Motor activity and mitotic spindle localization of the Drosophila kinesin-like protein KLP61F

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.

Kinesin-5 Is Dispensable for Bipolar Spindle Formation and Elongation in Candida albicans, but Simultaneous Loss of Kinesin-14 Activity Is Lethal

Mitotic spindles assume a bipolar architecture through the concerted actions of microtubules, motors, and cross-linking proteins. In most eukaryotes, kinesin-5 motors are essential to this process, and cells will fail to form a bipolar spindle without kinesin-5 activity. Remarkably, inactivation of kinesin-14 motors can rescue this kinesin-5 deficiency by reestablishing the balance of antagonistic forces needed to drive spindle pole separation and spindle assembly. We show that the yeast form of the opportunistic fungus Candida albicans assembles bipolar spindles in the absence of its sole kinesin-5, CaKip1, even though this motor exhibits stereotypical cell-cycle-dependent localization patterns within the mitotic spindle. However, cells lacking CaKip1 function have shorter metaphase spindles and longer and more numerous astral microtubules. They also show defective hyphal development. Interestingly, a small population of CaKip1-deficient spindles break apart and reform two bipolar spindles in a single nucleus. These spindles then separate, dividing the nucleus, and then elongate simultaneously in the mother and bud or across the bud neck, resulting in multinucleate cells. These data suggest that kinesin-5-independent mechanisms drive assembly and elongation of the mitotic spindle in C. albicans and that CaKip1 is important for bipolar spindle integrity. We also found that simultaneous loss of kinesin-5 and kinesin-14 (CaKar3Cik1) activity is lethal. This implies a divergence from the antagonistic force paradigm that has been ascribed to these motors, which could be linked to the high mitotic error rate that C. albicans experiences and often exploits as a generator of diversity.IMPORTANCE Candida albicans is one of the most prevalent fungal pathogens of humans and can infect a broad range of niches within its host. This organism frequently acquires resistance to antifungal agents through rapid generation of genetic diversity, with aneuploidy serving as a particularly important adaptive mechanism. This paper describes an investigation of the sole kinesin-5 in C. albicans, which is a major regulator of chromosome segregation. Contrary to other eukaryotes studied thus far, C. albicans does not require kinesin-5 function for bipolar spindle assembly or spindle elongation. Rather, this motor protein associates with the spindle throughout mitosis to maintain spindle integrity. Furthermore, kinesin-5 loss is synthetically lethal with loss of kinesin-14-canonically an opposing force producer to kinesin-5 in spindle assembly and anaphase. These results suggest a significant evolutionary rewiring of microtubule motor functions in the C. albicans mitotic spindle, which may have implications in the genetic instability of this pathogen.

A 'molecular guillotine' reveals the interphase function of Kinesin-5

Motor proteins are important for transport and force generation in a variety of cellular processes and in morphogenesis. Here, we describe a general strategy for conditional motor mutants by inserting a protease cleavage site into the 'neck' between the head domain and the stalk of the motor protein, making the protein susceptible to proteolytic cleavage at the neck by the corresponding protease. To demonstrate the feasibility of this approach, we inserted the cleavage site of the tobacco etch virus (TEV) protease into the neck of the tetrameric motor Kinesin-5. Application of TEV protease led to a specific depletion and functional loss of Kinesin-5 in Drosophila embryos. With our approach, we revealed that Kinesin-5 stabilizes the microtubule network during interphase in syncytial embryos. The 'molecular guillotine' can potentially be applied to many motor proteins because Kinesins and myosins have conserved structures with accessible neck regions.This article has an associated First Person interview with the first author of the paper.

New insights into the mechanism of force generation by kinesin-5 molecular motors

Kinesin-5 motors are members of a superfamily of microtubule-dependent ATPases and are widely conserved among eukaryotes. Kinesin-5s typically form homotetramers with pairs of motor domains located at either end of a dumbbell-shaped molecule. This quaternary structure enables cross-linking and ATP-driven sliding of pairs of microtubules, although the exact molecular mechanism of this activity is still unclear. Kinesin-5 function has been characterized in greatest detail in cell division, although a number of interphase roles have also been defined. The kinesin-5 ATPase is tuned for slow microtubule sliding rather than cellular transport and-in vertebrates-can be inhibited specifically by allosteric small molecules currently in cancer clinical trials. The biophysical and structural basis of kinesin-5 mechanochemistry is being elucidated and has provided further insight into kinesin-5 activities. However, it is likely that the precise mechanism of these important motors has evolved according to functional context and regulation in individual organisms.

Proteomic and functional analysis of the mitotic Drosophila centrosome

Regulation of centrosome structure, duplication and segregation is integrated into cellular pathways that control cell cycle progression and growth. As part of these pathways, numerous proteins with well-established non-centrosomal localization and function associate with the centrosome to fulfill regulatory functions. In turn, classical centrosomal components take up functional and structural roles as part of other cellular organelles and compartments. Thus, although a comprehensive inventory of centrosome components is missing, emerging evidence indicates that its molecular composition reflects the complexity of its functions. We analysed the Drosophila embryonic centrosomal proteome using immunoisolation in combination with mass spectrometry. The 251 identified components were functionally characterized by RNA interference. Among those, a core group of 11 proteins was critical for centrosome structure maintenance. Depletion of any of these proteins in Drosophila SL2 cells resulted in centrosome disintegration, revealing a molecular dependency of centrosome structure on components of the protein translation machinery, actin- and RNA-binding proteins. In total, we assigned novel centrosome-related functions to 24 proteins and confirmed 13 of these in human cells.

