Two Saccharomyces cerevisiae kinesin-related gene products required for mitotic spindle assembly

Phosphorylation-dependent protein interactions at the spindle midzone mediate cell cycle regulation of spindle elongation

The metaphase-to-anaphase transition is one of the most dramatic and highly regulated steps in cell division. At anaphase onset the protease separase dissolves sister chromatid cohesion. Simultaneously, the mitotic spindle elongates as interpolar microtubules (iMTs) slide apart at the spindle midzone, ensuring chromosome segregation. However, it remains unclear how spindle elongation is coordinated with cell cycle progression. Here we demonstrate that phosphorylation of the midzone organizer Ase1 controls localization and function of Cin8, a kinesin-5 that slides iMTs relative to each other. Phosphorylation of Ase1 by Cdk1 (cyclin-dependent kinase) inhibits Cin8 binding to iMTs, preventing bending and collapse of the metaphase spindle. In anaphase Ase1 dephosphorylation by the separase-activated phosphatase Cdc14 is necessary and sufficient for Cin8 recruitment to the midzone, where it drives spindle elongation. Our results reveal that sliding forces at the midzone are activated by separase and explain how spindle elongation is triggered with anaphase entry.

Phospho-regulated interaction between kinesin-6 Klp9p and microtubule bundler Ase1p promotes spindle elongation

The spindle midzone-composed of antiparallel microtubules, microtubule-associated proteins (MAPs), and motors-is the structure responsible for microtubule organization and sliding during anaphase B. In general, MAPs and motors stabilize the midzone and motors produce sliding. We show that fission yeast kinesin-6 motor klp9p binds to the microtubule antiparallel bundler ase1p at the midzone at anaphase B onset. This interaction depends upon the phosphorylation states of klp9p and ase1p. The cyclin-dependent kinase cdc2p phosphorylates and its antagonist phosphatase clp1p dephosphorylates klp9p and ase1p to control the position and timing of klp9p-ase1p interaction. Failure of klp9p-ase1p binding leads to decreased spindle elongation velocity. The ase1p-mediated recruitment of klp9p to the midzone accelerates pole separation, as suggested by computer simulation. Our findings indicate that a phosphorylation switch controls the spatial-temporal interactions of motors and MAPs for proper anaphase B, and suggest a mechanism whereby a specific motor-MAP conformation enables efficient microtubule sliding.

Efficient chromosome biorientation and the tension checkpoint in Saccharomyces cerevisiae both require Bir1

Accurate chromosome segregation requires the capture of sister kinetochores by microtubules from opposite spindle poles prior to the initiation of anaphase, a state termed chromosome biorientation. In the budding yeast Saccharomyces cerevisiae, the conserved protein kinase Ipl1 (Aurora B in metazoans) is critical for ensuring correct chromosomal alignment. Ipl1 associates with its activators Sli15 (INCENP), Nbl1 (Borealin), and Bir1 (Survivin), but while Sli15 clearly functions with Ipl1 to promote chromosome biorientation, the role of Bir1 has been uncertain. Using a temperature-sensitive bir1 mutant (bir1-17), we show that Bir1 is needed to permit efficient chromosome biorientation. However, once established, chromosome biorientation is maintained in bir1-17 cells at the restrictive temperature. Ipl1 is partially delocalized in bir1-17 cells, and its protein kinase activity is markedly reduced under nonpermissive conditions. bir1-17 cells arrest normally in response to microtubule depolymerization but fail to delay anaphase when sister kinetochore tension is reduced. Thus, Bir1 is required for the tension checkpoint. Despite their robust mitotic arrest in response to nocodazole, bir1-17 cells are hypersensitive to microtubule-depolymerizing drugs and show a more severe biorientation defect on recovery from nocodazole treatment. The role of Bir1 therefore may become more critical when spindle formation is delayed.

Cdc14 inhibition by the spindle assembly checkpoint prevents unscheduled centrosome separation in budding yeast

The spindle assembly checkpoint (SAC) is an evolutionarily conserved surveillance mechanism that delays anaphase onset and mitotic exit in response to the lack of kinetochore attachment. The target of the SAC is the E3 ubiquitin ligase anaphase-promoting complex (APC) bound to its Cdc20 activator. The Cdc20/APC complex is in turn required for sister chromatid separation and mitotic exit through ubiquitin-mediated proteolysis of securin, thus relieving inhibition of separase that unties sister chromatids. Separase is also involved in the Cdc-fourteen early anaphase release (FEAR) pathway of nucleolar release and activation of the Cdc14 phosphatase, which regulates several microtubule-linked processes at the metaphase/anaphase transition and also drives mitotic exit. Here, we report that the SAC prevents separation of microtubule-organizing centers (spindle pole bodies [SPBs]) when spindle assembly is defective. Under these circumstances, failure of SAC activation causes unscheduled SPB separation, which requires Cdc20/APC, the FEAR pathway, cytoplasmic dynein, and the actin cytoskeleton. We propose that, besides inhibiting sister chromatid separation, the SAC preserves the accurate transmission of chromosomes also by preventing SPBs to migrate far apart until the conditions to assemble a bipolar spindle are satisfied.

The selfish yeast plasmid uses the nuclear motor Kip1p but not Cin8p for its localization and equal segregation

The 2 micron plasmid of Saccharomyces cerevisiae uses the Kip1 motor, but not the functionally redundant Cin8 motor, for its precise nuclear localization and equal segregation. The timing and lifetime of Kip1p association with the plasmid partitioning locus STB are consistent with Kip1p being an authentic component of the plasmid partitioning complex. Kip1-STB association is not blocked by disassembling the mitotic spindle. Lack of Kip1p disrupts recruitment of the cohesin complex at STB and cohesion of replicated plasmid molecules. Colocalization of a 2 micron reporter plasmid with Kip1p in close proximity to the spindle pole body is reminiscent of that of a CEN reporter plasmid. Absence of Kip1p displaces the plasmid from this nuclear address, where it has the potential to tether to a chromosome or poach chromosome segregation factors. Exploiting Kip1p, which is subsidiary to Cin8p for chromosome segregation, to direct itself to a "partitioning center" represents yet another facet of the benign parasitism of the yeast plasmid.

Motile microtubule crosslinkers require distinct dynamic properties for correct functioning during spindle organization in Xenopus egg extract

The organization of the microtubule cytoskeleton depends crucially on crosslinking motors that arrange microtubules in space. Kinesin-5 is such an essential motile crosslinker. It is unknown whether its organizing capacity during bipolar spindle formation depends on its characteristic kinetic properties, or whether simply crosslinking combined with any plus-end-directed motility is sufficient for its function in a physiological context. To address this question, we replaced the motor domain of Xenopus Kinesin-5 by motor domains of kinesins belonging to other kinesin subfamilies, without changing the overall architecture of the molecule. This generated novel microtubule crosslinkers with altered kinetic properties. The chimeric crosslinkers mislocalized in spindles and consequently caused spindle collapse into tightly bundled microtubule arrays. This demonstrates that plus-end directionality and microtubule crosslinking are not the only characteristics required for proper functioning of Kinesin-5 during spindle assembly in Xenopus egg extract. Instead, its motor domain properties appear to be fine-tuned for the specific function of this kinesin.

