations of MPF activity and cyclin B1 expression were disturbed in Cd-exposed oocytes probably by affecting the spindle assembly checkpoint protein Bub3. In addition, Cd exposure induced oxidative stress as indicated by an increased level of reactive oxygen species and apoptosis in oocytes, leading to oocyte quality deterioration. Taken together, these data suggest that Cd exposure causes disrupted molecular events of meiotic progression and deterioration of oocyte quality via oxidative stress, leading to decrease of female fertility.

The copy-number and varied strengths of MELT motifs in Spc105 balance the strength and responsiveness of the spindle assembly checkpoint

During mitosis, the Spindle Assembly Checkpoint (SAC) maintains genome stability while also ensuring timely anaphase onset. To maintain genome stability, the SAC must be strong to delay anaphase even if just one chromosome is unattached, but for timely anaphase onset, it must promptly respond to silencing mechanisms. How the SAC meets these potentially antagonistic requirements is unclear. Here we show that the balance between SAC strength and responsiveness is determined by the number of ‘MELT’ motifs in the kinetochore protein Spc105/KNL1 and their Bub3-Bub1 binding affinities. Many strong MELT motifs per Spc105/KNL1 minimize chromosome missegregation, but too many delay anaphase onset. We demonstrate this by constructing a Spc105 variant that trades SAC responsiveness for much more accurate chromosome segregation. We propose that the necessity of balancing SAC strength and responsiveness drives the dual evolutionary trend of the amplification of MELT motif number, but degeneration of their functionally optimal amino acid sequence.

Yeast Sphingolipid Phospholipase Gene ISC1 Regulates the Spindle Checkpoint by a CDC55-Dependent Mechanism

Defects in the spindle assembly checkpoint (SAC) can lead to aneuploidy and cancer. Sphingolipids have important roles in many cellular functions, including cell cycle regulation and apoptosis. However, the specific mechanisms and functions of sphingolipids in cell cycle regulation have not been elucidated. Using analysis of concordance for synthetic lethality for the yeast sphingolipid phospholipase ISC1, we identified two groups of genes. The first comprises genes involved in chromosome segregation and stability (CSM3, CTF4, YKE2, DCC1, and GIM4) as synthetically lethal with ISC1 The second group, to which ISC1 belongs, comprises genes involved in the spindle checkpoint (BUB1, MAD1, BIM1, and KAR3), and they all share the same synthetic lethality with the first group. We demonstrate that spindle checkpoint genes act upstream of Isc1, and their deletion phenocopies that of ISC1 Reciprocally, ISC1 deletion mutants were sensitive to benomyl, indicating a SAC defect. Similar to BUB1 deletion, ISC1 deletion prevents spindle elongation in hydroxyurea-treated cells. Mechanistically, PP2A-Cdc55 ceramide-activated phosphatase was found to act downstream of Isc1, thus coupling the spindle checkpoint genes and Isc1 to CDC55-mediated nuclear functions.

Ensemble-Level Organization of Human Kinetochores and Evidence for Distinct Tension and Attachment Sensors

Kinetochores are multi-protein machines that form dynamic attachments to microtubules and control chromosome segregation. High fidelity is ensured because kinetochores can monitor attachment status and tension, using this information to activate checkpoints and error-correction mechanisms. To explore how kinetochores achieve this, we used two- and three-color subpixel fluorescence localization to define how proteins from six major complexes (CCAN, MIS12, NDC80, KNL1, RZZ, and SKA) and the checkpoint proteins Bub1, Mad1, and Mad2 are organized in the human kinetochore. This reveals how the outer kinetochore has a high nematic order and is largely invariant to the loss of attachment or tension, except for two mechanical sensors. First, Knl1 unravels to relay tension, and second, NDC80 undergoes jackknifing and loss of nematic order under microtubule detachment, with only the latter wired up to the checkpoint signaling system. This provides insight into how kinetochores integrate mechanical signals to promote error-free chromosome segregation.

