Accurate phosphoregulation of kinetochore-microtubule affinity requires unconstrained molecular interactions

SKAP binding to microtubules reduces friction at the kinetochore-microtubule interface and increases attachment stability under force

The kinetochore links chromosomes to spindle microtubules to drive chromosome segregation at cell division. We recently uncovered that the kinetochore complex Astrin-SKAP, which binds microtubules, reduces rather than increases friction at the mammalian kinetochore-microtubule interface. How it does so is not known. Astrin-SKAP could affect how other kinetochore complexes bind microtubules, reducing their friction along microtubules, or it could itself bind microtubules with similar affinity but lower friction than other attachment factors. Using SKAP mutants unable to bind microtubules, live imaging and laser ablation, we show that SKAP's microtubule binding is essential for sister kinetochore coordination, force dissipation at the interface and attachment responsiveness to force changes. Further, we show that SKAP's microtubule binding is essential to prevent chromosome detachment under both spindle forces and microneedle-generated forces. Together, our findings indicate that SKAP's microtubule binding reduces kinetochore friction and increases attachment responsiveness and stability under force. We propose that having complexes with both high and low sliding friction on microtubules, making a mechanically heterogeneous interface, is key to maintaining robust attachments under force and thus accurate segregation.

Measuring and modeling the dynamics of mitotic error correction

Error correction is central to many biological systems and is critical for protein function and cell health. During mitosis, error correction is required for the faithful inheritance of genetic material. When functioning properly, the mitotic spindle segregates an equal number of chromosomes to daughter cells with high fidelity. Over the course of spindle assembly, many initially erroneous attachments between kinetochores and microtubules are fixed through the process of error correction. Despite the importance of chromosome segregation errors in cancer and other diseases, there is a lack of methods to characterize the dynamics of error correction and how it can go wrong. Here, we present an experimental method and analysis framework to quantify chromosome segregation error correction in human tissue culture cells with live cell confocal imaging, timed premature anaphase, and automated counting of kinetochores after cell division. We find that errors decrease exponentially over time during spindle assembly. A coarse-grained model, in which errors are corrected in a chromosome-autonomous manner at a constant rate, can quantitatively explain both the measured error correction dynamics and the distribution of anaphase onset times. We further validated our model using perturbations that destabilized microtubules and changed the initial configuration of chromosomal attachments. Taken together, this work provides a quantitative framework for understanding the dynamics of mitotic error correction.

Higher-order protein assembly controls kinetochore formation

To faithfully segregate chromosomes during vertebrate mitosis, kinetochore-microtubule interactions must be restricted to a single site on each chromosome. Prior work on pair-wise kinetochore protein interactions has been unable to identify the mechanisms that prevent outer kinetochore formation in regions with a low density of CENP-A nucleosomes. To investigate the impact of higher-order assembly on kinetochore formation, we generated oligomers of the inner kinetochore protein CENP-T using two distinct, genetically engineered systems in human cells. Although individual CENP-T molecules interact poorly with outer kinetochore proteins, oligomers that mimic centromeric CENP-T density trigger the robust formation of functional, cytoplasmic kinetochore-like particles. Both in cells and in vitro, each molecule of oligomerized CENP-T recruits substantially higher levels of outer kinetochore components than monomeric CENP-T molecules. Our work suggests that the density dependence of CENP-T restricts outer kinetochore recruitment to centromeres, where densely packed CENP-A recruits a high local concentration of inner kinetochore proteins.

The RPA-RNF20-SNF2H cascade promotes proper chromosome segregation and homologous recombination repair

The human tumor suppressor Ring finger protein 20 (RNF20)-mediated histone H2B monoubiquitination (H2Bub) is essential for proper chromosome segregation and DNA repair. However, what is the precise function and mechanism of RNF20-H2Bub in chromosome segregation and how this pathway is activated to preserve genome stability remain unknown. Here, we show that the single-strand DNA-binding factor Replication protein A (RPA) interacts with RNF20 mainly in the S and G2/M phases and recruits RNF20 to mitotic centromeres in a centromeric R-loop-dependent manner. In parallel, RPA recruits RNF20 to chromosomal breaks upon DNA damage. Disruption of the RPA-RNF20 interaction or depletion of RNF20 increases mitotic lagging chromosomes and chromosome bridges and impairs BRCA1 and RAD51 loading and homologous recombination repair, leading to elevated chromosome breaks, genome instability, and sensitivities to DNA-damaging agents. Mechanistically, the RPA-RNF20 pathway promotes local H2Bub, H3K4 dimethylation, and subsequent SNF2H recruitment, ensuring proper Aurora B kinase activation at centromeres and efficient loading of repair proteins at DNA breaks. Thus, the RPA-RNF20-SNF2H cascade plays a broad role in preserving genome stability by coupling H2Bub to chromosome segregation and DNA repair.

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.

Strain stiffening of Ndc80 complexes attached to microtubule plus ends

In the mitotic spindle, microtubules attach to chromosomes via kinetochores. The microtubule-binding Ndc80 complex is an integral part of kinetochores, and is essential for kinetochores to attach to microtubules and to transmit forces from dynamic microtubule ends to the chromosomes. The Ndc80 complex has a rod-like appearance with globular domains at its ends that are separated by a long coiled coil. Its mechanical properties are considered important for the dynamic interaction between kinetochores and microtubules. Here, we present a novel method that allows us to time trace the effective stiffness of Ndc80 complexes following shortening microtubule ends against applied force in optical trap experiments. Applying this method to wild-type Ndc80 and three variants (calponin homology (CH) domains mutated or Hec1 tail unphosphorylated, phosphorylated, or truncated), we reveal that each variant exhibits strain stiffening; i.e., the effective stiffness increases under tension that is built up by a depolymerizing microtubule. The strain stiffening relation is roughly linear and independent of the state of the microtubule. We introduce structure-based models that show that the strain stiffening can be traced back to the specific architecture of the Ndc80 complex with a characteristic flexible kink, to thermal fluctuations of the microtubule, and to the bending elasticity of flaring protofilaments, which exert force to move the Ndc80 complexes. Our model accounts for changes in the amount of load-bearing attachments at various force levels and reproduces the roughly linear strain stiffening behavior, highlighting the importance of force-dependent binding affinity.

