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

Identification of isoforms of a mitotic motor in mammalian spermatogenesis

We have isolated the full-length coding sequence for mouse KIFC5A (kinesin family c-terminal 5A) cDNA, encoding a motor protein found in the testes. The complete sequence of the KIFC5A cDNA is homologous to a group of carboxyl-terminal motors, including hamster CHO2, human HSET, and mouse KIFC1 and KIFC4. The KIFC5A and KIFC1 cDNAs are nearly identical except for the presence of two additional sequence blocks in the 5'-end of KIFC5A and a number of single base-pair differences in their motor domains. Polymerase chain reaction amplification and sequencing of the 5'-end of KIFC5A identified 3 distinct RNA species in testes and other tissues. Sequence comparison and genetic mapping confirmed the existence of a small multi-gene family in the mouse and suggest possible mechanisms of alternative splicing, genetic duplication, and separate genetic loci in the generation of these motors. In order to examine the possible role of these motors in germ cells of the testes, an antibody to a shared epitope was used to localize this group of proteins to different spermatogenic cell types. These experiments suggest that KIFC5-like motor proteins are associated with multiple microtubule complexes in male germ cells, including the meiotic spindle, the manchette, and the flagella.

ch-TOGp is required for microtubule aster formation in a mammalian mitotic extract

Microtubules induced to polymerize with taxol in a mammalian mitotic extract organize into aster-like arrays in a centrosome-independent process that is driven by microtubule motors and structural proteins. These microtubule asters accurately reflect the noncentrosomal aspects of mitotic spindle pole formation. We show here that colonic-hepatic tumor-overexpressed gene (ch-TOGp) is an abundant component of these asters. We have prepared ch-TOGp-specific antibodies and show by immunodepletion that ch-TOGp is required for microtubule aster assembly. Microtubule polymerization is severely inhibited in the absence of ch-TOGp, and silver stain analysis of the ch-TOGp immunoprecipitate indicates that it is not present in a preformed complex and is the only protein removed from the extract during immunodepletion. Furthermore, the reduction in microtubule polymerization efficiency in the absence of ch-TOGp is dependent on ATP. These results demonstrate that ch-TOGp is a major constituent of microtubule asters assembled in a mammalian mitotic extract and that it is required for robust microtubule polymerization in an ATP-dependent manner in this system even though taxol is present. These data, coupled with biochemical and genetic data derived from analysis of ch-TOGp-related proteins in other organisms, indicate that ch-TOGp is a key factor regulating microtubule dynamics during mitosis.

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.

Identification of kinesin-C, a calmodulin-binding carboxy-terminal kinesin in animal (Strongylocentrotus purpuratus) cells

Several novel members of the kinesin superfamily, until now identified only in plants, are unique in their ability to bind calmodulin in the presence of Ca(2+). Here, we identify the first such kinesin in an animal system. Sequence analysis of this new motor, called kinesin-C, predicts that it is a large carboxy-terminal kinesin, 1624 amino acid residues in length, with a predicted molecular mass of 181 kDa. Kinesin-C is predicted to contain a kinesin motor domain at its carboxy terminus, linked to a segment of alpha-helical coiled-coil 950 amino acid residues long, ending with an amino-terminal proline-rich tail domain. A putative calmodulin-binding domain resides at the extreme carboxy terminus of the motor polypeptide, and recombinant kinesin-C binds to a calmodulin-affinity column in a Ca(2+)-dependent fashion. The presence of this novel calmodulin-binding motor in sea urchin embryos suggests that it plays a critical role in Ca(2+)-dependent events during early sea urchin development.

Function of a minus-end-directed kinesin-like motor protein in mammalian cells

CHO2 is a mammalian minus-end-directed kinesin-like motor protein present in interphase centrosomes/nuclei and mitotic spindle fibers/poles. Expression of HA- or GFP-tagged subfragments in transfected CHO cells revealed the presence of the nuclear localization site at the N-terminal tail. This domain becomes associated with spindle fibers during mitosis, indicating that the tail is capable of interaction with microtubules in vivo. While the central stalk diffusely distributes in the entire cytoplasm of cells, the motor domain co-localizes with microtubules throughout the cell cycle, which is eliminated by mutation of the ATP-binding consensus motif from GKT to AAA. Overexpression of the full-length CHO2 causes mitotic arrest and spindle abnormality. The effect of protein expression was first seen around the polar region where microtubule tended to be bundled together. A higher level of protein expression induces more elongated spindles which eventually become disorganized by loosing the structural integrity between microtubule bundles. Live cell observation demonstrated that GFP-labeled microtubule bundles underwent continuous changes in their relative position to one another through repeated attachment and detachment at one end; this results in the formation of irregular number of microtubule focal points in mitotic arrested cells. Thus the primary action of CHO2 appears to cross-link microtubules and move toward the minus-end direction to maintain association of the microtubule end at the pole. In contrast to the full-length of CHO2, overexpression of neither truncated nor mutant polypeptides resulted in significant effects on mitosis and mitotic spindles, suggesting that the function of CHO2 in mammalian cells may be redundant with other motor molecules during cell division.

