Mutations in the kinesin-like protein Eg5 disrupting localization to the mitotic spindle

Cdc2-mediated phosphorylation of Kid controls its distribution to spindle and chromosomes

The chromokinesin Kid is important in chromosome alignment at the metaphase plate. Here, we report that Kid function is regulated by phosphorylation. We identify Ser427 and Thr463 as M phase-specific phosphorylation sites and Cdc2-cyclin B as a Thr463 kinase. Kid with a Thr463 to alanine mutation fails to be localized on chromosomes and is only detected along spindles, although it retains the ability to bind DNA or chromosomes. Localization of rigor-type mutant Kid, which shows nucleotide-independent microtubule association, is also confined to the spindle, implying that strong association of Kid with the spindle can sequester it from chromosomes. T463A substitution in DNA-binding domain-truncated Kid consistently enhances its spindle localization. At physiological ionic strength, unphosphorylated Kid shows ATP-independent microtubule association, whereas Thr463-phosphorylated Kid shows ATP dependency. Moreover, the stalk region of unphosphorylated Kid interacts with microtubules and the interaction is weakened when Thr463 is phosphorylated. Our data suggest that phosphorylation on Thr463 of Kid downregulates its affinity for microtubules to ensure reversible association with spindles, allowing Kid to bind chromosomes and exhibit its function.

The protein tyrosine phosphatase PTP-BL associates with the midbody and is involved in the regulation of cytokinesis

PTP-BL is a highly modular protein tyrosine phosphatase of unknown function. It consists of an N-terminal FERM domain, five PDZ domains, and a C-terminally located tyrosine phosphatase domain. Here we show that PTP-BL is involved in the regulation of cytokinesis. We demonstrate localization of endogenous PTP-BL at the centrosomes during inter- and metaphase and at the spindle midzone during anaphase. Finally PTP-BL is concentrated at the midbody in cytokinesis. We show that PTP-BL is targeted to the midbody and centrosome by a specific splicing variant of the N-terminus characterized by an insertion of 182 amino acids. Moreover, we demonstrate that the FERM domain of PTP-BL is associated with the contractile ring and can be cosedimented with filamentous actin, whereas the N-terminus can be cosedimented with microtubules. We demonstrate that elevating the expression level of wild-type PTP-BL or expression of PTP-BL with an inactive tyrosine phosphatase domain leads to defects in cytokinesis and to the generation of multinucleate cells. We suggest that PTP-BL plays a role in the regulation of cytokinesis.

Selective loss of neurofilament expression in Cu/Zn superoxide dismutase (SOD1) linked amyotrophic lateral sclerosis

Neurofilament pathology is a hallmark of sporadic and familial amyotrophic lateral sclerosis (SALS and FALS). The disease mechanisms underlying this pathology are presently unclear, but recent evidence in SALS patients suggest that reductions in neurofilament light subunit (NFL) mRNA may contribute to the death of motor neurones. Mutations in the gene encoding Cu-Zn superoxide dismutase (SOD1) represent the best-studied cause of FALS, and a number of laboratory models of SOD1-mediated disease exist. Here we have used microdissected lumbar spinal cord motor neurones from human SOD1 FALS patients as well as G93A SOD1 transgenic mice and demonstrated that reduced NFL mRNA levels are seen in both. To probe the molecular mechanisms underpinning these observations, we generated NSC34 motor neurone-like cell lines expressing wild-type and mutant SOD1. NSC34 cells expressing G37R or G93A SOD1 showed selective reductions in NFL and NFM mRNA and protein. These data suggest that NFL mRNA reductions are common to SALS and FALS patients, and that cells and mice expressing mutant SOD1 may enable us to characterize the molecular mechanism(s) responsible for the loss of neurofilament mRNA.