Midzone organization restricts interpolar microtubule plus-end dynamics during spindle elongation

To study the dynamics of interpolar microtubules (iMTs) in Saccharomyces cerevisiae cells, we photobleached a considerable portion of the middle region of anaphase spindles in cells expressing tubulin-green fluorescent protein (GFP) and followed fluorescence recovery at the iMT plus-ends. We found that during anaphase, iMTs show phases of fast growth and shrinkage that are restricted to the iMT plus-ends. Our data indicate that iMT plus-end dynamics are regulated during mitosis, as fluorescence recovery was faster in intermediate anaphase (30 s) compared with long (100 s) and pre-anaphase (80 s) spindles. We also observed that deletion of Cin8, a microtubule-crosslinking kinesin-5 motor protein, reduced the recovery rate in anaphase spindles, indicating that Cin8 contributes to the destabilization of iMT plus-ends. Finally, we show that in cells lacking the midzone organizing protein Ase1, iMTs are highly dynamic and are exchangeable throughout most of their length, indicating that midzone organization is essential for restricting iMT dynamics.

Phosphorylation by Cdk1 increases the binding of Eg5 to microtubules in vitro and in Xenopus egg extract spindles

Background

Motor proteins from the kinesin-5 subfamily play an essential role in spindle assembly during cell division of most organisms. These motors crosslink and slide microtubules in the spindle. Kinesin-5 motors are phosphorylated at a conserved site by Cyclin-dependent kinase 1 (Cdk1) during mitosis. Xenopus laevis kinesin-5 has also been reported to be phosphorylated by Aurora A in vitro.

Methodology/Principal Findings

We investigate here the effect of these phosphorylations on kinesin-5 from Xenopus laevis, called Eg5. We find that phosphorylation at threonine 937 in the C-terminal tail of Eg5 by Cdk1 does not affect the velocity of Eg5, but strongly increases its binding to microtubules assembled in buffer. Likewise, this phosphorylation promotes binding of Eg5 to microtubules in Xenopus egg extract spindles. This enhancement of binding elevates the amount of Eg5 in spindles above a critical level required for bipolar spindle formation. We find furthermore that phosphorylation of Xenopus laevis Eg5 by Aurora A at serine 543 in the stalk is not required for spindle formation.

Conclusions/Significance

These results show that phosphorylation of Eg5 by Cdk1 has a direct effect on the interaction of this motor with microtubules. In egg extract, phosphorylation of Eg5 by Cdk1 ensures that the amount of Eg5 in the spindle is above a level that is required for spindle formation. This enhanced targeting to the spindle appears therefore to be, at least in part, a direct consequence of the enhanced binding of Eg5 to microtubules upon phosphorylation by Cdk1. These findings advance our understanding of the regulation of this essential mitotic motor protein.

Chromosome congression by Kinesin-5 motor-mediated disassembly of longer kinetochore microtubules

During mitosis, sister chromatids congress to the spindle equator and are subsequently segregated via attachment to dynamic kinetochore microtubule (kMT) plus ends. A major question is how kMT plus-end assembly is spatially regulated to achieve chromosome congression. Here we find in budding yeast that the widely conserved kinesin-5 sliding motor proteins, Cin8p and Kip1p, mediate chromosome congression by suppressing kMT plus-end assembly of longer kMTs. Of the two, Cin8p is the major effector and its activity requires a functional motor domain. In contrast, the depolymerizing kinesin-8 motor Kip3p plays a minor role in spatial regulation of yeast kMT assembly. Our analysis identified a model where kinesin-5 motors bind to kMTs, move to kMT plus ends, and upon arrival at a growing plus end promote net kMT plus-end disassembly. In conclusion, we find that length-dependent control of net kMT assembly by kinesin-5 motors yields a simple and stable self-organizing mechanism for chromosome congression.

Dynein, Lis1 and CLIP-170 counteract Eg5-dependent centrosome separation during bipolar spindle assembly

Bipolar spindle assembly critically depends on the microtubule plus-end-directed motor Eg5 that binds antiparallel microtubules and slides them in opposite directions. As such, Eg5 can produce the necessary outward force within the spindle that drives centrosome separation and inhibition of this antiparallel sliding activity results in the formation of monopolar spindles. Here, we show that upon depletion of the minus-end-directed motor dynein, or the dynein-binding protein Lis1, bipolar spindles can form in human cells with substantially less Eg5 activity, suggesting that dynein and Lis1 produce an inward force that counteracts the Eg5-dependent outward force. Interestingly, we also observe restoration of spindle bipolarity upon depletion of the microtubule plus-end-tracking protein CLIP-170. This function of CLIP-170 in spindle bipolarity seems to be mediated through its interaction with dynein, as loss of CLIP-115, a highly homologous protein that lacks the dynein-dynactin interaction domain, does not restore spindle bipolarity. Taken together, these results suggest that complexes of dynein, Lis1 and CLIP-170 crosslink and slide microtubules within the spindle, thereby producing an inward force that pulls centrosomes together.

Ustilago maydis, a new fungal model system for cell biology

The use of fungal model systems, such as Saccharomyces cerevisisae and Schizosaccharomyces pombe, has contributed enormously to our understanding of essential cellular processes in animals. Here, we introduce the corn smut fungus Ustilago maydis as a new model organism for studying cell biological processes. Genome-wide analysis demonstrates that U. maydis is more closely related to humans than to budding yeast, and numerous proteins are shared only by U. maydis and Homo sapiens. Growing evidence suggests that basic principles of long-distance transport, mitosis and motor-based microtubule organization are conserved between U. maydis and humans. The fungus U. maydis, therefore, offers a unique system for the study of certain mammalian processes.

Slk19-dependent mid-anaphase pause in kinesin-5-mutated cells

We examined spindle elongation in anaphase in Saccharomyces cerevisiae cells mutated for the kinesin-5 motor proteins Cin8 and Kip1. Cells were deleted for KIP1 and/or expressed one of two motor-domain Cin8 mutants (Cin8-F467A or Cin8-R196K, which differ in their ability to bind microtubules in vitro, with Cin8-F467A having the weakest ability). We found that, in kinesin-5-mutated cells, predominantly in kip1 Delta cin8-F467A cells, anaphase spindle elongation was frequently interrupted after the fast phase, resulting in a mid-anaphase pause. Expression of kinesin-5 mutants also caused an asymmetric midzone location and enlarged midzone size, suggesting that proper organization of the midzone is required for continuous spindle elongation. We also examined the effects of components of the FEAR pathway, which is involved in the early-anaphase activation of Cdc14 regulatory phosphatase, on anaphase spindle elongation in kip1 Delta cin8-F467A cells. Deletion of SLK19, but not SPO12, eliminated the mid-anaphase pause, caused premature anaphase onset and defects in DNA division during anaphase, and reduced viability in these cells. Finally, overriding of the pre-anaphase checkpoint by overexpression of Cdc20 also eliminated the mid-anaphase pause and caused DNA deformation during anaphase in kip1 Delta cin8-F467A cells. We propose that transient activation of the pre-anaphase checkpoint in kinesin-5-mutated cells induces a Slk19-dependent mid-anaphase pause, which might be important for proper DNA segregation.

