The kinesin-related protein MCAK is a microtubule depolymerase that forms an ATP-hydrolyzing complex at microtubule ends

The contribution of αβ-tubulin curvature to microtubule dynamics

Microtubules are dynamic polymers of αβ-tubulin that form diverse cellular structures, such as the mitotic spindle for cell division, the backbone of neurons, and axonemes. To control the architecture of microtubule networks, microtubule-associated proteins (MAPs) and motor proteins regulate microtubule growth, shrinkage, and the transitions between these states. Recent evidence shows that many MAPs exert their effects by selectively binding to distinct conformations of polymerized or unpolymerized αβ-tubulin. The ability of αβ-tubulin to adopt distinct conformations contributes to the intrinsic polymerization dynamics of microtubules. αβ-Tubulin conformation is a fundamental property that MAPs monitor and control to build proper microtubule networks.

Regulation of localization and activity of the microtubule depolymerase MCAK

Mitotic Centromere Associated Kinesin (MCAK) is a potent microtubule depolymerizing and catastrophe-inducing factor, which uses the energy of ATP hydrolysis to destabilize microtubule ends. MCAK is localized to inner centromeres, kinetochores and spindle poles of mitotic cells, and is also present in the cytoplasm. Both in interphase and in mitosis, MCAK can specifically accumulate at the growing microtubule ends. Here we discuss the mechanisms, which modulate subcellular localization and activity of MCAK through the interaction with the End Binding (EB) proteins and phosphorylation.

Kinesin-8s hang on by a tail

Accurate segregation of genetic material into two daughter cells is essential for organism reproduction, development, and survival. The cell assembles a macromolecular structure called the mitotic spindle, which is composed of dynamic microtubules (MTs) and many associated proteins that assemble the spindle and drive the segregation of the chromosomes. Members of the kinesin superfamily of MT associated proteins use the energy of ATP hydrolysis to help organize the spindle, to transport cargo within the spindle, and to regulate spindle MT dynamics. The Kinesin-8 and Kinesin-13 families are involved in controlling mitotic spindle morphology, spindle positioning, and chromosome movement. While both kinesin families are MT destabilizing enzymes, it is unclear whether their mechanisms of MT destabilization are mechanistically similar or how they act to destabilize MTs. Recently, three groups identified an additional MT binding domain within the tail of Kinesin-8s that is essential for their roles in regulating MT dynamics and chromosome positioning.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

XTACC3-XMAP215 association reveals an asymmetric interaction promoting microtubule elongation

chTOG is a conserved microtubule polymerase that catalyses the addition of tubulin dimers to promote microtubule growth. chTOG interacts with TACC3, a member of the transforming acidic coiled-coil (TACC) family. Here we analyse their association using the Xenopus homologues, XTACC3 (TACC3) and XMAP215 (chTOG), dissecting the mechanism by which their interaction promotes microtubule elongation during spindle assembly. Using SAXS, we show that the TACC domain (TD) is an elongated structure that mediates the interaction with the C terminus of XMAP215. Our data suggest that one TD and two XMAP215 molecules associate to form a four-helix coiled-coil complex. A hybrid methods approach was used to define the precise regions of the TACC heptad repeat and the XMAP215 C terminus required for assembly and functioning of the complex. We show that XTACC3 can induce the recruitment of larger amounts of XMAP215 by increasing its local concentration, thereby promoting efficient microtubule elongation during mitosis.

Rac1 and Aurora A regulate MCAK to polarize microtubule growth in migrating endothelial cells

A Rac1–Aurora A–MCAK signaling pathway mediates endothelial cell polarization and directional migration by promoting regional differences in microtubule dynamics in the leading and trailing cell edges.

Endothelial cells (ECs) migrate directionally during angiogenesis and wound healing by polarizing to extracellular cues to guide directional movement. EC polarization is controlled by microtubule (MT) growth dynamics, which are regulated by MT-associated proteins (MAPs) that alter MT stability. Mitotic centromere-associated kinesin (MCAK) is a MAP that promotes MT disassembly within the mitotic spindle, yet its function in regulating MT dynamics to promote EC polarity and migration has not been investigated. We used high-resolution fluorescence microscopy coupled with computational image analysis to elucidate the role of MCAK in regulating MT growth dynamics, morphology, and directional migration of ECs. Our results show that MCAK-mediated depolymerization of MTs is specifically targeted to the trailing edge of polarized wound-edge ECs. Regulation of MCAK function is dependent on Aurora A kinase, which is regionally enhanced by signaling from the small guanosine triphosphatase, Rac1. Thus, a Rac1–Aurora A–MCAK signaling pathway mediates EC polarization and directional migration by promoting regional differences in MT dynamics in the leading and trailing cell edges.

