Distinct regions of the kinesin-5 C-terminal tail are essential for mitotic spindle midzone localization and sliding force.

Kinesin-5 motor proteins play essential roles during mitosis in most organisms. Their tetrameric structure and plus-end-directed motility allow them to bind to and move along antiparallel microtubules, thereby pushing spindle poles apart to assemble a bipolar spindle. Recent work has shown that the C-terminal tail is particularly important to kinesin-5 function: The tail affects motor domain structure, ATP hydrolysis, motility, clustering, and sliding force measured for purified motors, as well as motility, clustering, and spindle assembly in cells. Because previous work has focused on presence or absence of the entire tail, the functionally important regions of the tail remain to be identified. We have therefore characterized a series of kinesin-5/Cut7 tail truncation alleles in fission yeast. Partial truncation causes mitotic defects and temperature-sensitive growth, while further truncation that removes the conserved BimC motif is lethal. We compared the sliding force generated by cut7 mutants using a kinesin-14 mutant background in which some microtubules detach from the spindle poles and are pushed into the nuclear envelope. These Cut7-driven protrusions decreased as more of the tail was truncated, and the most severe truncations produced no observable protrusions. Our observations suggest that the C-terminal tail of Cut7p contributes to both sliding force and midzone localization. In the context of sequential tail truncation, the BimC motif and adjacent C-terminal amino acids are particularly important for sliding force. In addition, moderate tail truncation increases midzone localization, but further truncation of residues N-terminal to the BimC motif decreases midzone localization.

The kinesin-5 protein Cut7 moves bidirectionally on fission yeast spindles with activity that increases in anaphase.

Kinesin-5 motors are essential to separate mitotic spindle poles and assemble a bipolar spindle in many organisms. These motors crosslink and slide apart antiparallel microtubules via microtubule plus-end-directed motility. However, kinesin-5 localization is enhanced away from antiparallel overlaps. Increasing evidence suggests this localization occurs due to bidirectional motility or trafficking. The purified fission-yeast kinesin-5 protein Cut7 moves bidirectionally, but bidirectionality has not been shown in cells, and the function of the minus-end-directed movement is unknown. Here, we characterized the motility of Cut7 on bipolar and monopolar spindles and observed movement toward both plus- and minus-ends of microtubules. Notably, the activity of the motor increased at anaphase B onset. Perturbations to microtubule dynamics only modestly changed Cut7 movement, whereas Cut7 mutation reduced movement. These results suggest that the directed motility of Cut7 contributes to the movement of the motor. Comparison of the Cut7 mutant and human Eg5 (also known as KIF11) localization suggest a new hypothesis for the function of minus-end-directed motility and spindle-pole localization of kinesin-5s.

Kinesin-5 Eg5 mediates centrosome separation to control spindle assembly in spermatocytes.

Timely and accurate centrosome separation is critical for bipolar spindle organization and faithful chromosome segregation during cell division. Kinesin-5 Eg5 is essential for centrosome separation and spindle organization in somatic cells; however, the detailed functions and mechanisms of Eg5 in spermatocytes remain unclear. In this study, we show that Eg5 proteins are located at spindle microtubules and centrosomes in spermatocytes both in vivo and in vitro. We reveal that the spermatocytes are arrested at metaphase I in seminiferous tubules after Eg5 inhibition. Eg5 ablation results in cell cycle arrest, the formation of monopolar spindle, and chromosome misalignment in cultured GC-2 spd cells. Importantly, we find that the long-term inhibition of Eg5 results in an increased number of centrosomes and chromosomal instability in spermatocytes. Our findings indicate that Eg5 mediates centrosome separation to control spindle assembly and chromosome alignment in spermatocytes, which finally contribute to chromosome stability and faithful cell division of the spermatocytes.

Self-organized optimal packing of kinesin-5-driven microtubule asters scales with cell size.