Hyper-active RAS/MAPK introduces cancer-specific mitotic vulnerabilities

Aneuploidy, the incorrect number of whole chromosomes, is a common feature of tumors that contributes to their initiation and evolution. Preventing aneuploidy requires properly functioning kinetochores, which are large protein complexes assembled on centromeric DNA that link mitotic chromosomes to dynamic spindle microtubules and facilitate chromosome segregation. The kinetochore leverages at least two mechanisms to prevent aneuploidy: error correction and the spindle assembly checkpoint (SAC). BubR1, a factor involved in both processes, was identified as a cancer dependency and therapeutic target in multiple tumor types; however, it remains unclear what specific oncogenic pressures drive this enhanced dependency on BubR1 and whether it arises from BubR1's regulation of the SAC or error-correction pathways. Here, we use a genetically controlled transformation model and glioblastoma tumor isolates to show that constitutive signaling by RAS or MAPK is necessary for cancer-specific BubR1 vulnerability. The MAPK pathway enzymatically hyperstimulates a network of kinetochore kinases that compromises chromosome segregation, rendering cells more dependent on two BubR1 activities: counteracting excessive kinetochore-microtubule turnover for error correction and maintaining the SAC. This work expands our understanding of how chromosome segregation adapts to different cellular states and reveals an oncogenic trigger of a cancer-specific defect.

Small changes in phospho-occupancy at the kinetochore-microtubule interface drive mitotic fidelity

Kinetochore protein phosphorylation promotes the correction of erroneous microtubule attachments to ensure faithful chromosome segregation during cell division. Determining how phosphorylation executes error correction requires an understanding of whether kinetochore substrates are completely (i.e., all-or-none) or only fractionally phosphorylated. Using quantitative mass spectrometry (MS), we measured phospho-occupancy on the conserved kinetochore protein Hec1 (NDC80) that directly binds microtubules. None of the positions measured exceeded ∼50% phospho-occupancy, and the cumulative phospho-occupancy changed by only ∼20% in response to changes in microtubule attachment status. The narrow dynamic range of phospho-occupancy is maintained, in part, by the ongoing phosphatase activity. Further, both Cdk1-Cyclin B1 and Aurora kinases phosphorylate Hec1 to enhance error correction in response to different types of microtubule attachment errors. The low inherent phospho-occupancy promotes microtubule attachment to kinetochores while the high sensitivity of kinetochore-microtubule attachments to small changes in phospho-occupancy drives error correction and ensures high mitotic fidelity.

Cell division: The science friction of chromosome attachment

During mitosis, chromosomes must bind spindle microtubules via kinetochores in a stable yet dynamic manner to ensure rapid frictionless movements. A recent study identifies the first complex that specifically reduces friction in the kinetochore-microtubule interface to ensure efficient chromosome segregation.

The Astrin-SKAP complex reduces friction at the kinetochore-microtubule interface

The kinetochore links chromosomes to spindle microtubules to drive chromosome segregation at cell division. While we know nearly all mammalian kinetochore proteins, how these give rise to the strong yet dynamic microtubule attachments required for function remains poorly understood. Here, we focus on the Astrin-SKAP complex, which localizes to bioriented kinetochores and is essential for chromosome segregation but whose mechanical role is unclear. Live imaging reveals that SKAP depletion dampens the movement and decreases the coordination of metaphase sister kinetochores and increases the tension between them. Using laser ablation to isolate kinetochores bound to polymerizing versus depolymerizing microtubules, we show that without SKAP, kinetochores move slower on both polymerizing and depolymerizing microtubules and that more force is needed to rescue microtubules to polymerize. Thus, in contrast to the previously described kinetochore proteins that increase the grip on microtubules under force, Astrin-SKAP reduces the grip, increasing attachment dynamics and force responsiveness and reducing friction. Together, our findings suggest a model where the Astrin-SKAP complex effectively "lubricates" correct, bioriented attachments to help preserve them.

The Role of Mitotic Kinases and the RZZ Complex in Kinetochore-Microtubule Attachments: Doing the Right Link

During mitosis, the interaction of kinetochores (KTs) with microtubules (MTs) drives chromosome congression to the spindle equator and supports the segregation of sister chromatids. Faithful genome partition critically relies on the ability of chromosomes to establish and maintain proper amphitelic end-on attachments, a configuration in which sister KTs are connected to robust MT fibers emanating from opposite spindle poles. Because the capture of spindle MTs by KTs is error prone, cells use mechanisms that sense and correct inaccurate KT-MT interactions before committing to segregate sister chromatids in anaphase. If left unresolved, these errors can result in the unequal distribution of chromosomes and lead to aneuploidy, a hallmark of cancer. In this review, we provide an overview of the molecular strategies that monitor the formation and fine-tuning of KT-MT attachments. We describe the complex network of proteins that operates at the KT-MT interface and discuss how AURORA B and PLK1 coordinate several concurrent events so that the stability of KT-MT attachments is precisely modulated throughout mitotic progression. We also outline updated knowledge on how the RZZ complex is regulated to ensure the formation of end-on attachments and the fidelity of mitosis.