Purification and assay of mitotic motors

To understand how mitotic kinesins contribute to the assembly and function of the mitotic spindle, we need to purify these motors and analyze their biochemical and ultrastructural properties. Here we briefly review our use of microtubule (MT) affinity and biochemical fractionation to obtain information about the oligomeric state of native mitotic kinesin holoenzymes from eggs and early embryos. We then detail the methods we use to purify full length recombinant Drosophila embryo mitotic kinesins, using the baculovirus expression system, in sufficient yields for detailed in vitro assays. These two approaches provide complementary biochemical information on the basic properties of these key mitotic proteins, and permit assays of critical motor activities, such as MT-MT crosslinking and sliding, that are not revealed by assaying motor domain subfragments.

Kinesin-5 in Drosophila embryo mitosis: sliding filament or spindle matrix mechanism?

The Drosophila syncytial embryo uses multiple astral mitotic spindles that are specialized for rapid mitosis. The homotetrameric kinesin-5, KLP61F contributes to various aspects of mitosis in this system, all of which are consistent with it exerting outward forces on spindle poles. In principle, kinesin-5 could accomplish this by (i) sliding microtubules (MTs), minus end leading, relative to a static spindle matrix or (ii) crosslinking and sliding apart adjacent pairs of antiparallel interpolar (ip) MTs. Here, I critically review data on the biochemistry of purified KLP61F, its localization and dynamic properties within spindles, and quantitative modeling of KLP61F function. While a matrix-based mechanism may operate in some systems, the work tends to support the latter "sliding filament" mechanism for KLP61F action in Drosophila embryo spindles. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Tyrosines in the kinesin-5 head domain are necessary for phosphorylation by Wee1 and for mitotic spindle integrity

Mitotic spindle assembly and maintenance relies on kinesin-5 motors that act as bipolar homotetramers to crosslink microtubules. Kinesin-5 motors have been the subject of extensive structure-function analysis, but the regulation of their activity in the context of mitotic progression remains less well understood. We report here that Drosophila kinesin-5 (KLP61F) is regulated by Drosophila Wee1 (dWee1). Wee1 tyrosine kinases are known to regulate mitotic entry via inhibitory phosphorylation of Cdk1. Recently, we showed that dWee1 also plays a role in mitotic spindle positioning through gamma-tubulin and spindle fidelity through an unknown mechanism. Here, we investigated whether a KLP61F-dWee1 interaction could explain the latter role of dWee1. We found that dWee1 phosphorylates KLP61F in vitro on three tyrosines within the head domain, the catalytic region that mediates movement along microtubules. In vivo, KLP61F with tyrosine-->phenylalanine mutations fails to complement a klp61f mutant and dominantly induces spindle defects similar to ones seen in dwee1 mutants. We propose that phosphorylation of the KLP61F catalytic domain by dWee1 is important for the motor's function. This study identifies a second substrate for a Wee1 kinase and provides evidence for phosphoregulation of a kinesin in the head domain.

MLN8054, a small-molecule inhibitor of Aurora A, causes spindle pole and chromosome congression defects leading to aneuploidy

Aurora A kinase plays an essential role in the proper assembly and function of the mitotic spindle, as its perturbation causes defects in centrosome separation, spindle pole organization, and chromosome congression. Moreover, Aurora A disruption leads to cell death via a mechanism that involves aneuploidy generation. However, the link between the immediate functional consequences of Aurora A inhibition and the development of aneuploidy is not clearly defined. In this study, we delineate the sequence of events that lead to aneuploidy following Aurora A inhibition using MLN8054, a selective Aurora A small-molecule inhibitor. Human tumor cells treated with MLN8054 show a high incidence of abnormal mitotic spindles, often with unseparated centrosomes. Although these spindle defects result in mitotic delays, cells ultimately divide at a frequency near that of untreated cells. We show that many of the spindles in the dividing cells are bipolar, although they lack centrosomes at one or more spindle poles. MLN8054-treated cells frequently show alignment defects during metaphase, lagging chromosomes in anaphase, and chromatin bridges during telophase. Consistent with the chromosome segregation defects, cells treated with MLN8054 develop aneuploidy over time. Taken together, these results suggest that Aurora A inhibition kills tumor cells through the development of deleterious aneuploidy.

A homotetrameric kinesin-5, KLP61F, bundles microtubules and antagonizes Ncd in motility assays

BACKGROUND

Mitosis depends upon the cooperative action of multiple microtubule (MT)-based motors. Among these, a kinesin-5, KLP61F, and the kinesin-14, Ncd, are proposed to generate antagonistic-sliding forces that control the spacing of the spindle poles. We tested whether purified KLP61F homotetramers and Ncd homodimers can generate a force balance capable of maintaining a constant spindle length in Drosophila embryos.

RESULTS

Using fluorescence microscopy and cryo-EM, we observed that purified full-length, motorless, and tailless KLP61F tetramers (containing a tetramerization domain) and Ncd dimers can all cross-link MTs into bundles in MgATP. In multiple-motor motility assays, KLP61F and Ncd drive plus-end and minus-end MT sliding at 0.04 and 0.1 microm/s, respectively, but the motility of either motor is decreased by increasing the mole fraction of the other. At the "balance point," the mean velocity was zero and MTs paused briefly and then oscillated, taking approximately 0.3 microm excursions at approximately 0.02 microm/s toward the MT plus end and then the minus end.

