Kinesin-13s form rings around microtubules

KLP-7/Kinesin-13 orchestrates axon-dendrite checkpoints for polarized trafficking in neurons

The polarized nature of neurons depends on their microtubule dynamics and orientation determined by both microtubule-stabilizing and destabilizing factors. The role of destabilizing factors in developing and maintaining neuronal polarity is unclear. We investigated the function of KLP-7, a microtubule depolymerizing motor of the Kinesin-13 family, in axon-dendrite compartmentalization using PVD neurons in Caenorhabditis elegans. Loss of KLP-7 caused a mislocalization of axonal proteins, including RAB-3, SAD-1, and their motor UNC-104, to dendrites. This is rescued by cell-autonomous expression of the KLP-7 or colchicine treatment, indicating the involvement of KLP-7-dependent microtubule depolymerization. The high mobility of KLP-7 is correlated to increased microtubule dynamics in the dendrites, which restricts the enrichment of UNC-44, an integral component of Axon Initial Segment (AIS) in these processes. Due to the loss of KLP-7, ectopic enrichment of UNC-44 in the dendrite potentially redirects axonal traffic into dendrites that include plus-end out microtubules, axonal motors, and cargoes. These observations indicate that KLP-7-mediated depolymerization defines the microtubule dynamics conducive to the specific enrichment of AIS components in dendrites. This further compartmentalizes dendritic and axonal microtubules, motors, and cargoes, thereby influencing neuronal polarity.

Doublet microtubule inner junction protein FAP20 recruits tubulin to the microtubule lattice

Doublet microtubules of eukaryotic cilia and flagella are made up of a complete A- and an incomplete B-tubule that are fused together. Of the two fusion points, the outer junction is made of tripartite tubulin connections, while the inner junction contains non-tubulin elements. The latter includes flagellar-associated protein 20 (FAP20) and Parkin co-regulated gene protein (PACRG) that together link the A- and B-tubule at the inner junction. While structures of doublet microtubules reveal molecular details, their assembly is poorly understood. In this study, we purified recombinant FAP20 and characterized its effects on microtubule dynamics. We use in vitro reconstitution and cryo-electron microscopy to show that FAP20 recruits free tubulin to the existing microtubule lattice. Our cryo-electron microscopy reconstruction of microtubule:FAP20:tubulin complex reveals the mode of tubulin recruitment by FAP20 onto microtubules, providing insights into assembly steps of B-tubule closure during doublet microtubule formation.

The minus-end depolymerase KIF2A drives flux-like treadmilling of γTuRC-uncapped microtubules

During mitosis, microtubules in the spindle turn over continuously. At spindle poles, where microtubule minus ends are concentrated, microtubule nucleation and depolymerization, the latter required for poleward microtubule flux, happen side by side. How these seemingly antagonistic processes of nucleation and depolymerization are coordinated is not understood. Here, we reconstitute this coordination in vitro combining different pole-localized activities. We find that the spindle pole-localized kinesin-13 KIF2A is a microtubule minus-end depolymerase, in contrast to its paralog MCAK. Due to its asymmetric activity, KIF2A still allows microtubule nucleation from the γ-tubulin ring complex (γTuRC), which serves as a protective cap shielding the minus end against KIF2A binding. Efficient γTuRC uncapping requires the combined action of KIF2A and a microtubule severing enzyme, leading to treadmilling of the uncapped microtubule driven by KIF2A. Together, these results provide insight into the molecular mechanisms by which a minimal protein module coordinates microtubule nucleation and depolymerization at spindle poles consistent with their role in poleward microtubule flux.

The Human UGT2B7 Nanodisc

The 20 uridine diphosphate glycosyl-transferases (UGTs) encoded in the human genome form an essential homeostatic network of overlapping catalytic functions that surveil and regulate the activity and clearance of scores of small molecule metabolites. Biochemical and biophysical UGT studies have been hampered by the inability to purify these membrane-bound proteins. Here, using cell-free expression and nanodisc technology, we assemble and purify to homogeneity the first UGT nanodisc-the human UGT2B7•nanodisc. The complex is readily isolated in milligram quantities. It is stable and its initial-rate parameters are identical within error to those associated with UGT2B7 in microsomal preparations (i.e., Supersomes). The high purity of the nanodisc preparation simplifies UGT assays, which allows complexities traditionally associated with microsomal assays (latency and the albumin effect) to be circumvented. Each nanodisc is shown to harbor a single UGT2B7 monomer. The methods described herein should be widely applicable to UGTs, and these findings are expected to set the stage for experimentalists to more freely explore the structure, function, and biology of this important area of phase II metabolism. SIGNIFICANCE STATEMENT: Lack of access to pure, catalytically competent human uridine diphosphate glucuronosyl-transferases (UGTs) has long been an impediment to biochemical and biophysical studies of this disease-relevant enzyme family. Here, we demonstrate this barrier can be removed using nanodisc technology-a human UGT2B7•nanodisc is assembled, purified to homogeneity, and shown to have activity comparable to microsomal UGT2B7.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Cryo-EM reveals the structural basis of microtubule depolymerization by kinesin-13s

Kinesin-13s constitute a distinct group within the kinesin superfamily of motor proteins that promote microtubule depolymerization and lack motile activity. The molecular mechanism by which kinesin-13s depolymerize microtubules and are adapted to perform a seemingly very different activity from other kinesins is still unclear. To address this issue, here we report the near atomic resolution cryo-electron microscopy (cryo-EM) structures of Drosophila melanogaster kinesin-13 KLP10A protein constructs bound to curved or straight tubulin in different nucleotide states. These structures show how nucleotide induced conformational changes near the catalytic site are coupled with movement of the kinesin-13-specific loop-2 to induce tubulin curvature leading to microtubule depolymerization. The data highlight a modular structure that allows similar kinesin core motor-domains to be used for different functions, such as motility or microtubule depolymerization.