VTT-006, an anti-mitotic compound, binds to the Ndc80 complex and suppresses cancer cell growth in vitro

Hec1 (Highly expressed in cancer 1) resides in the outer kinetochore where it works to facilitate proper kinetochore-microtubule interactions during mitosis. Hec1 is overexpressed in various cancers and its expression shows correlation with high tumour grade and poor patient prognosis. Chemical perturbation of Hec1 is anticipated to impair kinetochore-microtubule binding, activate the spindle assembly checkpoint (spindle checkpoint) and thereby suppress cell proliferation. In this study, we performed high-throughput screen to identify novel small molecules that target the Hec1 calponin homology domain (CHD), which is needed for normal microtubule attachments. 4 million compounds were first virtually fitted against the CHD, and the best hit molecules were evaluated in vitro. These approaches led to the identification of VTT-006, a 1,2-disubstituted-tetrahydro-beta-carboline derivative, which showed binding to recombinant Ndc80 complex and modulated Hec1 association with microtubules in vitro. VTT-006 treatment resulted in chromosome congression defects, reduced chromosome oscillations and induced loss of inter-kinetochore tension. Cells remained arrested in mitosis with an active spindle checkpoint for several hours before undergoing cell death. VTT-006 suppressed the growth of several cancer cell lines and enhanced the sensitivity of HeLa cells to Taxol. Our findings propose that VTT-006 is a potential anti-mitotic compound that disrupts M phase, impairs kinetochore-microtubule interactions, and activates the spindle checkpoint.

The Aurora B gradient sustains kinetochore stability in anaphase

Kinetochores assemble on chromosomes in mitosis to allow microtubules to attach and bring about accurate chromosome segregation. The kinases Cyclin B-Cdk1 and Aurora B are crucial for the formation of stable kinetochores. However, the activity of these two kinases appears to decline dramatically at centromeres during anaphase onset, precisely when microtubule attachments are required to move chromosomes toward opposite poles of the dividing cell. We find that, although Aurora B leaves centromeres at anaphase, a gradient of Aurora B activity centered on the central spindle is still able to phosphorylate kinetochore substrates such as Dsn1 to modulate kinetochore stability in anaphase and to regulate kinetochore disassembly as cells enter telophase. We provide a model to explain how Aurora B co-operates with Cyclin B-Cdk1 to maintain kinetochore function in anaphase.

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Central spindle Aurora B targets kinetochore substrates in anaphase

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Phosphorylation of Dsn1 by Aurora B stabilizes kinetochores in anaphase

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Dsn1 phosphorylation modulates chromosome movements in anaphase

A mathematical model of kinetochore-microtubule attachment regulated by Aurora A activity gradient describes chromosome oscillation and correction of erroneous attachments

Chromosome oscillation during metaphase is attenuated in cancer cell lines, concomitant with the reduction of Aurora A activity on kinetochores, which results in reduced mitotic fidelity. To verify the correlation between Aurora A activity, chromosome oscillation, and error correction efficiency, we developed a mathematical model of kinetochore-microtubule dynamics, based on stochastic attachment/detachment events regulated by Aurora A activity gradient centered at spindle poles. The model accurately reproduced the oscillatory movements of chromosomes, which were suppressed not only when Aurora A activity was inhibited, but also when it was upregulated, mimicking the situation in cancer cells. Our simulation also predicted efficient correction of erroneous attachments through chromosome oscillation, which was hampered by both inhibition and upregulation of Aurora A activity. Our model provides a framework to understand the physiological role of chromosome oscillation in the correction of erroneous attachments that is intrinsically related to Aurora A activity.

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.

Prometaphase

The establishment of a metaphase plate in which all chromosomes are attached to mitotic spindle microtubules and aligned at the cell equator is required for faithful chromosome segregation in metazoans. The achievement of this configuration relies on the precise coordination between several concurrent mechanisms that start upon nuclear envelope breakdown, mediate chromosome capture at their kinetochores during mitotic spindle assembly and culminate with the congression of all chromosomes to the spindle equator. This period is called 'prometaphase'. Because the nature of chromosome capture by mitotic spindle microtubules is error prone, the cell is provided of error correction mechanisms that sense and correct most erroneous kinetochore-microtubule attachments before committing to separate sister chromatids in anaphase. In this review, aimed for newcomers in the field, more than providing an exhaustive mechanistic coverage of each and every concurrent mechanism taking place during prometaphase, we provide an integrative overview of these processes that ultimately promote the subsequent faithful segregation of chromosomes during mitosis.

Chromosome oscillation promotes Aurora A-dependent Hec1 phosphorylation and mitotic fidelity

Most cancer cells show chromosomal instability, a condition where chromosome missegregation occurs frequently. We found that chromosome oscillation, an iterative chromosome motion during metaphase, is attenuated in cancer cell lines. We also found that metaphase phosphorylation of Hec1 at serine 55, which is mainly dependent on Aurora A on the spindle, is reduced in cancer cell lines. The Aurora A-dependent Hec1-S55 phosphorylation level was regulated by the chromosome oscillation amplitude and vice versa: Hec1-S55 and -S69 phosphorylation by Aurora A is required for efficient chromosome oscillation. Furthermore, enhancement of chromosome oscillation reduced the number of erroneous kinetochore-microtubule attachments and chromosome missegregation, whereas inhibition of Aurora A during metaphase increased such errors. We propose that Aurora A-mediated metaphase Hec1-S55 phosphorylation through chromosome oscillation, together with Hec1-S69 phosphorylation, ensures mitotic fidelity by eliminating erroneous kinetochore-microtubule attachments. Attenuated chromosome oscillation and the resulting reduced Hec1-S55 phosphorylation may be a cause of CIN in cancer cell lines.