Evidence that monastrol is an allosteric inhibitor of the mitotic kinesin Eg5

Monastrol, a cell-permeable inhibitor of the kinesin Eg5, has been used to probe the dynamic organization of the mitotic spindle. The mechanism by which monastrol inhibits Eg5 function is unknown. We found that monastrol inhibits both the basal and the microtubule-stimulated ATPase activity of the Eg5 motor domain. Unlike many ATPase inhibitors, monastrol does not compete with ATP binding to Eg5. Monastrol appears to inhibit microtubule-stimulated ADP release from Eg5 but does not compete with microtubule binding, suggesting that monastrol binds a novel allosteric site in the motor domain. Finally, we established that (S)-monastrol, as compared to the (R)-enantiomer, is a more potent inhibitor of Eg5 activity in vitro and in vivo. Future structural studies should help in designing more potent Eg5 inhibitors for possible use as anticancer drugs and cell biological reagents.

The centrosome cycle

The centrosome is the major microtubule-organizing center of animal cells. It influences cell shape and polarity and directs the formation of the bipolar mitotic spindle. Numerical and structural centrosome aberrations have been implicated in disease, notably cancer. In dividing cells, centrosomes need to be duplicated and segregated in synchrony with chromosomes. This centrosome cycle requires a series of structural and functional transitions that are regulated by both phosphorylation and proteolysis. Here we summarize recent information on the regulation of the centrosome cycle and its coordination with the chromosomal cell cycle.

Separating centrosomes interact in the absence of associated chromosomes during mitosis in cultured vertebrate cells

We detail here how "free" centrosomes, lacking associated chromosomes, behave during mitosis in PtK(2) homokaryons stably expressing GFP-alpha-tubulin. As free centrosomes separate during prometaphase, their associated astral microtubules (Mts) interact to form a spindle-shaped array that is enriched for cytoplasmic dynein and Eg5. Over the next 30 min, these arrays become progressively depleted of Mts until the two centrosomes are linked by a single bundle, containing 10-20 Mts, that persists for > 60 min. The overlapping astral Mts within this bundle are loosely organized, and their plus ends terminate near its midzone, which is enriched for an ill-defined matrix material. At this time, the distance between the centrosomes is not defined by external forces because these organelles remain stationary when the bundle connecting them is severed by laser microsurgery. However, since the centrosomes move towards one another in response to monastrol treatment, the kinesin-like motor protein Eg5 is involved. From these results, we conclude that separating asters interact during prometaphase of mitosis to form a spindle-shaped Mt array, but that in the absence of chromosomes this array is unstable. An analysis of the existing data suggests that the stabilization of spindle Mts during mitosis in vertebrates does not involve the chromatin (i.e., the RCC1/RanGTP pathway), but instead some other chromosomal component, e.g., kinetochores.

Drosophila Aurora A kinase is required to localize D-TACC to centrosomes and to regulate astral microtubules

Disruption of the function of the A-type Aurora kinase of Drosophila by mutation or RNAi leads to a reduction in the length of astral microtubules in syncytial embryos, larval neuroblasts, and cultured S2 cells. In neuroblasts, it can also lead to loss of an organized centrosome and its associated aster from one of the spindle poles, whereas the centrosome at the other pole has multiple centrioles. When centrosomes are present at the poles of aurA mutants or aurA RNAi spindles, they retain many antigens but are missing the Drosophila counterpart of mammalian transforming acidic coiled coil (TACC) proteins, D-TACC. We show that a subpopulation of the total Aurora A is present in a complex with D-TACC, which is a substrate for the kinase. We propose that one of the functions of Aurora A kinase is to direct centrosomal organization such that D-TACC complexed to the MSPS/XMAP215 microtubule-associated protein may be recruited, and thus modulate the behavior of astral microtubules.

Motor-cargo interactions: the key to transport specificity

Eukaryotic cells organize their cytoplasm by moving different organelles and macromolecular complexes along microtubules and actin filaments. These movements are powered by numerous motor proteins that must recognize their respective cargoes in order to function. Recently, several proteins that interact with motors have been identified by yeast two-hybrid and biochemical analyses, and their roles in transport are now being elucidated. In several cases, analysis of the binding partners helped to identify new transport pathways, new types of cargo, and transport regulated at the level of motor-cargo binding. We discuss here how different motors of the kinesin, dynein and myosin families recognize their cargo and how motor-cargo interactions are regulated.