Stu2, the budding yeast XMAP215/Dis1 homolog, promotes assembly of yeast microtubules by increasing growth rate and decreasing catastrophe frequency

Stu2 is the budding yeast member of the XMAP215/Dis1 family of microtubule polymerases. It is essential in cell division, allowing proper spindle orientation and metaphase chromosome alignment, as well as spindle elongation during anaphase. Despite Stu2 having a phenotype that suggests it promotes microtubule growth, like the other members of the XMAP215/Dis1 family, previous studies with purified Stu2 indicate only that it antagonizes microtubule growth. One potential explanation for these contradictory findings is that the assays were performed with mammalian brain tubulin, which may not be the right substrate to test the activity of Stu2 given that yeast and brain tubulins are quite divergent in sequence and that the vertebrate tubulins are subject to many post-translational modifications. To test this possibility, we reconstituted the activity of Stu2 with purified budding yeast tubulin. We found that Stu2 accelerated microtubule growth in yeast tubulin by severalfold, similar to the acceleration reported for XMAP215 in porcine brain tubulin. Furthermore, Stu2 accelerated polymerization in yeast tubulin to a much greater extent than in porcine brain tubulin, and the concentration of Stu2 required to reach 50% maximum activity in yeast tubulin was nearly 2 orders of magnitude lower than that in porcine brain tubulin. We conclude that Stu2 is a microtubule polymerase, like its relatives, and that its activity is considerably higher in yeast tubulin compared with mammalian brain tubulin. The biochemical properties of Stu2 reported here account for many of the phenotypes of Stu2 observed in cells.

The kinesin-13 KLP10A motor regulates oocyte spindle length and affects EB1 binding without altering microtubule growth rates

Kinesin-13 motors are unusual in that they do not walk along microtubules, but instead diffuse to the ends, where they remove tubulin dimers, regulating microtubule dynamics. Here we show that Drosophila kinesin-13 klp10A regulates oocyte meiosis I spindle length and is haplo-insufficient – KLP10A, reduced by RNAi or a loss-of-function P element insertion mutant, results in elongated and mispositioned oocyte spindles, and abnormal cortical microtubule asters and aggregates. KLP10A knockdown by RNAi does not significantly affect microtubule growth rates in oocyte spindles, but, unexpectedly, EB1 binding and unbinding are slowed, suggesting a previously unobserved role for kinesin-13 in mediating EB1 binding interactions with microtubules. Kinesin-13 may regulate spindle length both by disassembling subunits from microtubule ends and facilitating EB1 binding to plus ends. We also observe an increased number of paused microtubules in klp10A RNAi knockdown spindles, consistent with a reduced frequency of microtubule catastrophes. Overall, our findings indicate that reduced kinesin-13 decreases microtubule disassembly rates and affects EB1 interactions with microtubules, rather than altering microtubule growth rates, causing spindles to elongate and abnormal cortical microtubule asters and aggregates to form.

DHTP is an allosteric inhibitor of the kinesin-13 family of microtubule depolymerases

The kinesin-13 family of microtubule depolymerases is a major regulator of microtubule dynamics. RNA interference-induced knockdown studies have highlighted their importance in many cell division processes including spindle assembly and chromosome segregation. Since microtubule turnovers and most mitotic events are relatively rapid (in minutes or seconds), developing tools that offer faster control over protein functions is therefore essential to more effectively interrogate kinesin-13 activities in living cells. Here, we report the identification and characterization of a selective allosteric kinesin-13 inhibitor, DHTP. Using high resolution microscopy, we show that DHTP is cell permeable and can modulate microtubule dynamics in cells.

Regulation of microtubule minus-end dynamics by CAMSAPs and Patronin

The microtubule (MT) cytoskeleton plays an essential role in mitosis, intracellular transport, cell shape, and cell migration. The assembly and disassembly of MTs, which can occur through the addition or loss of subunits at the plus- or minus-ends of the polymer, is essential for MTs to carry out their biological functions. A variety of proteins act on MT ends to regulate their dynamics, including a recently described family of MT minus-end binding proteins called calmodulin-regulated spectrin-associated protein (CAMSAP)/Patronin/Nezha. Patronin, the single member of this family in Drosophila, was previously shown to stabilize MT minus-ends against depolymerization in vitro and in vivo. Here, we show that all three mammalian CAMSAP family members also bind specifically to MT minus-ends and protect them against kinesin-13-induced depolymerization. However, these proteins differ in their abilities to suppress tubulin addition at minus-ends and to dissociate from MTs. CAMSAP1 does not interfere with polymerization and tracks along growing minus-ends. CAMSAP2 and CAMSAP3 decrease the rate of tubulin incorporation and remain bound, thereby creating stretches of decorated MT minus-ends. By using truncation analysis, we find that somewhat different minimal domains of CAMSAP and Patronin are involved in minus-end localization. However, we find that, in both cases, a highly conserved C-terminal domain and a more variable central domain cooperate to suppress minus-end dynamics in vitro and that both regions are required to stabilize minus-ends in Drosophila S2 cells. These results show that members of the CAMSAP/Patronin family all localize to and protect minus-ends but have evolved distinct effects on MT dynamics.

Aurora B inhibits MCAK activity through a phosphoconformational switch that reduces microtubule association

BACKGROUND

Proper spindle assembly and chromosome segregation rely on precise microtubule dynamics, which are governed in part by the kinesin-13 MCAK. MCAK microtubule depolymerization activity is inhibited by Aurora B-dependent phosphorylation, but the mechanism of this inhibition is not understood.

RESULTS

Here, we develop the first Förster resonance energy transfer (FRET)-based biosensor for MCAK and show that MCAK in solution exists in a closed conformation mediated by an interaction between the C-terminal domain (CT) and the neck. Using fluorescence lifetime imaging (FLIM) we show that MCAK bound to microtubule ends is closed relative to MCAK associated with the microtubule lattice. Aurora B phosphorylation at S196 in the neck opens MCAK conformation and diminishes the interaction between the CT and the neck. Using FLIM and TIRF imaging, we find that changes in MCAK conformation are associated with a decrease in MCAK affinity for the microtubule.