CONCLUSIONS

The results, combined with quantitative analysis, suggest that these motors could act as mutual brakes to modulate the rate of pole-pole separation and could maintain a prometaphase spindle displaying small fluctuations in its steady-state length.

Cell cycle-dependent dynamics and regulation of mitotic kinesins in Drosophila S2 cells

Constructing a mitotic spindle requires the coordinated actions of several kinesin motor proteins. Here, we have visualized the dynamics of five green fluorescent protein (GFP)-tagged mitotic kinesins (class 5, 6, 8, 13, and 14) in live Drosophila Schneider cell line (S2), after first demonstrating that the GFP-tag does not interfere with the mitotic functions of these kinesins using an RNA interference (RNAi)-based rescue strategy. Class 8 (Klp67A) and class 14 (Ncd) kinesin are sequestered in an active form in the nucleus during interphase and engage their microtubule targets upon nuclear envelope breakdown (NEB). Relocalization of Klp67A to the cytoplasm using a nuclear export signal resulted in the disassembly of the interphase microtubule array, providing support for the hypothesis that this kinesin class possesses microtubule-destabilizing activity. The interactions of Kinesin-5 (Klp61F) and -6 (Pavarotti) with microtubules, on the other hand, are activated and inactivated by Cdc2 phosphorylation, respectively, as shown by examining localization after mutating Cdc2 consensus sites. The actions of microtubule-destabilizing kinesins (class 8 and 13 [Klp10A]) seem to be controlled by cell cycle-dependent changes in their localizations. Klp10A, concentrated on microtubule plus ends in interphase and prophase, relocalizes to centromeres and spindle poles upon NEB and remains at these sites throughout anaphase. Consistent with this localization, RNAi analysis showed that this kinesin contributes to chromosome-to-pole movement during anaphase A. Klp67A also becomes kinetochore associated upon NEB, but the majority of the population relocalizes to the central spindle by the timing of anaphase A onset, consistent with our RNAi result showing no effect of depleting this motor on anaphase A. These results reveal a diverse spectrum of regulatory mechanisms for controlling the localization and function of five mitotic kinesins at different stages of the cell cycle.

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.

Novel nuclear defects in KLP61F-deficient mutants in Drosophila are partially suppressed by loss of Ncd function

KLP61F in Drosophila and other BimC kinesins are essential for spindle bipolarity across species; loss of BimC function generates high frequencies of monopolar spindles. Concomitant loss of Kar3 kinesin function increases the frequency of bipolar spindles although the underlying mechanism is not known. Recent studies raise the question of whether BimC kinesins interact with a non-microtubule spindle matrix rather than spindle microtubules. Here we present cytological evidence that loss of KLP61F function generates novel defects during M-phase in the organization and integrity of the nuclear lamina, an integral component of the nuclear matrix. Larval neuroblasts and spermatocytes of klp61F mutants showed deep involutions in the nuclear lamina extending toward the centrally located centrosomes. Repositioning of centrosomes to form monopolar spindles probably does not cause invaginations as similar invaginations formed in spermatocytes lacking centrosomes entirely. Immunofluorescence microscopy indicated that non-claret disjunctional (Ncd) is a component of the nuclear matrix in somatic cells and spermatocytes. Loss of Ncd function increases the frequency of bipolar spindles in klp61F mutants. Nuclear defects were incompletely suppressed; micronuclei formed near telophase at the poles of bipolar spindle in klp61F ncd spermatocytes. Our results are consistent with a model in which KLP61F prevents Ncd-mediated collapse of a nonmicrotubule matrix derived from the interphase nucleus.

Toucan protein is essential for the assembly of syncytial mitotic spindles in Drosophila melanogaster

The toc gene of Drosophila melanogaster encodes a 235-kD polypeptide with a coiled-coil domain, which is highly expressed during oogenesis (Grammont et al., 1997, 2000). We now report the localization of the Toucan protein during early embryonic development. The Toucan protein is present only during the syncytial stages and is associated with the nuclear envelope and the cytoskeletal structures of the syncytial embryo. In anaphase A, Toucan is concentrated at the spindle poles near the minus end of microtubules. This microtubule association is very dynamic during the nuclear cell cycle. Mutant embryos lacking the Toucan protein are blocked in a metaphase-like state. They display abnormal and nonfunctional spindles, characterized by broad poles, detachment of the centrosomes, and failure of migration of the chromosomes. These results strongly suggest that Toucan represents a factor essential for the assembly and the function of the syncytial mitotic spindles.

Physical properties determining self-organization of motors and microtubules

In eukaryotic cells, microtubules and their associated motor proteins can be organized into various large-scale patterns. Using a simplified experimental system combined with computer simulations, we examined how the concentrations and kinetic parameters of the motors contribute to their collective behavior. We observed self-organization of generic steady-state structures such as asters, vortices, and a network of interconnected poles. We identified parameter combinations that determine the generation of each of these structures. In general, this approach may become useful for correlating the morphogenetic phenomena taking place in a biological system with the biophysical characteristics of its constituents.