A common molecular mechanism underlies the role of Mps1 in chromosome biorientation and the spindle assembly checkpoint

The Mps1 kinase corrects improper kinetochore-microtubule attachments, thereby ensuring chromosome biorientation. Yet, its critical phosphorylation targets in this process remain largely elusive. Mps1 also controls the spindle assembly checkpoint (SAC), which halts chromosome segregation until biorientation is attained. Its role in SAC activation is antagonised by the PP1 phosphatase and involves phosphorylation of the kinetochore scaffold Knl1/Spc105, which in turn recruits the Bub1 kinase to promote assembly of SAC effector complexes. A crucial question is whether error correction and SAC activation are part of a single or separable pathways. Here, we isolate and characterise a new yeast mutant, mps1-3, that is severely defective in chromosome biorientation and SAC signalling. Through an unbiased screen for extragenic suppressors, we found that mutations lowering PP1 levels at Spc105 or forced association of Bub1 with Spc105 reinstate both chromosome biorientation and SAC signalling in mps1-3 cells. Our data argue that a common mechanism based on Knl1/Spc105 phosphorylation is critical for Mps1 function in error correction and SAC signalling, thus supporting the idea that a single sensory apparatus simultaneously elicits both pathways.

Interactions between N-terminal Modules in MPS1 Enable Spindle Checkpoint Silencing

Faithful chromosome segregation relies on the ability of the spindle assembly checkpoint (SAC) to delay anaphase onset until chromosomes are attached to the mitotic spindle via their kinetochores. MPS1 kinase is recruited to kinetochores to initiate SAC signaling and is removed from kinetochores once stable microtubule attachments have been formed to allow normal mitotic progression. Here, we show that a helical fragment within the kinetochore-targeting N-terminal extension (NTE) module of MPS1 is required for interactions with kinetochores and forms intramolecular interactions with its adjacent tetratricopeptide repeat (TPR) domain. Bypassing this NTE-TPR interaction results in high MPS1 levels at kinetochores due to loss of regulatory input into MPS1 localization, inefficient MPS1 delocalization upon microtubule attachment, and SAC silencing defects. These results show that SAC responsiveness to attachments relies on regulated intramolecular interactions in MPS1 and highlight the sensitivity of mitosis to perturbations in the dynamics of the MPS1-NDC80-C interactions.

Deregulation of Chromosome Segregation and Cancer

The mitotic spindle assembly checkpoint (SAC) is an intricate cell signaling system that ensures the high fidelity and timely segregation of chromosomes during cell division. Mistakes in this process can lead to the loss, gain, or rearrangement of the genetic material. Gross chromosomal aberrations are usually lethal but can cause birth and development defects as well as cancer. Despite advances in the identification of SAC protein components, important details of the interactions underpinning chromosome segregation regulation remain to be established. This review discusses the current understanding of the function, structure, mode of regulation, and dynamics of the assembly and disassembly of SAC subcomplexes, which ultimately safeguard the accurate transmission of a stable genome to descendants. We also discuss how diverse oncoviruses take control of human cell division by exploiting the SAC and the potential of this signaling circuitry as a pool of drug targets to develop effective cancer therapies.

A PP2A-B56-Centered View on Metaphase-to-Anaphase Transition in Mouse Oocyte Meiosis I

Meiosis is required to reduce to haploid the diploid genome content of a cell, generating gametes-oocytes and sperm-with the correct number of chromosomes. To achieve this goal, two specialized cell divisions without intermediate S-phase are executed in a time-controlled manner. In mammalian female meiosis, these divisions are error-prone. Human oocytes have an exceptionally high error rate that further increases with age, with significant consequences for human fertility. To understand why errors in chromosome segregation occur at such high rates in oocytes, it is essential to understand the molecular players at work controlling these divisions. In this review, we look at the interplay of kinase and phosphatase activities at the transition from metaphase-to-anaphase for correct segregation of chromosomes. We focus on the activity of PP2A-B56, a key phosphatase for anaphase onset in both mitosis and meiosis. We start by introducing multiple roles PP2A-B56 occupies for progression through mitosis, before laying out whether or not the same principles may apply to the first meiotic division in oocytes, and describing the known meiosis-specific roles of PP2A-B56 and discrepancies with mitotic cell cycle regulation.