Aurora/Ipl1p-related kinases, a new oncogenic family of mitotic serine-threonine kinases

During the past five years, a growing number of serine-threonine kinases highly homologous to the Saccharomyces cerevisiae Ipl1p kinase have been isolated in various organisms. A Drosophila melanogaster homologue, aurora, was the first to be isolated from a multicellular organism. Since then, several related kinases have been found in mammalian cells. They localise to the mitotic apparatus: in the centrosome, at the poles of the bipolar spindle or in the midbody. The kinases are necessary for completion of mitotic events such as centrosome separation, bipolar spindle assembly and chromosome segregation. Extensive research is now focusing on these proteins because the three human homologues are overexpressed in various primary cancers. Furthermore, overexpression of one of these kinases transforms cells. Because of the myriad of kinases identified, we suggest a generic name: Aurora/Ipl1p-related kinase (AIRK). We denote AIRKs with a species prefix and a number, e.g. HsAIRK1.

The kinesin-related protein, HSET, opposes the activity of Eg5 and cross-links microtubules in the mammalian mitotic spindle

We have prepared antibodies specific for HSET, the human homologue of the KAR3 family of minus end-directed motors. Immuno-EM with these antibodies indicates that HSET frequently localizes between microtubules within the mammalian metaphase spindle consistent with a microtubule cross-linking function. Microinjection experiments show that HSET activity is essential for meiotic spindle organization in murine oocytes and taxol-induced aster assembly in cultured cells. However, inhibition of HSET did not affect mitotic spindle architecture or function in cultured cells, indicating that centrosomes mask the role of HSET during mitosis. We also show that (acentrosomal) microtubule asters fail to assemble in vitro without HSET activity, but simultaneous inhibition of HSET and Eg5, a plus end-directed motor, redresses the balance of forces acting on microtubules and restores aster organization. In vivo, centrosomes fail to separate and monopolar spindles assemble without Eg5 activity. Simultaneous inhibition of HSET and Eg5 restores centrosome separation and, in some cases, bipolar spindle formation. Thus, through microtubule cross-linking and oppositely oriented motor activity, HSET and Eg5 participate in spindle assembly and promote spindle bipolarity, although the activity of HSET is not essential for spindle assembly and function in cultured cells because of centrosomes.

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-based motor function in mitosis

Microtubule-based motors are essential both for the proper assembly of the mitotic spindle and for chromosome segregation. Mitotic motors in the yeast Saccharomyces cerevisiae exhibit either overlapping or opposing activities in order to achieve proper spindle function, whereas the analysis of motors using vertebrate cytoplasmic extracts has revealed less functional redundancy. In several systems, biochemical, genetic and two-hybrid approaches have been used both to identify associated nonmotor proteins and to address the molecular mechanisms behind kinetochore movements during chromosome alignment and segregation.

Kin I kinesins are microtubule-destabilizing enzymes

Using in vitro assays with purified proteins, we show that XKCM1 and XKIF2, two distinct members of the internal catalytic domain (Kin I) kinesin subfamily, catalytically destabilize microtubules using a novel mechanism. Both XKCM1 and XKIF2 influence microtubule stability by targeting directly to microtubule ends where they induce a destabilizing conformational change. ATP hydrolysis recycles XKCM1/XKIF2 for multiple rounds of action by dissociating a XKCM1/ XKIF2-tubulin dimer complex released upon microtubule depolymerization. These results establish Kin I kinesins as microtubule-destabilizing enzymes, distinguish them mechanistically from kinesin superfamily members that use ATP hydrolysis to translocate along microtubules, and have important implications for the regulation of microtubule dynamics and for the intracellular functions and evolution of the kinesin superfamily.

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.

Interaction of human Arp2/3 complex and the Listeria monocytogenes ActA protein in actin filament nucleation

Actin filament assembly at the cell surface of the pathogenic bacterium Listeria monocytogenes requires the bacterial ActA surface protein and the host cell Arp2/3 complex. Purified Arp2/3 complex accelerated the nucleation of actin polymerization in vitro, but pure ActA had no effect. However, when combined, the Arp2/3 complex and ActA synergistically stimulated the nucleation of actin filaments. This mechanism of activating the host Arp2/3 complex at the L. monocytogenes surface may be similar to the strategy used by cells to control Arp2/3 complex activity and hence the spatial and temporal distribution of actin polymerization.