The Xenopus laevis aurora/Ip11p-related kinase pEg2 participates in the stability of the bipolar mitotic spindle

The Xenopus laevis aurora/Ip11p-related kinase pEg2 is required for centrosome separation, which is a prerequisite for bipolar mitotic spindle formation. Here, we report that the inhibition of pEg2 by addition of either an inactive kinase or a monoclonal antibody destabilizes bipolar spindles previously assembled in Xenopus egg extracts. The bipolar spindles collapse to form structures such as microtubule asters with chromosome rosettes, monopolar spindles, and multipolar spindles. In collapsed spindles, chromosomes remain attached to the microtubules plus ends. The destabilization of the bipolar spindle is reminiscent of the destabilization observed after inhibition of cross-linking activities which maintain parallel and anti-parallel microtubules linked together. We have previously reported that pEg2 phosphorylates the kinesin-related protein XlEg5 which is involved in centrosome separation but which was also reported to be involved in spindle stability. The collapse of the bipolar spindle observed after inhibition of pEg2 suggests that the kinase might regulate the cross-linking activity of XlEg5. We do not exclude the possibility that pEg2 also regulates other microtubule-based motor proteins involved in bipolar spindle stability. To our knowledge, this is the first evidence that aurora/Ip11p-related kinase activity actually participates not only in mitotic spindle formation by regulating centrosome separation but also in mitotic spindle stabilization.

Orbit, a novel microtubule-associated protein essential for mitosis in Drosophila melanogaster

We describe a Drosophila gene, orbit, that encodes a conserved 165-kD microtubule-associated protein (MAP) with GTP binding motifs. Hypomorphic mutations in orbit lead to a maternal effect resulting in branched and bent mitotic spindles in the syncytial embryo. In the larval central nervous system, such mutants have an elevated mitotic index with some mitotic cells showing an increase in ploidy. Amorphic alleles show late lethality and greater frequencies of hyperploid mitotic cells. The presence of cells in the hypomorphic mutant in which the chromosomes can be arranged, either in a circular metaphase or an anaphase-like configuration on monopolar spindles, suggests that polyploidy arises through spindle and chromosome segregation defects rather than defects in cytokinesis. A role for the Orbit protein in regulating microtubule behavior in mitosis is suggested by its association with microtubules throughout the spindle at all mitotic stages, by its copurification with microtubules from embryonic extracts, and by the finding that the Orbit protein directly binds to MAP-free microtubules in a GTP-dependent manner.

Mitotic motors in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae provides a unique opportunity for study of the microtubule-based motor proteins that participate in mitotic spindle function. The genome of Saccharomyces encodes a relatively small and genetically tractable set of microtubule-based motor proteins. The single cytoplasmic dynein and five of the six kinesin-related proteins encoded have been implicated in mitotic spindle function. Each motor protein is unique in amino acid sequence. On account of functional overlap, no single motor is uniquely required for cell viability, however. The ability to create and analyze multiple mutants has allowed experimental dissection of the roles performed by each mitotic motor. Some of the motors operate within the nucleus to assemble and elongate the bipolar spindle (kinesin-related Cin8p, Kip1p, Kip3p and Kar3p). Others operate on the cytoplasmic microtubules to effect spindle and nuclear positioning within the cell (dynein and kinesin-related Kip2p, Kip3p and Kar3p). The six motors apparently contribute three fundamental activities to spindle function: motility, microtubule cross-linking and regulation of microtubule dynamics.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

mini spindles: A gene encoding a conserved microtubule-associated protein required for the integrity of the mitotic spindle in Drosophila

We describe a new Drosophila gene, mini spindles (msps) identified in a cytological screen for mitotic mutant. Mutation in msps disrupts the structural integrity of the mitotic spindle, resulting in the formation of one or more small additional spindles in diploid cells. Nucleation of microtubules from centrosomes, metaphase alignment of chromosomes, or the focusing of spindle poles appears much less affected. The msps gene encodes a 227-kD protein with high similarity to the vertebrate microtubule-associated proteins (MAPs), human TOGp and Xenopus XMAP215, and with limited similarity to the Dis1 and STU2 proteins from fission yeast and budding yeast. Consistent with their sequence similarity, Msps protein also associates with microtubules in vitro. In the embryonic division cycles, Msps protein localizes to centrosomal regions at all mitotic stages, and spreads over the spindles during metaphase and anaphase. The absence of centrosomal staining in interphase of the cellularized embryos suggests that the interactions between Msps protein and microtubules or centrosomes may be regulated during the cell cycle.

The Xenopus laevis aurora-related protein kinase pEg2 associates with and phosphorylates the kinesin-related protein XlEg5

We have previously reported on the cloning of XlEg5, a Xenopus laevis kinesin-related protein from the bimC family (Le Guellec, R., Paris, J., Couturier, A., Roghi, C., and Philippe, M. (1991) Mol. Cell. Biol. 11, 3395-3408) as well as pEg2, an Aurora-related serine/threonine kinase (Roghi, C., Giet, R., Uzbekov, R., Morin, N., Chartrain, I., Le Guellec, R., Couturier, A., Dorée, M., Philippe, M., and Prigent, C. (1998) J. Cell Sci. 111, 557-572). Inhibition of either XlEg5 or pEg2 activity during mitosis in Xenopus egg extract led to monopolar spindle formation. Here, we report that in Xenopus XL2 cells, pEg2 and XlEg5 are both confined to separated centrosomes in prophase, and then to the microtubule spindle poles. We also show that pEg2 co-immunoprecipitates with XlEg5 from egg extracts and XL2 cell lysates. Both proteins can directly interact in vitro, but also through the two-hybrid system. Furthermore immunoprecipitated pEg2 were found to remain active when bound to the beads and phosphorylate XlEg5 present in the precipitate. Two-dimensional mapping of XlEg5 tryptic peptides phosphorylated in vivo first confirmed that XlEg5 was phosphorylated by p34(cdc2) and next revealed that in vitro pEg2 kinase phosphorylated XlEg5 on the same stalk domain serine residue that was phosphorylated in metabolically labeled XL2 cells. The kinesin-related XlEg5 is to our knowledge the first in vivo substrate ever reported for an Aurora-related kinase.