Radial microtubule (MT) arrays or asters determine cell geometry in animal cells. Multiple asters interacting with motors, such as those in syncytia, form intracellular patterns, but the mechanical principles behind this are not clear. Here, we report that oocytes of the marine ascidian Phallusia mammillata treated with the drug BI-D1870 spontaneously form cytoplasmic MT asters, or cytasters. These asters form steady state segregation patterns in a shell just under the membrane. Cytaster centers tessellate the oocyte cytoplasm, that is divide it into polygonal structures, dominated by hexagons, in a kinesin-5-dependent manner, while inter-aster MTs form 'mini-spindles'. A computational model of multiple asters interacting with kinesin-5 can reproduce both tessellation patterns and mini-spindles in a manner specific to the number of MTs per aster, MT lengths and kinesin-5 density. Simulations predict that the hexagonal tessellation patterns scale with increasing cell size, when the packing fraction of asters in cells is ∼1.6. This self-organized in vivo tessellation by cytasters is comparable to the 'circle packing problem', suggesting that there is an intrinsic mechanical pattern-forming module that is potentially relevant to understanding the role of collective mechanics of cytoskeletal elements in embryogenesis. This article has an associated First Person interview with the first author of the paper.

Crumbs, Galla and Xpd are required for Kinesin-5 regulation in mitosis and organ growth in Drosophila.

Xeroderma Pigmentosum D (XPD, also known as ERCC2) is a multi-functional protein involved in transcription, DNA repair and chromosome segregation. In Drosophila, Xpd interacts with Crumbs (Crb) and Galla to regulate mitosis during embryogenesis. It is unknown how these proteins are linked to mitosis. Here, we show that Crb, Galla-2 and Xpd regulate nuclear division in the syncytial embryo by interacting with Klp61F, the Drosophila mitotic Kinesin-5 associated with bipolar spindles. Crb, Galla-2 and Xpd physically interact with Klp61F and colocalize to mitotic spindles. Knockdown of any of these proteins results in similar mitotic defects. These phenotypes are restored by overexpression of Klp61F, suggesting that Klp61F is a major effector. Mitotic defects of galla-2 RNAi are suppressed by Xpd overexpression but not vice versa. Depletion of Crb, Galla-2 or Xpd results in a reduction of Klp61F levels. Reducing proteasome function restores Klp61F levels and suppresses mitotic defects caused by knockdown of Crb, Galla-2 or Xpd. Furthermore, eye growth is regulated by Xpd and Klp61F. Hence, we propose that Crb, Galla-2 and Xpd interact to maintain the level of Klp61F during mitosis and organ growth.

Flexible microtubule anchoring modulates the bi-directional motility of the kinesin-5 Cin8.

Two modes of motility have been reported for bi-directional kinesin-5 motors: (a) context-dependent directionality reversal, a mode in which motors undergo persistent minus-end directed motility at the single-molecule level and switch to plus-end directed motility in different assays or under different conditions, such as during MT gliding or antiparallel sliding or as a function of motor clustering; and (b) bi-directional motility, defined as movement in two directions in the same assay, without persistent unidirectional motility. Here, we examine how modulation of motor-microtubule (MT) interactions affects these two modes of motility for the bi-directional kinesin-5, Cin8. We report that the large insert in loop 8 (L8) within the motor domain of Cin8 increases the MT affinity of Cin8 in vivo and in vitro and is required for Cin8 intracellular functions. We consistently found that recombinant purified L8 directly binds MTs and L8 induces single Cin8 motors to behave according to context-dependent directionality reversal and bi-directional motility modes at intermediate ionic strength and according to a bi-directional motility mode in an MT surface-gliding assay under low motor density conditions. We propose that the largely unstructured L8 facilitates flexible anchoring of Cin8 to the MTs. This flexible anchoring enables the direct observation of bi-directional motility in motility assays. Remarkably, although L8-deleted Cin8 variants exhibit a strong minus-end directed bias at the single-molecule level, they also exhibit plus-end directed motility in an MT-gliding assay. Thus, L8-induced flexible MT anchoring is required for bi-directional motility of single Cin8 molecules but is not necessary for context-dependent directionality reversal of Cin8 in an MT-gliding assay.

Complementation of fission yeast kinesin-5/Cut7 with human Eg5 provides a versatile platform for screening of anticancer compounds.

Kinesin-5 family proteins are essential for bipolar spindle assembly to ensure mitotic fidelity. Here, we demonstrate evolutionary functional conservation of kinesin-5 between human and fission yeast. Human Eg5 expressed in the nucleus replaces fission yeast counterpart Cut7. Intriguingly, Eg5 overproduction results in cytotoxicity. This phenotype provides a useful platform for the development of novel kinesin-5 inhibitors as anticancer drugs.

