Attachment issues: kinetochore transformations and spindle checkpoint silencing

Stable expression and multi-site location of Odf2 in mouse oocytes, sperm and early embryos

In mammals, centriole is degenerated during early oogenesis, but it is still not known about the expression and function of centriolar structural components in oocyte meiosis. Here we found that Odf2 (outer dense fiber of sperm tails 2), a key centriolar appendage protein, was stably expressed in mouse oocytes during meiotic progression. Distinct from its single location at centrosomes in somatic mitosis, Odf2 was multiply located at microtubule organizing centers (MTOCs), chromosome centromeres and vesicles in oocyte meiosis. In addition, the vesicle-associated Odf2 disappeared in oocytes treated with the vesicle inhibitor Brefeldin A. Odf2 was mainly co-localized with the mitochondrial sheath in the sperm tail and presented as double spots, similar to γ-tubulin, in the sperm neck region. After fertilization, Odf2 remained on vesicles in embryos from 1-cell to 4-cell stage but was only detected on centrosomes at blastocyst stage. Taken together, Odf2 is expressed precisely in mouse oocytes even in the absence of intact centriole structure, and may regulate oocyte spindle assembly and positioning, additionally, the sperm motility and early embryo development.

A Yeast Mitotic Tale for the Nucleus and the Vacuoles to Embrace

The morphology of the nucleus is roughly spherical in most eukaryotic cells. However, this organelle shape needs to change as the cell travels through narrow intercellular spaces during cell migration and during cell division in organisms that undergo closed mitosis, i.e., without dismantling the nuclear envelope, such as yeast. In addition, the nuclear morphology is often modified under stress and in pathological conditions, being a hallmark of cancer and senescent cells. Thus, understanding nuclear morphological dynamics is of uttermost importance, as pathways and proteins involved in nuclear shaping can be targeted in anticancer, antiaging, and antifungal therapies. Here, we review how and why the nuclear shape changes during mitotic blocks in yeast, introducing novel data that associate these changes with both the nucleolus and the vacuole. Altogether, these findings suggest a close relationship between the nucleolar domain of the nucleus and the autophagic organelle, which we also discuss here. Encouragingly, recent evidence in tumor cell lines has linked aberrant nuclear morphology to defects in lysosomal function.

The role of kinetochore dynein in checkpoint silencing is restricted to disassembly of the corona

During mitosis, kinetochore-microtubule attachments are monitored by a molecular surveillance system known as the spindle assembly checkpoint. The prevailing model posits that dynein evicts checkpoint proteins (e.g., Mad1, Mad2) from stably attached kinetochores by transporting them away from kinetochores, thus contributing to checkpoint silencing. However, the mechanism by which dynein performs this function, and its precise role in checkpoint silencing remain unresolved. Here, we find that dynein's role in checkpoint silencing is restricted to evicting checkpoint effectors from the fibrous corona, and not the outer kinetochore. Dynein evicts these molecules from the corona in a manner that does not require stable, end-on microtubule attachments. Thus, by disassembling the corona through indiscriminate microtubule encounters, dynein primes the checkpoint signaling apparatus so it can respond to stable end-on microtubule attachments and permit cells to progress through mitosis. Accordingly, we find that dynein function in checkpoint silencing becomes largely dispensable in cells in which checkpoint effectors are excluded from the corona.

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.

Separase Control and Cohesin Cleavage in Oocytes: Should I Stay or Should I Go?

The key to gametogenesis is the proper execution of a specialized form of cell division named meiosis. Prior to the meiotic divisions, the recombination of maternal and paternal chromosomes creates new genetic combinations necessary for fitness and adaptation to an ever-changing environment. Two rounds of chromosome segregation -meiosis I and II- have to take place without intermediate S-phase and lead to the creation of haploid gametes harboring only half of the genetic material. Importantly, the segregation patterns of the two divisions are fundamentally different and require adaptation of the mitotic cell cycle machinery to the specificities of meiosis. Separase, the enzyme that cleaves Rec8, a subunit of the cohesin complex constituting the physical connection between sister chromatids, has to be activated twice: once in meiosis I and immediately afterwards, in meiosis II. Rec8 is cleaved on chromosome arms in meiosis I and in the centromere region in meiosis II. This step-wise cohesin removal is essential to generate gametes of the correct ploidy and thus, embryo viability. Hence, separase control and Rec8 cleavage must be perfectly controlled in time and space. Focusing on mammalian oocytes, this review lays out what we know and what we still ignore about this fascinating mechanism.

The cell cycle, cancer development and therapy

The process of cell division plays a vital role in cancer progression. Cell proliferation and error-free chromosomes segregation during mitosis are central events in life cycle. Mistakes during cell division generate changes in chromosome content and alter the balances of chromosomes number. Any defects in expression of TIF1 family proteins, SAC proteins network, mitotic checkpoint proteins involved in chromosome mis-segregation and cancer development. Here we discuss the function of organelles deal with the chromosome segregation machinery, proteins and correction mechanisms involved in the accurate chromosome segregation during mitosis.