Microtubule motors in spindle and chromosome motility

Many of the kinesin microtubule motor proteins discovered during the past 8-9 years have roles in spindle assembly and function or chromosome movement during meiosis or mitosis. The discovery of kinesin motor proteins with a clear involvement in spindle and chromosome motility, together with recent evidence that cytoplasmic dynein plays a role in chromosome distribution, has attracted great interest. The identification of microtubule motors that function in chromosome distribution represents a major advance in understanding the forces that underlie chromosome and spindle movements during cell division.

Motile properties of the kinesin-related Cin8p spindle motor extracted from Saccharomyces cerevisiae cells

We have developed microtubule binding and motility assays for Cin8p, a kinesin-related mitotic spindle motor protein from Saccharomyces cerevisiae. The methods examine Cin8p rapidly purified from crude yeast cell extracts. We created a recombinant form of CIN8 that fused the biotin carrying polypeptide from yeast pyruvate carboxylase to the carboxyl terminus of Cin8p. This form was biotinated in yeast cells and provided Cin8p activity in vivo. Avidin-coated glass surfaces were used to specifically bind biotinated Cin8p from crude extracts. Microtubules bound to the Cin8p-coated surfaces and moved at 3.4 +/- 0.5 micrometer/min in the presence of ATP. Force production by Cin8p was directed toward the plus ends of microtubules. A mutation affecting the microtubule-binding site within the motor domain (cin8-F467A) decreased Cin8p's ability to bind microtubules to the glass surface by >10-fold, but reduced gliding velocity by only 35%. The cin8-3 mutant form, affecting the alpha2 helix of the motor domain, caused a moderate defect in microtubule binding, but motility was severely affected. cin8-F467A cells, but not cin8-3 cells, were greatly impaired in bipolar spindle forming ability. We conclude that microtubule binding by Cin8p is more important than motility for proper spindle formation.

Cell cycle analysis and synchronization of the Xenopus laevis XL2 cell line: study of the kinesin related protein XlEg5

Cell free extracts prepared from Xenopus eggs are one of the most powerful in vitro systems to analyze cell cycle-regulated mechanisms such as DNA replication, nuclear assembly, chromosome condensation, or spindle formation. Xenopus embryos can complete several synchronous cell cycles in the absence of transcription, consequently Xenopus extracts are very helpful to study the molecular level of cellular mechanisms. Many key cell cycle regulators like p34cdc2 and cdk2 have been discovered and characterized using those extracts, but their regulation during somatic cell cycles have only been studied in mammalian cultured cells. In this paper, we describe optimized conditions to obtain cell cycle arrested Xenopus XL2 cultured cells. Synchronization of XL2 cells at different stages of the cell cycle was achieved by serum starvation and drug treatments such as aphidicolin, nocodazole, and ALLN. The degree of synchronization was assessed by indirect fluorescence microscopy and FACS analysis. This method was used to study the cell cycle expression of the Xenopus kinesin-related protein, XlEg5, a microtubule-based motor protein involved in movement and cell division in early development. We found that the expression of the protein was maximum in mitosis and minimum in G1, which correlated with the expression of its messenger RNA. XL2 cultured cells were also used to analyze the ultrastructural sub-cellular localization of XlEg5. During mitosis, the protein was found around the centrosome in prophase, on the spindle microtubules in metaphase, and, interestingly, around the minus end of the midbody microtubules in telophase.

Abnormal spindle protein, Asp, and the integrity of mitotic centrosomal microtubule organizing centers

The product of the abnormal spindle (asp) gene was found to be an asymmetrically localized component of the centrosome during mitosis, required to focus the poles of the mitotic spindle in vivo. Removing Asp protein function from Drosophila melanogaster embryo extracts, either by mutation or immunodepletion, resulted in loss of their ability to restore microtubule-organizing center activity to salt-stripped centrosome preparations. This was corrected by addition of purified Asp protein. Thus, Asp appears to hold together the microtubule-nucleating gamma-tubulin ring complexes that organize the mitotic centrosome.

The bipolar kinesin, KLP61F, cross-links microtubules within interpolar microtubule bundles of Drosophila embryonic mitotic spindles

Previous genetic and biochemical studies have led to the hypothesis that the essential mitotic bipolar kinesin, KLP61F, cross-links and slides microtubules (MTs) during spindle assembly and function. Here, we have tested this hypothesis by immunofluorescence and immunoelectron microscopy (immunoEM). We show that Drosophila embryonic spindles at metaphase and anaphase contain abundant bundles of MTs running between the spindle poles. These interpolar MT bundles are parallel near the poles and antiparallel in the midzone. We have observed that KLP61F motors, phosphorylated at a cdk1/cyclin B consensus domain within the BimC box (BCB), localize along the length of these interpolar MT bundles, being concentrated in the midzone region. Nonphosphorylated KLP61F motors, in contrast, are excluded from the spindle and display a cytoplasmic localization. Immunoelectron microscopy further suggested that phospho-KLP61F motors form cross-links between MTs within interpolar MT bundles. These bipolar KLP61F MT-MT cross-links should be capable of organizing parallel MTs into bundles within half spindles and sliding antiparallel MTs apart in the spindle midzone. Thus we propose that bipolar kinesin motors and MTs interact by a "sliding filament mechanism" during the formation and function of the mitotic spindle.