Mechanisms by Which Kinesin-5 Motors Perform Their Multiple Intracellular Functions.

Bipolar kinesin-5 motor proteins perform multiple intracellular functions, mainly during mitotic cell division. Their specialized structural characteristics enable these motors to perform their essential functions by crosslinking and sliding apart antiparallel microtubules (MTs). In this review, we discuss the specialized structural features of kinesin-5 motors, and the mechanisms by which these features relate to kinesin-5 functions and motile properties. In addition, we discuss the multiple roles of the kinesin-5 motors in dividing as well as in non-dividing cells, and examine their roles in pathogenetic conditions. We describe the recently discovered bidirectional motility in fungi kinesin-5 motors, and discuss its possible physiological relevance. Finally, we also focus on the multiple mechanisms of regulation of these unique motor proteins.

Effect of Kinesin-5 Tail Domain on Motor Dynamics for Antiparallel Microtubule Sliding.

Kinesin-5 motor consists of two pairs of heads and tail domains, which are situated at the opposite ends of a common stalk. The two pairs of heads can bind to two antiparallel microtubules (MTs) and move on the two MTs independently towards the plus ends, sliding apart the two MTs, which is responsible for chromosome segregation during mitosis. Prior experimental data showed that the tails of kinesin-5 Eg5 can modulate the dynamics of single motors and are critical for multiple motors to generate high steady forces to slide apart two antiparallel MTs. To understand the molecular mechanism of the tails modulating the ability of Eg5 motors, based on our proposed model the dynamics of the single Eg5 with the tails and that without the tails moving on single MTs is studied analytically and compared. Furthermore, the dynamics of antiparallel MT sliding by multiple Eg5 motors with the tails and that without the tails is studied numerically and compared. Both the analytical results for single motors and the numerical results for multiple motors are consistent with the available experimental data.

Pivoting of microtubules driven by minus-end-directed motors leads to spindle assembly.

BACKGROUND: At the beginning of mitosis, the cell forms a spindle made of microtubules and associated proteins to segregate chromosomes. An important part of spindle architecture is a set of antiparallel microtubule bundles connecting the spindle poles. A key question is how microtubules extending at arbitrary angles form an antiparallel interpolar bundle. RESULTS: Here, we show in fission yeast that microtubules meet at an oblique angle and subsequently rotate into antiparallel alignment. Our live-cell imaging approach provides a direct observation of interpolar bundle formation. By combining experiments with theory, we show that microtubules from each pole search for those from the opposite pole by performing random angular movement. Upon contact, two microtubules slide sideways along each other in a directed manner towards the antiparallel configuration. We introduce the contour length of microtubules as a measure of activity of motors that drive microtubule sliding, which we used together with observation of Cut7/kinesin-5 motors and our theory to reveal the minus-end-directed motility of this motor in vivo. CONCLUSION: Random rotational motion helps microtubules from the opposite poles to find each other and subsequent accumulation of motors allows them to generate forces that drive interpolar bundle formation.

Cryo-EM structure of the Ustilago maydis kinesin-5 motor domain bound to microtubules.

In many eukaryotes, kinesin-5 motors are essential for mitosis, and small molecules that inhibit human kinesin-5 disrupt cell division. To investigate whether fungal kinesin-5s could be targets for novel fungicides, we studied kinesin-5 from the pathogenic fungus Ustilago maydis. We used cryo-electron microscopy to determine the microtubule-bound structure of its motor domain with and without the N-terminal extension. The ATP-like conformations of the motor in the presence or absence of this N-terminus are very similar, suggesting this region is structurally disordered and does not directly influence the motor ATPase. The Ustilago maydis kinesin-5 motor domain adopts a canonical ATP-like conformation, thereby allowing the neck linker to bind along the motor domain towards the microtubule plus end. However, several insertions within this motor domain are structurally distinct. Loop2 forms a non-canonical interaction with α-tubulin, while loop8 may bridge between two adjacent protofilaments. Furthermore, loop5 - which in human kinesin-5 is involved in binding allosteric inhibitors - protrudes above the nucleotide binding site, revealing a distinct binding pocket for potential inhibitors. This work highlights fungal-specific elaborations of the kinesin-5 motor domain and provides the structural basis for future investigations of kinesins as targets for novel fungicides.