Role of spatial patterns and kinetochore architecture in spindle morphogenesis

Mitotic spindle is a self-assembling macromolecular machine responsible for the faithful segregation of chromosomes during cell division. Assembly of the spindle is believed to be governed by the 'Search & Capture' (S&C) principle in which dynamic microtubules explore space in search of kinetochores while the latter capture microtubules and thus connect chromosomes to the spindle. Due to the stochastic nature of the encounters between kinetochores and microtubules, the time required for incorporating all chromosomes into the spindle is profoundly affected by geometric constraints, such as the size and shape of kinetochores as well as their distribution in space at the onset of spindle assembly. In recent years, several molecular mechanisms that control these parameters have been discovered. It is now clear that stochastic S&C takes place in structured space, where components are optimally distributed and oriented to minimize steric hindrances. Nucleation of numerous non-centrosomal microtubules near kinetochores accelerates capture, while changes in the kinetochore architecture at various stages of spindle assembly promote proper connection of sister kinetochores to the opposite spindle poles. Here we discuss how the concerted action of multiple facilitating mechanisms ensure that the spindle assembles rapidly yet with a minimal number of errors.

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.

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.

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.

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.

Killing two birds with one stone: how budding yeast Mps1 controls chromosome segregation and spindle assembly checkpoint through phosphorylation of a single kinetochore protein

During mitosis, the identical sister chromatids of each chromosome must attach through their kinetochores to microtubules emanating from opposite spindle poles. This process, referred to as chromosome biorientation, is essential for equal partitioning of the genetic information to the two daughter cells. Defects in chromosome biorientation can give rise to aneuploidy, a hallmark of cancer and genetic diseases. A conserved surveillance mechanism called spindle assembly checkpoint (SAC) prevents the onset of anaphase until biorientation is attained. Key to chromosome biorientation is an error correction mechanism that allows kinetochores to establish proper bipolar attachments by disengaging faulty kinetochore-microtubule connections. Error correction relies on the Aurora B and Mps1 kinases that also promote SAC signaling, raising the possibility that they are part of a single sensory device responding to improper attachments and concomitantly controlling both their disengagement and a temporary mitotic arrest. In budding yeast, Aurora B and Mps1 promote error correction independently from one another, but while the substrates of Aurora B in this process are at least partially known, the mechanism underlying the involvement of Mps1 in the error correction pathway is unknown. Through the characterization of a novel mps1 mutant and an unbiased genetic screen for extragenic suppressors, we recently gained evidence that a common mechanism based on Mps1-dependent phosphorylation of the Knl1/Spc105 kinetochore scaffold and subsequent recruitment of the Bub1 kinase is critical for the function of Mps1 in chromosome biorientation as well as for SAC activation (Benzi et al. EMBO Rep, 2020).

The Hec1/Ndc80 tail domain is required for force generation at kinetochores, but is dispensable for kinetochore-microtubule attachment formation and Ska complex recruitment

The conserved kinetochore-associated NDC80 complex (composed of Hec1/Ndc80, Nuf2, Spc24, and Spc25) has well-documented roles in mitosis including 1) connecting mitotic chromosomes to spindle microtubules to establish force-transducing kinetochore-microtubule attachments and 2) regulating the binding strength between kinetochores and microtubules such that correct attachments are stabilized and erroneous attachments are released. Although the NDC80 complex plays a central role in forming and regulating attachments to microtubules, additional factors support these processes as well, including the spindle and kinetochore-associated (Ska) complex. Multiple lines of evidence suggest that Ska complexes strengthen attachments by increasing the ability of NDC80 complexes to bind microtubules, especially to depolymerizing microtubule plus ends, but how this is accomplished remains unclear. Using cell-based and in vitro assays, we demonstrate that the Hec1 tail domain is dispensable for Ska complex recruitment to kinetochores and for generation of kinetochore-microtubule attachments in human cells. We further demonstrate that Hec1 tail phosphorylation regulates kinetochore-microtubule attachment stability independently of the Ska complex. Finally, we map the location of the Ska complex in cells to a region near the coiled-coil domain of the NDC80 complex and demonstrate that this region is required for Ska complex recruitment to the NDC80 complex--microtubule interface.

Protein Complex NDC80: Properties, Functions, and Possible Role in Pathophysiology of Cell Division

Mitotic division maintains genetic identity of any multicellular organism throughout an entire lifetime. Each time a parent cell divides, chromosomes are equally distributed between the daughter cells due to the action of mitotic spindle. Mitotic spindle is formed by the microtubules that represent dynamic polymers of tubulin protein. Spindle microtubules are attached end-on to kinetochores - large multi-protein complexes on chromosomes. This review focuses on the four-subunit NDC80 complex, one of the most important kinetochore elements that plays a major role in the attachment of assembling/disassembling microtubule ends to the chromosomes. Here, we summarize published data on the structure, properties, and regulation of the NDC80 complex and discuss possible relationship between changes in the expression of genes coding for the NDC80 complex components, mitotic disorders, and oncogenesis with special emphasis on the diagnostic and therapeutic potential of NDC80.