Aurora B Tension Sensing Mechanisms in the Kinetochore Ensure Accurate Chromosome Segregation

The accurate segregation of chromosomes is essential for the survival of organisms and cells. Mistakes can lead to aneuploidy, tumorigenesis and congenital birth defects. The spindle assembly checkpoint ensures that chromosomes properly align on the spindle, with sister chromatids attached to microtubules from opposite poles. Here, we review how tension is used to identify and selectively destabilize incorrect attachments, and thus serves as a trigger of the spindle assembly checkpoint to ensure fidelity in chromosome segregation. Tension is generated on properly attached chromosomes as sister chromatids are pulled in opposing directions but resisted by centromeric cohesin. We discuss the role of the Aurora B kinase in tension-sensing and explore the current models for translating mechanical force into Aurora B-mediated biochemical signals that regulate correction of chromosome attachments to the spindle.

The right place at the right time: Aurora B kinase localization to centromeres and kinetochores

The fidelity of chromosome segregation during mitosis is intimately linked to the function of kinetochores, which are large protein complexes assembled at sites of centromeric heterochromatin on mitotic chromosomes. These key "orchestrators" of mitosis physically connect chromosomes to spindle microtubules and transduce forces through these connections to congress chromosomes and silence the spindle assembly checkpoint. Kinetochore-microtubule attachments are highly regulated to ensure that incorrect attachments are not prematurely stabilized, but instead released and corrected. The kinase activity of the centromeric protein Aurora B is required for kinetochore-microtubule destabilization during mitosis, but how the kinase acts on outer kinetochore substrates to selectively destabilize immature and erroneous attachments remains debated. Here, we review recent literature that sheds light on how Aurora B kinase is recruited to both centromeres and kinetochores and discuss possible mechanisms for how kinase interactions with substrates at distinct regions of mitotic chromosomes are regulated.

Spindle Architectural Features Must Be Considered Along With Cell Size to Explain the Timing of Mitotic Checkpoint Silencing

Mitosis proceeds through a defined series of events that is largely conserved, but the amount of time needed for their completion can vary in different cells and organisms. In many systems, mitotic duration depends on the time required to satisfy and silence the spindle assembly checkpoint (SAC), also known as the mitotic checkpoint. Because SAC silencing involves trafficking SAC molecules among kinetochores, spindle, and cytoplasm, the size and geometry of the spindle relative to cell volume are expected to affect mitotic duration by influencing the timing of SAC silencing. However, the relationship between SAC silencing, cell size, and spindle dimensions is unclear. To investigate this issue, we used four DLD-1 tetraploid (4N) clones characterized by small or large nuclear and cell size. We found that the small 4N clones had longer mitotic durations than the parental DLD-1 cells and that this delay was due to differences in their metaphase duration. Leveraging a previous mathematical model for spatiotemporal regulation of SAC silencing, we show that the difference in metaphase duration, i.e., SAC silencing time, can be explained by the distinct spindle microtubule densities and sizes of the cell, spindle, and spindle poles in the 4N clones. Lastly, we demonstrate that manipulating spindle geometry can alter mitotic and metaphase duration, consistent with a model prediction. Our results suggest that spindle size does not always scale with cell size in mammalian cells and cell size is not sufficient to explain the differences in metaphase duration. Only when a number of spindle architectural features are considered along with cell size can the kinetics of SAC silencing, and hence mitotic duration, in the different clones be explained.

Functional reciprocity of proteins involved in mitosis and endocytosis

Mitosis and endocytosis are two fundamental cellular processes essential for maintaining a eukaryotic life. Mitosis partitions duplicated chromatin enveloped in the nuclear membrane into two new cells, whereas endocytosis takes in extracellular substances through membrane invagination. These two processes are spatiotemporally separated and seemingly unrelated. However, recent studies have uncovered that endocytic proteins have moonlighting functions in mitosis, and mitotic complexes manifest additional roles in endocytosis. In this review, we summarize important proteins or protein complexes that participate in both processes, compare their mechanism of action, and discuss the rationale behind this multifunctionality. We also speculate on the possible origin of the functional reciprocity from an evolutionary perspective.

Silencing the spindle assembly checkpoint: Let's play Polo!

Silencing of the spindle assembly checkpoint involves two protein phosphatases, PP1 and PP2A-B56, that are thought to extinguish checkpoint signaling through dephosphorylation of a checkpoint scaffold at kinetochores. In this issue, Cordeiro et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.202002020) now show that a critical function of these phosphatases in checkpoint silencing is removal of Polo kinase at kinetochores, which would otherwise autonomously sustain the checkpoint.