CONCLUSIONS

Unlike motile kinesins, which are open when doing work, the high-affinity binding state for microtubule-depolymerizing kinesins is in a closed conformation. Phosphorylation switches MCAK conformation, which inhibits its ability to interact with microtubules and reduces its microtubule depolymerization activity. This work shows that the conformational model proposed for regulating kinesin activity is not universal and that microtubule-depolymerizing kinesins utilize a distinct conformational mode to regulate affinity for the microtubule, thus controlling their catalytic efficiency. Furthermore, our work provides a mechanism by which the robust microtubule depolymerization activity of kinesin-13s can be rapidly modulated to control cellular microtubule dynamics.

Direct regulation of microtubule dynamics by KIF17 motor and tail domains

KIF17 is a kinesin-2 family motor that interacts with EB1 at microtubule (MT) plus-ends and contributes to MT stabilization in epithelial cells. The mechanism by which KIF17 affects MTs and how its activity is regulated are not yet known. Here, we show that EB1 and the KIF17 autoinhibitory tail domain (KIF17-Tail) interacted competitively with the KIF17 catalytic motor domain (K370). Both EB1 and KIF17-Tail decreased the K0.5MT of K370, with opposing effects on MT-stimulated ATPase activity. Importantly, K370 had independent effects on MT dynamic instability, resulting in formation of long MTs without affecting polymerization rate or total polymer mass. K370 also inhibited MT depolymerization induced by dilution in vitro and by nocodazole in cells, suggesting that it acts by protecting MT plus-ends. Interestingly, KIF17-Tail bound MTs and tubulin dimers, delaying initial MT polymerization in vitro and MT regrowth in cells. However, neither EB1 nor KIF17-Tail affected K370-mediated MT polymerization or stabilization significantly in vitro, and EB1 was dispensable for MT stabilization by K370 in cells. Thus, although EB1 and KIF17-Tail may coordinate KIF17 catalytic activity, our data reveal a novel and direct role for KIF17 in regulating MT dynamics.

Centrosomal protein CEP104 (Chlamydomonas FAP256) moves to the ciliary tip during ciliary assembly

The ciliary tip has been implicated in ciliary assembly and disassembly, and signaling, yet information on its protein composition is limited. Using comparative, quantitative proteomics based on the fact that tip proteins will be approximately twice as concentrated in half-length compared with full-length flagella, we have identified FAP256 as a tip protein in Chlamydomonas. FAP256 localizes to the tips of both central pair and outer doublet microtubules (MTs) and it remains at the tip during flagellar assembly and disassembly. Similarly, its vertebrate counterpart, CEP104, localizes on the distal ends of both centrioles of nondividing cells until the mother centriole forms a cilium and then localizes at the tip of the elongating cilium. A null mutant of FAP256 in Chlamydomonas and RNAi in vertebrate cells showed that FAP256/CEP104 is required for ciliogenesis in a high percentage of cells. In those cells that could form cilia, there were structural deformities at the ciliary tips.

Lateral to end-on conversion of chromosome-microtubule attachment requires kinesins CENP-E and MCAK

BACKGROUND

Proper attachment of chromosomes to microtubules is crucial for the accurate segregation of chromosomes. Human chromosomes attach initially to lateral walls of microtubules. Subsequently, attachments to lateral walls disappear and attachments to microtubule ends (end-on attachments) predominate. While it is known in yeasts that lateral to end-on conversion of attachments occurs through a multistep process, equivalent conversion steps in humans remain unknown.

RESULTS

By developing a high-resolution imaging assay to visualize intermediary steps of the lateral to end-on conversion process, we show that the mechanisms that bring a laterally bound chromosome and its microtubule end closer to each other are indispensable for proper end-on attachment because laterally attached chromosomes seldom detach. We show that end-on conversion requires (1) the plus-end-directed motor CENP-E to tether the lateral kinetochore onto microtubule walls and (2) the microtubule depolymerizer MCAK to release laterally attached microtubules after a partial end-on attachment is formed.

CONCLUSIONS

By uncovering a CENP-E mediated wall-tethering event and a MCAK-mediated wall-removing event, we establish that human chromosome-microtubule attachment is achieved through a set of deterministic sequential events rather than stochastic direct capture of microtubule ends.

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.

Intracellular targets for a phosphotyrosine peptidomimetic include the mitotic kinesin, MCAK

SH2 domains are attractive targets for chemotherapeutic agents due to their involvement in the formation of protein-protein interactions critical to many signal transduction cascades. Little is known, however, about how synthetic SH2 domain ligands would influence the growth properties of tumor cells or with which intracellular proteins they would interact due to their highly charged nature and enzymatic lability. In this study, a prodrug delivery strategy was used to introduce an enzymatically stable, phosphotyrosine peptidomimetic into tumor cells. When tested in a human tumor cell panel, the prodrug exhibited a preference for inhibiting the growth of leukemia and lymphoma cells. In these cells, it was largely cytostatic and induced endoreduplication and the appearance of midbodies. Proteomic analyses identified multiple targets that included mitotic centromere-associated kinesin (MCAK). Molecular modeling studies suggested the ATP-binding site on MCAK as the likely site of drug interaction. Consistent with this, ATP inhibited the drug-MCAK interaction and the drug inhibited MCAK ATPase activity. Accordingly, the effects of the prodrug on the assembly of the mitotic spindle and alignment of chromosomes were consistent with the identification of MCAK as an important intracellular target.