Expression of the mitotic motor protein Eg5 in postmitotic neurons: implications for neuronal development

It is well established that the microtubules of the mitotic spindle are organized by a variety of motor proteins, and it appears that the same motors or closely related variants organize microtubules in the postmitotic neuron. Specifically, cytoplasmic dynein and the kinesin-related motor known as CHO1/MKLP1 are used within the mitotic spindle, and recent studies suggest that they are also essential for the establishment of the axonal and dendritic microtubule arrays of the neuron. Other motors are required to tightly regulate microtubule behaviors in the mitotic spindle, and it is attractive to speculate that these motors might also help to regulate microtubule behaviors in the neuron. Here we show that a homolog of the mitotic kinesin-related motor known as Eg5 continues to be expressed in rodent neurons well after their terminal mitotic division. In neurons, Eg5 is directly associated with the microtubule array and is enriched within the distal regions of developing processes. This distal enrichment is transient, and typically lost after a process has been clearly defined as an axon or a dendrite. Strong expression can resume later in development, and if so, the protein concentrates within newly forming sprouts at the distal tips of dendrites. We suggest that Eg5 generates forces that help to regulate microtubule behaviors within the distal tips of developing axons and dendrites.

Expression of the mitotic motor protein Eg5 in postmitotic neurons: implications for neuronal development

It is well established that the microtubules of the mitotic spindle are organized by a variety of motor proteins, and it appears that the same motors or closely related variants organize microtubules in the postmitotic neuron. Specifically, cytoplasmic dynein and the kinesin-related motor known as CHO1/MKLP1 are used within the mitotic spindle, and recent studies suggest that they are also essential for the establishment of the axonal and dendritic microtubule arrays of the neuron. Other motors are required to tightly regulate microtubule behaviors in the mitotic spindle, and it is attractive to speculate that these motors might also help to regulate microtubule behaviors in the neuron. Here we show that a homolog of the mitotic kinesin-related motor known as Eg5 continues to be expressed in rodent neurons well after their terminal mitotic division. In neurons, Eg5 is directly associated with the microtubule array and is enriched within the distal regions of developing processes. This distal enrichment is transient, and typically lost after a process has been clearly defined as an axon or a dendrite. Strong expression can resume later in development, and if so, the protein concentrates within newly forming sprouts at the distal tips of dendrites. We suggest that Eg5 generates forces that help to regulate microtubule behaviors within the distal tips of developing axons and dendrites.

Purification of novel kinesins from embryonic systems

Several kinesin holoenzymes, including the heterotrimeric kinesin-II and bipolar KLP61F complexes described here, are being purified in our laboratory using microtubule affinity precipitation and conventional biochemical fractionation procedures. These protocols have been optimized by using pan-kinesin peptide antibodies and subunit-specific antibodies to monitor the enrichment of kinesin-related polypeptides in particular fractions by immunoblotting. Protein purification represents the most direct route available for determining the oligomeric state and subunit composition of a kinesin holoenzyme, for identifying tightly associated accessory subunits such as SpKAP115, and for determining the molecular architecture and functional properties of native kinesin motors. Protein purification methods therefore represent an important complementary approach to molecular genetic approaches that are being pursued in many other laboratories.

A model for the proposed roles of different microtubule-based motor proteins in establishing spindle bipolarity

BACKGROUND

In eukaryotes, assembly of the mitotic spindle requires the interaction of chromosomes with microtubules. During this process, several motor proteins that move along microtubules promote formation of a bipolar microtubule array, but the precise mechanism is unclear. In order to examine the roles of different motor proteins in building a bipolar spindle, we have used a simplified system in which spindles assemble around beads coated with plasmid DNA and incubated in extracts from Xenopus eggs. Using this system, we can study spindle assembly in the absence of paired cues, such as centrosomes and kinetochores, whose microtubule-organizing properties might mask the action of motor proteins.

RESULTS

We blocked the function of individual motor proteins in the Xenopus extracts using specific antibodies. Inhibition of Xenopus kinesin-like protein 1 (Xklp1) led either to the dissociation of chromatin beads from microtubule arrays, or to collapsed microtubule bundles on beads. Inhibition of Eg5 resulted in monopolar microtubule arrays emanating from chromatin beads. Addition of antibodies against dynein inhibited the focusing of microtubule ends into spindle poles in a dose-dependent manner. Inhibition of Xenopus carboxy-terminal kinesin 2 (XCTK2) affected both pole formation and spindle stability. Co-inhibition of XCTK2 and dynein dramatically increased the severity of spindle pole defects. Inhibition of Xklp2 caused only minor spindle pole defects.

CONCLUSIONS

Multiple microtubule-based motor activities are required for the bipolar organization of microtubules around chromatin beads, and we propose a model for the roles of the individual motor proteins in this process.