Inactivation of Cdh1 by synergistic action of Cdk1 and polo kinase is necessary for proper assembly of the mitotic spindle

Separation of duplicated centrosomes (spindle-pole bodies or SPBs in yeast) is a crucial step in the biogenesis of the mitotic spindle. In vertebrates, centrosome separation requires the BimC family kinesin Eg5 and the activities of Cdk1 and polo kinase; however, the roles of these kinases are not fully understood. In Saccharomyces cerevisiae, SPB separation also requires activated Cdk1 and the plus-end kinesins Cin8 (homologous to vertebrate Eg5) and Kip1. Here we report that polo kinase has a role in the separation of SPBs. We show that adequate accumulation of Cin8 and Kip1 requires inactivation of the anaphase-promoting complex-activator Cdh1 through sequential phosphorylation by Cdk1 and polo kinase. In this process, Cdk1 functions as a priming kinase in that Cdk1-mediated phosphorylation creates a binding site for polo kinase,which further phosphorylates Cdh1. Thus, Cdh1 inactivation through the synergistic action of Cdk1 and polo kinase provides a new model for inactivation of cell-cycle effectors.

Intrinsic and cyclin-dependent kinase-dependent control of spindle pole body duplication in budding yeast

Centrosome duplication must be tightly controlled so that duplication occurs only once each cell cycle. Accumulation of multiple centrosomes can result in the assembly of a multipolar spindle and lead to chromosome mis-segregation and genomic instability. In metazoans, a centrosome-intrinsic mechanism prevents reduplication until centriole disengagement. Mitotic cyclin/cyclin-dependent kinases (CDKs) prevent reduplication of the budding yeast centrosome, called a spindle pole body (SPB), in late S-phase and G2/M, but the mechanism remains unclear. How SPB reduplication is prevented early in the cell cycle is also not understood. Here we show that, similar to metazoans, an SPB-intrinsic mechanism prevents reduplication early in the cell cycle. We also show that mitotic cyclins can inhibit SPB duplication when expressed before satellite assembly in early G1, but not later in G1, after the satellite had assembled. Moreover, electron microscopy revealed that SPBs do not assemble a satellite in cells expressing Clb2 in early G1. Finally, we demonstrate that Clb2 must localize to the cytoplasm in order to inhibit SPB duplication, suggesting the possibility for direct CDK inhibition of satellite components. These two mechanisms, intrinsic and extrinsic control by CDK, evoke two-step system that prevents SPB reduplication throughout the cell cycle.

Disruption of the mitotic kinesin Eg5 gene (Knsl1) results in early embryonic lethality

Eg5, a member of the widely conserved kinesin-5 family, is a plus-end-directed motor involved in separation of centrosomes, and in bipolar spindle formation and maintenance during mitosis in vertebrates. To investigate the requirement for Eg5 in mammalian development, we have generated Eg5 deficient mice by gene targeting. Heterozygous mice are healthy, fertile, and show no detectable phenotype, whereas Eg5(-/-) embryos die during early embryogenesis, prior to the implantation stage. This result shows that Eg5 is essential during early mouse development and cannot be compensated by another molecular motor.

Chromosome 20q amplification regulates in vitro response to Kinesin-5 inhibitor

We identified gene expression signatures predicting responsiveness to a Kinesin-5 (KIF11) inhibitor (Kinesin-5i) in cultured colon tumor cell lines. Genes predicting resistance to Kinesin-5i were enriched for those from chromosome 20q, a region of frequent amplification in a number of tumor types. siRNAs targeting genes in this chromosomal region identified AURKA, TPX2 and MYBL2 as genes whose disruption enhances response to Kinesin-5i. Taken together, our results show functional interaction between these genes, and suggest that their overexpression is involved in resistance to Kinesin-5i. Furthermore, our results suggest that patients whose tumors overexpress AURKA due to amplification of 20q will more likely resist treatment with Kinesin-5 inhibitor, and that inactivation of AURKA may sensitize these patients to treatment.

The Saccharomyces cerevisiae homolog of p24 is essential for maintaining the association of p150Glued with the dynactin complex

Stu1 is the Saccharomyces cerevisiae member of the CLASP family of microtubule plus-end tracking proteins and is essential for spindle formation. A genomewide screen for gene deletions that are lethal in combination with the temperature-sensitive stu1-5 allele identified ldb18Delta. ldb18Delta cells exhibit defects in spindle orientation similar to those caused by a block in the dynein pathway. Consistent with this observation, ldb18Delta is synthetic lethal with mutations affecting the Kar9 spindle orientation pathway, but not with those affecting the dynein pathway. We show that Ldb18 is a component of dynactin, a complex required for dynein activity in yeast and mammalian cells. Ldb18 shares modest sequence and structural homology with the mammalian dynactin component p24. It interacts with dynactin proteins in two-hybrid and co-immunoprecipitation assays, and comigrates with them as a 20 S complex during sucrose gradient sedimentation. In ldb18Delta cells, the interaction between Nip100 (p150(Glued)) and Jnm1 (dynamitin) is disrupted, while the interaction between Jnm1 and Arp1 is not affected. These results indicate that p24 is required for attachment of the p150(Glued) arm to dynamitin and the remainder of the dynactin complex. The genetic interaction of ldb18Delta with stu1-5 also supports the notion that dynein/dynactin helps to generate a spindle pole separating force.

Getting in sync with dimeric Eg5. Initiation and regulation of the processive run

Eg5/KSP is the kinesin-related motor protein that generates the major plus-end directed force for mitotic spindle assembly and dynamics. Recent work using a dimeric form of Eg5 has found it to be a processive motor; however, its mechanochemical cycle is different from that of conventional Kinesin-1. Dimeric Eg5 appears to undergo a conformational change shortly after collision with the microtubule that primes the motor for its characteristically short processive runs. To better understand this conformational change as well as head-head communication during processive stepping, equilibrium and transient kinetic approaches have been used. By contrast to the mechanism of Kinesin-1, microtubule association triggers ADP release from both motor domains of Eg5. One motor domain releases ADP rapidly, whereas ADP release from the other occurs after a slow conformational change at approximately 1 s(-1). Therefore, dimeric Eg5 begins its processive run with both motor domains associated with the microtubule and in the nucleotide-free state. During processive stepping however, ATP binding and potentially ATP hydrolysis signals rearward head advancement 16 nm forward to the next microtubule-binding site. This alternating cycle of processive stepping is proposed to terminate after a few steps because the head-head communication does not sufficiently control the timing to prevent both motor domains from entering the ADP-bound state simultaneously.