BuGZ facilitates loading of spindle assembly checkpoint proteins to kinetochores in early mitosis

BuGZ is a kinetochore component that binds to and stabilizes Bub3, a key player in mitotic spindle assembly checkpoint signaling. Bub3 is required for kinetochore recruitment of Bub1 and BubR1, two proteins that have essential and distinct roles in the checkpoint. Both Bub1 and BubR1 localize to kinetochores through interactions with Bub3, which are mediated through conserved GLEBS domains in both Bub1 and BubR1. BuGZ also has a GLEBS domain, which is required for its kinetochore localization as well, presumably mediated through Bub3 binding. Although much is understood about the requirements for Bub1 and BubR1 interaction with Bub3 and kinetochores, much less is known regarding BuGZ's requirements. Here, we used a series of mutants to demonstrate that BuGZ kinetochore localization requires only its core GLEBS domain, which is distinct from the requirements for both Bub1 and BubR1. Furthermore, we found that the kinetics of Bub1, BubR1, and BuGZ loading to kinetochores differ, with BuGZ localizing prior to BubR1 and Bub1. To better understand how complexes containing Bub3 and its binding partners are loaded to kinetochores, we carried out size-exclusion chromatography and analyzed Bub3-containing complexes from cells under different spindle assembly checkpoint signaling conditions. We found that prior to kinetochore formation, Bub3 is complexed with BuGZ but not Bub1 or BubR1. Our results point to a model in which BuGZ stabilizes Bub3 and promotes Bub3 loading onto kinetochores in early mitosis, which, in turn, facilitates Bub1 and BubR1 kinetochore recruitment and spindle assembly checkpoint signaling.

Aurora B kinase is recruited to multiple discrete kinetochore and centromere regions in human cells

Aurora B kinase has a critical role in regulating attachments between kinetochores and spindle microtubules during mitosis. Early in mitosis, kinase activity at kinetochores is high to promote attachment turnover, and in later mitosis, activity decreases to ensure attachment stabilization. Aurora B localizes prominently to inner centromeres, and a population of the kinase is also detected at kinetochores. How Aurora B is recruited to and evicted from these regions to regulate kinetochore-microtubule attachments remains unclear. Here, we identified and investigated discrete populations of Aurora B at the centromere/kinetochore region. An inner centromere pool is recruited by Haspin phosphorylation of histone H3, and a kinetochore-proximal outer centromere pool is recruited by Bub1 phosphorylation of histone H2A. Finally, a third pool resides ~20 nm outside of the inner kinetochore protein CENP-C in early mitosis and does not require either the Bub1/pH2A/Sgo1 or Haspin/pH3 pathway for localization or activity. Our results suggest that distinct molecular pathways are responsible for Aurora B recruitment to centromeres and kinetochores.

Kinetochore proteins suppress neuronal microtubule dynamics and promote dendrite regeneration

Kinetochores connect centromeric chromatin to spindle microtubules during mitosis. Neurons are postmitotic, so it was surprising to identify transcripts of structural kinetochore (KT) proteins and regulatory chromosome passenger complex (CPC) and spindle assembly checkpoint (SAC) proteins in Drosophila neurons after dendrite injury. To test whether these proteins function during dendrite regeneration, postmitotic RNA interference (RNAi) was performed and dendrites or axons were removed using laser microsurgery. Reduction of KT, CPC, and SAC proteins decreased dendrite regeneration without affecting axon regeneration. To understand whether neuronal functions of these proteins rely on microtubules, we analyzed microtubule behavior in uninjured neurons. The number of growing plus, but not minus, ends increased in dendrites with reduced KT, CPC, and SAC proteins, while axonal microtubules were unaffected. Increased dendritic microtubule dynamics was independent of dual leucine zipper kinase (DLK)-mediated stress but was rescued by concurrent reduction of γ-tubulin, the core microtubule nucleation protein. Reduction of γ-tubulin also rescued dendrite regeneration in backgrounds containing kinetochore RNAi transgenes. We conclude that kinetochore proteins function postmitotically in neurons to suppress dendritic microtubule dynamics by inhibiting nucleation.

The Modified Phenanthridine PJ34 Unveils an Exclusive Cell-Death Mechanism in Human Cancer Cells

This overview summarizes recent data disclosing the efficacy of the PARP inhibitor PJ34 in exclusive eradication of a variety of human cancer cells without impairing healthy proliferating cells. Its cytotoxic activity in cancer cells is attributed to the insertion of specific un-repairable anomalies in the structure of their mitotic spindle, leading to mitotic catastrophe cell death. This mechanism paves the way to a new concept of cancer therapy.

Cyclin B1 scaffolds MAD1 at the kinetochore corona to activate the mitotic checkpoint

Cyclin B:CDK1 is the master kinase regulator of mitosis. We show here that, in addition to its kinase functions, mammalian Cyclin B also scaffolds a localised signalling pathway to help preserve genome stability. Cyclin B1 localises to an expanded region of the outer kinetochore, known as the corona, where it scaffolds the spindle assembly checkpoint (SAC) machinery by binding directly to MAD1. In vitro reconstitutions map the key binding interface to a few acidic residues in the N-terminal region of MAD1, and point mutations in this sequence abolish MAD1 corona localisation and weaken the SAC. Therefore, Cyclin B1 is the long-sought-after scaffold that links MAD1 to the corona, and this specific pool of MAD1 is needed to generate a robust SAC response. Robustness arises because Cyclin B1:MAD1 localisation loses dependence on MPS1 kinase after the corona has been established, ensuring that corona-localised MAD1 can still be phosphorylated when MPS1 activity is low. Therefore, this study explains how corona-MAD1 generates a robust SAC signal, and it reveals a scaffolding role for the key mitotic kinase, Cyclin B1:CDK1, which ultimately helps to inhibit its own degradation.