Identification of 15 New Bypassable Essential Genes of Fission Yeast.

Every organism has a different set of genes essential for its viability. This indicates that an organism can become tolerant to the loss of an essential gene under certain circumstances during evolution, via the manifestation of 'masked' alternative mechanisms. In our quest to systematically uncover masked mechanisms in eukaryotic cells, we developed an extragenic suppressor screening method using haploid spores deleted of an essential gene in the fission yeast Schizosaccharomyces pombe. We screened for the 'bypass' suppressors of lethality of 92 randomly selected genes that are essential for viability in standard laboratory culture conditions. Remarkably, extragenic mutations bypassed the essentiality of as many as 20 genes (22%), 15 of which have not been previously reported. Half of the bypass-suppressible genes were involved in mitochondria function; we also identified multiple genes regulating RNA processing. 18 suppressible genes were conserved in the budding yeast Saccharomyces cerevisiae, but 13 of them were non-essential in that species. These trends suggest that essentiality bypass is not a rare event and that each organism may be endowed with secondary or backup mechanisms that can substitute for primary mechanisms in various biological processes. Furthermore, the robustness of our simple spore-based methodology paves the way for genome-scale screening.Key words: Schizosaccharomyces pombe, extragenic suppressor screening, bypass of essentiality (BOE), cut7 (kinesin-5), hul5 (E3 ubiquitin ligase).

A potential physiological role for bi-directional motility and motor clustering of mitotic kinesin-5 Cin8 in yeast mitosis.

The bipolar kinesin-5 Cin8 switches from minus- to plus-end-directed motility under various conditions in vitro The mechanism and physiological significance of this switch remain unknown. Here, we show that under high ionic strength conditions, Cin8 moves towards and concentrates in clusters at the minus ends of stable and dynamic microtubules. Clustering of Cin8 induces a switch from fast minus- to slow plus-end-directed motility and forms sites that capture antiparallel microtubules (MTs) and induces their sliding apart through plus-end-directed motility. In early mitotic cells with monopolar spindles, Cin8 localizes near the spindle poles at microtubule minus ends. This localization is dependent on the minus-end-directed motility of Cin8. In cells with assembled bipolar spindles, Cin8 is distributed along the spindle microtubules. We propose that minus-end-directed motility is required for Cin8 clustering near the spindle poles before spindle assembly. Cin8 clusters promote the capture of microtubules emanating from the neighboring spindle poles and mediate their antiparallel sliding. This activity is essential to maximize microtubule crosslinking before bipolar spindle assembly and to induce the initial separation of the spindle poles.

Bidirectional motility of kinesin-5 motor proteins: structural determinants, cumulative functions and physiological roles.

Mitotic kinesin-5 bipolar motor proteins perform essential functions in mitotic spindle dynamics by crosslinking and sliding antiparallel microtubules (MTs) apart within the mitotic spindle. Two recent studies have indicated that single molecules of Cin8, the Saccharomyces cerevisiae kinesin-5 homolog, are minus end-directed when moving on single MTs, yet switch directionality under certain experimental conditions (Gerson-Gurwitz et al., EMBO J 30:4942-4954, 2011; Roostalu et al., Science 332:94-99, 2011). This finding was unexpected since the Cin8 catalytic motor domain is located at the N-terminus of the protein, and such kinesins have been previously thought to be exclusively plus end-directed. In addition, the essential intracellular functions of kinesin-5 motors in separating spindle poles during mitosis can only be accomplished by plus end-directed motility during antiparallel sliding of the spindle MTs. Thus, the mechanism and possible physiological role of the minus end-directed motility of kinesin-5 motors remain unclear. Experimental and theoretical studies from several laboratories in recent years have identified additional kinesin-5 motors that are bidirectional, revealed structural determinants that regulate directionality, examined the possible mechanisms involved and have proposed physiological roles for the minus end-directed motility of kinesin-5 motors. Here, we summarize our current understanding of the remarkable ability of certain kinesin-5 motors to switch directionality when moving along MTs.

Schizosaccharomyces pombe kinesin-5 switches direction using a steric blocking mechanism.