Recent advances in understanding the role of Cdk1 in the Spindle Assembly Checkpoint

The goal of mitosis is to form two daughter cells each containing one copy of each mother cell chromosome, replicated in the previous S phase. To achieve this, sister chromatids held together back-to-back at their primary constriction, the centromere, have to interact with microtubules of the mitotic spindle so that each chromatid takes connections with microtubules emanating from opposite spindle poles (we will refer to this condition as bipolar attachment). Only once all replicated chromosomes have reached bipolar attachments can sister chromatids lose cohesion with each other, at the onset of anaphase, and move toward opposite spindle poles, being segregated into what will soon become the daughter cell nucleus. Prevention of errors in chromosome segregation is granted by a safeguard mechanism called Spindle Assembly Checkpoint (SAC). Until all chromosomes are bipolarly oriented at the equator of the mitotic spindle, the SAC prevents loss of sister chromatid cohesion, thus anaphase onset, and maintains the mitotic state by inhibiting inactivation of the major M phase promoting kinase, the cyclin B-cdk1 complex (Cdk1). Here, we review recent mechanistic insights about the circuitry that links Cdk1 to the SAC to ensure correct achievement of the goal of mitosis.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

Accurate chromosome segregation during cell division is essential to maintain genome integrity in all eukaryotic cells, and chromosome missegregation leads to aneuploidy and therefore represents a hallmark of many cancers. Accurate segregation requires sister kinetochores to attach to microtubules emanating from opposite spindle poles, known as bipolar attachment or biorientation. Recent studies have uncovered several mechanisms critical to chromosome bipolar attachment. First, a mechanism exists to ensure that the conformation of sister centromeres is biased toward bipolar attachment. Second, the phosphorylation of some kinetochore proteins destabilizes kinetochore attachment to facilitate error correction, but a protein phosphatase reverses this phosphorylation. Moreover, the activity of the spindle assembly checkpoint is regulated by kinases and phosphatases at the kinetochore, and this checkpoint prevents anaphase entry in response to faulty kinetochore attachment. The fine-tuned kinase/phosphatase balance at kinetochores is crucial for faithful chromosome segregation during both mitosis and meiosis. Here, we discuss the function and regulation of protein phosphatases in the establishment of chromosome bipolar attachment with a focus on the model organism budding yeast.

Delineating the contribution of Spc105-bound PP1 to spindle checkpoint silencing and kinetochore microtubule attachment regulation

Roy et al. highlight a harmful cross-talk that can arise between spindle assembly checkpoint silencing and chromosome biorientation due to the involvement of protein phosphatase 1 in both the processes.

Accurate chromosome segregation during cell division requires the spindle assembly checkpoint (SAC), which detects unattached kinetochores, and an error correction mechanism that destabilizes incorrect kinetochore–microtubule attachments. While the SAC and error correction are both regulated by protein phosphatase 1 (PP1), which silences the SAC and stabilizes kinetochore–microtubule attachments, how these distinct PP1 functions are coordinated remains unclear. Here, we investigate the contribution of PP1, docked on its conserved kinetochore receptor Spc105/Knl1, to SAC silencing and attachment regulation. We find that Spc105-bound PP1 is critical for SAC silencing but dispensable for error correction; in fact, reduced PP1 docking on Spc105 improved chromosome segregation and viability of mutant/stressed states. We additionally show that artificially recruiting PP1 to Spc105/Knl1 before, but not after, chromosome biorientation interfered with error correction. These observations lead us to propose that recruitment of PP1 to Spc105/Knl1 is carefully regulated to ensure that chromosome biorientation precedes SAC silencing, thereby ensuring accurate chromosome segregation.

Mammalian kinetochores count attached microtubules in a sensitive and switch-like manner

Kinetochores monitor their attachment to spindle microtubules to control spindle assembly checkpoint (SAC) signaling and cell cycle progression. Kuhn and Dumont show that individual mammalian kinetochores monitor the number of attached microtubules as a single unit in a sensitive and switch-like manner.

The spindle assembly checkpoint (SAC) prevents anaphase until all kinetochores attach to the spindle. Each mammalian kinetochore binds many microtubules, but how many attached microtubules are required to turn off the checkpoint, and how the kinetochore monitors microtubule numbers, are not known and are central to understanding SAC mechanisms and function. To address these questions, here we systematically tune and fix the fraction of Hec1 molecules capable of microtubule binding. We show that Hec1 molecules independently bind microtubules within single kinetochores, but that the kinetochore does not independently process attachment information from different molecules. Few attached microtubules (20% occupancy) can trigger complete Mad1 loss, and Mad1 loss is slower in this case. Finally, we show using laser ablation that individual kinetochores detect changes in microtubule binding, not in spindle forces that accompany attachment. Thus, the mammalian kinetochore responds specifically to the binding of each microtubule and counts microtubules as a single unit in a sensitive and switch-like manner. This may allow kinetochores to rapidly react to early attachments and maintain a robust SAC response despite dynamic microtubule numbers.