Hec1/Ndc80 Tail Domain Function at the Kinetochore-Microtubule Interface

Successful mitotic cell division is critically dependent on the formation of correct attachments between chromosomes and spindle microtubules. Microtubule attachments are mediated by kinetochores, which are large proteinaceous structures assembled on centromeric chromatin of mitotic chromosomes. These attachments must be sufficiently stable to transduce force; however, the strength of these attachments are also tightly regulated to ensure timely, error-free progression through mitosis. The highly conserved, kinetochore-associated NDC80 complex is a core component of the kinetochore-microtubule attachment machinery in eukaryotic cells. A small, disordered region within the Hec1 subunit of the NDC80 complex – the N-terminal “tail” domain – has been actively investigated during the last decade due to its roles in generating and regulating kinetochore-microtubule attachments. In this review, we discuss the role of the NDC80 complex, and specifically the Hec1 tail domain, at the kinetochore-microtubule interface, and how recent studies provide a more unified view of Hec1 tail domain function.

A comparative analysis of methods to measure kinetochore-microtubule attachment stability

During mitosis, spindle microtubules dynamically attach to and detach from kinetochores in a precise and regulated fashion. To ensure mitotic fidelity, kinetochore-microtubule (k-MT) attachments must be stable enough to satisfy the spindle assembly checkpoint (SAC), but sufficiently unstable to facilitate the correction of maloriented attachments. Different methods are available to assess k-MT stability in both live and fixed cells, but a comparative survey of these methods has not yet been reported. Here, we evaluate several quantitative and semiquantitative methods for determining k-MT stability and apply each technique to illustrate changes in spindle microtubule dynamics upon perturbation with physiologically relevant concentrations of microtubule stabilizing (Taxol) and destabilizing (UMK57 and nocodazole) compounds. We discuss the utility of each technique for defining specific features of spindle microtubule dynamics and k-MT attachment stability.

Molecular determinants of the Ska-Ndc80 interaction and their influence on microtubule tracking and force-coupling

Errorless chromosome segregation requires load-bearing attachments of the plus ends of spindle microtubules to chromosome structures named kinetochores. How these end-on kinetochore attachments are established following initial lateral contacts with the microtubule lattice is poorly understood. Two microtubule-binding complexes, the Ndc80 and Ska complexes, are important for efficient end-on coupling and may function as a unit in this process, but precise conditions for their interaction are unknown. Here, we report that the Ska-Ndc80 interaction is phosphorylation-dependent and does not require microtubules, applied force, or several previously identified functional determinants including the Ndc80-loop and the Ndc80-tail. Both the Ndc80-tail, which we reveal to be essential for microtubule end-tracking, and Ndc80-bound Ska stabilize microtubule ends in a stalled conformation. Modulation of force-coupling efficiency demonstrates that the duration of stalled microtubule disassembly predicts whether a microtubule is stabilized and rescued by the kinetochore, likely reflecting a structural transition of the microtubule end.

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.

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.

Tight bending of the Ndc80 complex provides intrinsic regulation of its binding to microtubules

Regulation of the outer kinetochore complex Ndc80 is essential to ensure correct kinetochore-microtubule attachments during mitosis. Here, we present a novel mechanism of regulation that is intrinsic to its structure; tight bending of the Ndc80 complex inhibits its microtubule binding. Using single molecule Förster resonance energy transfer (FRET), we show that the Saccharomyces cerevisiae Ndc80 complex can fluctuate between straight and bent forms, and that binding of the complex to microtubules selects for straightened forms. The loop region of the complex enables its bent conformation, as deletion of the loop promotes straightening. In addition, the kinetochore complex MIND enhances microtubule binding by opposing the tightly bent, auto-inhibited conformation of the Ndc80 complex. We suggest that prior to its assembly at the kinetochore, the Ndc80 complex interchanges between bent (auto-inhibited) and open conformations. Once assembled, its association with MIND stabilizes the Ndc80 complex in a straightened form for higher affinity microtubule binding.

Microtubule end conversion mediated by motors and diffusing proteins with no intrinsic microtubule end-binding activity

Accurate chromosome segregation relies on microtubule end conversion, the ill-understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus-ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are elusive. We reconstituted end conversion in vitro using two kinetochore components: the plus end-directed kinesin CENP-E and microtubule-binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENP-E is to ensure close proximity between Ndc80 complexes and the microtubule plus-end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus-end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip-binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall-binding proteins with finely tuned force-dependent motility characteristics.

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.

Phosphatases in Mitosis: Roles and Regulation

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

Correcting aberrant kinetochore microtubule attachments: a hidden regulation of Aurora B on microtubules

For equal chromosome segregation, a pair of kinetochores on each duplicated chromosome must attach to microtubules connecting to opposite poles. The protein kinase Aurora B plays a critical role in destabilizing microtubules attached in a wrong orientation through phosphorylating kinetochore proteins. The mechanism behind this selective destabilization of aberrant attachments remains elusive. While Aurora B is most enriched on the centromere from prophase to metaphase, emerging evidence suggests the importance of Aurora B on microtubules in this process. Here I discuss two hypothetical models that could explain the requirement of Aurora B on microtubules for selective destabilization of aberrant attachments; microtubule-induced substrate masking and treadmill-removal of Aurora B on microtubules proximal to polymerizing ends.

The kinetoplastid kinetochore protein KKT4 is an unconventional microtubule tip-coupling protein

The evolutionarily divergent class of kinetoplastid organisms has a set of unconventional kinetochore proteins that drive chromosome segregation, but it is unclear which components interact with spindle microtubules. Llauró et al. now identify KKT4 as the first microtubule-binding kinetochore protein in Trypanosoma brucei, a major human pathogenic parasite.