Kinesin-13s are microtubule depolymerases that lack motile activity. Here the authors present the cryo-EM structures of kinesin-13 microtubule complexes in different nucleotide bound states, which reveal how ATP hydrolysis is linked to conformational changes and propose a model for kinesin induced depolymerisation.

Microtubule Tip Tracking by the Spindle and Kinetochore Protein Ska1 Requires Diverse Tubulin-Interacting Surfaces

The macromolecular kinetochore functions to generate interactions between chromosomal DNA and spindle microtubules [1]. To facilitate chromosome movement and segregation, kinetochores must maintain associations with both growing and shrinking microtubule ends. It is critical to define the proteins and their properties that allow kinetochores to associate with dynamic microtubules. The kinetochore-localized human Ska1 complex binds to microtubules and tracks with depolymerizing microtubule ends [2]. We now demonstrate that the Ska1 complex also autonomously tracks with growing microtubule ends in vitro, a key property that would allow this complex to act at kinetochores to mediate persistent associations with dynamic microtubules. To define the basis for Ska1 complex interactions with dynamic microtubules, we investigated the tubulin-binding properties of the Ska1 microtubule binding domain. In addition to binding to the microtubule lattice and dolastatin-induced protofilament-like structures, we demonstrate that the Ska1 microtubule binding domain can associate with soluble tubulin heterodimers and promote assembly of oligomeric ring-like tubulin structures. We generated mutations on distinct surfaces of the Ska1 microtubule binding domain that disrupt binding to soluble tubulin but do not prevent microtubule binding. These mutants display compromised microtubule tracking activity in vitro and result in defective chromosome alignment and mitotic progression in cells using a CRISPR/Cas9-based replacement assay. Our work supports a model in which multiple surfaces of Ska1 interact with diverse tubulin substrates to associate with dynamic microtubule polymers and facilitate optimal chromosome segregation.

Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles

The separation of sister chromatids during anaphase is the culmination of mitosis and one of the most strikingly beautiful examples of cellular movement. It consists of two distinct processes: Anaphase A, the movement of chromosomes toward spindle poles via shortening of the connecting fibers, and anaphase B, separation of the two poles from one another via spindle elongation. I focus here on anaphase A chromosome-to-pole movement. The chapter begins by summarizing classical observations of chromosome movements, which support the current understanding of anaphase mechanisms. Live cell fluorescence microscopy studies showed that poleward chromosome movement is associated with disassembly of the kinetochore-attached microtubule fibers that link chromosomes to poles. Microtubule-marking techniques established that kinetochore-fiber disassembly often occurs through loss of tubulin subunits from the kinetochore-attached plus ends. In addition, kinetochore-fiber disassembly in many cells occurs partly through 'flux', where the microtubules flow continuously toward the poles and tubulin subunits are lost from minus ends. Molecular mechanistic models for how load-bearing attachments are maintained to disassembling microtubule ends, and how the forces are generated to drive these disassembly-coupled movements, are discussed.

Distinct Interaction Modes of the Kinesin-13 Motor Domain with the Microtubule

Kinesins-13s are members of the kinesin superfamily of motor proteins that depolymerize microtubules (MTs) and have no motile activity. Instead of generating unidirectional movement over the MT lattice, like most other kinesins, kinesins-13s undergo one-dimensional diffusion (ODD) and induce depolymerization at the MT ends. To understand the mechanism of ODD and the origin of the distinct kinesin-13 functionality, we used ensemble and single-molecule fluorescence polarization microscopy to analyze the behavior and conformation of Drosophila melanogaster kinesin-13 KLP10A protein constructs bound to the MT lattice. We found that KLP10A interacts with the MT in two coexisting modes: one in which the motor domain binds with a specific orientation to the MT lattice and another where the motor domain is very mobile and able to undergo ODD. By comparing the orientation and dynamic behavior of mutated and deletion constructs we conclude that 1) the Kinesin-13 class specific neck domain and loop-2 help orienting the motor domain relative to the MT. 2) During ODD the KLP10A motor-domain changes orientation rapidly (rocks or tumbles). 3) The motor domain alone is capable of undergoing ODD. 4) A second tubulin binding site in the KLP10A motor domain is not critical for ODD. 5) The neck domain is not the element preventing KLP10A from binding to the MT lattice like motile kinesins.

The Human Centriolar Protein CEP135 Contains a Two-Stranded Coiled-Coil Domain Critical for Microtubule Binding

Centrioles are microtubule-based structures that play important roles notably in cell division and cilium biogenesis. CEP135/Bld10p family members are evolutionarily conserved microtubule-binding proteins important for centriole formation. Here, we analyzed in detail the microtubule-binding activity of human CEP135 (HsCEP135). X-ray crystallography and small-angle X-ray scattering in combination with molecular modeling revealed that the 158 N-terminal residues of HsCEP135 (HsCEP135-N) form a parallel two-stranded coiled-coil structure. Biochemical, cryo-electron, and fluorescence microscopy analyses revealed that in vitro HsCEP135-N interacts with tubulin, protofilaments, and microtubules and induces the formation of microtubule bundles. We further identified a 13 amino acid segment spanning residues 96-108, which represents a major microtubule-binding site in HsCEP135-N. Within this segment, we identified a cluster of three lysine residues that contribute to the microtubule bundling activity of HsCEP135-N. Our results provide the first structural information on CEP135/Bld10p proteins and offer insights into their microtubule-binding mechanism.