Design features of a mitotic spindle: balancing tension and compression at a single microtubule kinetochore interface in budding yeast

Accurate segregation of duplicated chromosomes ensures that daughter cells get one and only one copy of each chromosome. Errors in chromosome segregation result in aneuploidy and have severe consequences on human health. Incorrect chromosome number and chromosomal instability are hallmarks of tumor cells. Hence, segregation errors are thought to be a major cause of tumorigenesis. A study of the physical mechanical basis of chromosome segregation is essential to understand the processes that can lead to errors. Tremendous progress has been made in recent years in identifying the proteins necessary for chromosome movement and segregation, but the mechanism and structure of critical force generating components and the molecular basis of centromere stiffness remain poorly understood.

Overexpression of Eg5 causes genomic instability and tumor formation in mice

Proper chromosome segregation in eukaryotes is driven by a complex superstructure called the mitotic spindle. Assembly, maintenance, and function of the spindle depend on centrosome migration, organization of microtubule arrays, and force generation by microtubule motors. Spindle pole migration and elongation are controlled by the unique balance of forces generated by antagonistic molecular motors that act upon microtubules of the mitotic spindle. Defects in components of this complex structure have been shown to lead to chromosome missegregation and genomic instability. Here, we show that overexpression of Eg5, a member of the Bim-C class of kinesin-related proteins, leads to disruption of normal spindle development, as we observe both monopolar and multipolar spindles in Eg5 transgenic mice. Our findings show that perturbation of the mitotic spindle leads to chromosomal missegregation and the accumulation of tetraploid cells. Aging of these mice revealed a higher incidence of tumor formation with a mixed array of tumor types appearing in mice ages 3 to 30 months with the mean age of 20 months. Analysis of the tumors revealed widespread aneuploidy and genetic instability, both hallmarks of nearly all solid tumors. Together with previous findings, our results indicate that Eg5 overexpression disrupts the unique balance of forces associated with normal spindle assembly and function, and thereby leads to the development of spindle defects, genetic instability, and tumors.

Mitch a rapidly evolving component of the Ndc80 kinetochore complex required for correct chromosome segregation in Drosophila

We identified an essential kinetochore protein, Mitch, from a genetic screen in D. melanogaster. Mitch localizes to the kinetochore, and its targeting is independent of microtubules (MTs) and several other known kinetochore components. Animals carrying mutations in mitch die as late third-instar larvae; mitotic neuroblasts in larval brains exhibit high levels of aneuploidy. Analysis of fixed D. melanogaster brains and mitch RNAi in cultured cells, as well as video recordings of cultured mitch mutant neuroblasts, reveal that chromosome alignment in mitch mutants is compromised during spindle formation, with many chromosomes displaying persistent mono-orientation. These misalignments lead to aneuploidy during anaphase. Mutations in mitch also disrupt chromosome behavior during both meiotic divisions in spermatocytes: the entire chromosome complement often moves to only one spindle pole. Mutant mitotic cells exhibit contradictory behavior with respect to the spindle assembly checkpoint (SAC). Anaphase onset is delayed in untreated cells, probably because incorrect kinetochore attachment maintains the SAC. However, mutant brain cells and mitch RNAi cells treated with MT poisons prematurely disjoin their chromatids, and exit mitosis. These data suggest that Mitch participates in SAC signaling that responds specifically to disruptions in spindle microtubule dynamics. The mitch gene corresponds to the transcriptional unit CG7242, and encodes a protein that is a possible ortholog of the Spc24 or Spc25 subunit of the Ndc80 kinetochore complex. Despite the crucial role of Mitch in cell division, the mitch gene has evolved very rapidly among species in the genus Drosophila.

A pathway containing the Ipl1/aurora protein kinase and the spindle midzone protein Ase1 regulates yeast spindle assembly

It is critical to elucidate the pathways that mediate spindle assembly and therefore ensure accurate chromosome segregation during cell division. Our studies of a unique allele of the budding yeast Ipl1/Aurora protein kinase revealed that it is required for centrosome-mediated spindle assembly in the absence of the BimC motor protein Cin8. In addition, we found that the Ase1 spindle midzone-associated protein is required for bipolar spindle assembly. The cin8 ipl1 and cin8 ase1 double mutant cells exhibit similar defects, and Ase1 overexpression completely restores spindle assembly in cin8 ipl1 strains. Consistent with the possibility that Ipl1 regulates Ase1, an ase1 mutant lacking the Ipl1 consensus phosphorylation sites cannot assemble spindles in the absence of Cin8. In addition, Ase1 phosphorylation and localization were altered in an ipl1 mutant. We therefore propose that Ipl1/Aurora and Ase1 constitute a previously unidentified spindle assembly pathway that becomes essential in the absence of Cin8.

Effective killing of the human pathogen Candida albicans by a specific inhibitor of non-essential mitotic kinesin Kip1p

Kinesins from the bipolar (Kinesin-5) family are conserved in eukaryotic organisms and play critical roles during the earliest stages of mitosis to mediate spindle pole body separation and formation of a bipolar mitotic spindle. To date, genes encoding bipolar kinesins have been reported to be essential in all organisms studied. We report the characterization of CaKip1p, the sole member of this family in the human pathogenic yeast Candida albicans. C. albicans Kip1p appears to localize to the mitotic spindle and loss of CaKip1p function interferes with normal progression through mitosis. Inducible excision of CaKIP1 revealed phenotypes unique to C. albicans, including viable homozygous Cakip1 mutants and an aberrant spindle morphology in which multiple spindle poles accumulate in close proximity to each other. Expression of the C. albicans Kip1 motor domain in Escherichia coli produced a protein with microtubule-stimulated ATPase activity that was inhibited by an aminobenzothiazole (ABT) compound in an ATP-competitive fashion. This inhibition results in ‘rigor-like’, tight association with microtubules in vitro. Upon treatment of C. albicans cells with the ABT compound, cells were killed, and terminal phenotype analysis revealed an aberrant spindle morphology similar to that induced by loss of the CaKIP1 gene. The ABT compound discovered is the first example of a fungal spindle inhibitor targeted to a mitotic kinesin. Our results also show that the non-essential nature and implementation of the bipolar motor in C. albicans differs from that seen in other organisms, and suggest that inhibitors of a non-essential mitotic kinesin may offer promise as cidal agents for antifungal drug discovery.