Two kinesin light chain genes in mice. Identification and characterization of the encoded proteins

Native kinesin consists of two light chains and two heavy chains in a 1:1 stoichiometric ratio. To date, only one gene for kinesin light chain has been characterized, while a second gene was identified in a genomic sequencing study but not analyzed biochemically. Here we describe new genes encoding kinesin light chains in mouse. One of these light chains is neuronally enriched, while another shows ubiquitous expression. The presence of multiple kinesin light chain genes in mice is especially interesting, since there are two kinesin heavy chain genes in humans (Niclas, J., Navone, F., Hom-Booher, N., and Vale, R. D. (1994) Neuron 12, 1059-1072). To assess the selectivity of kinesin light chain interaction with the heavy chains, we performed immunoprecipitation experiments. The data suggested that the light chains form homodimers with no specificity in their interaction with the two heavy chains. Immunofluorescence and biochemical subfractionation suggested differences in the subcellular localization of the two kinesin light chain gene products. Although both kinesin light chains are distributed throughout the central and peripheral nervous systems, there is enrichment of one in sciatic nerve axons, while the other shows elevated levels in olfactory bulb glomeruli. These results indicate that the mammalian nervous system contains multiple kinesin light chain gene products with potentially distinct functions.

Kinesin light chains are essential for axonal transport in Drosophila

Kinesin is a heterotetramer composed of two 115-kD heavy chains and two 58-kD light chains. The microtubule motor activity of kinesin is performed by the heavy chains, but the functions of the light chains are poorly understood. Mutations were generated in the Drosophila gene Kinesin light chain (Klc), and the phenotypic consequences of loss of Klc function were analyzed at the behavioral and cellular levels. Loss of Klc function results in progressive lethargy, crawling defects, and paralysis followed by death at the end of the second larval instar. Klc mutant axons contain large aggregates of membranous organelles in segmental nerve axons. These aggregates, or organelle jams (Hurd, D.D., and W.M. Saxton. 1996. Genetics. 144: 1075-1085), contain synaptic vesicle precursors as well as organelles that may be transported by kinesin, kinesin-like protein 68D, and cytoplasmic dynein, thus providing evidence that the loss of Klc function blocks multiple pathways of axonal transport. The similarity of the Klc and Khc (. Cell 64:1093-1102; Hurd, D.D., and W.M. Saxton. 1996. Genetics 144: 1075-1085) mutant phenotypes indicates that KLC is essential for kinesin function, perhaps by tethering KHC to intracellular cargos or by activating the kinesin motor.

Mutations in the bimC box of Cut7 indicate divergence of regulation within the bimC family of kinesin related proteins

Members of the bimC family of kinesin related proteins (KRPs) play vital roles in the formation and function of the mitotic spindle. Although they share little amino acid homology outside the highly conserved microtubule motor domain, several family members do contain a ‘bimC box’, a sequence motif around a p34(cdc2) consensus phosphorylation site in their carboxy-terminal ‘tail’ region. One family member, Eg5, requires phosphorylation at this site for association with the mitotic spindle. We show that mutations in the Schizosaccharomyces pombe cut7+ gene that change the bimC box p34(cdc2) consensus phosphorylation site at position 1,011 and a neighbouring MAP kinase consensus phosphorylation site at position 1,020 to non-phosphorylatable residues did not affect the ability of S. pombe cut7 genes to complement temperature sensitive cut7 mutants. Phosphorylation site mutants expressed as fusions to green fluorescent protein associated with the mitotic spindle with a localisation indistinguishable from similarly expressed wild-type Cut7. Cells in which cut7.T1011A replaced the genomic copy of cut7+ were viable and formed normal spindles. Deletion of the entire carboxy-terminal tail region did not affect the ability of Cut7 to associate with the mitotic spindle but did inhibit normal spindle formation. Thus, unlike Eg5, neither the p34(cdc2) consensus phosphorylation site in the bimC box nor the entire tail region of Cut7 are required for association with the mitotic spindle.

Characterization of the KIF3C neural kinesin-like motor from mouse

Proteins of the kinesin superfamily define a class of microtubule-dependent motors that play crucial roles in cell division and intracellular transport. To study the molecular mechanism of axonal transport, a cDNA encoding a new kinesin-like protein called KIF3C was cloned from a mouse brain cDNA library. Sequence and secondary structure analysis revealed that KIF3C is a member of the KIF3 family. In contrast to KIF3A and KIF3B, Northern and Western analysis indicated that KIF3C expression is highly enriched in neural tissues such as brain, spinal cord, and retina. When anti-KIF3C antibodies were used to stain the cerebellum, the strongest signal came from the cell bodies and dendrites of Purkinje cells. In retina, anti-KIF3C mainly stains the ganglion cells. Immunolocalization showed that the KIF3C motor in spinal cord and sciatic nerve is mainly localized in cytoplasm. In spinal cord, the KIF3C staining was punctate; double labeling with anti-giantin and anti-KIF3C showed a clear concentration of the motor protein in the Golgi complex. Staining of ligated sciatic nerves demonstrated that the KIF3C motor accumulated at the proximal side of the ligated nerve, which suggests that KIF3C is an anterograde motor. Immunoprecipitation experiments revealed that KIF3C and KIF3A, but not KIF3B, were coprecipitated. These data, combined with previous data from other labs, indicate that KIF3C and KIF3B are "variable" subunits that associate with a common KIF3A subunit, but not with each other. Together these results suggest that KIF3 family members combinatorially associate to power anterograde axonal transport.