Cell-Cycle Cross Talk with Caspases and Their Substrates

Caspases play central roles in mediating both cell death and inflammation. It has more recently become evident that caspases also drive other biological processes. Most prominently, caspases have been shown to be involved in differentiation. Several stem and progenitor cell types rely on caspases to initiate and execute their differentiation processes. These range from neural and glial cells, to skeletal myoblasts and osteoblasts, and several cell types of the hematopoietic system. Beyond differentiation, caspases have also been shown to play roles in other "noncanonical" processes, including cell proliferation, arrest, and senescence, thereby contributing to the mechanisms that regulate tissue homeostasis at multiple levels. Remarkably, caspases directly influence the course of the cell cycle in both a positive and negative manner. Caspases both cleave elements of the cell-cycle machinery and are themselves substrates of cell-cycle kinases. Here we aim to summarize the breadth of interactions between caspases and cell-cycle regulators. We also highlight recent developments in this area.

Crowning the Kinetochore: The Fibrous Corona in Chromosome Segregation

The kinetochore is at the heart of chromosome segregation in mitosis and meiosis. Rather than a static linker complex for chromatin and spindle microtubules, it is highly dynamic in composition, size, and shape. While known for decades that it can expand and grow a fibrous meshwork known as the corona, it was until recently unclear what constitutes this 'crown' and what its relevance is for kinetochore function. Here, we highlight recent discoveries in fibrous corona biology, and place them in the context of the processes that orchestrate high-fidelity chromosome segregation.

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.

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.

The Mitotic Apparatus and Kinetochores in Microcephaly and Neurodevelopmental Diseases

Regulators of mitotic division, when dysfunctional or expressed in a deregulated manner (over- or underexpressed) in somatic cells, cause chromosome instability, which is a predisposing condition to cancer that is associated with unrestricted proliferation. Genes encoding mitotic regulators are growingly implicated in neurodevelopmental diseases. Here, we briefly summarize existing knowledge on how microcephaly-related mitotic genes operate in the control of chromosome segregation during mitosis in somatic cells, with a special focus on the role of kinetochore factors. Then, we review evidence implicating mitotic apparatus- and kinetochore-resident factors in the origin of congenital microcephaly. We discuss data emerging from these works, which suggest a critical role of correct mitotic division in controlling neuronal cell proliferation and shaping the architecture of the central nervous system.

The anaphase-promoting complex: A key mitotic regulator associated with somatic mutations occurring in cancer

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that helps control chromosome separation and exit from mitosis in many different kinds of organisms, including yeast, flies, worms, and humans. This review represents a new perspective on the connection between APC/C subunit mutations and cancer. The complex nature of APC/C and limited mutation analysis of its subunits has made it difficult to determine the relationship of each subunit to cancer. In this work, cancer genomic data were examined to identify APC/C subunits with a greater than 5% alteration frequency in 11 representative cancers using the cBioPortal database. Using the Genetic Determinants of Cancer Patient Survival database, APC/C subunits were also studied and found to be significantly associated with poor patient prognosis in several cases. In comparing these two kinds of cancer genomics data to published large-scale genomic analyses looking for cancer driver genes, ANAPC1 and ANAPC3/CDC27 stood out as being represented in all three types of analyses. Seven other subunits were found to be associated both with >5% alteration frequency in certain cancers and being associated with an effect on cancer patient prognosis. The aim of this review is to provide new approaches for investigators conducting in vivo studies of APC/C subunits and cancer progression. In turn, a better understanding of these APC/C subunits and their role in different cancers will help scientists design drugs that are more precisely targeted to certain cancers, using APC/C mutation status as a biomarker.

The Kinetochore Protein Spc105, a Novel Interaction Partner of LaeA, Regulates Development and Secondary Metabolism in Aspergillus flavus