The kinesin related motor protein, Eg5, is essential for maintenance of pre-implantation embryogenesis

Eg5 is a plus end directed kinesin related motor protein (KRP) previously shown to be involved in the assembly and maintenance of the mitotic spindle. KRPs are molecular motors capable of generating forces upon microtubules (MTs) in dividing cells and driving structural rearrangements necessary in the developing spindle. In vitro experiments demonstrate that loss of Eg5 results in cell cycle arrest and defective centrosome separation resulting in the development of monopolar spindles. Here we describe mice with a genetrap insertion in Eg5. Heterozygous mutant mice appear phenotypically normal. In contrast, embryos homozygous for the Eg5 null allele recovered at embryonic days 2.5-3.5 display signs of a proliferation defect as reduced cell numbers and failure of compaction and progression to the blastocyst stage was observed. These data, in conjunction with previous in vitro data, suggest that loss of Eg5 results in abnormal spindle structure, cell cycle arrest and thereby reduced cell proliferation of early cleavage pre-implantation embryos. These observations further support the conclusion that Eg5 is essential for cell division early in mouse development, and that maternal contribution may sustain the embryo through the maternal to zygotic transition at which point supplies of functional Eg5 are exhausted, preventing further cell cleavage.

Katanin controls mitotic and meiotic spindle length

Accurate control of spindle length is a conserved feature of eukaryotic cell division. Lengthening of mitotic spindles contributes to chromosome segregation and cytokinesis during mitosis in animals and fungi. In contrast, spindle shortening may contribute to conservation of egg cytoplasm during female meiosis. Katanin is a microtubule-severing enzyme that is concentrated at mitotic and meiotic spindle poles in animals. We show that inhibition of katanin slows the rate of spindle shortening in nocodazole-treated mammalian fibroblasts and in untreated Caenorhabditis elegans meiotic embryos. Wild-type C. elegans meiotic spindle shortening proceeds through an early katanin-independent phase marked by increasing microtubule density and a second, katanin-dependent phase that occurs after microtubule density stops increasing. In addition, double-mutant analysis indicated that gamma-tubulin-dependent nucleation and microtubule severing may provide redundant mechanisms for increasing microtubule number during the early stages of meiotic spindle assembly.

Eh Klp5 is a divergent member of the kinesin 5 family that regulates genome content and microtubular assembly in Entamoeba histolytica

Earlier studies have established two unusual features in the cell division cycle of Entamoeba histolytica. First, microtubules form a radial assembly instead of a bipolar mitotic spindle, and second, the genome content of E. histolytica cells varied from 1x to 6x or more. In this study, Eh Klp5 was identified as a divergent member of the BimC kinesin family that is known to regulate formation and stabilization of the mitotic spindle in other eukaryotes. In contrast to earlier studies, we show here that bipolar microtubular spindles were formed in E. histolytica but were visible only in 8-12% of the cells after treatment with taxol. The number of bipolar spindles was significantly increased in Eh Klp5 stable transformants (20-25%) whereas Eh Klp5 double-stranded RNA (dsRNA) transformants did not show any spindles (< 1%). The genome content of Eh Klp5 stable transformants was regulated between 1x and 2x unlike control cells. Binucleated cells accumulated in Eh Klp5 dsRNA transformants and after inhibition of Eh Klp5 with small molecule inhibitors in control cells, suggesting that cytokinesis was delayed in the absence of Eh Klp5. Taken together, our results indicate that Eh Klp5 regulates microtubular assembly, genome content and cell division in E. histolytica. Additionally, Eh Klp5 showed alterations in its drug-binding site compared with its human homologue, Hs Eg5 and this was reflected in its reduced sensitivity to Eg5 inhibitors - monastrol and HR22C16 analogues.

Tracks for traffic: microtubules in the plant pathogen Ustilago maydis

Pathogenic development of the corn smut fungus Ustilago maydis depends on the ability of the hypha to grow invasively. Extended hyphal growth and mitosis require microtubules, as revealed by recent studies on the microtubule cytoskeleton. Surprisingly, hyphal tip growth involves only two out of 10 kinesins. Kinesin-3 is responsible for tip-directed (anterograde) endosome motility of early endosomes, which are thought to support hyphal elongation by apical membrane recycling. In addition, kinesin-3, together with kinesin-1 and myosin-5, appear to deliver secretory vesicles to the hyphal tip. Kinesin-1 also affects endosome motility by targeting cytoplasmic dynein to microtubule plus ends. This plus-end localization of dynein is essential for cell body-directed (retrograde) endosome motility, but also allows force generation during spindle elongation in mitosis. Furthermore, kinesin-1 and dynein participate in the organization of the microtubule array, thereby building their own network of tracks for intracellular motility. The recent progress in understanding microtubule-based processes in U. maydis has revealed an unexpected complexity of motor functions essential for the virulence of this pathogen. Further studies on structural and regulatory requirements for motor activity should help identify novel targets for fungicide development.

Dynein-mediated pulling forces drive rapid mitotic spindle elongation in Ustilago maydis

Spindle elongation segregates chromosomes and occurs in anaphase, an essential step in mitosis. Dynein-mediated pulling forces position the spindle, but their role in anaphase is a matter of debate. Here, we demonstrate that dynein is responsible for rapid spindle elongation in the model fungus Ustilago maydis. We show that initial slow elongation is supported by kinesin-5, which is located in the spindle mid-zone. When the spindle reaches approximately 2 microm in length, the elongation rate increases four-fold. This coincides with the appearance of long and less-dynamic microtubules (MTs) at each pole that accumulate dynein at their tips. Laser-mediated nanosurgery revealed that these MTs exert pulling forces in control cells, but not in dynein mutants. In addition, dynein mutants undergo initial slow anaphase, but fail to establish less-dynamic MTs and do not perform rapid spindle elongation, suggesting that dynein drives anaphase B. This is most likely mediated by cortical sliding of astral MTs along stationary dynein, which is off-loaded from the MT plus-end to the cortex.

Deletion of RNQ1 gene reveals novel functional relationship between divergently transcribed Bik1p/CLIP-170 and Sfi1p in spindle pole body separation

Spindle pole body (SPB; the microtubule organizing center in yeast) duplication is essential to form a bipolar spindle. The duplicated SPBs must then separate and migrate to opposite sides of the nucleus. We identified a novel functional relationship in SPB separation between the microtubule stabilizing protein Bik1p/CLIP-170 and the SPB half-bridge protein Sfi1p. A genetic interaction between BIK1 and SFI1 was discovered in a synthetic lethal screen using a strain deficient in the prion protein gene RNQ1. RNQ1 deletion reduced expression from the divergently transcribed BIK1, allowing us to identify genetic interactors with bik1. The sfi1-1 bik1 synthetic lethality was suppressed by over-expression of CIK1, KAR1, and PPH21. Genetic analysis indicated that the sfi1-1 bik1 synthetic lethality was unlikely related to the function of Bik1p in the dynein pathway or to defects in spindle position. Furthermore, a sfi1-1 Deltakip2 mutant was viable, suggesting that the Bik1p pool at the cytoplasmic microtubule plus-ends may not be required in sfi1-1. Microscopic examination indicated the sfi1-1 mutant was delayed in SPB duplication, SPB separation, or spindle elongation and the sfi-1 Deltabik1 double mutant arrested with duplicated but unseparated SPBs. These results suggest that Bik1p has a previously uncharacterized function in the separation of duplicated SPBs.