Aurora at the pole and equator: overlapping functions of Aurora kinases in the mitotic spindle

The correct assembly and timely disassembly of the mitotic spindle is crucial for the propagation of the genome during cell division. Aurora kinases play a central role in orchestrating bipolar spindle establishment, chromosome alignment and segregation. In most eukaryotes, ranging from amoebas to humans, Aurora activity appears to be required both at the spindle pole and the kinetochore, and these activities are often split between two different Aurora paralogues, termed Aurora A and B. Polar and equatorial functions of Aurora kinases have generally been considered separately, with Aurora A being mostly involved in centrosome dynamics, whereas Aurora B coordinates kinetochore attachment and cytokinesis. However, double inactivation of both Aurora A and B results in a dramatic synergy that abolishes chromosome segregation. This suggests that these two activities jointly coordinate mitotic progression. Accordingly, recent evidence suggests that Aurora A and B work together in both spindle assembly in metaphase and disassembly in anaphase. Here, we provide an outlook on these shared functions of the Auroras, discuss the evolution of this family of mitotic kinases and speculate why Aurora kinase activity may be required at both ends of the spindle microtubules.

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.

Aurora B kinase is required for cytokinesis through effecting spindle structure

The spindle, composed of microtubules and associated proteins, serves as major machinery for cell division. Aurora B is a chromosome passenger protein that has important functions in mitosis. Its dynamic distribution at mitosis goes along with spindle structure and dynamics. We used the siRNA technique to knockdown protein expression and immunofluorescence technique to follow Aurora B during mitosis. Aurora B regulates microtubule plus ends as mitosis progresses, including both kinetochore and polar microtubules. Interactions between Aurora B and polar microtubule plus ends lead to failure in cytokinesis and abnormal midbody structure. We think Aurora B may not only play a role as kinase, but regulate microtubule plus ends.

Building complexity: insights into self-organized assembly of microtubule-based architectures

Successful completion of diverse cellular functions, such as mitosis, positioning organelles, and assembling cilia, depends on the proper assembly of microtubule-based structures. While essentially all of the proteins needed to assemble these structures are now known, we cannot explain how even simple features such as size and shape are determined. As steps toward filling this knowledge gap, there have been several recent efforts toward reconstituting, with purified proteins, the basic structural motifs that recur in diverse cytoskeletal arrays. We discuss these studies and highlight how they shed light on the self-organized assembly of complex and dynamic cytoskeleton-based cellular structures.

KIF19A is a microtubule-depolymerizing kinesin for ciliary length control

Cilia control homeostasis of the mammalian body by generating fluid flow. It has long been assumed that ciliary length-control mechanisms are essential for proper flow generation, because fluid flow generation is a function of ciliary length. However, the molecular mechanisms of ciliary length control in mammals remain elusive. Here, we suggest that KIF19A, a member of the kinesin superfamily, regulates ciliary length by depolymerizing microtubules at the tips of cilia. Kif19a(-/-) mice displayed hydrocephalus and female infertility phenotypes due to abnormally elongated cilia that cannot generate proper fluid flow. KIF19A localized to cilia tips, and recombinant KIF19A controlled the length of microtubules polymerized from axonemes in vitro. KIF19A had ATP-dependent microtubule-depolymerizing activity mainly at the plus end of microtubules. Our results indicated a molecular mechanism of ciliary length regulation in mammals, which plays an important role in the maintenance of the mammalian body.

Microtubule plus-ends within a mitotic cell are 'moving platforms' with anchoring, signalling and force-coupling roles

The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.

Mitotic centromere-associated kinase (MCAK/Kif2C) regulates cellular senescence in human primary cells through a p53-dependent pathway

Mitotic centromere-associated kinase (MCAK/Kif2C) plays a critical role in chromosome movement and segregation with ATP-dependent microtubule depolymerase activity. However, its role in cellular senescence remains unclear. MCAK/Kif2C expression decreased in human primary cells under replicative and premature senescence. MCAK/Kif2C down-regulation in young cells induced premature senescence. MCAK/Kif2C overexpression in old cells partially reversed cell senescence. Senescence phenotypes by MCAK/Kif2C knockdown were observed in p16-knockdown cells, but not in p53-knockdown cells. These results suggest that MCAK/Kif2C plays an important role in the regulation of cellular senescence through a p53-dependent pathway and might contribute to tissue/organism aging and protection of cellular transformation.

Move in for the kill: motile microtubule regulators

The stereotypical function of kinesin superfamily motors is to transport cargo along microtubules. However, some kinesins also shape the microtubule track by regulating microtubule assembly and disassembly. Recent work has shown that the kinesin-8 family of motors emerge as key regulators of cellular microtubule length. The studied kinesin-8s are highly processive motors that walk towards the microtubule plus-end. Once at plus-ends, they have complex effects on polymer dynamics; kinesin-8s either destabilize or stabilize microtubules, depending on the context. This review focuses on the mechanisms underlying kinesin-8-microtubule interactions and microtubule length control. We compare and contrast kinesin-8s with the other major microtubule-regulating kinesins (kinesin-4 and kinesin-13), to survey the current understanding of the diverse ways that kinesins control microtubule dynamics.