New Insights into the Coupling between Microtubule Depolymerization and ATP Hydrolysis by Kinesin-13 Protein Kif2C

Kinesin-13 proteins depolymerize microtubules in an ATP hydrolysis-dependent manner. The coupling between these two activities remains unclear. Here, we first studied the role of the kinesin-13 subfamily-specific loop 2 and of the KVD motif at the tip of this loop. Shortening the loop, the lysine/glutamate interchange and the additional Val to Ser substitution all led to Kif2C mutants with decreased microtubule-stimulated ATPase and impaired depolymerization capability. We rationalized these results based on a structural model of the Kif2C-ATP-tubulin complex derived from the recently determined structures of kinesin-1 bound to tubulin. In this model, upon microtubule binding Kif2C undergoes a conformational change governed in part by the interaction of the KVD motif with the tubulin interdimer interface. Second, we mutated to an alanine the conserved glutamate residue of the switch 2 nucleotide binding motif. This mutation blocks motile kinesins in a post-conformational change state and inhibits ATP hydrolysis. This Kif2C mutant still depolymerized microtubules and yielded complexes of one Kif2C with two tubulin heterodimers. These results demonstrate that the structural change of Kif2C-ATP upon binding to microtubule ends is sufficient for tubulin release, whereas ATP hydrolysis is not required. Overall, our data suggest that the conformation reached by kinesin-13s upon tubulin binding is similar to that of tubulin-bound, ATP-bound, motile kinesins but that this conformation is adapted to microtubule depolymerization.

HX-MS2 for high performance conformational analysis of complex protein states

Water-mediated hydrogen exchange (HX) processes involving the protein main chain are sensitive to structural dynamics and molecular interactions. Measuring deuterium uptake in amide bonds provides information on conformational states, structural transitions and binding events. Increasingly, deuterium levels are measured by mass spectrometry (MS) from proteolytically generated peptide fragments of large molecular systems. However, this bottom-up method has limited spectral capacity and requires a burdensome manual validation exercise, both of which restrict analysis of protein systems to generally less than 150 kDa. In this study, we present a bottom-up HX-MS(2) method that improves peptide identification rates, localizes high-quality HX data and simplifies validation. The method combines a new peptide scoring algorithm (WUF, weighted unique fragment) with data-independent acquisition of peptide fragmentation data. Scoring incorporates the validation process and emphasizes identification accuracy. The HX-MS(2) method is illustrated using data from a conformational analysis of microtubules treated with dimeric kinesin MCAK. When compared to a conventional Mascot-driven HX-MS method, HX-MS(2) produces two-fold higher α/β-tubulin sequence depth at a peptide utilization rate of 74%. A Mascot approach delivers a utilization rate of 44%. The WUF score can be constrained by false utilization rate (FUR) calculations to return utilization values exceeding 90% without serious data loss, indicating that automated validation should be possible. The HX-MS(2) data confirm that N-terminal MCAK domains anchor kinesin force generation in kinesin-mediated depolymerization, while the C-terminal tails regulate MCAK-tubulin interactions.

Mass spec studio for integrative structural biology

The integration of biophysical data from multiple sources is critical for developing accurate structural models of large multiprotein systems and their regulators. Mass spectrometry (MS) can be used to measure the insertion location for a wide range of topographically sensitive chemical probes, and such insertion data provide a rich, but disparate set of modeling restraints. We have developed a software platform that integrates the analysis of label-based MS and tandem MS (MS(2)) data with protein modeling activities (Mass Spec Studio). Analysis packages can mine any labeling data from any mass spectrometer in a proteomics-grade manner, and link labeling methods with data-directed protein interaction modeling using HADDOCK. Support is provided for hydrogen/deuterium exchange (HX) and covalent labeling chemistries, including novel acquisition strategies such as targeted HX-MS(2) and data-independent HX-MS(2). The latter permits the modeling of highly complex systems, which we demonstrate by the analysis of microtubule interactions.

Nucleotide exchange in dimeric MCAK induces longitudinal and lateral stress at microtubule ends to support depolymerization

Members of the kinesin-13 subfamily use motor domains in an unconventional fashion to initiate microtubule (MT) depolymerization at MT ends, suggesting unique conformational transitions for lattice engagement, end adaptation, or both. Using hydrogen-deuterium exchange and electron microscopy, we explored conformational changes in free dimeric mitotic centromere-associated kinesin (MCAK) and when bound to a depolymerization intermediate. ATP hydrolysis relaxes the conformation of the dimer, notably in the neck and N-terminal domain. Exchanging ADP in dimeric MCAK with ATP at the MT plus end induces outward curvature in α/β-tubulin, accompanied by a restructuring of the MCAK neck and N terminus, as it returns to a closed state. Reestablishing a closed dimer induces lateral separation of paired tubulin dimers, which may assist in depolymerization. Thus, full-length ADP-MCAK transitions from an open diffusion-competent configuration to a closed state upon plus end-mediated nucleotide exchange, which is mediated by conformational changes in the N-terminal domains of the dimer.

Effects of tubulin acetylation and tubulin acetyltransferase binding on microtubule structure

Acetylation of α-tubulin is uniquely located on the microtubule lumen and away from most MAPs. It does not affect microtubule structure and must affect only proteins that bind directly to the lumen. αTAT1 can interact with the tubulin C-termini, which could facilitate access to its luminal site and make it responsive to other modifications.