Cut7, the sole kinesin-5 in Schizosaccharomyces pombe, is essential for mitosis. Like other yeast kinesin-5 motors, Cut7 can reverse its stepping direction, by mechanisms that are currently unclear. Here we show that for full-length Cut7, the key determinant of stepping direction is the degree of motor crowding on the microtubule lattice, with greater crowding converting the motor from minus end-directed to plus end-directed stepping. To explain how high Cut7 occupancy causes this reversal, we postulate a simple proximity sensing mechanism that operates via steric blocking. We propose that the minus end-directed stepping action of Cut7 is selectively inhibited by collisions with neighbors under crowded conditions, whereas its plus end-directed action, being less space-hungry, is not. In support of this idea, we show that the direction of Cut7-driven microtubule sliding can be reversed by crowding it with non-Cut7 proteins. Thus, crowding by either dynein microtubule binding domain or Klp2, a kinesin-14, converts Cut7 from net minus end-directed to net plus end-directed stepping. Biochemical assays confirm that the Cut7 N terminus increases Cut7 occupancy by binding directly to microtubules. Direct observation by cryoEM reveals that this occupancy-enhancing N-terminal domain is partially ordered. Overall, our data point to a steric blocking mechanism for directional reversal through which collisions of Cut7 motor domains with their neighbors inhibit their minus end-directed stepping action, but not their plus end-directed stepping action. Our model can potentially reconcile a number of previous, apparently conflicting, observations and proposals for the reversal mechanism of yeast kinesins-5.

Three Cdk1 sites in the kinesin-5 Cin8 catalytic domain coordinate motor localization and activity during anaphase.

The bipolar kinesin-5 motors perform essential functions in mitotic spindle dynamics. We previously demonstrated that phosphorylation of at least one of the Cdk1 sites in the catalytic domain of the Saccharomyces cerevisiae kinesin-5 Cin8 (S277, T285, S493) regulates its localization to the anaphase spindle. The contribution of these three sites to phospho-regulation of Cin8, as well as the timing of such contributions, remains unknown. Here, we examined the function and spindle localization of phospho-deficient (serine/threonine to alanine) and phospho-mimic (serine/threonine to aspartic acid) Cin8 mutants. In vitro, the three Cdk1 sites undergo phosphorylation by Clb2-Cdk1. In cells, phosphorylation of Cin8 affects two aspects of its localization to the anaphase spindle, translocation from the spindle-pole bodies (SPBs) region to spindle microtubules (MTs) and the midzone, and detachment from the mitotic spindle. We found that phosphorylation of S277 is essential for the translocation of Cin8 from SPBs to spindle MTs and the subsequent detachment from the spindle. Phosphorylation of T285 mainly affects the detachment of Cin8 from spindle MTs during anaphase, while phosphorylation at S493 affects both the translocation of Cin8 from SPBs to the spindle and detachment from the spindle. Only S493 phosphorylation affected the anaphase spindle elongation rate. We conclude that each phosphorylation site plays a unique role in regulating Cin8 functions and postulate a model in which the timing and extent of phosphorylation of the three sites orchestrates the anaphase function of Cin8.

Clinical Development of Anti-mitotic Drugs in Cancer.

Mitosis is one of the most fundamental processes of life by which a mammalian cell divides into two daughter cells. Mitosis has been an attractive target for anticancer therapies since fast proliferation was identified as one of the hallmarks of cancer cells. Despite efforts into developing specific inhibitors for mitotic kinases and kinesins, very few drugs have shown the efficiency of microtubule targeting-agents in cancer cells with paclitaxel being the most successful. A deeper translational research accompanying clinical trials of anti-mitotic drugs will help in identifying potent biomarkers predictive for response. Here, we review the current knowledge of mitosis targeting agents that have been tested so far in the clinics.

Deletion of the Tail Domain of the Kinesin-5 Cin8 Affects Its Directionality.