The Ndc80 kinetochore complex directly modulates microtubule dynamics

The conserved Ndc80 complex is an essential microtubule-binding component of the kinetochore. Recent findings suggest that the Ndc80 complex influences microtubule dynamics at kinetochores in vivo. However, it was unclear if the Ndc80 complex mediates these effects directly, or by affecting other factors localized at the kinetochore. Using a reconstituted system in vitro, we show that the human Ndc80 complex directly stabilizes the tips of disassembling microtubules and promotes rescue (the transition from microtubule shortening to growth). In vivo, an N-terminal domain in the Ndc80 complex is phosphorylated by the Aurora B kinase. Mutations that mimic phosphorylation of the Ndc80 complex prevent stable kinetochore-microtubule attachment, and mutations that block phosphorylation damp kinetochore oscillations. We find that the Ndc80 complex with Aurora B phosphomimetic mutations is defective at promoting microtubule rescue, even when robustly coupled to disassembling microtubule tips. This impaired ability to affect dynamics is not simply because of weakened microtubule binding, as an N-terminally truncated complex with similar binding affinity is able to promote rescue. Taken together, these results suggest that in addition to regulating attachment stability, Aurora B controls microtubule dynamics through phosphorylation of the Ndc80 complex.

A designed ankyrin repeat protein selected to bind to tubulin caps the microtubule plus end

Microtubules are cytoskeleton filaments consisting of αβ-tubulin heterodimers. They switch between phases of growth and shrinkage. The underlying mechanism of this property, called dynamic instability, is not fully understood. Here, we identified a designed ankyrin repeat protein (DARPin) that interferes with microtubule assembly in a unique manner. The X-ray structure of its complex with GTP-tubulin shows that it binds to the β-tubulin surface exposed at microtubule (+) ends. The details of the structure provide insight into the role of GTP in microtubule polymerization and the conformational state of tubulin at the very microtubule end. They show in particular that GTP facilitates the tubulin structural switch that accompanies microtubule assembly but does not trigger it in unpolymerized tubulin. Total internal reflection fluorescence microscopy revealed that the DARPin specifically blocks growth at the microtubule (+) end by a selective end-capping mechanism, ultimately favoring microtubule disassembly from that end. DARPins promise to become designable tools for the dissection of microtubule dynamic properties selective for either of their two different ends.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

MCAK activity at microtubule tips regulates spindle microtubule length to promote robust kinetochore attachment

The kinesin MCAK binds to end-binding proteins and antagonizes centrosome separation and promotes robust kinetochore attachments to spindle microtubules.

Mitotic centromere-associated kinesin (MCAK) is a microtubule-depolymerizing kinesin-13 member that can track with polymerizing microtubule tips (hereafter referred to as tip tracking) during both interphase and mitosis. MCAK tracks with microtubule tips by binding to end-binding proteins (EBs) through the microtubule tip localization signal SKIP, which lies N terminal to MCAK’s neck and motor domain. The functional significance of MCAK’s tip-tracking behavior during mitosis has never been explained. In this paper, we identify and define a mitotic function specific to the microtubule tip–associated population of MCAK: negative regulation of microtubule length within the assembling bipolar spindle. This function depends on MCAK’s ability to bind EBs and track with polymerizing nonkinetochore microtubule tips. Although this activity antagonizes centrosome separation during bipolarization, it ultimately benefits the dividing cell by promoting robust kinetochore attachments to the spindle microtubules.

Coupling of kinesin ATP turnover to translocation and microtubule regulation: one engine, many machines

The cycle of ATP turnover is integral to the action of motor proteins. Here we discuss how variation in this cycle leads to variation of function observed amongst members of the kinesin superfamily of microtubule associated motor proteins. Variation in the ATP turnover cycle among superfamily members can tune the characteristic kinesin motor to one of the range of microtubule-based functions performed by kinesins. The speed at which ATP is hydrolysed affects the speed of translocation. The ratio of rate constants of ATP turnover in relation to association and dissociation from the microtubule influence the processivity of translocation. Variation in the rate-limiting step of the cycle can reverse the way in which the motor domain interacts with the microtubule producing non-motile kinesins. Because the ATP turnover cycle is not fully understood for the majority of kinesins, much work remains to show how the kinesin engine functions in such a wide variety of molecular machines.

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.

S. pombe kinesins-8 promote both nucleation and catastrophe of microtubules

The kinesins-8 were originally thought to be microtubule depolymerases, but are now emerging as more versatile catalysts of microtubule dynamics. We show here that S. pombe Klp5-436 and Klp6-440 are non-processive plus-end-directed motors whose in vitro velocities on S. pombe microtubules at 7 and 23 nm s(-1) are too slow to keep pace with the growing tips of dynamic interphase microtubules in living S. pombe. In vitro, Klp5 and 6 dimers exhibit a hitherto-undescribed combination of strong enhancement of microtubule nucleation with no effect on growth rate or catastrophe frequency. By contrast in vivo, both Klp5 and Klp6 promote microtubule catastrophe at cell ends whilst Klp6 also increases the number of interphase microtubule arrays (IMAs). Our data support a model in which Klp5/6 bind tightly to free tubulin heterodimers, strongly promoting the nucleation of new microtubules, and then continue to land as a tubulin-motor complex on the tips of growing microtubules, with the motors then dissociating after a few seconds residence on the lattice. In vivo, we predict that only at cell ends, when growing microtubule tips become lodged and their growth slows down, will Klp5/6 motor activity succeed in tracking growing microtubule tips. This mechanism would allow Klp5/6 to detect the arrival of microtubule tips at cells ends and to amplify the intrinsic tendency for microtubules to catastrophise in compression at cell ends. Our evidence identifies Klp5 and 6 as spatial regulators of microtubule dynamics that enhance both microtubule nucleation at the cell centre and microtubule catastrophe at the cell ends.