Tubulin undergoes posttranslational modifications proposed to specify microtubule subpopulations for particular functions. Most of these modifications occur on the C-termini of tubulin and may directly affect the binding of microtubule-associated proteins (MAPs) or motors. Acetylation of Lys-40 on α-tubulin is unique in that it is located on the luminal surface of microtubules, away from the interaction sites of most MAPs and motors. We investigate whether acetylation alters the architecture of microtubules or the conformation of tubulin, using cryo–electron microscopy (cryo-EM). No significant changes are observed based on protofilament distributions or microtubule helical lattice parameters. Furthermore, no clear differences in tubulin structure are detected between cryo-EM reconstructions of maximally deacetylated or acetylated microtubules. Our results indicate that the effect of acetylation must be highly localized and affect interaction with proteins that bind directly to the lumen of the microtubule. We also investigate the interaction of the tubulin acetyltransferase, αTAT1, with microtubules and find that αTAT1 is able to interact with the outside of the microtubule, at least partly through the tubulin C-termini. Binding to the outside surface of the microtubule could facilitate access of αTAT1 to its luminal site of action if microtubules undergo lateral opening between protofilaments.

A second tubulin binding site on the kinesin-13 motor head domain is important during mitosis

Kinesin-13s are microtubule (MT) depolymerases different from most other kinesins that move along MTs. Like other kinesins, they have a motor or head domain (HD) containing a tubulin and an ATP binding site. Interestingly, kinesin-13s have an additional binding site (Kin-Tub-2) on the opposite side of the HD that contains several family conserved positively charged residues. The role of this site in kinesin-13 function is not clear. To address this issue, we investigated the in-vitro and in-vivo effects of mutating Kin-Tub-2 family conserved residues on the Drosophila melanogaster kinesin-13, KLP10A. We show that the Kin-Tub-2 site enhances tubulin cross-linking and MT bundling properties of KLP10A in-vitro. Disruption of the Kin-Tub-2 site, despite not having a deleterious effect on MT depolymerization, results in abnormal mitotic spindles and lagging chromosomes during mitosis in Drosophila S2 cells. The results suggest that the additional Kin-Tub-2 tubulin biding site plays a direct MT attachment role in-vivo.

Microtubule-depolymerizing kinesins

The microtubule (MT) cytoskeleton supports a broad range of cellular functions, from providing tracks for intracellular transport, to supporting movement of cilia and flagella, to segregating chromosomes in mitosis. These functions are facilitated by the organizational and dynamic plasticity of MT networks. An important class of enzymes that alters MT dynamics is the depolymerizing kinesin-like proteins, which use their catalytic activities to regulate MT end dynamics. In this review, we discuss four topics surrounding these MT-depolymerizing kinesins. We provide a historical overview of studies focused on these motors and discuss their phylogeny. In the second half, we discuss their enzymology and biophysics and give an overview of their known cellular functions. This discussion highlights the fact that MT-depolymerizing kinesins exhibit a diverse range of design principles, which in turn increases their functional versatility in cells.

Structural model for tubulin recognition and deformation by kinesin-13 microtubule depolymerases

To elucidate the structural basis of the mechanism of microtubule depolymerization by kinesin-13s, we analyzed complexes of tubulin and the Drosophila melanogaster kinesin-13 KLP10A by electron microscopy (EM) and fluorescence polarization microscopy. We report a nanometer-resolution (1.1 nm) cryo-EM three-dimensional structure of the KLP10A head domain (KLP10AHD) bound to curved tubulin. We found that binding of KLP10AHD induces a distinct tubulin configuration with displacement (shear) between tubulin subunits in addition to curvature. In this configuration, the kinesin-binding site differs from that in straight tubulin, providing an explanation for the distinct interaction modes of kinesin-13s with the microtubule lattice or its ends. The KLP10AHD-tubulin interface comprises three areas of interaction, suggesting a crossbow-type tubulin-bending mechanism. These areas include the kinesin-13 family conserved KVD residues, and as predicted from the crossbow model, mutating these residues changes the orientation and mobility of KLP10AHDs interacting with the microtubule.

Kif2C minimal functional domain has unusual nucleotide binding properties that are adapted to microtubule depolymerization

The kinesin-13 Kif2C hydrolyzes ATP and uses the energy released to disassemble microtubules. The mechanism by which this is achieved remains elusive. Here we show that Kif2C-(sN+M), a monomeric construct consisting of the motor domain with the proximal part of the N-terminal Neck extension but devoid of its more distal, unstructured, and highly basic part, has a robust depolymerase activity. When detached from microtubules, the Kif2C-(sN+M) nucleotide-binding site is occupied by ATP at physiological concentrations of adenine nucleotides. As a consequence, Kif2C-(sN+M) starts its interaction with microtubules in that state, which differentiates kinesin-13s from motile kinesins. Moreover, in this ATP-bound conformational state, Kif2C-(sN+M) has a higher affinity for soluble tubulin compared with microtubules. We propose a mechanism in which, in the first step, the specificity of ATP-bound Kif2C for soluble tubulin causes it to stabilize a curved conformation of tubulin heterodimers at the ends of microtubules. Data from an ATPase-deficient Kif2C mutant suggest that, then, ATP hydrolysis precedes and is required for tubulin release to take place. Finally, comparison with Kif2C-Motor indicates that the binding specificity for curved tubulin and, accordingly, the microtubule depolymerase activity are conferred to the motor domain by its N-terminal Neck extension.