Cell cycle-dependent degradation of the Saccharomyces cerevisiae spindle motor Cin8p requires APC(Cdh1) and a bipartite destruction sequence

Saccharomyces cerevisiae Cin8p belongs to the BimC family of kinesin-related motor proteins that are essential for spindle assembly. Cin8p levels were found to oscillate in the cell cycle due in part to a high rate of degradation imposed from the end of mitosis through the G1 phase. Cin8p degradation required the anaphase-promoting complex ubiquitin ligase and its late mitosis regulator Cdh1p but not the early mitosis regulator Cdc20p. Cin8p lacks a functional destruction box sequence that is found in the majority of anaphase-promoting complex substrates. We carried out an extensive mutagenesis study to define the cis-acting sequence required for Cin8p degradation in vivo. The C terminus of Cin8p contains two elements required for its degradation: 1) a bipartite destruction sequence composed of a KEN-box plus essential residues within the downstream 22 amino acids and 2) a nuclear localization signal. The bipartite destruction sequence appears in other BimC kinesins as well. Expression of nondegradable Cin8p showed very mild phenotypic effects, with an increase in the fraction of mitotic cells with broken spindles.

Centrosome cohesion is regulated by a balance of kinase and phosphatase activities

Centrosome cohesion and separation are regulated throughout the cell cycle, but the underlying mechanisms are not well understood. Since overexpression of a protein kinase, Nek2, is able to trigger centrosome splitting (the separation of parental centrioles), we have surveyed a panel of centrosome-associated kinases for their ability to induce a similar phenotype. Cdk2, in association with either cyclin A or E, was as effective as Nek2, but several other kinases tested did not significantly interfere with centrosome cohesion. Centrosome splitting could also be triggered by inhibition of phosphatases, and protein phosphatase 1 alpha (PP1 alpha) was identified as a likely physiological antagonist of Nek2. Furthermore, we have revisited the role of the microtubule network in the control of centrosome cohesion. We could confirm that microtubule depolymerization by nocodazole causes centrosome splitting. Surprisingly, however, this drug-induced splitting also required kinase activity and could specifically be suppressed by a dominant-negative mutant of Nek2. These studies highlight the importance of protein phosphorylation in the control of centrosome cohesion, and they point to Nek2 and PP1 alpha as critical regulators of centrosome structure.

Eg5 is static in bipolar spindles relative to tubulin: evidence for a static spindle matrix

We used fluorescent speckle microscopy to probe the dynamics of the mitotic kinesin Eg5 in Xenopus extract spindles, and compared them to microtubule dynamics. We found significant populations of Eg5 that were static over several seconds while microtubules flux towards spindle poles. Eg5 dynamics are frozen by adenylimidodiphosphate. Bulk turnover experiments showed that Eg5 can exchange between the spindle and the extract with a half life of <55 s. Eg5 distribution in spindles was not perturbed by inhibition of its motor activity with monastrol, but was perturbed by inhibition of dynactin with p50 dynamitin. We interpret these data as revealing the existence of a static spindle matrix that promotes Eg5 targeting to spindles, and transient immobilization of Eg5 within spindles. We discuss alternative interpretations of the Eg5 dynamics we observe, ideas for the biochemical nature of a spindle matrix, and implications for Eg5 function.

Cloning and characterization of hMAP126, a new member of mitotic spindle-associated proteins

One novel gene product, hMAP126, was demonstrated to interact with p29 in the yeast two-hybrid assay. The full-length cDNA of hMAP126 has been obtained and encodes a protein of 1120 amino acids. Multiple tissue Northern blot analysis showed that hMAP126 was abundantly expressed in the testis. Polyclonal antiserum against hMAP126 was raised and affinity-purification of anti-hMAP126 antibodies was performed. The subcellular distribution of hMAP126 was localized to the mitotic spindle. Furthermore, hMAP126 was identified to be post-translationally modified and phosphorylated by p34(cdc2) kinase in vitro. Taken together, we have isolated a novel protein, hMAP126, which may be involved in the functional and dynamic regulation of mitotic spindles.