Nuclear protein LaeA is known as the global regulator of secondary metabolism in Aspergillus. LaeA connects with VeA and VelB to form a heterotrimeric complex, which coordinates fungal development and secondary metabolism. Here, we describe a new interaction partner of LaeA, the kinetochore protein Spc105, from the aflatoxin-producing fungus Aspergillus flavus. We showed that in addition to involvement in nuclear division, Spc105 is required for normal conidiophore development and sclerotia production of A. flavus. Moreover, Spc105 positively regulates the production of secondary metabolites such as aflatoxin and kojic acid, and negatively regulates the production of cyclopiazonic acid. Transcriptome analysis of the Δspc105 strain revealed that 23 backbone genes were differentially expressed, corresponding to 19 of the predicted 56 secondary metabolite gene clusters, suggesting a broad regulatory role of Spc105 in secondary metabolism. Notably, the reduced expression of laeA in our transcriptome data led to the discovery of the correlation between Spc105 and LaeA, and double mutant analysis indicated a functional interdependence between Spc105 and LaeA. Further, in vitro and in vivo protein interaction assays revealed that Spc105 interacts directly with the S-adenosylmethionine (SAM)-binding domain of LaeA, and that the leucine zipper motif in Spc105 is required for this interaction. The Spc105-LaeA interaction identified in our study indicates a cooperative interplay of distinct regulators in A. flavus, providing new insights into fungal secondary metabolism regulation networks.

PP1 and PP2A Use Opposite Phospho-dependencies to Control Distinct Processes at the Kinetochore

PP1 and PP2A-B56 are major serine/threonine phosphatase families that achieve specificity by colocalizing with substrates. At the kinetochore, however, both phosphatases localize to an almost identical molecular space and yet they still manage to regulate unique pathways and processes. By switching or modulating the positions of PP1/PP2A-B56 at kinetochores, we show that their unique downstream effects are not due to either the identity of the phosphatase or its precise location. Instead, these phosphatases signal differently because their kinetochore recruitment can be either inhibited (PP1) or enhanced (PP2A) by phosphorylation inputs. Mathematical modeling explains how these inverse phospho-dependencies elicit unique forms of cross-regulation and feedback, which allows otherwise indistinguishable phosphatases to produce distinct network behaviors and control different mitotic processes. Furthermore, our genome-wide analysis suggests that these major phosphatase families may have evolved to respond to phosphorylation inputs in opposite ways because many other PP1 and PP2A-B56-binding motifs are also phospho-regulated.

Post-metaphase correction of aberrant kinetochore-microtubule attachments in mammalian eggs

The accuracy of the two sequential meiotic divisions in oocytes is essential for creating a haploid gamete with a normal chromosomal content. Here, we have analysed the 3D dynamics of chromosomes during the second meiotic division in live mouse oocytes. We find that chromosomes form stable kinetochore-microtubule attachments at the end of prometaphase II stage that are retained until anaphase II onset. Remarkably, we observe that more than 20% of the kinetochore-microtubule attachments at the metaphase II stage are merotelic or lateral. However, < 1% of all chromosomes at onset of anaphase II are found to lag at the spindle equator and < 10% of the laggards missegregate and give rise to aneuploid gametes. Our results demonstrate that aberrant kinetochore-microtubule attachments are not corrected at the metaphase stage of the second meiotic division. Thus, the accuracy of the chromosome segregation process in mouse oocytes during meiosis II is ensured by an efficient correction process acting at the anaphase stage.

Spindle checkpoint silencing at kinetochores with submaximal microtubule occupancy

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

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.

AMPK-mediated activation of MCU stimulates mitochondrial Ca2+ entry to promote mitotic progression

The capacity of cells to alter bioenergetics in response to the demands of various biological processes is essential for normal physiology. The coordination of energy sensing and production with highly energy-demanding cellular processes, such as cell division, is poorly understood. Here, we show that a cell cycle-dependent mitochondrial Ca²⁺ transient connects energy sensing to mitochondrial activity for mitotic progression. The mitochondrial Ca²⁺ uniporter (MCU) mediates a rapid mitochondrial Ca²⁺ transient during mitosis. Inhibition of mitochondrial Ca²⁺ transients via MCU depletion causes spindle checkpoint-dependent mitotic delay. Cellular ATP levels drop during early mitosis, and the mitochondrial Ca²⁺ transients boost mitochondrial respiration to restore energy homeostasis. This is achieved through mitosis-specific MCU phosphorylation and activation by the mitochondrial translocation of energy sensor AMP-activated protein kinase (AMPK). Our results establish a critical role for AMPK- and MCU-dependent mitochondrial Ca²⁺ signalling in mitosis and reveal a mechanism of mitochondrial metabolic adaptation to acute cellular energy stress.

Coordination of Timers and Sensors in Cell Signaling

Timers and sensors are common devices that make our daily life safer, more convenient, and more efficient. In a cellular context, they arguably play an even more crucial role as they ensure the survival of cells in the presence of various extrinsic and intrinsic stresses. Biological timers and sensors generate distinct signaling profiles, enabling them to produce different types of cellular responses. Recent data suggest that they can work together to guarantee correct timing and responsiveness. By exploring examples of cellular stress signaling from mitosis, DNA damage, and hypoxia, the authors discuss the common architecture of timer-sensor integration, and how its added features contribute to the generation of desired signaling profiles when dealing with stresses of variable duration and strength. The authors propose timer-sensor integration as a widespread mechanism with profound biological implications and therapeutic potential.