Microtubules cut loose at the cell cortex

The ability of the microtubule cytoskeleton to rapidly and locally reorganize itself in response to intra- and extracellular signals is essential to its wide range of functions. A site of tightly regulated microtubule dynamics--and the major interface between the microtubule cytoskeleton and the extracellular environment--is the cell cortex, where the selective stabilization and destabilization of microtubule plus-ends is required for normal cell division, morphogenesis and migration. In a recent study, we found that the cortex of Drosophila S2 and D17 cells is coated with the microtubule severing enzyme and plus-end depolymerase, Kat-60, which actively suppresses microtubule growth and stability along the cell edge. We have proposed that cortical Kat-60 functions by uncapping plus-ends, thereby activating another microtubule depolymerase, KLP10A, preloaded onto the end. The localized destruction of microtubule plus-ends at a specific cortical could feed into larger regulatory pathways, such as those in control of the actin cytoskeleton, to influence cell polarization and motility.

Klp10A, a microtubule-depolymerizing kinesin-13, cooperates with CP110 to control Drosophila centriole length

Klp10A is a kinesin-13 of Drosophila melanogaster that depolymerizes cytoplasmic microtubules. In interphase, it promotes microtubule catastrophe; in mitosis, it contributes to anaphase chromosome movement by enabling tubulin flux. Here we show that Klp10A also acts as a microtubule depolymerase on centriolar microtubules to regulate centriole length. Thus, in both cultured cell lines and the testes, absence of Klp10A leads to longer centrioles that show incomplete 9-fold symmetry at their ends. These structures and associated pericentriolar material undergo fragmentation. We also show that in contrast to mammalian cells where depletion of CP110 leads to centriole elongation, in Drosophila cells it results in centriole length diminution that is overcome by codepletion of Klp10A to give longer centrioles than usual. We discuss how loss of centriole capping by CP110 might have different consequences for centriole length in mammalian and insect cells and also relate these findings to the functional interactions between mammalian CP110 and another kinesin-13, Kif24, that in mammalian cells regulates cilium formation.

Mitotic centromere-associated kinesin (MCAK): a potential cancer drug target

The inability to faithfully segregate chromosomes in mitosis results in chromosome instability, a hallmark of solid tumors. Disruption of microtubule dynamics contributes highly to mitotic chromosome instability. The kinesin-13 family is critical in the regulation of microtubule dynamics and the best characterized member of the family, the mitotic centromere-associated kinesin (MCAK), has recently been attracting enormous attention. MCAK regulates microtubule dynamics as a potent depolymerizer of microtubules by removing tubulin subunits from the polymer end. This depolymerizing activity plays pivotal roles in spindle formation, in correcting erroneous attachments of microtubule-kinetochore and in chromosome movement. Thus, the accurate regulation of MCAK is important for ensuring the faithful segregation of chromosomes in mitosis and for safeguarding chromosome stability. In this review we summarize recent data concerning the regulation of MCAK by mitotic kinases, Aurora A/B, Polo-like kinase 1 and cyclin-dependent kinase 1. We propose a molecular model of the regulation of MCAK by these mitotic kinases and relevant phosphatases throughout mitosis. An ever-increasing quantity of data indicates that MCAK is aberrantly regulated in cancer cells. This deregulation is linked to increased malignance, invasiveness, metastasis and drug resistance, most probably due to increased chromosomal instability and remodeling of the microtubule cytoskeleton in cancer cells. Most interestingly, recent observations suggest that MCAK could be a novel molecular target for cancer therapy, as a new cancer antigen or as a mitotic regulator. This collection of new data indicates that MCAK could be a new star in the cancer research sky due to its critical roles in the control of genome stability and the cytoskeleton. Further investigations are required to dissect the fine details of the regulation of MCAK throughout mitosis and its involvements in oncogenesis.

The kinesin-13 MCAK has an unconventional ATPase cycle adapted for microtubule depolymerization

Most kinesins move directionally along microtubules, but MCAK instead depolymerizes them. This study analyses the ATPase cycle of MCAK, identifying unusual kinetic features that fit with its unconventional activity.

Unlike other kinesins, members of the kinesin-13 subfamily do not move directionally along microtubules but, instead, depolymerize them. To understand how kinesins with structurally similar motor domains can have such dissimilar functions, we elucidated the ATP turnover cycle of the kinesin-13, MCAK. In contrast to translocating kinesins, ATP cleavage, rather than product release, is the rate-limiting step for ATP turnover by MCAK; unpolymerized tubulin and microtubules accelerate this step. Further, microtubule ends fully activate the ATPase by accelerating the exchange of ADP for ATP. This tuning of the cycle adapts MCAK for its depolymerization activity: lattice-stimulated ATP cleavage drives MCAK into a weakly bound nucleotide state that reaches microtubule ends by diffusion, and end-specific acceleration of nucleotide exchange drives MCAK into a strongly bound state that promotes depolymerization. This altered cycle accounts well for the different mechanical behaviour of this kinesin, which depolymerizes microtubules from their ends, compared to translocating kinesins that walk along microtubules. Thus, the kinesin motor domain is a nucleotide-dependent engine that can be differentially tuned for transport or depolymerization functions.

Mitotic force generators and chromosome segregation

The mitotic spindle uses dynamic microtubules and mitotic motors to generate the pico-Newton scale forces that are needed to drive the mitotic movements that underlie chromosome capture, alignment and segregation. Here, we consider the biophysical and molecular basis of force-generation for chromosome movements in the spindle, and, with reference to the Drosophila embryo mitotic spindle, we briefly discuss how mathematical modeling can complement experimental analysis to illuminate the mechanisms of chromosome-to-pole motility during anaphase A and spindle elongation during anaphase B.