The bipolar kinesin-5 motors are one of the major players that govern mitotic spindle dynamics. Their bipolar structure enables them to cross-link and slide apart antiparallel microtubules (MTs) emanating from the opposing spindle poles. The budding yeast kinesin-5 Cin8 was shown to switch from fast minus-end- to slow plus-end-directed motility upon binding between antiparallel MTs. This unexpected finding revealed a new dimension of cellular control of transport, the mechanism of which is unknown. Here we have examined the role of the C-terminal tail domain of Cin8 in regulating directionality. We first constructed a stable dimeric Cin8/kinesin-1 chimera (Cin8Kin), consisting of head and neck linker of Cin8 fused to the stalk of kinesin-1. As a single dimeric motor, Cin8Kin switched frequently between plus and minus directionality along single MTs, demonstrating that the Cin8 head domains are inherently bidirectional, but control over directionality was lost. We next examined the activity of a tetrameric Cin8 lacking only the tail domains (Cin8Δtail). In contrast to wild-type Cin8, the motility of single molecules of Cin8Δtail in high ionic strength was slow and bidirectional, with almost no directionality switches. Cin8Δtail showed only a weak ability to cross-link MTs in vitro. In vivo, Cin8Δtail exhibited bias toward the plus-end of the MTs and was unable to support viability of cells as the sole kinesin-5 motor. We conclude that the tail of Cin8 is not necessary for bidirectional processive motion, but is controlling the switch between plus- and minus-end-directed motility.

Distinctions between dynamic characteristics of the single EG5 motor protein along neural vs. cancerous microtubules.

The kinesin 5 motor contributes critically to mitosis, and is often upregulated in cancer. In vitro motility studies of kinesin 5 moving along bovine brain microtubules indicate that the motors have limited processivity. Cancer cells have abnormal mitotic behavior, so one might wonder whether the functional properties of kinesin 5 change in such a background. Because there could be multiple unknown changes in cancerous vs normal cells, we chose to address this question in a controlled in vitro environment. Specifically, through a series of parallel experiments along bovine brain vs. breast cancer microtubules, we quantified the in vitro motility characteristics of single Eg5 molecular motors along these two types of microtubules, combining the utilization of an optical trapping technique with a study of motion in the unloaded regime. The obtained values indicate that Eg5 processivity is 40% less along MCF7 microtubules, compared to that measured on bovine brain MTs. Interestingly, not all single-molecule properties are altered, as the velocity of the single motor doesn't show any significant changes on either track, though the binding time along MCF7 microtubules is almost 25% shorter. The current results, in conjunction with our previously reported outcomes of the evaluation of the Eg5's characteristics under external load, show that in transition from no-load to high-load regime, the Eg5 binding time has less sensitivity on MCF7 as compared to bovine brain MTs. This finding is intriguing, as it suggests that, potentially, groups of Eg5 motors function more effectively in the cancer background of a large ensemble, possibly contributing to faster mitosis in cancer cells.

Kinesin-5 Kip1 is a bi-directional motor that stabilizes microtubules and tracks their plus-ends in vivo.

In this study, we examined the anaphase functions of the S. cerevisiae kinesin-5 homolog Kip1. We show that Kip1 is attached to the mitotic spindle midzone during late anaphase. This attachment is essential to stabilize interpolar microtubule (iMTs) plus-ends. By detailed examination of iMT dynamics we show that at the end of anaphase, iMTs depolymerize in two stages: during the first stage, one pair of anti-parallel iMTs depolymerizes at a velocity of 7.7 µm/minute; during the second stage, ∼90 seconds later, the remaining pair of iMTs depolymerizes at a slower velocity of 5.4 µm/minute. We show that upon the second depolymerization stage, which coincides with spindle breakdown, Kip1 follows the plus-ends of depolymerizing iMTs and translocates toward the spindle poles. This movement is independent of mitotic microtubule motor proteins or the major plus-end binding or tracking proteins. In addition, we show that Kip1 processively tracks the plus-ends of growing and shrinking MTs, both inside and outside the nucleus. The plus-end tracking activity of Kip1 requires its catalytic motor function, because a rigor mutant of Kip1 does not exhibit this activity. Finally, we show that Kip1 is a bi-directional motor: in vitro, at high ionic strength conditions, single Kip1 molecules move processively in the minus-end direction of the MTs, whereas in a multi-motor gliding assay, Kip1 is plus-end directed. The bi-directionality and plus-end tracking activity of Kip1, properties revealed here for the first time, allow Kip1 to perform its multiple functions in mitotic spindle dynamics and to partition the 2-micron plasmid.