JWA enhances As₂O₃-induced tubulin polymerization and apoptosis via p38 in HeLa and MCF-7 cells

Arsenic trioxide (As₂O₃) has potential anti-cancer activity against a wide range of carcinomas via apoptosis induction or oncoprotein degradation. The mechanisms involved are not fully elucidated. Here, we demonstrated that As₂O₃ induced-apoptosis in HeLa and MCF-7 cancer cells was in part triggered by tubulin polymerization. High expression of JWA promoted tubulin polymerization and increased the sensitivity of the cancer cells to As₂O₃. The activation of the p38 MAPK (mitogen-activated protein kinases) signaling pathway was found to contribute to JWA-promoted tubulin polymerization. Our results suggest that JWA may serve as an effective enhancer of microtubule-targeted As₂O₃ anti-cancer therapy.

Depolymerizing kinesins Kip3 and MCAK shape cellular microtubule architecture by differential control of catastrophe

Microtubules are dynamic filaments whose ends alternate between periods of slow growth and rapid shortening as they explore intracellular space and move organelles. A key question is how regulatory proteins modulate catastrophe, the conversion from growth to shortening. To study this process, we reconstituted microtubule dynamics in the absence and presence of the kinesin-8 Kip3 and the kinesin-13 MCAK. Surprisingly, we found that, even in the absence of the kinesins, the microtubule catastrophe frequency depends on the age of the microtubule, indicating that catastrophe is a multistep process. Kip3 slowed microtubule growth in a length-dependent manner and increased the rate of aging. In contrast, MCAK eliminated the aging process. Thus, both kinesins are catastrophe factors; Kip3 mediates fine control of microtubule length by narrowing the distribution of maximum lengths prior to catastrophe, whereas MCAK promotes rapid restructuring of the microtubule cytoskeleton by making catastrophe a first-order random process.

Phosphatidylinositol 4-phosphate 5-kinase alpha (PIPKα) regulates neuronal microtubule depolymerase kinesin, KIF2A and suppresses elongation of axon branches

Neuronal morphology is regulated by cytoskeletons. Kinesin superfamily protein 2A (KIF2A) depolymerizes microtubules (MTs) at growth cones and regulates axon pathfinding. The factors controlling KIF2A in neurite development remain totally elusive. Here, using immunoprecipitation with an antibody specific to KIF2A, we identified phosphatidylinositol 4-phosphate 5-kinase alpha (PIPKα) as a candidate membrane protein that regulates the activity of KIF2A. Yeast two-hybrid and biochemical assays demonstrated direct binding between KIF2A and PIPKα. Partial colocalization of the clusters of punctate signals for these two molecules was detected by confocal microscopy and photoactivated localization microscopy. Additionally, the MT-depolymerizing activity of KIF2A was enhanced in the presence of PIPKα in vitro and in vivo. PIPKα suppressed the elongation of axon branches in a KIF2A-dependent manner, suggesting a unique PIPK-mediated mechanism controlling MT dynamics in neuronal development.

Islands containing slowly hydrolyzable GTP analogs promote microtubule rescues

Microtubules are dynamic polymers of GTP- and GDP-tubulin that undergo stochastic transitions between growing and shrinking phases. Rescues, the conversion from shrinking to growing, have recently been proposed to be to the result of regrowth at GTP-tubulin islands within the lattice of growing microtubules. By introducing mixed GTP/GDP/GMPCPP (GXP) regions within the lattice of dynamic microtubules, we reconstituted GXP islands in vitro (GMPCPP is the slowly hydrolyzable GTP analog guanosine-5′-[(α,β)-methyleno]triphosphate). We found that such islands could reproducibly induce rescues and that the probability of rescue correlated with both the size of the island and the percentage of GMPCPP-tubulin within the island. The islands slowed the depolymerization rate of shortening microtubules and promoted regrowth more readily than GMPCPP seeds. Together, these findings provide new mechanistic insights supporting the possibility that rescues could be triggered by enriched GTP-tubulin regions and present a new tool for studying such rescue events in vitro.

K-fibre minus ends are stabilized by a RanGTP-dependent mechanism essential for functional spindle assembly

Chromosome segregation requires the formation of K-fibres, microtubule bundles that attach sister kinetochores to spindle poles. Most K-fibre microtubules originate around the chromosomes through a non-centrosomal RanGTP-dependent pathway and become oriented with the plus ends attached to the kinetochore and the minus ends focused at the spindle poles. The capture and stabilization of microtubule plus ends at the kinetochore has been extensively studied but very little is known on how their minus-end dynamics are controlled. Here we show that MCRS1 is a RanGTP-regulated factor essential for non-centrosomal microtubule assembly. MCRS1 localizes to the minus ends of chromosomal microtubules and K-fibres, where it protects them from depolymerization. Our data reveal the existence of a mechanism that stabilizes the minus ends of chromosomal microtubules and K-fibres, and is essential for the assembly of a functional bipolar spindle.

A non-motor microtubule binding site is essential for the high processivity and mitotic function of kinesin-8 Kif18A

Background

Members of the kinesin-8 subfamily are plus end-directed molecular motors that accumulate at the plus-ends of kinetochore-microtubules (kt-MTs) where they regulate MT dynamics. Loss of vertebrate kinesin-8 function induces hyperstable MTs and elongated mitotic spindles accompanied by severe chromosome congression defects. It has been reported that the motility of human kinesin-8, Kif18A, is required for its accumulation at the plus tips of kt-MTs.