Kinetochores are multiprotein machines that drive chromosome segregation by maintaining persistent, load-bearing linkages between chromosomes and dynamic microtubule tips. Kinetochores in commonly studied eukaryotes bind microtubules through widely conserved components like the Ndc80 complex. However, in evolutionarily divergent kinetoplastid species such as Trypanosoma brucei, which causes sleeping sickness, the kinetochores assemble from a unique set of proteins lacking homology to any known microtubule-binding domains. Here, we show that the T. brucei kinetochore protein KKT4 binds directly to microtubules and maintains load-bearing attachments to both growing and shortening microtubule tips. The protein localizes both to kinetochores and to spindle microtubules in vivo, and its depletion causes defects in chromosome segregation. We define a microtubule-binding domain within KKT4 and identify several charged residues important for its microtubule-binding activity. Thus, despite its lack of significant similarity to other known microtubule-binding proteins, KKT4 has key functions required for driving chromosome segregation. We propose that it represents a primary element of the kinetochore–microtubule interface in kinetoplastids.

Reconstitution of a 26-Subunit Human Kinetochore Reveals Cooperative Microtubule Binding by CENP-OPQUR and NDC80

The approximately thirty core subunits of kinetochores assemble on centromeric chromatin containing the histone H3 variant CENP-A and connect chromosomes with spindle microtubules. The chromatin proximal 16-subunit CCAN (constitutive centromere associated network) creates a mechanically stable bridge between CENP-A and the kinetochore’s microtubule-binding machinery, the 10-subunit KMN assembly. Here, we reconstituted a stoichiometric 11-subunit human CCAN core that forms when the CENP-OPQUR complex binds to a joint interface on the CENP-HIKM and CENP-LN complexes. The resulting CCAN particle is globular and connects KMN and CENP-A in a 26-subunit recombinant particle. The disordered, basic N-terminal tail of CENP-Q binds microtubules and promotes accurate chromosome alignment, cooperating with KMN in microtubule binding. The N-terminal basic tail of the NDC80 complex, the microtubule-binding subunit of KMN, can functionally replace the CENP-Q tail. Our work dissects the connectivity and architecture of CCAN and reveals unexpected functional similarities between CENP-OPQUR and the NDC80 complex.

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The kinetochore CENP-OPQUR complex is reconstituted and functionally dissected

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A kinetochore particle with 26 subunits and defined stoichiometry is reconstituted

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EM structure of an 11-subunit inner kinetochore complex reveals globular shape

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CENP-Q and the Ndc80 complex bind microtubules cooperatively

Measuring NDC80 binding reveals the molecular basis of tension-dependent kinetochore-microtubule attachments

Proper kinetochore-microtubule attachments, mediated by the NDC80 complex, are required for error-free chromosome segregation. Erroneous attachments are corrected by the tension dependence of kinetochore-microtubule interactions. Here, we present a method, based on fluorescence lifetime imaging microscopy and Förster resonance energy transfer, to quantitatively measure the fraction of NDC80 complexes bound to microtubules at individual kinetochores in living human cells. We found that NDC80 binding is modulated in a chromosome autonomous fashion over prometaphase and metaphase, and is predominantly regulated by centromere tension. We show that this tension dependency requires phosphorylation of the N-terminal tail of Hec1, a component of the NDC80 complex, and the proper localization of Aurora B kinase, which modulates NDC80 binding. Our results lead to a mathematical model of the molecular basis of tension-dependent NDC80 binding to kinetochore microtubules in vivo.

When a cell divides, each new cell that forms needs to contain a complete set of DNA, which is stored in structures called chromosomes. So first, the chromosomes duplicate, and the two copies are held together. A protein structure known as a kinetochore then forms on each copy of the chromosome. The kinetochores act as a pair of hands that pull the chromosome copies apart and toward opposite sides of the dividing cell. They do this by grabbing protein ‘ropes’ called microtubules that extend toward the chromosomes from each side of the cell.

Kinetochores grip the microtubule ropes more tightly when the connection is under greater tension. This helps the kinetochores to remain attached to the microtubules that will separate the chromosome copies while releasing the microtubules that would pull both copies to the same side. Previous research has shown that hundreds of finger-like structures made out of a protein group called NDC80 extend from each kinetochore ‘hand’ and attach to the microtubules. What remains a mystery is whether and how the NDC80 fingers grip the microtubules more tightly when tension is greater in cells.

Yoo et al. developed a technique for counting how many of the available NDC80 fingers of a single kinetochore are attached to microtubules within a living human cell. The new technique combines genetic engineering, fluorescence imaging and statistical methods to quantify the attachment of NDC80 to microtubules over time and space.

Yoo et al. found that more NDC80 bound to microtubules when there was greater tension. This relationship between binding and tension depends on an enzyme called Aurora B, which modifies the tip of each NDC80 finger and consequently changes the binding of NDC80 to microtubules. Yoo et al. further showed that Aurora B needs to be properly placed between two kinetochore hands to make NDC80-microtubule binding dependent on tension. Without this tension dependency, chromosomes could segregate unevenly into the newly formed cells – a problem that can lead to cancer, infertility and birth defects. The results presented by Yoo et al. therefore expand our understanding of how these diseases originate and may eventually help researchers to develop new treatments for them.

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.