Crystal structure of the mitotic spindle kinesin Eg5 reveals a novel conformation of the neck-linker

Success of mitosis depends upon the coordinated and regulated activity of many cellular factors, including kinesin motor proteins, which are required for the assembly and function of the mitotic spindle. Eg5 is a kinesin implicated in the formation of the bipolar spindle and its movement prior to and during anaphase. We have determined the crystal structure of the Eg5 motor domain with ADP-Mg bound. This structure revealed a new intramolecular binding site of the neck-linker. In other kinesins, the neck-linker has been shown to be a critical mechanical element for force generation. The neck-linker of conventional kinesin is believed to undergo an ordered-to-disordered transition as it translocates along a microtubule. The structure of Eg5 showed an ordered neck-linker conformation in a position never observed previously. The docking of the neck-linker relies upon residues conserved only in the Eg5 subfamily of kinesin motors. Based on this new information, we suggest that the neck-linker of Eg5 may undergo an ordered-to-ordered transition during force production. This ratchet-like mechanism is consistent with the biological activity of Eg5.

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.

Phosphorylation by cdc2-CyclinB1 kinase releases cytoplasmic dynein from membranes

Movement of various cargoes toward microtubule minus ends is driven by the microtubule motor cytoplasmic dynein (CD). Many cargoes are motile only during certain cell cycle phases, suggesting that CD function may be under cell cycle control. Phosphorylation of the CD light intermediate chain (DLIC) has been suggested to play a crucial role in modulating CD function during the Xenopus embryonic cell cycle, where CD-driven organelle movement is active in interphase but greatly reduced in metaphase. This down-regulation correlates with hyperphosphorylation of DLIC and release of CD from the membrane. Here we investigate the role of the key mitotic kinase, cdc2-cyclinB1, in this process. We show that DLIC within the native Xenopus CD complex is an excellent substrate for purified Xenopus cdc2-glutathione S-transferase (GST) cyclinB1 (cdc2-GSTcyclinB1) kinase. Mass spectrometry of native DLIC revealed that a conserved cdc2 site (Ser-197) previously implicated in the metaphase modulation of CD remains phosphorylated in interphase and so is unlikely to be the key regulatory site. We also demonstrate that incubating interphase membranes with cdc2-GSTcyclinB1 kinase results in substantial release of CD from the membrane. These data suggest that phosphorylation of DLIC by cdc2 kinase leads directly to the loss of membrane-associated CD and an inhibition of organelle movement.

Ran stimulates spindle assembly by altering microtubule dynamics and the balance of motor activities

The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindles. However, it is not known what aspects of the microtubule cytoskeleton are subject to regulation by Ran in mitosis. Here we show that Ran-GTP stimulates microtubule assembly by increasing the rescue frequency of microtubules three- to eightfold. In addition to changing microtubule dynamics, Ran-GTP also alters the balance of motor activities, partly as a result of an increase in the amount of motile Eg5, a plus-end-directed microtubule motor that is essential for spindle formation. Thus, Ran regulates multiple processes that are involved in spindle assembly.

Boursin, a sea urchin bimC kinesin protein, plays a role in anaphase and cytokinesis

We have isolated and characterized Boursin, a kinesin-related protein of the bimC family, from Paracentrotus lividus sea urchin eggs. Boursin is expressed at high levels in eggs and embryos during early cleavage stages. Boursin was found to be associated with different parts of the mitotic spindle from early prophase to telophase. Expression of a form of the protein predicted to act as a dominant negative mutant caused severe defects in cell division and resulted in the formation of embryos with polyploid and multiastral blastomeres. Immunofluorescence analysis indicated that these defects did not arise from failure in either centrosome separation or bipolar spindle formation. Time-lapse observations showed rather that these perturbations in cell division resulted from abnormal anaphase and failure to complete cytokinesis. These phenotypes differ from the phenotype described following perturbation of the function of bimC family members in other organisms. Our study has thus uncovered roles for a bimC kinesin in late stages of cell division.

Mitotic kinases as regulators of cell division and its checkpoints

Mitosis and cytokinesis are undoubtedly the most spectacular parts of the cell cycle. Errors in the choreography of these processes can lead to aneuploidy or genetic instability, fostering cell death or disease. Here, I give an overview of the many mitotic kinases that regulate cell division and the fidelity of chromosome transmission.