Sumoylation of the budding yeast kinetochore protein Ndc10 is required for Ndc10 spindle localization and regulation of anaphase spindle elongation

Posttranslational modification by the ubiquitin-like protein SUMO (small ubiquitin-like modifier) is emerging as an important regulator in many cellular processes, including genome integrity. In this study, we show that the kinetochore proteins Ndc10, Bir1, Ndc80, and Cep3, which mediate the attachment of chromosomes to spindle microtubules, are sumoylated substrates in budding yeast. Furthermore, we show that Ndc10, Bir1, and Cep3 but not Ndc80 are desumoylated upon exposure to nocodazole, highlighting the possibility of distinct roles for sumoylation in modulating kinetochore protein function and of a potential link between the sumoylation of kinetochore proteins and mitotic checkpoint function. We find that lysine to arginine mutations that eliminate the sumoylation of Ndc10 cause chromosome instability, mislocalization of Ndc10 from the mitotic spindle, abnormal anaphase spindles, and a loss of Bir1 sumoylation. These data suggest that sumoylation of Ndc10 and other kinetochore proteins play a critical role during the mitotic process.

Homotetrameric form of Cin8p, a Saccharomyces cerevisiae kinesin-5 motor, is essential for its in vivo function

Kinesin-5 motor proteins are evolutionarily conserved and perform essential roles in mitotic spindle assembly and spindle elongation during anaphase. Previous studies demonstrated a specialized homotetrameric structure with two pairs of catalytic domains, one at each end of a dumbbell-shaped molecule. This suggests that they perform their spindle roles by cross-linking and sliding antiparallel spindle microtubules. However, the exact kinesin-5 sequence elements that are important for formation of the tetrameric complexes have not yet been identified. In addition, it has not been demonstrated that the homotetrameric form of these proteins is essential for their biological functions. Thus, we investigated a series of Saccharomyces cerevisiae Cin8p truncations and internal deletions, in order to identify structural elements in the Cin8p sequence that are required for Cin8p functionality, spindle localization, and multimerization. We found that all variants of Cin8p that are functional in vivo form tetrameric complexes. The first coiled-coil domain in the stalk of Cin8p, a feature that is shared by all kinesin-5 homologues, is required for its dimerization, and sequences in the last part of the stalk, specifically those likely involved in coiled-coil formation, are required for Cin8p tetramerization. We also found that dimeric forms of Cin8p that are nonfunctional in vivo can nonetheless bind to microtubules. These findings suggest that binding of microtubules is not sufficient for the functionality of Cin8p and that microtubule cross-linking by the tetrameric complex is essential for Cin8p mitotic functions.

Deciphering protein function during mitosis in PtK cells using RNAi

Background

Studying mitosis requires a system in which the dramatic movements of chromosomes and spindle microtubules can be visualized. PtK cells, due to their flat morphology and their small number of large chromosomes, allow microscopic visualizations to be readily performed.

Results

By performing RNAi in PtK cells, we can explore the function of many proteins important for spindle assembly and chromosome segregation. Although it is difficult to transfect DNA into PtK cells (efficiency ~ 10%), we have transfected a fluorescent siRNA at nearly 100% efficiency. Using a cDNA expression library, we then isolated a complete PtK MCAK (P-MCAK) cDNA. P-MCAK shares 81% identity to Human-MCAK (H-MCAK) protein and 66% identity to H-MCAK DNA. Knockdown of P-MCAK by RNAi caused defects in chromosome congression and defective spindle organization. Live imaging revealed that chromosomes had defects in congression and segregation, similar to what we found after microinjection of inhibitory anti-MCAK antibodies. Because it is laborious to isolate full-length clones, we explored using RT-PCR with degenerate primers to yield cDNA fragments from PtK cells from which to design siRNAs. We isolated a cDNA fragment of the mitotic kinesin Eg5 from PtK cells. This fragment is 93% identical to H-Eg5 protein and 87% identical to H-Eg5 DNA. A conserved 21 bp siRNA was used for RNAi in both HeLa and PtK cells in which Eg5 knockdown resulted in an increased mitotic index and cells with monopolar spindles. In addition, we used RT-PCR to isolate fragments of 5 additional genes, whose sequence identity ranged from 76 to 90% with human, mouse, or rat genes, suggesting that this strategy is feasible to apply to any gene of interest.

Conclusion

This approach will allow us to effectively probe mitotic defects from protein knockdowns by combining genomic information from other organisms with the tractable morphology of PtK cells.

Disjunction of conjoined twins: Cdk1, Cdh1 and separation of centrosomes

Accurate transmission of chromosomes from parent to progeny cell requires assembly of a bipolar spindle. Centrosomes (spindle pole body in yeast) are critical for the biogenesis of this complex mitotic apparatus since they confer bipolarity on the spindle and serve as the site of microtubule polymerization. In each division cycle, the centrosome is duplicated and the sister-centrosomes move away from each other, forming the two poles of the spindle. While the structure and the duplication of centrosomes have been investigated extensively, the understanding of the control of their segregation remains scant. Recent findings are beginning to yield insights into the regulation of centrosome segregation in yeast and its link to the mitotic kinase.

Cdk1 regulates centrosome separation by restraining proteolysis of microtubule-associated proteins

In yeast, separation of duplicated spindle pole bodies (SPBs) (centrosomes in higher eukaryotes) is an indispensable step in the assembly of mitotic spindle and is triggered by severing of the bridge that connects the sister SPBs. This process requires Cdk1 (Cdc28) activation by Tyrosine 19 dephosphorylation. We show that cells that fail to activate Cdk1 are devoid of spindles due to persistently active APCCdh1, which targets microtubule-associated proteins Cin8, Kip1 and Ase1 for degradation. Tyrosine 19 dephosphorylation of Cdk1 is necessary to specifically prevent proteolysis of these proteins. Interestingly, SPB separation is dependent on the microtubule-bundling activity of Cin8 but not on its motor function. Since ectopic expression of proteolysis-resistant Cin8, Kip1 or Ase1 is sufficient for SPB separation even in the absence of Cdc28-Clb activity, we suggest that stabilization of these mechanical force-generating proteins is the predominant role of Cdc28-Clb in centrosome separation.