Complete microtubule-kinetochore occupancy favours the segregation of merotelic attachments

Kinetochores are multi-protein complexes that power chromosome movements by tracking microtubules plus-ends in the mitotic spindle. Human kinetochores bind up to 20 microtubules, even though single microtubules can generate sufficient force to move chromosomes. Here, we show that high microtubule occupancy at kinetochores ensures robust chromosome segregation by providing a strong mechanical force that favours segregation of merotelic attachments during anaphase. Using low doses of the microtubules-targeting agent BAL27862 we reduce microtubule occupancy and observe that spindle morphology is unaffected and bi-oriented kinetochores can still oscillate with normal intra-kinetochore distances. Inter-kinetochore stretching is, however, dramatically reduced. The reduction in microtubule occupancy and inter-kinetochore stretching does not delay satisfaction of the spindle assembly checkpoint or induce microtubule detachment via Aurora-B kinase, which was so far thought to release microtubules from kinetochores under low stretching. Rather, partial microtubule occupancy slows down anaphase A and increases incidences of lagging chromosomes due to merotelically attached kinetochores.

Single microtubules (MTs) can move chromosomes, but it is unclear why kinetochores bind up to 20 MTs. Here, the authors decrease the number of kinetochore MTs with BAL27862 and see lagging chromosomes, suggesting that numerous kinetochore MTs provide force ensuring robust chromosomal segregation.

Molecular force spectroscopy of kinetochore-microtubule attachment in silico: Mechanical signatures of an unusual catch bond and collective effects

Measurement of the lifetime of attachments formed by a single microtubule (MT) with a single kinetochore (kt) in vitro under force-clamp conditions had earlier revealed a catch-bond-like behavior. In the past, the physical origin of this apparently counterintuitive phenomenon was traced to the nature of the force dependence of the (de)polymerization kinetics of the MTs. Here, first the same model MT-kt attachment is subjected to external tension that increases linearly with time until rupture occurs. In our force-ramp experiments in silico, the model displays the well known "mechanical signatures" of a catch bond probed by molecular force spectroscopy. Exploiting this evidence, we have further strengthened the analogy between MT-kt attachments and common ligand-receptor bonds in spite of the crucial differences in their underlying physical mechanisms. We then extend the formalism to model the stochastic kinetics of an attachment formed by a bundle of multiple parallel microtubules with a single kt considering the effect of rebinding under force-clamp and force-ramp conditions. From numerical studies of the model we predict the trends of variation of the mean lifetime and mean rupture force with the increasing number of MTs in the bundle. Both the mean lifetime and the mean rupture force display nontrivial nonlinear dependence on the maximum number of MTs that can attach simultaneously to the same kt.

Aurora A Kinase Function at Kinetochores

One of the most important regulatory aspects of chromosome segregation is the ability of kinetochores to precisely control their attachment strength to spindle microtubules. Central to this regulation is Aurora B, a mitotic kinase that phosphorylates kinetochore substrates to promote microtubule turnover. A critical target of Aurora B is the kinetochore protein Ndc80/Hec1, which is a component of the NDC80 complex, the primary force-transducing link between kinetochores and microtubules. Although Aurora B is regarded as the "master regulator" of kinetochore-microtubule attachment, it is becoming clear that this kinase is not solely responsible for phosphorylating Hec1 and other kinetochore substrates to facilitate microtubule turnover. In particular, there is growing evidence that Aurora A kinase, whose activities at spindle poles have been extensively described, has additional roles at kinetochores in regulating the kinetochore-microtubule interface.

Human Ska complex and Ndc80 complex interact to form a load-bearing assembly that strengthens kinetochore-microtubule attachments

Microtubules are dynamic, tube-like structures that drive the segregation of duplicated chromosomes during cell division. The Ska complex is part of a molecular machine that forms force-bearing connections between chromosomes and microtubule ends. Depletion of the Ska complex destabilizes these connections and disrupts cell division. The Ska complex binds microtubules, but it is unknown whether it directly holds force at microtubules or indirectly stabilizes the connections. Here, we show that the Ska complex makes a direct force-bearing linkage with microtubule ends and assembles with another microtubule binding component, the Ndc80 complex, to strengthen its ability to withstand force. Our results suggest that the Ska and Ndc80 complexes work together to maintain the connections between chromosomes and microtubule ends.

Accurate segregation of chromosomes relies on the force-bearing capabilities of the kinetochore to robustly attach chromosomes to dynamic microtubule tips. The human Ska complex and Ndc80 complex are outer-kinetochore components that bind microtubules and are required to fully stabilize kinetochore–microtubule attachments in vivo. While purified Ska complex tracks with disassembling microtubule tips, it remains unclear whether the Ska complex–microtubule interaction is sufficiently strong to make a significant contribution to kinetochore–microtubule coupling. Alternatively, Ska complex might affect kinetochore coupling indirectly, through recruitment of phosphoregulatory factors. Using optical tweezers, we show that the Ska complex itself bears load on microtubule tips, strengthens Ndc80 complex-based tip attachments, and increases the switching dynamics of the attached microtubule tips. Cross-linking mass spectrometry suggests the Ska complex directly binds Ndc80 complex through interactions between the Ska3 unstructured C-terminal region and the coiled-coil regions of each Ndc80 complex subunit. Deletion of the Ska complex microtubule-binding domain or the Ska3 C terminus prevents Ska complex from strengthening Ndc80 complex-based attachments. Together, our results indicate that the Ska complex can directly strengthen the kinetochore–microtubule interface and regulate microtubule tip dynamics by forming an additional connection between the Ndc80 complex and the microtubule.