Two kinesin-related proteins associated with the cold-stable cytoskeleton of carrot cells: characterization of a novel kinesin, DcKRP120-2

We have previously described the biochemical isolation of 65 kDa and 120 kDa microtubule-associated proteins from carrot cytoskeletons. The 65 kDa MAPs have subsequently been shown to be structural MAPs that reconstitute 30 nm cross-bridges of the kind that maintain cortical microtubules in parallel groups. By exploiting its avid binding to microtubules, we have now devised a method for isolating MAP120 from protoplast extracts, and shown that it has properties of a kinesin-related protein. MAP120 segregates with the cold stable pool of microtubules in carrot cytoskeletons, whilst the 65 kDa MAPs are also associated with the cold-sensitive microtubules. On gradient gels, MAP120 resolves as two kinesin-like bands. We report the isolation of a carrot cDNA, DcKRP120-2, corresponding to a novel kinesin of the BimC class known to move to the plus ends of microtubules. Antibodies raised against specific expressed sequences recognize the upper band, while the lower band is recognized by antibodies to the tobacco kinesin-related protein, TKRP125. We have also isolated a partial genomic carrot DNA, DcKRP120-1, homologous to the motor region of tobacco TKRP125. Immunofluorescence of the two proteins produces different staining patterns. Anti-TKRP125 labels the cortical microtubules and the pre-prophase band, but anti-DcKRP120-2 does so only weakly. Both clearly stain the spindle and the phragmoplast, but in a proportion of cells anti-DcKRP120-2 strongly decorates the phragmoplast mid-line where the plus ends of the microtubules overlap. We discuss the potential roles of these proteins during the microtubule cycle.

Microtubule motors in mitosis

The mitotic spindle uses microtubule-based motor proteins to assemble itself and to segregate sister chromatids. It is becoming clear that motors invoke several distinct mechanisms to generate the forces that drive mitosis. Moreover, in carrying out its function, the spindle appears to pass through a series of transient steady-state structures, each established by a delicate balance of forces generated by multiple complementary and antagonistic motors. Transitions from one steady state to the next can occur when a change in the activity of a subset of mitotic motors tips the balance.

Rigor-type mutation in the kinesin-related protein HsEg5 changes its subcellular localization and induces microtubule bundling

HsEg5 is a human kinesin-related motor protein essential for the formation of a bipolar mitotic spindle. It interacts with the mitotic centrosomes in a phosphorylation-dependent manner. To investigate further the mechanisms involved in targetting HsEg5 to the spindle apparatus, we expressed various mutants of HsEg5 in HeLa cells. All these mutants share a mutation of Thr-112 in the N-terminal motor domain, resulting in the inactivation of the ATP binding domain. In vitro, the HsEg5-T112N mutant motor domain showed a nucleotide-independent microtubule association, typical of a kinesin protein binding to microtubules in a rigor state. In vivo, overexpression of the HsEg5 rigor mutant in HeLa cells induced, in interphase, microtubule bundling, and, in mitosis, the formation of monopolar mitotic spindles similar to those observed after microinjection of anti-HsEg5 antibodies. Localization of the HsEg5 rigor mutant on cytoplasmic microtubules did not require the C-terminal tail domain but was lost when the stalk domain was also deleted. Sucrose gradient centrifugation experiments showed that microtubule bundling was most likely caused by the binding of HsEg5 mutants in a dimeric state. These results demonstrate that the precise subcellular localization of HsEg5 in vivo is regulated not only by the phosphorylation of the tail domain but also by the oligomeric state of the protein.