The Drosophila kinesin-13, KLP59D, impacts Pacman- and Flux-based chromosome movement

Chromosome movements are linked to the active depolymerization of spindle microtubule (MT) ends. Here we identify the kinesin-13 family member, KLP59D, as a novel and uniquely important regulator of spindle MT dynamics and chromosome motility in Drosophila somatic cells. During prometaphase and metaphase, depletion of KLP59D, which targets to centrosomes and outer kinetochores, suppresses the depolymerization of spindle pole-associated MT minus ends, thereby inhibiting poleward tubulin Flux. Subsequently, during anaphase, loss of KLP59D strongly attenuates chromatid-to-pole motion by suppressing the depolymerization of both minus and plus ends of kinetochore-associated MTs. The mechanism of KLP59D's impact on spindle MT plus and minus ends appears to differ. Our data support a model in which KLP59D directly depolymerizes kinetochore-associated plus ends during anaphase, but influences minus ends indirectly by localizing the pole-associated MT depolymerase KLP10A. Finally, electron microscopy indicates that, unlike the other Drosophila kinesin-13s, KLP59D is largely incapable of oligomerizing into MT-associated rings in vitro, suggesting that such structures are not a requisite feature of kinetochore-based MT disassembly and chromosome movements.

Motor domain phosphorylation and regulation of the Drosophila kinesin 13, KLP10A

Microtubule (MT)-destabilizing kinesin 13s perform fundamental roles throughout the cell cycle. In this study, we show that the Drosophila melanogaster kinesin 13, KLP10A, is phosphorylated in vivo at a conserved serine (S573) positioned within the alpha-helix 5 of the motor domain. In vitro, a phosphomimic KLP10A S573E mutant displays a reduced capacity to depolymerize MTs but normal affinity for the MT lattice. In cells, replacement of endogenous KLP10A with KLP10A S573E dampens MT plus end dynamics throughout the cell cycle, whereas a nonphosphorylatable S573A mutant apparently enhances activity during mitosis. Electron microscopy suggests that KLP10A S573 phosphorylation alters its association with the MT lattice, whereas molecular dynamics simulations reveal how KLP10A phosphorylation can alter the kinesin-MT interface without changing important structural features within the motor's core. Finally, we identify casein kinase 1alpha as a possible candidate for KLP10A phosphorylation. We propose a model in which phosphorylation of the KLP10A motor domain provides a regulatory switch controlling the time and place of MT depolymerization.

A new model for binding of kinesin 13 to curved microtubule protofilaments

Kinesin motor proteins use adenosine triphosphate hydrolysis to do work on microtubules (MTs). Most kinesins walk along the MT, but class 13 kinesins instead uniquely recognize MT ends and depolymerize MT protofilaments. We have used electron microscopy (EM) to understand the molecular interactions by which kinesin 13 performs these tasks. Although a construct of only the motor domain of kinesin 13 binds to every heterodimer of a tubulin ring, a construct containing the neck and the motor domain occupies alternate binding sites. Likewise, EM maps of the dimeric full-length (FL) protein exhibit alternate site binding but reveal density for only one of two motor heads. These results indicate that the second head of dimeric kinesin 13 does not have access to adjacent binding sites on the curved protofilament and suggest that the neck alone is sufficient to obstruct access. Additionally, the FL construct promotes increased stacking of rings compared with other constructs. Together, these data suggest a model for kinesin 13 depolymerization in which increased efficiency is achieved by binding of one kinesin 13 molecule to adjacent protofilaments.

ATPase cycle of the nonmotile kinesin NOD allows microtubule end tracking and drives chromosome movement

Segregation of nonexchange chromosomes during Drosophila melanogaster meiosis requires the proper function of NOD, a nonmotile kinesin-10. We have determined the X-ray crystal structure of the NOD catalytic domain in the ADP- and AMPPNP-bound states. These structures reveal an alternate conformation of the microtubule binding region as well as a nucleotide-sensitive relay of hydrogen bonds at the active site. Additionally, a cryo-electron microscopy reconstruction of the nucleotide-free microtubule-NOD complex shows an atypical binding orientation. Thermodynamic studies show that NOD binds tightly to microtubules in the nucleotide-free state, yet other nucleotide states, including AMPPNP, are weakened. Our pre-steady-state kinetic analysis demonstrates that NOD interaction with microtubules occurs slowly with weak activation of ADP product release. Upon rapid substrate binding, NOD detaches from the microtubule prior to the rate-limiting step of ATP hydrolysis, which is also atypical for a kinesin. We propose a model for NOD's microtubule plus-end tracking that drives chromosome movement.

Structure of the kinesin13-microtubule ring complex

To investigate the mechanism of kinesin13-induced microtubule depolymerization, we have calculated a three-dimensional (3D) map of the kinesin13-microtubule ring complex, using cryo-electron microscopy (cryo-EM) and image analysis. An atomic model of the complex was produced by docking the crystal structures of tubulin and a kinesin13 motor domain (MD) into the 3D map. The model reveals a snapshot of the depolymerization mechanism by providing a 3D view of the complex formed between the kinesin13 MD and a curved tubulin protofilament (pf). It suggests that contacts mediated by kinesin13 class-specific residues in the putative microtubule-binding site stabilize intra-dimer tubulin curvature. In addition, a tubulin-binding site on the kinesin13 MD was identified. Mutations at this class-conserved site selectively disrupt the formation of microtubule-associated ring complexes.