A guide to classifying mitotic stages and mitotic defects in fixed cells

Cell division is fundamental to life and its perturbation can disrupt organismal development, alter tissue homeostasis, and cause disease. Analysis of mitotic abnormalities provides insight into how certain perturbations affect the fidelity of cell division and how specific cellular structures, molecules, and enzymatic activities contribute to the accuracy of this process. However, accurate classification of mitotic defects is instrumental for correct interpretation of data and formulation of new hypotheses. In this article, we provide guidelines for identifying specific mitotic stages and for classifying normal and deviant mitotic phenotypes. We hope this will clarify confusion about how certain defects are classified and help investigators avoid misnomers, misclassification, and/or misinterpretation, thus leading to a unified and standardized system to classify mitotic defects.

Aurora A kinase phosphorylates Hec1 to regulate metaphase kinetochore-microtubule dynamics

Precise regulation of kinetochore-microtubule attachments is essential for successful chromosome segregation. Central to this regulation is Aurora B kinase, which phosphorylates kinetochore substrates to promote microtubule turnover. A critical target of Aurora B is the N-terminal "tail" domain of Hec1, which is a component of the NDC80 complex, a force-transducing link between kinetochores and microtubules. Although Aurora B is regarded as the "master regulator" of kinetochore-microtubule attachment, other mitotic kinases likely contribute to Hec1 phosphorylation. In this study, we demonstrate that Aurora A kinase regulates kinetochore-microtubule dynamics of metaphase chromosomes, and we identify Hec1 S69, a previously uncharacterized phosphorylation target site in the Hec1 tail, as a critical Aurora A substrate for this regulation. Additionally, we demonstrate that Aurora A kinase associates with inner centromere protein (INCENP) during mitosis and that INCENP is competent to drive accumulation of the kinase to the centromere region of mitotic chromosomes. These findings reveal that both Aurora A and B contribute to kinetochore-microtubule attachment dynamics, and they uncover an unexpected role for Aurora A in late mitosis.

Src family kinase phosphorylation of the motor domain of the human kinesin-5, Eg5

Spindle formation in mammalian cells requires precise spatial and temporal regulation of the kinesin-5, Eg5, which generates outward force to establish spindle bipolarity. Our results demonstrate that Eg5 is phosphorylated in cultured cells by Src family kinases (SFKs) at three sites in the motor head: Y125, Y211, and Y231. Mutation of these sites diminishes motor activity in vitro, and replacement of endogenous Eg5 with phosphomimetic Y211 in LLC-Pk1 cells results in monopolar spindles, consistent with loss of Eg5 activity. Cells treated with SFK inhibitors show defects in spindle formation, similar to those in cells expressing the nonphosphorylatable Y211 mutant, and distinct from inhibition of other mitotic kinases. We propose that this phosphoregulatory mechanism tunes Eg5 enzymatic activity for optimal spindle morphology.

Hec1 Tail Phosphorylation Differentially Regulates Mammalian Kinetochore Coupling to Polymerizing and Depolymerizing Microtubules

The kinetochore links chromosomes to dynamic spindle microtubules and drives both chromosome congression and segregation. To do so, the kinetochore must hold on to depolymerizing and polymerizing microtubules. At metaphase, one sister kinetochore couples to depolymerizing microtubules, pulling its sister along polymerizing microtubules [1, 2]. Distinct kinetochore-microtubule interfaces mediate these behaviors: active interfaces transduce microtubule depolymerization into mechanical work, and passive interfaces generate friction as the kinetochore moves along microtubules [3, 4]. Despite a growing understanding of the molecular components that mediate kinetochore binding [5-7], we do not know how kinetochores physically interact with polymerizing versus depolymerizing microtubule bundles, and whether they use the same mechanisms and regulation to do so. To address this question, we focus on the mechanical role of the essential load-bearing protein Hec1 [8-11] in mammalian cells. Hec1's affinity for microtubules is regulated by Aurora B phosphorylation on its N-terminal tail [12-15], but its role at the interface with polymerizing versus depolymerizing microtubules remains unclear. Here we use laser ablation to trigger cellular pulling on mutant kinetochores and decouple sisters in vivo, and thereby separately probe Hec1's role on polymerizing versus depolymerizing microtubules. We show that Hec1 tail phosphorylation tunes friction along polymerizing microtubules and yet does not compromise the kinetochore's ability to grip depolymerizing microtubules. Together, the data suggest that kinetochore regulation has differential effects on engagement with growing and shrinking microtubules. Through this mechanism, the kinetochore can modulate its grip on microtubules over mitosis and yet retain its ability to couple to microtubules powering chromosome movement.

Spindle assembly checkpoint satisfaction occurs via end-on but not lateral attachments under tension

To ensure accurate chromosome segregation, the spindle assembly checkpoint (SAC) prevents anaphase until all kinetochores attach to the spindle. What signals the SAC monitors remains unclear. We do not know the contributions of different microtubule attachment features or tension from biorientation to SAC satisfaction nor how these possible cues change during attachment. In this study, we quantify concurrent Mad1 intensity and report on SAC silencing, real-time attachment geometry, occupancy, and tension at individual mammalian kinetochores. We show that Mad1 loss from the kinetochore is switch-like with robust kinetics and that tension across sister kinetochores is established just before Mad1 loss events at the first sister. We demonstrate that CenpE-mediated lateral attachment of the second sister can persistently generate this metaphase-like tension before biorientation, likely stabilizing sister end-on attachment, yet cannot induce Mad1 loss from that kinetochore. Instead, Mad1 loss begins after several end-on microtubules attach. Thus, end-on attachment provides geometry-specific molecular cues or force on specific kinetochore linkages that other attachment geometries cannot provide.