Interaction of a kinesin-like calmodulin-binding protein with a protein kinase

Kinesin-like calmodulin-binding protein (KCBP) is a novel member of the kinesin superfamily that is involved in cell division and trichome morphogenesis. KCBP is unique among all known kinesins in having a myosin tail homology-4 region in the N-terminal tail and a calmodulin-binding region following the motor domain. Calcium, through calmodulin, has been shown to negatively regulate the interaction of KCBP with microtubules. Here we have used the yeast two-hybrid system to identify the proteins that interact with the tail region of KCBP. A protein kinase (KCBP-interacting protein kinase (KIPK)) was found to interact specifically with the tail region of KCBP. KIPK is related to a group of protein kinases specific to plants that has an additional sequence between subdomains VII and VIII of the conserved C-terminal catalytic domain and an extensive N-terminal region. The catalytic domain alone of KIPK interacted weakly with the N-terminal KCBP protein but strongly with full-length KCBP, whereas the noncatalytic region did not interact with either protein. The interaction of KCBP with KIPK was confirmed using coprecipitation assays. Using bacterially expressed full-length and truncated proteins, we have shown that the catalytic domain is capable of phosphorylating itself. The association of KIPK with KCBP suggests regulation of KCBP or KCBP-associated proteins by phosphorylation and/or that KCBP is involved in targeting KIPK to its proper cellular location.

Relationships between the central spindle and the contractile ring during cytokinesis in animal cells

During late anaphase and telophase, animal cells develop a bundle of antiparallel, interdigitating microtubules between the two daughter nuclei. Recent data indicate that this structure, called the central spindle, plays an essential role during cytokinesis. Studies in Drosophila and on vertebrate cells strongly suggest that the molecular signals for cytokinesis specifically emanate from the central spindle midzone. Moreover, the analysis of Drosophila mutants defective in cytokinesis has revealed a cooperative interaction between the central spindle microtubules and the contractile ring: when either of these structures is perturbed, the proper assembly of the other is disrupted. Based on these results we propose a model for the role of the central spindle during cytokinesis. We suggest that the interaction between central spindle microtubules and cortical actin filaments leads to two early events crucial for cytokinesis: the positioning of the contractile ring, and the stabilization of the plus ends of the interdigitating microtubules that comprise the central spindle. The latter event would provide the cell with a specialized microtubule scaffold that could mediate the translocation of plus-end-directed molecular motors to the cell's equator. Among the cargoes transported by these motors could be proteins involved in the regulation and execution of cytokinesis.

Progesterone regulates the accumulation and the activation of Eg2 kinase in Xenopus oocytes

Xenopus prophase oocytes reenter meiotic division in response to progesterone. The signaling pathway leading to Cdc2 activation depends on neosynthesized proteins and a decrease in PKA activity. We demonstrate that Eg2 protein, a Xenopus member of the Aurora/Ipl1 family of protein kinases, accumulates in response to progesterone and is degraded after parthenogenetic activation. The polyadenylation and cap ribose methylation of Eg2 mRNA are not needed for the protein accumulation. Eg2 protein accumulation is induced by progesterone through a decrease in PKA activity, upstream of Cdc2 activation. Eg2 kinase activity is undetectable in prophase and is raised in parallel with Cdc2 activation. In contrast to Eg2 protein accumulation, Eg2 kinase activation is under Cdc2 control. Furthermore, by using an anti-sense strategy, we show that Eg2 accumulation is not required in the transduction pathway leading to Cdc2 activation. Altogether, our results strongly suggest that Eg2 is not necessary for Cdc2 activation, though it could participate in the organization of the meiotic spindles, in agreement with the well-conserved roles of the members of the Aurora family, from yeast to man.

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.

Regulation of cell division and the cytoskeleton by mitogen-activated protein kinases in higher plants

The microtubule-associated protein 2 kinase (MAP2-kinase), now better known as mitogen-activated protein kinase (MAPK), was initially discovered in association with the cytoskeleton, and was later also implicated in cell division. The importance of mitogenic stimulation in plant development roused interest in finding the plant homologues of MAPKs. However, data on plant MAPKs in cell division are rather sparse and fragmentary. Therefore we place the available information on cell cycle control of MAPKs in plants into a broader context. We discuss four aspects of cell division control: cell proliferation and the G1/S-phase transition, G2-phase and mitosis, cytokinesis, and cytoskeletal reorganisation. Future work will reveal to what extent plants use signalling pathways that are similar or different to those of animal or yeast cells in regulating cell divisions.