Mitotic live-cell imaging at different timescales

Mitosis is a highly dynamic and choreographed process in which chromosomes are captured by the mitotic spindle and physically segregated into the two daughter cells to ensure faithful transmission of the genetic material. Live-cell fluorescence microscopy enables these dynamics to be analyzed over diverse temporal scales. Here we present the methodologies to study chromosome segregation at three timescales: we first show how automated tracking of kinetochores enables investigation of mitotic spindle and chromosome dynamics in the seconds-to-minutes timescale; next we highlight how new DNA live dyes allow the study of chromosome segregation over a period of several hours in any cell line; finally, we demonstrate how image sequences acquired over several days can reveal the fate of whole cell populations over several consecutive cell divisions.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

Chromosome structural anomalies due to aberrant spindle forces exerted at gene editing sites in meiosis

Acentrosomal spindle assembly in mouse oocytes depends on chromosomes and acentriolar microtubule-organizing centers (aMTOCs). Manil-Ségalen et al. observe that Plk4-induced perturbation of aMTOCs coupled to Cre-mediated gene editing generates fragile chromosomes that break when subjected to forces exerted by altered meiosis I spindles.

Mouse female meiotic spindles assemble from acentriolar microtubule-organizing centers (aMTOCs) that fragment into discrete foci. These are further sorted and clustered to form spindle poles, thus providing balanced forces for faithful chromosome segregation. To assess the impact of aMTOC biogenesis on spindle assembly, we genetically induced their precocious fragmentation in mouse oocytes using conditional overexpression of Plk4, a master microtubule-organizing center regulator. Excessive microtubule nucleation from these fragmented aMTOCs accelerated spindle assembly dynamics. Prematurely formed spindles promoted the breakage of three different fragilized bivalents, generated by the presence of recombined Lox P sites. Reducing the density of microtubules significantly diminished the extent of chromosome breakage. Thus, improper spindle forces can lead to widely described yet unexplained chromosomal structural anomalies with disruptive consequences on the ability of the gamete to transmit an uncorrupted genome.

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.

Dynamic kinetochore size regulation promotes microtubule capture and chromosome biorientation in mitosis

Faithful chromosome segregation depends on the ability of sister kinetochores to attach to spindle microtubules. The outer layer of kinetochores transiently expands in early mitosis to form a fibrous corona, and compacts following microtubule capture. Here we show that the dynein adaptor Spindly and the RZZ (ROD-Zwilch-ZW10) complex drive kinetochore expansion in a dynein-independent manner. C-terminal farnesylation and MPS1 kinase activity cause conformational changes of Spindly that promote oligomerization of RZZ-Spindly complexes into a filamentous meshwork in cells and in vitro. Concurrent with kinetochore expansion, Spindly potentiates kinetochore compaction by recruiting dynein via three conserved short linear motifs. Expanded kinetochores unable to compact engage in extensive, long-lived lateral microtubule interactions that persist to metaphase, and result in merotelic attachments and chromosome segregation errors in anaphase. Thus, dynamic kinetochore size regulation in mitosis is coordinated by a single, Spindly-based mechanism that promotes initial microtubule capture and subsequent correct maturation of attachments.

Purification of kinetochores from the budding yeast Saccharomyces cerevisiae

Chromosome segregation relies on forces generated by spindle microtubules that are translated into chromosome movement through interactions with kinetochores, highly conserved macromolecular machines that assemble on a specialized centromeric chromatin structure. Kinetochores not only have to stably attach to growing and shrinking microtubules, but they also need to recruit spindle assembly checkpoint proteins to halt cell cycle progression when there are attachment defects. Even the simplest kinetochore in budding yeast contains more than 50 unique components that are present in multiple copies, totaling more than 250 proteins in a single kinetochore. The complex nature of kinetochores makes it challenging to elucidate the contributions of individual components to its various functions. In addition, it is difficult to manipulate forces in vivo to understand how they regulate kinetochore-microtubule attachments and the checkpoint. To address these issues, we developed a technique to purify kinetochores from budding yeast that can be used to analyze kinetochore functions and composition as well as to reconstitute kinetochore-microtubule attachments in vitro.

USP9X Limits Mitotic Checkpoint Complex Turnover to Strengthen the Spindle Assembly Checkpoint and Guard against Chromosomal Instability

Faithful chromosome segregation during mitosis depends on the spindle assembly checkpoint (SAC), which delays progression through mitosis until every chromosome has stably attached to spindle microtubules via the kinetochore. We show here that the deubiquitinase USP9X strengthens the SAC by antagonizing the turnover of the mitotic checkpoint complex produced at unattached kinetochores. USP9X thereby opposes activation of anaphase-promoting complex/cyclosome (APC/C) and specifically inhibits the mitotic degradation of SAC-controlled APC/C substrates. We demonstrate that depletion or loss of USP9X reduces the effectiveness of the SAC, elevates chromosome segregation defects, and enhances chromosomal instability (CIN). These findings provide a rationale to explain why loss of USP9X could be either pro- or anti-tumorigenic depending on the existing level of CIN.