Autophosphorylation is sufficient to release Mps1 kinase from native kinetochores

Accurate mitosis depends on a surveillance system called the spindle assembly checkpoint. This checkpoint acts at kinetochores, which attach chromosomes to the dynamic tips of spindle microtubules. When a kinetochore is unattached or improperly attached, the protein kinase Mps1 phosphorylates kinetochore components, catalyzing the generation of a diffusible "wait" signal that delays anaphase and gives the cell time to correct the error. When a kinetochore becomes properly attached, its checkpoint signal is silenced to allow progression into anaphase. Recently, microtubules were found to compete directly against recombinant human Mps1 fragments for binding to the major microtubule-binding kinetochore element Ndc80c, suggesting a direct competition model for silencing the checkpoint signal at properly attached kinetochores. Here, by developing single-particle fluorescence-based assays, we tested whether such direct competition occurs in the context of native kinetochores isolated from yeast. Mps1 levels were not reduced on kinetochore particles bound laterally to the sides of microtubules or on particles tracking processively with disassembling tips. Instead, we found that Mps1 kinase activity was sufficient to promote its release from the isolated kinetochores. Mps1 autophosphorylation, rather than phosphorylation of other kinetochore components, was responsible for this dissociation. Our findings suggest that checkpoint silencing in yeast does not arise from a direct competition between Mps1 and microtubules, and that phosphoregulation of Mps1 may be a critical aspect of the silencing mechanism.

The Bub1-TPR Domain Interacts Directly with Mad3 to Generate Robust Spindle Checkpoint Arrest

The spindle checkpoint monitors kinetochore-microtubule interactions and generates a “wait anaphase” delay when any defects are apparent [1, 2, 3]. This provides time for cells to correct chromosome attachment errors and ensure high-fidelity chromosome segregation. Checkpoint signals are generated at unattached chromosomes during mitosis. To activate the checkpoint, Mps1^Mph1 kinase phosphorylates the kinetochore component KNL1^Spc105/Spc7 on conserved MELT motifs to recruit Bub3-Bub1 complexes [4, 5, 6] via a direct Bub3 interaction with phospho-MELT motifs [7, 8]. Mps1^Mph1 then phosphorylates Bub1, which strengthens its interaction with Mad1-Mad2 complexes to produce a signaling platform [9, 10]. The Bub1-Mad1 platform is thought to recruit Mad3, Cdc20, and Mad2 to produce the mitotic checkpoint complex (MCC), which is the diffusible wait anaphase signal [9, 11, 12]. The MCC binds and inhibits the mitotic E3 ubiquitin ligase, known as Cdc20-anaphase promoting complex/cyclosome (APC/C), and stabilizes securin and cyclin to delay anaphase onset [13, 14, 15, 16, 17]. Here we demonstrate, in both budding and fission yeast, that kinetochores and KNL1^Spc105/Spc7 can be bypassed; simply inducing heterodimers of Mps1^Mph1 kinase and Bub1 is sufficient to trigger metaphase arrest that is dependent on Mad1, Mad2, and Mad3. We use this to dissect the domains of Bub1 necessary for arrest, highlighting the need for Bub1-CD1, which binds Mad1 [9], and Bub1’s highly conserved N-terminal tetratricopeptide repeat (TPR) domain [18, 19]. We demonstrate that the Bub1 TPR domain is both necessary and sufficient to bind and recruit Mad3. We propose that this brings Mad3 into close proximity to Mad1-Mad2 and Mps1^Mph1 kinase, enabling efficient generation of MCC complexes.