Mitotic phosphorylation of the dynein light intermediate chain is mediated by cdc2 kinase

Cytoplasmic dynein, a large minus-end-directed microtubule motor, performs multiple functions during the cell cycle. In interphase, dynein moves membrane organelles, while in mitosis it moves chromosomes and helps to form the mitotic spindle. The cell-cycle regulation of dynein activity may be controlled, at least in part, by the phosphorylation of its light intermediate chains (DLIC), since a 10-fold increase in light intermediate chain phosphorylation correlates with a decrease in dynein-based membrane transport of similar magnitude in mitosis. In this study, we sought to identify the kinase responsible for this potentially important phosphorylation event. We show that bacterially-expressed chicken light intermediate chain (chDLIC) will undergo mitosis-specific phosphorylation when added to Xenopus egg extracts. Mutation of a conserved cdc2 kinase consensus site (Ser197) abolishes this phosphorylation event, and mass spectroscopy analysis confirms that the wild-type DLIC is stoichiometrically phosphorylated at this site when incubated with metaphase but not interphase extracts. We also show that purified cdc2 kinase phosphorylates purified DLICs at Ser197 in vitro and that Ser197 phosphorylation is dramatically reduced in metaphase extracts depleted of cdc2 kinase. These results indicate that cdc2 kinase directly phosphorylates dynein and thus may be an important regulator of dynein activity in the cell cycle.

Dr. Dolittle and the making of the mitotic spindle

The intrinsic polarity of microtubules within cells is exploited each time cells divide. Kinesins, microtubule-associated motor proteins, are required to execute the dramatic events of mitosis: bipolar spindle assembly, metaphase chromosome alignment, anaphase chromosome segregation, and separation of spindle poles prior to cytokinesis. Surprisingly, kinesin-related proteins have been found to move in either "plus-ward" or "minus-ward" directions along microtubules. Evidence from genetic analyses of simple eukaryotes and in vitro activity assays supports the notion that certain subfamilies of kinesin-related proteins provide antagonistic activities necessary to balance mitotic forces. A recent study by Sharp et al.((1)) sheds further light on the subject by exploiting the genetics and cytology of the fruit fly embryo.

The kinesin-related protein Kip1p of Saccharomyces cerevisiae is bipolar

Kip1p is a mitotic spindle-associated kinesin-related protein in Saccharomyces cerevisiae that participates in spindle pole separation. Here, we define the domain arrangement and polypeptide composition of the Kip1p holoenzyme. Electron microscopy of rotary shadowed Kip1p molecules revealed two globular domains 14 nm in diameter connected by a 73-nm long stalk. When the Kip1p domain homologous to the kinesin motor domain was decorated with an unrelated protein, the diameter of the globular domains at both ends of the stalk increased, indicating that Kip1p is bipolar. Soluble Kip1p isolated from S. cerevisiae cells was homomeric, based on the similarity of the sedimentation coefficients of native Kip1p from S. cerevisiae and Kip1p which was purified after expression in insect cells. The holoenzyme molecular weight was estimated using the sedimentation coefficient and Stokes radius, and was most consistent with a tetrameric composition. Kip1p exhibited an ionic strength-dependent transition in its sedimentation coefficient, revealing a potential regulatory mechanism. The position of kinesin motor-related domains at each end of the stalk may allow Kip1p to cross-link either parallel or antiparallel microtubules during mitotic spindle assembly and pole separation.

Allele-specific activators and inhibitors for kinesin

Members of the kinesin superfamily are force-generating ATPases that drive movement and influence cytoskeleton organization in cells. Often, more than one kinesin is implicated in a cellular process, and many kinesins are proposed to have overlapping functions. By using conventional kinesin as a model system, we have developed an approach to activate or inhibit a specific kinesin allele in the presence of other similar motor proteins. Modified ATP analogs are described that do not activate either conventional kinesin or another superfamily member, Eg5. However, a kinesin allele with Arg-14 in its nucleotide binding pocket mutated to alanine can use a subset of these nucleotide analogs to drive microtubule gliding. Cyclopentyl-ATP is one such analog. Cyclopentyl-adenylylimidodiphosphate, a nonhydrolyzable form of this analog, inhibits the mutant allele in microtubule-gliding assays, but not wild-type kinesin or Eg5. We anticipate that the incorporation of kinesin mutants and allele-specific activators and inhibitors in in vitro assays should clarify the role of individual motor proteins in complex cellular processes.