Analysis of kinesin motor function at budding yeast kinetochores

Accurate chromosome segregation during mitosis requires biorientation of sister chromatids on the microtubules (MT) of the mitotic spindle. Chromosome–MT binding is mediated by kinetochores, which are multiprotein structures that assemble on centromeric (CEN) DNA. The simple CENs of budding yeast are among the best understood, but the roles of kinesin motor proteins at yeast kinetochores have yet to be determined, despite evidence of their importance in higher eukaryotes. We show that all four nuclear kinesins in Saccharomyces cerevisiae localize to kinetochores and function in three distinct processes. Kip1p and Cin8p, which are kinesin-5/BimC family members, cluster kinetochores into their characteristic bilobed metaphase configuration. Kip3p, a kinesin-8,-13/KinI kinesin, synchronizes poleward kinetochore movement during anaphase A. The kinesin-14 motor Kar3p appears to function at the subset of kinetochores that become detached from spindle MTs. These data demonstrate roles for structurally diverse motors in the complex processes of chromosome segregation and reveal important similarities and intriguing differences between higher and lower eukaryotes.

Inhibition of the mitotic kinesin Eg5 up-regulates Hsp70 through the phosphatidylinositol 3-kinase/Akt pathway in multiple myeloma cells

The microtubule-dependent motor protein Eg5 plays a critical role in spindle assembly and maintenance in mitosis. Herein we show that the suppression of Eg5 by a specific inhibitor arrested mitosis, induced apoptosis, and up-regulated Hsp70 in human multiple myeloma cells. Mechanistically, Hsp70 induction occurred at the transcriptional level via a cis-regulatory DNA element in Hsp70 promoter and was mediated by the phosphatidylinositol 3-kinase/Akt pathway. Eg5 inhibitor-mediated Hsp70 up-regulation is cytoprotective because blocking Hsp70 induction directly by antisense or small interfering RNA or indirectly by inhibiting the phosphatidylinositol 3-kinase/Akt pathway significantly increased Eg5 inhibitor-induced apoptosis. Furthermore, a farnesyltransferase inhibitor interacted synergistically with the Eg5 inhibitor in inducing apoptosis through disrupting the Akt/Hsp70 signaling axis. These findings provide the first evidence for Eg5 inhibitor activity in hematologic malignancy and identify Hsp70 up-regulation as a critical mechanism responsible for modulating myeloma cell sensitivity to Eg5 inhibitors. In addition, these findings suggest that a combination of Eg5 inhibitors with agents abrogating Hsp70 induction would be useful for myeloma therapy in the clinic.

Mutations in the yeast kinesin-like Cin8p are alleviated by osmotic support

Loss of function of Cin8p (a yeast kinesin-like motor protein) in the absence of either Kip1p (a motor of the same family) or Dyn1p (the dynein heavy chain) is lethal. We report that cin8 mutants are sensitive to the cell wall disrupting agents calcofluor white and SDS. Conditionally lethal double mutants containing the temperature sensitive allele cin8-3 in a background deletion of either kip1 or dyn1 grew normally at the restrictive temperature when osmolytes such as sorbitol were added to the medium. Sorbitol could not alleviate the sensitivity of cin8 mutants to calcofluor and SDS. However, it rendered cells more resistant to the microtubule depolymerizing drugs benomyl and thiabendazole (TBZ). Our findings reveal a novel interaction between mitotic motor proteins and the cell wall and suggest that the induction of signaling pathways aimed at maintaining the cell wall suppresses phenotypes of mutations in microtubule-associated motor proteins through stabilization of microtubules.

Induction of the Candida albicans filamentous growth program by relief of transcriptional repression: a genome-wide analysis

Candida albicans, the major human fungal pathogen, undergoes a reversible morphological transition from blastospores (round budding cells) to filaments (elongated cells attached end-to-end). This transition, which is induced upon exposure of C. albicans cells to a number of host conditions, including serum and body temperature (37 degrees C), is required for virulence. Using whole-genome DNA microarray analysis, we describe 61 genes that are significantly induced (> or =2-fold) during the blastospore to filament transition that takes place in response to exposure to serum and 37 degrees C. We next show that approximately half of these genes are transcriptionally repressed in the blastospore state by three transcriptional repressors, Rfg1, Nrg1, and Tup1. We conclude that the relief of this transcriptional repression plays a key role in bringing the C. albicans filamentous growth program into play, and we describe the framework of this transcriptional circuit.

The rate of bipolar spindle assembly depends on the microtubule-gliding velocity of the mitotic kinesin Eg5

During early embryonic cycles, the time required for mitotic spindle assembly must match the autonomous cell cycle oscillations because a lack of coordination between these two processes will result in chromosome segregation errors. Members of the widely conserved BimC kinesin family are essential for spindle formation in all eukaryotes, and complete loss of BimC function results in monopolar spindles that have two spindle poles that are not separated. However, the precise roles of BimC motor activity in the spindle assembly process are not known. To examine the contribution of BimC kinesin's motor activity to spindle assembly, we generated and characterized mutants of Eg5, a vertebrate BimC kinesin, with reduced in vitro microtubule-gliding velocities. In Xenopus egg extracts, we replaced endogenous Eg5 with recombinant wild-type or mutant motor proteins. By using centrosome-dependent and centrosome-independent spindle assembly assays, we found that mechanisms that determine spindle size and shape were robust to approximately 6-fold reductions in Eg5 motility. However, the spindle assembly process was slower when Eg5 motor function was impaired. This role of Eg5 was independent of its contribution to centrosome separation. We provide evidence that Eg5 is a rate-limiting component of the cellular machinery that drives spindle assembly in vertebrates.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

The Caenorhabditis elegans Aurora B kinase AIR-2 phosphorylates and is required for the localization of a BimC kinesin to meiotic and mitotic spindles

BimC kinesins are required for mitotic spindle assembly in a variety of organisms. These proteins are localized to centrosomes, spindle microtubules, and the spindle midzone. We have previously shown that the Caenorhabditis elegans Aurora B kinase AIR-2 is required for the localization of the ZEN-4 kinesin protein to midzone microtubules. To determine whether the association of BimC kinesins with spindle microtubules is also dependent on AIR-2, we examined the expression pattern of BMK-1, a C. elegans BimC kinesin, in wild-type and AIR-2-deficient embryos. BMK-1 is highly expressed in the hermaphrodite gonad and is localized to meiotic spindle microtubules in the newly fertilized embryo. In mitotic embryos, BMK-1 is associated with spindle microtubules from prophase through anaphase and is concentrated at the spindle midzone during anaphase and telophase. In the absence of AIR-2, BMK-1 localization to meiotic and mitotic spindles is greatly reduced. This is not a consequence of loss of ZEN-4 localization because BMK-1 is appropriately localized in ZEN-4-deficient embryos. Furthermore, AIR-2 and BMK-1 directly interact with one another and the C-terminal tail domain of BMK-1 is specifically phosphorylated by AIR-2 in vitro. Together with our previous data, these results suggest that at least one function of the Aurora B kinases is to recruit spindle-associated motor proteins to their sites of action.