Methodology/Findings

Here, we investigate how Kif18A localizes to the plus-ends of kt-MTs. We find that Kif18A lacking its C-terminus does not accumulate on the tips of kt-MTs and fails to fulfill its mitotic function. In vitro studies reveal that Kif18A possesses a non-motor MT binding site located within its C-proximal 121 residues. Using single molecule measurements we find that Kif18A is a highly processive motor and, furthermore, that the C-terminal tail is essential for the high processivity of Kif18A.

Conclusion/Significance

These results show that Kif18A like its yeast orthologue is a highly processive motor. The ability of Kif18A to walk on MTs for a long distance without dissociating depends on a non-motor MT binding site located at the C-terminus of Kif18A. This C-proximal tail of Kif18A is essential for its plus-end accumulation and mitotic function. These findings advance our understanding of how Kif18A accumulates at the tips of kt-MTs to fulfill its function in mitosis.

Aurora-A inactivation causes mitotic spindle pole fragmentation by unbalancing microtubule-generated forces

BACKGROUND

Aurora-A is an oncogenic kinase playing well-documented roles in mitotic spindle organisation. We previously found that Aurora-A inactivation yields the formation of spindles with fragmented poles that can drive chromosome mis-segregation. Here we have addressed the mechanism through which Aurora-A activity regulates the structure and cohesion of spindle poles.

RESULTS

We inactivated Aurora-A in human U2OS osteosarcoma cells either by RNA-interference-mediated silencing or treating cultures with the specific inhibitor MLN8237. We show that mitotic spindle pole fragmentation induced by Aurora-A inactivation is associated with microtubule hyperstabilisation. Silencing of the microtubule-stabilising factor ch-TOG prevents spindle pole fragmentation caused by inactivation of Aurora-A alone and concomitantly reduces the hyperstabilisation of microtubules. Furthermore, decreasing pole-directed spindle forces by inhibition of the Eg5 kinesin, or by destabilisation of microtubule-kinetochore attachments, also prevents pole fragmentation in Aurora-A-inactivated mitoses.

CONCLUSIONS

Our findings indicate that microtubule-generated forces are imbalanced in Aurora-A-defective cells and exert abnormal pressure at the level of spindle poles, ultimately causing their fragmentation. This study therefore highlights a novel role of the Aurora-A kinase in regulating the balance between microtubule forces during bipolar spindle assembly.

Elucidating the functionality of kinesins: an overview of small molecule inhibitors

Kinesin motor proteins are ubiquitously involved in multiple fundamental cellular processes, coordinating transport and mediating changes to cellular architecture. Thus, specific small molecule kinesin inhibitors can shed new light on the functions of kinesins and the dynamic roles in which they participate. Here we review the range of known inhibitors, their key characteristics, and specificity, and discuss their potential suitability for chemical genetics as starting points to further investigate complex kinesin-mediated processes.

Regulation of microtubule dynamics by kinesins

The simple mechanistic and functional division of the kinesin family into either active translocators or non-motile microtubule depolymerases was initially appropriate but is now proving increasingly unhelpful, given evidence that several translocase kinesins can affect microtubule dynamics, whilst non-translocase kinesins can promote microtubule assembly and depolymerisation. Such multi-role kinesins act either directly on microtubule dynamics, by interaction with microtubules and tubulin, or indirectly, through the transport of other factors along the lattice to the microtubule tip. Here I review recent progress on the mechanisms and roles of these translocase kinesins.

Rapid microtubule self-assembly kinetics

Microtubule assembly is vital for many fundamental cellular processes. Current models for microtubule assembly kinetics assume that the subunit dissociation rate from a microtubule tip is independent of free subunit concentration. Total-Internal-Reflection-Fluorescence (TIRF) microscopy experiments and data from a laser tweezers assay that measures in vitro microtubule assembly with nanometer resolution, provides evidence that the subunit dissociation rate from a microtubule tip increases as the free subunit concentration increases. These data are consistent with a two-dimensional model for microtubule assembly, and are explained by a shift in microtubule tip structure from a relatively blunt shape at low free concentrations to relatively tapered at high free concentrations. We find that because both the association and the dissociation rates increase at higher free subunit concentrations, the kinetics of microtubule assembly are an order-of-magnitude higher than currently estimated in the literature.

Regulation of cell migration by dynamic microtubules

Microtubules define the architecture and internal organization of cells by positioning organelles and activities, as well as by supporting cell shape and mechanics. One of the major functions of microtubules is the control of polarized cell motility. In order to support the asymmetry of polarized cells, microtubules have to be organized asymmetrically themselves. Asymmetry in microtubule distribution and stability is regulated by multiple molecular factors, most of which are microtubule-associated proteins that locally control microtubule nucleation and dynamics. At the same time, the dynamic state of microtubules is key to the regulatory mechanisms by which microtubules regulate cell polarity, modulate cell adhesion and control force-production by the actin cytoskeleton. Here, we propose that even small alterations in microtubule dynamics can influence cell migration via several different microtubule-dependent pathways. We discuss regulatory factors, potential feedback mechanisms due to functional microtubule-actin crosstalk and implications for cancer cell motility.

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.

Kif18B interacts with EB1 and controls astral microtubule length during mitosis

Kif18B is a newly discovered plus-tip-tracking protein that is enriched on astral microtubule (MT) ends during early mitosis. Kif18B binds directly to EB1, and this interaction is required for proper localization of Kif18B and to control astral MT length.