•

Heterodimers of Mps1 and Bub1 generate robust spindle checkpoint arrest in yeasts

•

This arrest is independent of kinetochores but requires Bub1-CD1 and the Bub1-TPR

•

The Bub1-TPR is both necessary and sufficient for Mad3 interaction and recruitment

•

Recombinant fission yeast Bub1-TPR and Mad3 form a stable complex

Protein Complexes in the Nucleus: The Control of Chromosome Segregation

Mistakes in the process of cell division can lead to the loss, gain or rearrangement of chromosomes. Significant chromosomal abnormalities are usually lethal to the cells and cause spontaneous miscarriages. However, in some cases, defects in the spindle assembly checkpoint lead to severe diseases, such as cancer and birth and development defects, including Down's syndrome. The timely and accurate control of chromosome segregation in mitosis relies on the spindle assembly checkpoint (SAC), an evolutionary conserved, self-regulated signalling system present in higher organisms. The spindle assembly checkpoint is orchestrated by dynamic interactions between spindle microtubules and the kinetochore , a multiprotein complex that constitutes the site for attachment of chromosomes to microtubule polymers to pull sister chromatids apart during cell division. This chapter discusses the current molecular understanding of the essential, highly dynamic molecular interactions underpinning spindle assembly checkpoint signalling and how the complex choreography of interactions can be coordinated in time and space to finely regulate the process. The potential of targeting this signalling pathway to interfere with the abnormal segregation of chromosomes, which occurs in diverse malignancies and the new opportunities that recent technological developments are opening up for a deeper understanding of the spindle assembly checkpoint are also discussed.

The mammalian kinetochore-microtubule interface: robust mechanics and computation with many microtubules

The kinetochore drives chromosome segregation at cell division. It acts as a physical link between chromosomes and dynamic microtubules, and as a signaling hub detecting and processing microtubule attachments to control anaphase onset. The mammalian kinetochore is a large macromolecular machine that forms a dynamic interface with the many microtubules that it binds. While we know most of the kinetochore's component parts, how they work together to give rise to its robust functions remains poorly understood. Here we highlight recent findings that shed light on this question, driven by an expanding physical and molecular toolkit. We present emerging principles that underlie the kinetochore's robust microtubule grip, such as redundancy, specialization, and dynamicity, and present signal processing principles that connect this microtubule grip to robust computation. Throughout, we identify open questions, and define simple engineering concepts that provide insight into kinetochore function.

A Kinase-Phosphatase Network that Regulates Kinetochore-Microtubule Attachments and the SAC

The KMN network (for KNL1, MIS12 and NDC80 complexes) is a hub for signalling at the outer kinetochore. It integrates the activities of two kinases (MPS1 and Aurora B) and two phosphatases (PP1 and PP2A-B56) to regulate kinetochore-microtubule attachments and the spindle assembly checkpoint (SAC). We will first discuss each of these enzymes separately, to describe how they are regulated at kinetochores and why this is important for their primary function in controlling either microtubule attachments or the SAC. We will then discuss why inhibiting any one of them individually produces secondary effects on all the others. This cross-talk may help to explain why all enzymes have been linked to both processes, even though the direct evidence suggests they each control only one. This chapter therefore describes how a network of kinases and phosphatases work together to regulate two key mitotic processes.

The Power of Xenopus Egg Extract for Reconstitution of Centromere and Kinetochore Function

Faithful transmission of genetic information during cell division requires attachment of chromosomes to the mitotic spindle via the kinetochore. In vitro reconstitution studies are beginning to uncover how the kinetochore is assembled upon the underlying centromere, how the kinetochore couples chromosome movement to microtubule dynamics, and how cells ensure the site of kinetochore assembly is maintained from one generation to the next. Here we give special emphasis to advances made in Xenopus egg extract, which provides a unique, biochemically tractable in vitro system that affords the complexity of cytoplasm and nucleoplasm to permit reconstitution of the dynamic, cell cycle-regulated functions of the centromere and kinetochore.

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.

Leader of the SAC: molecular mechanisms of Mps1/TTK regulation in mitosis

Discovered in 1991 in a screen for genes involved in spindle pole body duplication, the monopolar spindle 1 (Mps1) kinase has since claimed a central role in processes that ensure error-free chromosome segregation. As a result, Mps1 kinase activity has become an attractive candidate for pharmaceutical companies in the search for compounds that target essential cellular processes to eliminate, for example, tumour cells or pathogens. Research in recent decades has offered many insights into the molecular function of Mps1 and its regulation. In this review, we integrate the latest knowledge regarding the regulation of Mps1 activity and its spatio-temporal distribution, highlight gaps in our understanding of these processes and propose future research avenues to address them.

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.