Analysis of a spindle pole body mutant reveals a defect in biorientation and illuminates spindle forces

The spindle pole body (SPB) is the microtubule organizing center in Saccharomyces cerevisiae. An essential task of the SPB is to ensure assembly of the bipolar spindle, which requires a proper balancing of forces on the microtubules and chromosomes. The SPB component Spc110p connects the ends of the spindle microtubules to the core of the SPB. We previously reported the isolation of a mutant allele spc110-226 that causes broken spindles and SPB disintegration 30 min after spindle formation. By live cell imaging of mutant cells with green fluorescent protein (GFP)-Tub1p or Spc97p-GFP, we show that spc110-226 mutant cells have early defects in spindle assembly. Short spindles form but do not advance to the 1.5-microm stage and frequently collapse. Kinetochores are not arranged properly in the mutant cells. In 70% of the cells, no stable biorientation occurs and all kinetochores are associated with only one SPB. Examination of the SPB remnants by electron microscopy tomography and fluorescence microscopy revealed that the Spc110-226p/calmodulin complex is stripped off of the central plaque of the SPB and coalesces to from a nucleating structure in the nucleoplasm. The central plaque components Spc42p and Spc29p remain behind in the nuclear envelope. The delamination is likely due to a perturbed interaction between Spc42p and Spc110-226p as detected by fluorescence resonance energy transfer analysis. We suggest that the force exerted on the SPB by biorientation of the chromosomes pulls the Spc110-226p out of the SPB; removal of force exerted by coherence of the sister chromatids reduced fragmentation fourfold. Removal of the forces exerted by the cytoplasmic microtubules had no effect on fragmentation. Our results provide insights into the relative contributions of the kinetochore and cytoplasmic microtubules to the forces involved in formation of a bipolar spindle.

Mechanistic analysis of the Saccharomyces cerevisiae kinesin Kar3

Kar3 is a minus-end-directed microtubule motor that is implicated in meiotic and mitotic spindle function in Saccharomyces cerevisiae. To date, the only truncated protein of Kar3 that has been reported to promote unidirectional movement in vitro is GSTKar3. This motor contains an NH2-terminal glutathione S-transferase (GST) tag followed by the Kar3 sequence that is predicted to form an extended alpha-helical coiled-coil. The alpha-helical domain leads into the neck linker and COOH-terminal motor domain. Kar3 does not homodimerize with itself but forms a heterodimer with either Cik1 or Vik1, both of which are non-motor polypeptides. We evaluated the microtubule-GSTKar3 complex in comparison to the microtubule-Kar3 motor domain complex to determine the distinctive mechanistic features required for GSTKar3 motility. Our results indicate that ATP binding was significantly faster for GSTKar3 than that observed previously for the Kar3 motor domain. In addition, microtubule-activated ADP release resulted in an intermediate that bound ADP weakly in contrast to the Kar3 motor domain, suggesting that after ADP release, the microtubule-GSTKar3 motor binds ATP in preference to ADP. The kinetics also showed that GST-Kar3 readily detached from the microtubule rather than remaining bound for multiple ATP turnovers. These results indicate that the extended alpha-helical domain NH2-terminal to the catalytic core provides the structural transitions in response to the ATPase cycle that are critical for motility and that dimerization is not specifically required. This study provides the foundation to define the mechanistic contributions of Cik1 and Vik1 for Kar3 force generation and function in vivo.

Mechanistic analysis of the mitotic kinesin Eg5

Eg5 is a slow, plus-end-directed microtubule-based motor of the BimC kinesin family that is essential for bipolar spindle formation during eukaryotic cell division. We have analyzed two human Eg5/KSP motors, Eg5-367 and Eg5-437, and both are monomeric based on results from sedimentation velocity and sedimentation equilibrium centrifugation as well as analytical gel filtration. The steady-state parameters were: for Eg5-367: k(cat) = 5.5 s(-1), K(1/2,Mt) = 0.7 microm, and K(m,ATP) = 25 microm; and for Eg5-437: k(cat) = 2.9 s(-1), K(1/2,Mt) = 4.5 microm, and K(m,ATP) = 19 microm. 2'(3')-O-(N-Methylanthraniloyl)-ATP (mantATP) binding was rapid at 2-3 microm(-1)s(-1), followed immediately by ATP hydrolysis at 15 s(-1). ATP-dependent Mt.Eg5 dissociation was relatively slow and rate-limiting at 8 s(-1) with mantADP release at 40 s(-1). Surprisingly, Eg5-367 binds microtubules more effectively (11 microm(-1)s(-1)) than Eg5-437 (0.7 microm(-1)s(-1)), consistent with the steady-state K(1/2,Mt) and the mantADP release K(1/2,Mt). These results indicate that the ATPase pathway for monomeric Eg5 is more similar to conventional kinesin than the spindle motors Ncd and Kar3, where ADP product release is rate-limiting for steady-state turnover.

Antitumor activity of a kinesin inhibitor

Several members of the kinesin family of microtubule motor proteins play essential roles in mitotic spindle function and are potential targets for the discovery of novel antimitotic cancer therapies. KSP, also known as HsEg5, is a kinesin that plays an essential role in formation of a bipolar mitotic spindle and is required for cell cycle progression through mitosis. We identified a potent inhibitor of KSP, CK0106023, which causes mitotic arrest and growth inhibition in several human tumor cell lines. Here we show that CK0106023 is an allosteric inhibitor of KSP motor domain ATPase with a Ki of 12 nM. Among five kinesins tested, CK0106023 was specific for KSP. In tumor-bearing mice, CK0106023 exhibited antitumor activity comparable to or exceeding that of paclitaxel and caused the formation of monopolar mitotic figures identical to those produced in cultured cells. KSP was most abundant in proliferating human tissues and was absent from cultured postmitotic neurons. These findings are the first to demonstrate the feasibility of targeting mitotic kinesins for the treatment of cancer.

Domain analysis of the tubulin cofactor system: a model for tubulin folding and dimerization

Background

The correct folding and dimerization of tubulins, before their addition to the microtubular structure, needs a group of conserved proteins called cofactors A to E. The biochemical analysis of cofactors gave an insight to their general functions, however not much is known about the domain structure and detailed, molecular function of these proteins.

Results

Combining modelling and fold prediction tools, we present 3D models of all cofactors, including several previously unannotated domains of cofactors B-E. Apart from the new HEAT and Armadillo domains in cofactor D and an unusual spectrin-like domain in cofactor C, we have identified a new subfamily of ubiquitin-like domains in cofactors B and E. Together, these observations provide a reliable, molecular level model of cofactor complex.

Conclusion

Distant homology searches allowed the identification of unknown regions of cofactors as self-reliant domains and allow us to present a detailed hypothesis of how a cofactor complex performs its function.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.