Analysis of cortical arrays from Tradescantia virginiana at high resolution reveals discrete microtubule subpopulations and demonstrates that confocal images of arrays can be misleading

Cortical microtubule arrays are highly organized networks involved in directing cellulose microfibril deposition within the cell wall. Their organization results from complex interactions between individual microtubules and microtubule-associated proteins. The precise details of these interactions are often not evident using optical microscopy. Using high-resolution scanning electron microscopy, we analyzed extensive regions of cortical arrays and identified two spatially discrete microtubule subpopulations that exhibited different stabilities. Microtubules that lay adjacent to the plasma membrane were often bundled and more stable than the randomly aligned, discordant microtubules that lay deeper in the cytoplasm. Immunolabeling revealed katanin at microtubule ends, on curves, or at sites along microtubules in line with neighboring microtubule ends. End binding 1 protein also localized along microtubules, at microtubule ends or junctions between microtubules, and on the plasma membrane in direct line with microtubule ends. We show fine bands in vivo that traverse and may encircle microtubules. Comparing confocal and electron microscope images of fluorescently tagged arrays, we demonstrate that optical images are misleading, highlighting the fundamental importance of studying cortical microtubule arrays at high resolution.

Kinetochore-microtubule interactions: the means to the end

Kinetochores are proteinaceous complexes containing dozens of components; they are assembled at centromeric DNA regions and provide the major microtubule attachment site on chromosomes during cell division. Recent studies have defined the kinetochore components comprising the direct interface with spindle microtubules, primarily through structural and functional analysis of the Ndc80 and Dam1 complexes. These studies have facilitated our understanding of how kinetochores remain attached to the end of dynamic microtubules and how proper orientation of a kinetochore-microtubule attachment is promoted on the mitotic spindle. In this article, we review these recent studies and summarize their key findings.

Mechanisms of mitotic spindle assembly and function

The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. The major structural elements of the spindle are microtubule polymers, whose intrinsic polarity and dynamic properties are critical for bipolar spindle organization and function. In most cell types, spindle microtubule nucleation occurs primarily at two centrosomes, which define the spindle poles, but microtubules can also be generated by the chromosomes and within the spindle itself. Many associated factors help organize the spindle, including molecular motors and regulators of microtubule dynamics. The past decade has provided a wealth of information on the molecular players that are critical for spindle assembly as well as a high-resolution view of the intricate movements and dynamics of the spindle microtubules and the chromosomes. In this chapter we provide a historical account of the key observations leading to current models of spindle assembly, as well as an up-to-date status report on this exciting field.

Rings, bracelets, sleeves, and chevrons: new structures of kinetochore proteins

Electron microscopy has recently revealed striking structural orderliness in kinetochore proteins and protein complexes that associate with microtubules. In addition to their astonishing appearance and intrinsic beauty, the structures are functionally informative. The Dam1 and Ndc80 complexes bind to the microtubule lattice as rings and chevrons, respectively. These structures give insight into how the kinetochore couples to dynamic microtubules, a process crucial to the accurate segregation of chromosomes. HURP and kinesin-13 arrange tubulin into sleeves and bracelets surrounding the microtubule lattice. These structures might reflect the ability of these proteins to modulate microtubule dynamics by interacting with specialized tubulin configurations. In this review, we compare and contrast the structure of these proteins and their interactions with microtubules to illustrate how they attach to and modulate the dynamics of microtubules.

Kinesin-13 regulates flagellar, interphase, and mitotic microtubule dynamics in Giardia intestinalis

Microtubule depolymerization dynamics in the spindle are regulated by kinesin-13, a nonprocessive kinesin motor protein that depolymerizes microtubules at the plus and minus ends. Here we show that a single kinesin-13 homolog regulates flagellar length dynamics, as well as other interphase and mitotic dynamics in Giardia intestinalis, a widespread parasitic diplomonad protist. Both green fluorescent protein-tagged kinesin-13 and EB1 (a plus-end tracking protein) localize to the plus ends of mitotic and interphase microtubules, including a novel localization to the eight flagellar tips, cytoplasmic anterior axonemes, and the median body. The ectopic expression of a kinesin-13 (S280N) rigor mutant construct caused significant elongation of the eight flagella with significant decreases in the median body volume and resulted in mitotic defects. Notably, drugs that disrupt normal interphase and mitotic microtubule dynamics also affected flagellar length in Giardia. Our study extends recent work on interphase and mitotic kinesin-13 functioning in metazoans to include a role in regulating flagellar length dynamics. We suggest that kinesin-13 universally regulates both mitotic and interphase microtubule dynamics in diverse microbial eukaryotes and propose that axonemal microtubules are subject to the same regulation of microtubule dynamics as other dynamic microtubule arrays. Finally, the present study represents the first use of a dominant-negative strategy to disrupt normal protein function in Giardia and provides important insights into giardial microtubule dynamics with relevance to the development of antigiardial compounds that target critical functions of kinesins in the giardial life cycle.

Architecture of the Dam1 kinetochore ring complex and implications for microtubule-driven assembly and force-coupling mechanisms

The Dam1 kinetochore complex is essential for chromosome segregation in budding yeast. This ten-protein complex self-assembles around microtubules, forming ring-like structures that move with depolymerizing microtubule ends, a mechanism with implications for cellular function. Here we used EM-based single-particle and helical analyses to define the architecture of the Dam1 complex at 30-A resolution and the self-assembly mechanism. Ring oligomerization seems to be facilitated by a conformational change upon binding to microtubules, suggesting that the Dam1 ring is not preformed, but self-assembles around kinetochore microtubules. The C terminus of the Dam1p protein, where most of the Aurora kinase Ipl1 phosphorylation sites reside, is in a strategic location to affect oligomerization and interactions with the microtubule. One of Ipl1's roles might be to fine-tune the coupling of the microtubule interaction with the conformational change required for oligomerization, with phosphorylation resulting in ring breakdown.