Intersectin-Cdc42 interaction is required for orderly meiosis in porcine oocytes

Intersectins (ITSNs) have been shown to act as adaptor proteins that govern multiple cellular events via regulating Cdc42 activity. However, it remains to be determined whether the ITSN-Cdc42 pathway is functional in porcine oocytes. To address this question, we used a small molecule, ZCL278, to selectively disrupt the ITSN2-Cdc42 interaction. In the present study, we find that porcine oocytes exposed to ZCL278 are unable to completely progress through meiosis. Meanwhile, the spindle defects and chromosomal congression failure are frequently detected in these oocytes. In support of this, we observed the accumulated distribution of vesicle-like ITSN2 signals around the chromosome/spindle region during porcine oocyte maturation. In addition, our results also showed that inhibition of the ITSN-Cdc42 interaction impairs the actin polymerization in porcine oocytes. In summary, the findings support a model where ITSNs, through the interaction with Cdc42, modulates the assembly of meiotic apparatus and actin polymerization, consequently ensuring the orderly meiotic progression during porcine oocyte maturation.

Emerging Contributions of Cancer/Testis Antigens to Neoplastic Behaviors

Tumors of nearly every origin activate the expression of genes normally restricted to gametogenic cells. These genes encode proteins termed cancer/testis (CT) antigens, since expression outside of their naturally immune-privileged site can evoke an immune response. Despite extensive efforts to exploit CT antigens as immunotherapeutic targets, investigation of whether these proteins participate in tumorigenic processes has lagged. Here, we discuss emerging evidence that demonstrates that CT antigens can confer a selective advantage to tumor cells by promoting oncogenic processes or permitting evasion of tumor-suppressive mechanisms. These advances indicate the inherent flexibility of tumor cell regulatory networks to engage aberrantly expressed proteins to promote neoplastic behaviors, which could ultimately present novel therapeutic entry points.

A simple and inexpensive quantitative technique for determining chemical sensitivity in Saccharomyces cerevisiae

Chemical sensitivity, growth inhibition in response to a chemical, is a powerful phenotype that can reveal insight into diverse cellular processes. Chemical sensitivity assays are used in nearly every model system, however the yeast Saccharomyces cerevisiae provides a particularly powerful platform for discovery and mechanistic insight from chemical sensitivity assays. Here we describe a simple and inexpensive approach to determine chemical sensitivity quantitatively in yeast in the form of half maximal inhibitory concentration (IC50) using common laboratory equipment. We demonstrate the utility of this method using chemicals commonly used to monitor changes in membrane traffic. When compared to traditional agar-based plating methods, this method is more sensitive and can detect defects not apparent using other protocols. Additionally, this method reduces the experimental protocol from five days to 18 hours for the toxic amino acid canavanine. Furthermore, this method provides reliable results using lower amounts of chemicals. Finally, this method is easily adapted to additional chemicals as demonstrated with an engineered system that activates the spindle assembly checkpoint in response to rapamycin with differing efficiencies. This approach provides researchers with a cost-effective method to perform chemical genetic profiling without specialized equipment.

How Kinetochore Architecture Shapes the Mechanisms of Its Function

The eukaryotic kinetochore is a sophisticated multi-protein machine that segregates chromosomes during cell division. To ensure accurate chromosome segregation, it performs three major functions using disparate molecular mechanisms. It operates a mechanosensitive signaling cascade known as the spindle assembly checkpoint (SAC) to detect and signal the lack of attachment to spindle microtubules, and delay anaphase onset in response. In addition, after attaching to spindle microtubules, the kinetochore generates the force necessary to move chromosomes. Finally, if the two sister kinetochores on a chromosome are both attached to microtubules emanating from the same spindle pole, they activate another mechanosensitive mechanism to correct the monopolar attachments. All three of these functions maintain genome stability during cell division. The outlines of the biochemical activities responsible for these functions are now available. How the kinetochore integrates the underlying molecular mechanisms is still being elucidated. In this Review, we discuss how the nanoscale protein organization in the kinetochore, which we refer to as kinetochore 'architecture', organizes its biochemical activities to facilitate the realization and integration of emergent mechanisms underlying its three major functions. For this discussion, we will use the relatively simple budding yeast kinetochore as a model, and extrapolate insights gained from this model to elucidate functional roles of the architecture of the much more complex human kinetochore.

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.