Regulation of microtubule (MT) dynamics is essential for proper spindle assembly and organization. Kinesin-8 family members are plus-end-directed motors that modulate plus-end MT dynamics by acting as MT depolymerases or as MT plus-end capping proteins. In this paper, we show that the human kinesin-8 Kif18B functions during mitosis to control astral MT organization. Kif18B is a MT plus-tip-tracking protein that localizes to the nucleus in interphase and is enriched at astral MT plus ends during early mitosis. Knockdown of Kif18B caused spindle defects, resulting in an increased number and length of MTs. A yeast two-hybrid screen identified an interaction of the C-terminal domain of Kif18B with the plus-end MT-binding protein EB1. EB1 knockdown disrupted Kif18B targeting to MT plus ends, indicating that EB1/Kif18B interaction is physiologically important. This interaction is direct, as the far C-terminal end of Kif18B is sufficient for binding to EB1 in vitro. Overexpression of this domain is sufficient for plus-end MT targeting in cells; however, targeting is enhanced by the motor domain, which cooperates with the tail to achieve proper Kif18B localization at MT plus ends. Our results suggest that Kif18B is a new MT dynamics regulatory protein that interacts with EB1 to control astral MT length.

How do microtubules affect deposition of cell wall polysaccharides in the pollen tube?

Callose is the primary polysaccharide present in the so-called secondary layer of the pollen tube cell wall while the content of cellulose in such layer is usually lower. Despite its lower quantity, cellulose might be potentially able to establish the growth direction of pollen tubes. Microtubules have been shown to regulate the deposition of callose synthase in the distal regions of pollen tubes related to the synthesis of callose plugs. However, the interplay between microtubules and cellulose synthase in the pollen tube is unclear. Here, the hypothetical role of microtubules and microtubule-based motor proteins in controlling the insertion of cellulose synthase in relation with growth and directionality of pollen tubes is discussed.

DDA3 associates with MCAK and controls chromosome congression

DDA3 regulates spindle microtubule (MT) dynamics and chromosome movement in mitosis through its interaction with and subsequent recruitment of Kif2a, a minus end-MT depolymerase. Depletion of DDA3 causes a hyper-stabilization of spindle MT, a loss of inter-kinetochore tension, and a defect in chromosome congression, leading to unaligned chromosomes at metaphase. We report here that DDA3 is also localized at kinetochores and interacts with MCAK. Furthermore, CENP-E, a plus end-motor protein, accumulates at kinetochores in unaligned chromosomes in mitotic cells depleted of DDA3. On the other hand, the localization of chromosomal passenger complex (CPC) and the kinase activity of Aurora B are normal in DDA3-depleted cells. We conclude that MCAK and CENP-E are involved in DDA3-mediated chromosome congression.

Drugs that target dynamic microtubules: a new molecular perspective

Microtubules have long been considered an ideal target for anticancer drugs because of the essential role they play in mitosis, forming the dynamic spindle apparatus. As such, there is a wide variety of compounds currently in clinical use and in development that act as antimitotic agents by altering microtubule dynamics. Although these diverse molecules are known to affect microtubule dynamics upon binding to one of the three established drug domains (taxane, vinca alkaloid, or colchicine site), the exact mechanism by which each drug works is still an area of intense speculation and research. In this study, we review the effects of microtubule-binding chemotherapeutic agents from a new perspective, considering how their mode of binding induces conformational changes and alters biological function relative to the molecular vectors of microtubule assembly or disassembly. These "biological vectors" can thus be used as a spatiotemporal context to describe molecular mechanisms by which microtubule-targeting drugs work.

Kinesins and protein kinases: key players in the regulation of microtubule dynamics and organization

Microtubule dynamics is controlled and amplified in vivo by complex sets of regulators. Among these regulatory proteins, molecular motors from the kinesin superfamily are taking an increasing importance. Here we review how microtubule disassembly or assembly into interphase microtubules, mitotic spindle or cilia may involve kinesins and how protein kinases may participate in these kinesin-dependent regulations.

PLK1 phosphorylates mitotic centromere-associated kinesin and promotes its depolymerase activity

During cell division, interaction between kinetochores and dynamic spindle microtubules governs chromosome movements. The microtubule depolymerase mitotic centromere-associated kinesin (MCAK) is a key regulator of mitotic spindle assembly and dynamics. However, the regulatory mechanisms underlying its depolymerase activity during the cell cycle remain elusive. Here, we showed that PLK1 is a novel regulator of MCAK in mammalian cells. MCAK interacts with PLK1 in vitro and in vivo. The neck and motor domain of MCAK associates with the kinase domain of PLK1. MCAK is a novel substrate of PLK1, and the phosphorylation stimulates its microtubule depolymerization activity of MCAK in vivo. Overexpression of a polo-like kinase 1 phosphomimetic mutant MCAK causes a dramatic increase in misaligned chromosomes and in multipolar spindles in mitotic cells, whereas overexpression of a nonphosphorylatable MCAK mutant results in aberrant anaphase with sister chromatid bridges, suggesting that precise regulation of the MCAK activity by PLK1 phosphorylation is critical for proper microtubule dynamics and essential for the faithful chromosome segregation. We reasoned that dynamic regulation of MCAK phosphorylation by PLK1 is required to orchestrate faithful cell division, whereas the high levels of PLK1 and MCAK activities seen in cancer cells may account for a mechanism underlying the pathogenesis of genomic instability.