Molecular mechanisms of microtubule-dependent kinetochore transport toward spindle poles

In mitosis, kinetochores are initially captured by the lateral sides of single microtubules and are subsequently transported toward spindle poles. Mechanisms for kinetochore transport are not yet known. We present two mechanisms involved in microtubule-dependent poleward kinetochore transport in Saccharomyces cerevisiae. First, kinetochores slide along the microtubule lateral surface, which is mainly and probably exclusively driven by Kar3, a kinesin-14 family member that localizes at kinetochores. Second, kinetochores are tethered at the microtubule distal ends and pulled poleward as microtubules shrink (end-on pulling). Kinetochore sliding is often converted to end-on pulling, enabling more processive transport, but the opposite conversion is rare. The establishment of end-on pulling is partly hindered by Kar3, and its progression requires the Dam1 complex. We suggest that the Dam1 complexes, which probably encircle a single microtubule, can convert microtubule depolymerization into the poleward kinetochore-pulling force. Thus, microtubule-dependent poleward kinetochore transport is ensured by at least two distinct mechanisms.

The kinesin-13 proteins Kif2a, Kif2b, and Kif2c/MCAK have distinct roles during mitosis in human cells

The human genome has three unique genes coding for kinesin-13 proteins called Kif2a, Kif2b, and MCAK (Kif2c). Kif2a and MCAK have documented roles in mitosis, but the function of Kif2b has not been defined. Here, we show that Kif2b is expressed at very low levels in cultured cells and that GFP-Kif2b localizes predominately to centrosomes and midbodies, but also to spindle microtubules and transiently to kinetochores. Kif2b-deficient cells assemble monopolar or disorganized spindles. Chromosomes in Kif2b-deficient cells show typical kinetochore-microtubule attachments, but the velocity of movement is reduced approximately 80% compared with control cells. Some Kif2b-deficient cells attempt anaphase, but the cleavage furrow regresses and cytokinesis fails. Like Kif2a-deficient cells, bipolar spindle assembly can be restored to Kif2b-deficient cells by simultaneous deficiency of MCAK or Nuf2 or treatment with low doses of nocodazole. However, Kif2b-deficient cells are unique in that they assemble bipolar spindles when the pole focusing activities of NuMA and HSET are perturbed. These data demonstrate that Kif2b function is required for spindle assembly and chromosome movement and that the microtubule depolymerase activities of Kif2a, Kif2b, and MCAK fulfill distinct functions during mitosis in human cells.

The Ndc80 complex: hub of kinetochore activity

Kinetochores are protein scaffolds coordinating the process of chromosome segregation in mitosis. Kinetochore components are organized in functionally and topologically distinct domains that are designed to connect the sister chromatids to the mitotic spindle. The inner kinetochore proteins are in direct contact with the centromeric DNA, whilst the outer kinetochore proteins are responsible for binding to spindle microtubules. The conserved Ndc80 complex is implicated in several essential outer kinetochore functions, including microtubule binding and control of a safety device known as the spindle assembly checkpoint. Here, we describe how current work is contributing to unravel the complex endeavors of this essential kinetochore complex.

The interplay of the N- and C-terminal domains of MCAK control microtubule depolymerization activity and spindle assembly

Spindle assembly and accurate chromosome segregation require the proper regulation of microtubule dynamics. MCAK, a Kinesin-13, catalytically depolymerizes microtubules, regulates physiological microtubule dynamics, and is the major catastrophe factor in egg extracts. Purified GFP-tagged MCAK domain mutants were assayed to address how the different MCAK domains contribute to in vitro microtubule depolymerization activity and physiological spindle assembly activity in egg extracts. Our biochemical results demonstrate that both the neck and the C-terminal domain are necessary for robust in vitro microtubule depolymerization activity. In particular, the neck is essential for microtubule end binding, and the C-terminal domain is essential for tight microtubule binding in the presence of excess tubulin heterodimer. Our physiological results illustrate that the N-terminal domain is essential for regulating microtubule dynamics, stimulating spindle bipolarity, and kinetochore targeting; whereas the C-terminal domain is necessary for robust microtubule depolymerization activity, limiting spindle bipolarity, and enhancing kinetochore targeting. Unexpectedly, robust MCAK microtubule (MT) depolymerization activity is not needed for sperm-induced spindle assembly. However, high activity is necessary for proper physiological MT dynamics as assayed by Ran-induced aster assembly. We propose that MCAK activity is spatially controlled by an interplay between the N- and C-terminal domains during spindle assembly.

Microtubule polymerases and depolymerases

The variety of shapes and sizes of the microtubule cytoskeleton is as great as the number of different cell types. This large variety is a consequence of the dynamic properties of microtubules, which allow them to adopt distributions of arbitrary size and form. How is the distribution of microtubule lengths controlled? Recent work suggests that the length distribution is controlled, at least in part, by the activity of microtubule polymerases and depolymerases, which accelerate microtubule growth and shrinkage. Specifically, biochemical and single-molecule studies have shown how MCAK (kinesin-13) and Kip3p (kinesin-8) accelerate depolymerization and how XMAP215 may accelerate growth. Studies on the yeast Dam1 complex have shown how proteins can couple a cellular structure, the kinetochore, to the ends of polymerizing and depolymerizing microtubules.