Characterization of KIFC2, a neuronal kinesin superfamily member in mouse

Members of the kinesin superfamily of microtubule-associated proteins are involved in a variety of intracellular processes including cell division and organelle transport. In the case of axonal transport, all kinesin superfamily members reported thus far appear to play a role in anterograde transport, while a different type of microtubule motor, dynein, appears to function in retrograde transport. To better understand the role of kinesins in axonal transport, we cloned and characterized KIFC2, a novel kinesin superfamily member from mouse brain. KIFC2 encodes a 792 amino acid protein, which contains the conserved motor domain at the C-terminal end of the protein and is most similar to members of the KAR3 family involved in cell division. However, expression analysis localized KIFC2 mRNA to nonproliferative neuronal cells in the central nervous system, and immunolocalization studies demonstrated that KIFC2 is present in axons and dendrites of neurons in the central and peripheral nervous systems. Immunolocalization and biochemical fractionation studies suggest that KIFC2 localizes with some, but not all, axonally transported organelles. Finally, ligation of mouse peripheral nerves showed that KIFC2 accumulates at the proximal and distal sides of an axonal ligature. Taken together, the data suggest that, unlike other C-terminal motor proteins that appear to be involved in cell division, KIFC2 may play a role in retrograde axonal transport.

XCTK2: a kinesin-related protein that promotes mitotic spindle assembly in Xenopus laevis egg extracts

We used a peptide antibody to a conserved sequence in the motor domain of kinesins to screen a Xenopus ovary cDNA expression library. Among the clones isolated were two that encoded a protein we named XCTK2 for Xenopus COOH-terminal kinesin 2. XCTK2 contains an NH2-terminal globular domain, a central alpha-helical stalk, and a COOH-terminal motor domain. XCTK2 is similar to CTKs in other organisms and is most homologous to CHO2. Antibodies raised against XCTK2 recognize a 75-kD protein in Xenopus egg extracts that cosediments with microtubules. In Xenopus tissue culture cells, the anti-XCTK2 antibodies stain mitotic spindles as well as a subset of interphase nuclei. To probe the function of XCTK2, we have used an in vitro assay for spindle assembly in Xenopus egg extracts. Addition of antibodies to cytostatic factor-arrested extracts causes a 70% reduction in the percentage of bipolar spindles formed. XCTK2 is not required for maintenance of bipolar spindles, as antibody addition to preformed spindles has no effect. To further evaluate the function of XCTK2, we expressed XCTK2 in insect Sf-9 cells using the baculovirus expression system. When purified (recombinant XCTK2 is added to Xenopus egg extracts at a fivefold excess over endogenous levels) there is a stimulation in both the rate and extent of bipolar spindle formation. XCTK2 exists in a large complex in extracts and can be coimmunoprecipitated with two other proteins from extracts. XCTK2 likely plays an important role in the establishment and structural integrity of mitotic spindles.

Monastral bipolar spindles: implications for dynamic centrosome organization

Implicit to all models for mitotic spindle assembly is the view that centrosomes are essentially permanent structures. Yet, immunofluorescence revealed that spindles in larval brains of urchin mutants in Drosophila were frequently monastral but bipolar; the astral pole contained a centrosome while the opposing anastral pole showed neither gamma tubulin nor a radial array of astral microtubules. Thus, mutations in the urchin gene seem to uncouple centrosome organization and spindle bipolarity in mitotic cells. Hypomorphic mutants showed a high frequency of monastral bipolar spindles but low frequencies of polyploidy, suggesting that monastral bipolar spindles might be functional. To test this hypothesis, we performed pedigree analysis of centrosome distribution and spindle structure in the four mitotic divisions of gonial cells. Prophase gonial cells showed two centrosomes, suggesting cells entered mitosis with the normal number of centrosomes and that centrosomes separated during prophase. Despite a high frequency of monastral bipolar spindles, the end products of the four mitotic divisions were equivalent in size and chromatin content. These results indicate that monastral bipolar spindles are functional and that the daughter cell derived from the anastral pole can assemble a functional bipolar spindle in the subsequent cell cycle. Cell proliferation despite high frequencies of monastral bipolar spindles can be explained if centrosome structure in mitotic cells is dynamic, allowing transient and benign disorganization of pericentriolar components. Since urchin proved to be allelic to KLP61F which encodes a kinesin related motor protein (Heck et al. (1993) J. Cell Biol. 123, 665–671), our results suggest that motors influence the dynamic organization of centrosomes.

Genetic analysis of the mitotic spindle

Much of our understanding of the molecular basis of mitotic spindle function has been achieved within the past decade. Studies utilizing genetically tractable organisms have made important contributions to this field and these studies form the basis of this review. We focus upon three areas of spindle research: spindle poles, centromeres, and spindle motors. The structure and duplication mechanisms of spindle poles are considered as well as their roles in organizing spindle microtubules. Centromeres vary considerably in their size and complexity. We describe recent progress in our understanding of the relatively simple centromeres of the yeast Saccharomyces cerevisiae and the complex centromeres that are more typical of eukaryotic cells. Microtubule-based motor proteins that generate the characteristic spindle movements have been identified in recent years and can be grouped into families defined by conserved primary sequence and mitotic function.

A bipolar kinesin

Chromosome segregation during mitosis depends on the action of the mitotic spindle, a self-organizing, bipolar protein machine which uses microtubules (MTs) and their associated motors. Members of the BimC subfamily of kinesin-related MT-motor proteins are believed to be essential for the formation and functioning of a normal bipolar spindle. Here we report that KRP130, a homotetrameric BimC-related kinesin purified from Drosophila melanogaster embryos, has an unusual ultrastructure. It consists of four kinesin-related polypeptides assembled into a bipolar aggregate with motor domains at opposite ends, analogous to a miniature myosin filament. Such a bipolar 'minifilament' could crosslink spindle MTs and slide them relative to one another. We do not know of any other MT motors that have a bipolar structure.