Focusing on spindle poles

Spindle poles are discernible by light microscopy as the sites where microtubules converge at the ends of both mitotic and meiotic spindles. In most cell types centrosomes are present at spindle poles due to their dominant role in microtubule nucleation. However, in some specialized cell types microtubules converge into spindle poles in the absence of centrosomes. Thus, spindle poles in centrosomal and acentrosomal cell types are structurally different, and it is this structural dichotomy that has created confusion as to the mechanism by which microtubules are organized into spindle poles. This review summarizes a series of recent articles that begin to resolve this confusion by demonstrating that spindle poles are organized through a common mechanism by a conserved group of non-centrosomal proteins in the presence or absence of centrosomes.

Chromophore-assisted light inactivation and self-organization of microtubules and motors

Chromophore-assisted light inactivation (CALI) offers the only method capable of modulating specific protein activities in localized regions and at particular times. Here, we generalize CALI so that it can be applied to a wider range of tasks. Specifically, we show that CALI can work with a genetically inserted epitope tag; we investigate the effectiveness of alternative dyes, especially fluorescein, comparing them with the standard CALI dye, malachite green; and we study the relative efficiencies of pulsed and continuous-wave illumination. We then use fluorescein-labeled hemagglutinin antibody fragments, together with relatively low-power continuous-wave illumination to examine the effectiveness of CALI targeted to kinesin. We show that CALI can destroy kinesin activity in at least two ways: it can either result in the apparent loss of motor activity, or it can cause irreversible attachment of the kinesin enzyme to its microtubule substrate. Finally, we apply this implementation of CALI to an in vitro system of motor proteins and microtubules that is capable of self-organized aster formation. In this system, CALI can effectively perturb local structure formation by blocking or reducing the degree of aster formation in chosen regions of the sample, without influencing structure formation elsewhere.

Kinesin and dynein superfamily proteins in organelle transport and cell division

Microtubule-associated motor proteins of the kinesin and dynein superfamilies play important roles in cellular mechanisms such as organelle transport and mitosis. Identification and characterization of new family members (in particular KIFC2, 16 new KIFs, XKlp2 and XKCM1 of the kinesin superfamily, and DHC2 and DHC3 of the dynein superfamily) and further characterization of known family members have improved our understanding of these cellular mechanisms. Sophisticated biophysical and structural analyses of monomeric and dimeric motor proteins have contributed to elucidating the mechanisms behind motor protein motility and polarity.

The design plan of kinesin motors

The kinesin superfamily comprises a large and structurally diverse group of microtubule-based motor proteins that produce a variety of force-generating activities within cells. This review addresses how the structures of kinesin proteins provide clues as to their biological functions and motile properties. We discuss structural features common to all kinesin motors, as well as specialized features that enable subfamilies of related motors to carry out specialized activities. We also discuss how the kinesin motor domain uses chemical energy from ATP hydrolysis to move along microtubules.

Mitotic spindle poles are organized by structural and motor proteins in addition to centrosomes

The focusing of microtubules into mitotic spindle poles in vertebrate somatic cells has been assumed to be the consequence of their nucleation from centrosomes. Contrary to this simple view, in this article we show that an antibody recognizing the light intermediate chain of cytoplasmic dynein (70.1) disrupts both the focused organization of microtubule minus ends and the localization of the nuclear mitotic apparatus protein at spindle poles when injected into cultured cells during metaphase, despite the presence of centrosomes. Examination of the effects of this dynein-specific antibody both in vitro using a cell-free system for mitotic aster assembly and in vivo after injection into cultured cells reveals that in addition to its direct effect on cytoplasmic dynein this antibody reduces the efficiency with which dynactin associates with microtubules, indicating that the antibody perturbs the cooperative binding of dynein and dynactin to microtubules during spindle/aster assembly. These results indicate that microtubule minus ends are focused into spindle poles in vertebrate somatic cells through a mechanism that involves contributions from both centrosomes and structural and microtubule motor proteins. Furthermore, these findings, together with the recent observation that cytoplasmic dynein is required for the formation and maintenance of acentrosomal spindle poles in extracts prepared from Xenopus eggs (Heald, R., R. Tournebize, T. Blank, R. Sandaltzopoulos, P. Becker, A. Hyman, and E. Karsenti. 1996. Nature (Lond.). 382: 420-425) demonstrate that there is a common mechanism for focusing free microtubule minus ends in both centrosomal and acentrosomal spindles. We discuss these observations in the context of a search-capture-focus model for spindle assembly.