Guiding Mitotic Progression by Crosstalk between Post-translational Modifications

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

Live Cell Imaging of Chromosome Segregation During Mitosis

Chromosomes must be reliably and uniformly segregated into daughter cells during mitotic cell division. Fidelity of chromosomal segregation is controlled by multiple mechanisms that include the Spindle Assembly Checkpoint (SAC). The SAC is part of a complex feedback system that is responsible for prevention of a cell progress through mitosis unless all chromosomal kinetochores have attached to spindle microtubules. Chromosomal lagging and abnormal chromosome segregation is an indicator of dysfunctional cell cycle control checkpoints and can be used to measure the genomic stability of dividing cells. Deregulation of the SAC can result in the transformation of a normal cell into a malignant cell through the accumulation of errors during chromosomal segregation. Implementation of the SAC and the formation of the kinetochore complex are tightly regulated by interactions between kinases and phosphatase such as Protein Phosphatase 2A (PP2A). This protocol describes live cell imaging of lagging chromosomes in mouse embryonic fibroblasts isolated from mice that had a knockout of the PP2A-B56γ regulatory subunit. This method overcomes the shortcomings of other cell cycle control imaging techniques such as flow cytometry or immunocytochemistry that only provide a snapshot of a cell cytokinesis status, instead of a dynamic spatiotemporal visualization of chromosomes during mitosis.

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

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

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Mps1 autophosphorylation at the NTE promotes activity independent of localization

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NTE phosphorylation relieves an NTE-dependent autoinhibition

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

Phosphorylation of EB1 regulates the recruitment of CLIP-170 and p150glued to the plus ends of astral microtubules

Phosphorylation of end-binding protein 1 (EB1), a key member of microtubule plus end-tracking proteins (+TIPs), by apoptosis signal-regulating kinase 1 (ASK1) has been demonstrated to promote the stability of astral microtubules during mitosis by stimulating the binding of EB1 to microtubule plus ends. However, the roles of other members of the +TIPs family in ASK1/EB1-mediated regulation of astral microtubules are unknown. Herein, we show that ASK1-mediated phosphorylation of EB1 enhances the localization of cytoplasmic linker protein 170 (CLIP-170) and p150^glued to the plus ends of astral microtubules. Depletion of ASK1 or expression of phospho-deficient or phospho-mimetic EB1 mutants results in changes in the levels of plus-end localized CLIP-170 or p150^glued. Mechanistic studies reveal that EB1 phosphorylation promotes its interactions with CLIP-170 and p150^glued, thereby recruiting these +TIPs to microtubules. Structural analysis suggests that serine-40 is the primary phosphorylation site on EB1 that exerts these effects. Together, these findings provide novel insight into the molecular mechanisms that regulate the interactions of EB1 with other +TIPs.

Mps1 promotes chromosome meiotic chromosome biorientation through Dam1

During meiosis, chromosomes attach to microtubules at their kinetochores and are moved by microtubule depolymerization. The Mps1 kinase is essential for this process. Phosphorylation of Dam1 by Mps1 allows kinetochores to move processively poleward along microtubules during the biorientation process.

In budding yeast meiosis, homologous chromosomes become linked by chiasmata and then move back and forth on the spindle until they are bioriented, with the kinetochores of the partners attached to microtubules from opposite spindle poles. Certain mutations in the conserved kinase, Mps1, result in catastrophic meiotic segregation errors but mild mitotic defects. We tested whether Dam1, a known substrate of Mps1, was necessary for its critical meiotic role. We found that kinetochore–microtubule attachments are established even when Dam1 is not phosphorylated by Mps1, but that Mps1 phosphorylation of Dam1 sustains those connections. But the meiotic defects when Dam1 is not phosphorylated are not nearly as catastrophic as when Mps1 is inactivated. The results demonstrate that one meiotic role of Mps1 is to stabilize connections that have been established between kinetochores and microtubles by phosphorylating Dam1.

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.

Non-equilibrium assembly of microtubules: from molecules to autonomous chemical robots

Biological systems have evolved to harness non-equilibrium processes from the molecular to the macro scale. It is currently a grand challenge of chemistry, materials science, and engineering to understand and mimic biological systems that have the ability to autonomously sense stimuli, process these inputs, and respond by performing mechanical work. New chemical systems are responding to the challenge and form the basis for future responsive, adaptive, and active materials. In this article, we describe a particular biochemical-biomechanical network based on the microtubule cytoskeletal filament - itself a non-equilibrium chemical system. We trace the non-equilibrium aspects of the system from molecules to networks and describe how the cell uses this system to perform active work in essential processes. Finally, we discuss how microtubule-based engineered systems can serve as testbeds for autonomous chemical robots composed of biological and synthetic components.

DNA damage induces a kinetochore-based ATM/ATR-independent SAC arrest unique to the first meiotic division in mouse oocytes

Mouse oocytes carrying DNA damage arrest in meiosis I, thereby preventing creation of embryos with deleterious mutations. The arrest is dependent on activation of the spindle assembly checkpoint, which results in anaphase-promoting complex (APC) inhibition. However, little is understood about how this checkpoint is engaged following DNA damage. Here, we find that within minutes of DNA damage checkpoint proteins are assembled at the kinetochore, not at damage sites along chromosome arms, such that the APC is fully inhibited within 30 min. Despite this robust response, there is no measurable loss in k-fibres, or tension across the bivalent. Through pharmacological inhibition we observed that the response is dependent on Mps1 kinase, aurora kinase and Haspin. Using oocyte-specific knockouts we find the response does not require the DNA damage response kinases ATM or ATR. Furthermore, checkpoint activation does not occur in response to DNA damage in fully mature eggs during meiosis II, despite the divisions being separated by just a few hours. Therefore, mouse oocytes have a unique ability to sense DNA damage rapidly by activating the checkpoint at their kinetochores.

Playing polo during mitosis: PLK1 takes the lead

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

Shrinking Daughters: Rlm1-Dependent G1/S Checkpoint Maintains Saccharomyces cerevisiae Daughter Cell Size and Viability

The Rlm1 transcription factor is a target of the cell wall integrity pathway. We report that an rlm1Δ mutant grown on a nonfermentable carbon source at low osmolarity forms cell groups in which a mother cell is surrounded by smaller "satellite-daughter" cells. Mother cells in these groups progressed through repeated rounds of cell division with normal rates of bud growth and genetic stability; however, these cells underwent precocious START relative to wild-type mothers. Thus, once activated, Rlm1 delays the transition from G₁ to S, a mechanism we term the cell wall/START (CW/START) checkpoint. The rlm1Δ satellite-cell phenotype is suppressed by deletion of either SLT2, which encodes the kinase that activates Rlm1, or SWI4, which is also activated by Slt2; suggesting that Slt2 can have opposing roles in regulating the START transition. Consistent with an Rlm1-dependent CW/START checkpoint, rlm1Δ satellite daughters were unable to grow or divide further even after transfer to rich medium, but UV irradiation in G₁ could partially rescue rlm1Δ satellite daughters in the next division. Indeed, after cytokinesis, these satellite daughters shrank rapidly, displayed amorphous actin staining, and became more permeable. As a working hypothesis, we propose that duplication of an "actin-organizing center" in late G₁ may be required both to progress through START and to reestablish the actin cytoskeleton in daughter cells.

Merotelic kinetochore attachment in oocyte meiosis II causes sister chromatids segregation errors in aged mice

Mammalian oocyte chromosomes undergo 2 meiotic divisions to generate haploid gametes. The frequency of chromosome segregation errors during meiosis I increase with age. However, little attention has been paid to the question of how aging affects sister chromatid segregation during oocyte meiosis II. More importantly, how aneuploid metaphase II (MII) oocytes from aged mice evade the spindle assembly checkpoint (SAC) mechanism to complete later meiosis II to form aneuploid embryos remains unknown. Here, we report that MII oocytes from naturally aged mice exhibited substantial errors in chromosome arrangement and configuration compared with young MII oocytes. Interestingly, these errors in aged oocytes had no impact on anaphase II onset and completion as well as 2-cell formation after parthenogenetic activation. Further study found that merotelic kinetochore attachment occurred more frequently and could stabilize the kinetochore-microtubule interaction to ensure SAC inactivation and anaphase II onset in aged MII oocytes. This orientation could persist largely during anaphase II in aged oocytes, leading to severe chromosome lagging and trailing as well as delay of anaphase II completion. Therefore, merotelic kinetochore attachment in oocyte meiosis II exacerbates age-related genetic instability and is a key source of age-dependent embryo aneuploidy and dysplasia.

PP2A-B56γ is required for an efficient spindle assembly checkpoint

The Spindle Assembly Checkpoint (SAC) is part of a complex feedback system designed to ensure that cells do not proceed through mitosis unless all chromosomal kinetochores have attached to spindle microtubules. The formation of the kinetochore complex and the implementation of the SAC are regulated by multiple kinases and phosphatases. BubR1 is a phosphoprotein that is part of the Cdc20 containing mitotic checkpoint complex that inhibits the APC/C so that Cyclin B1 and Securin are not degraded, thus preventing cells going into anaphase. In this study, we found that PP2A in association with its B56γ regulatory subunit, are needed for the stability of BubR1 during nocodazole induced cell cycle arrest. In primary cells that lack B56γ, BubR1 is prematurely degraded and the cells proceed through mitosis. The reduced SAC efficiency results in cells with abnormal chromosomal segregation, a hallmark of transformed cells. Previous studies on PP2A's role in the SAC and kinetochore formation were done using siRNAs to all 5 of the B56 family members. In our study we show that inactivation of only the PP2A-B56γ subunit can affect the efficiency of the SAC. We also provide data that show the intracellular locations of the B56 subunits varies between family members, which is consistent with the hypothesis that they are not completely functionally redundant.