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.

Molecular characteristics of the centrosome

As an organizer of the microtubule cytoskeleton in animals, the centrosome has an important function. From the early light microscopic observation of the centrosome to examination by electron microscopy, the centrosome field is now in an era of molecular identification and precise functional analyses. Tables compiling centrosomal proteins and reviews on the centrosome are presented here and demonstrate how active the field is. However, despite this intense research activity, many classical questions are still unanswered. These include those regarding the precise function of centrioles, the mechanism of centrosome duplication and assembly, the origin of the centrosome, and the regulation and mechanism of the centrosomal microtubule nucleation activity. Fortunately, these questions are becoming elucidated based on experimental data discussed here. Given the fact that the centrosome is primarily a site of microtubule nucleation, special focus is placed on the process of microtubule nucleation and on the regulation of centrosomal microtubule nucleation capacity during the cell cycle and in some tissues.

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.

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.

Stu-7/air-2 is a C. elegans aurora homologue essential for chromosome segregation during embryonic and post-embryonic development

We have isolated a new sterile uncoordinated C. elegans mutant, stu-7, which is defective in post-embryonic cell divisions in a regionally-specific fashion. The anterior of the worm is relatively unaffected whereas the mid-body and/or posterior are markedly thin, often resulting in worms having a central 'waist'. We have cloned stu-7 and found that it encodes a member of the recently expanding aurora sub-family of serine/threonine kinases. Elimination of maternal as well as zygotic stu-7 expression reveals that stu-7 is essential for mitosis from the first embryonic cell cycle onwards and is required for chromosome segregation though not for centrosome separation or for setting up a bipolar spindle. Multicopy expression of stu-7 also causes mitotic defects, suggesting that the level of this protein must be tightly controlled in order to maintain genetic stability during development.

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.

Cytoplasmic dynein and dynactin in cell division and intracellular transport

Since the initial discovery of cytoplasmic dynein, it has become apparent that this microtubule-based motor is involved in several cellular functions including cell division and intracellular transport. Another multisubunit complex, dynactin, may be required for most, if not all, cytoplasmic dynein-driven activities and may provide clues to dynein's functional diversity. Recent genetic and biochemical findings have illuminated the cellular roles of dynein and dynactin and provided insight into the functional mechanism of this complex motor.

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.

Kinetics of induction of DNA adducts, cell proliferation and gene mutations in the liver of MutaMice treated with 5,9-dimethyldibenzo[c,g]carbazole

5,9-Dimethyldibenzo[c,g]carbazole (DMDBC) is a synthetic derivative of the environmental pollutant 7H-dibenzo[c,g]carbazole. DMDBC is a potent genotoxic carcinogen specific for mouse liver. Using the MutaMouse lacZ transgenic mouse model and a positive selection assay, we measured lacZ mutant frequency (MF) in the liver 28 days after a single s.c. administration of DMDBC at 3, 10, 30, 90 or 180 mg/kg. MF remained low at 3 and 10 mg/kg, but increased markedly from 30 mg/kg onwards. To investigate the reason for this non-linear response, we examined mechanisms potentially involved in mutation induction in the liver. Genotoxic effects such as DNA adduct formation were detected in 32P-post-labelling studies. Liver sections were examined for microscopic changes and cell proliferation. These parameters, and MF, were studied 2, 4, 7, 14, 21 and 28 days after a single s.c. administration of 10 or 90 mg/kg DMDBC. At 10 mg/kg, a dose found to double the MF on day 28, DNA adducts reached a level of 200-600 adducts per 10(8) nucleotides from day 4 to day 28. No changes in histology or cell proliferation were detected at this low dose. At 90 mg/kg, MF increased gradually from day 7 to day 28 (maximum 44-fold). The DNA adduct level ranged from 400 to 4500 adducts per 10(8) nucleotides on day 2, then stabilized at approximately 400 adducts per 10(8) nucleotides on day 4. An early cytotoxic effect was detected microscopically in centrilobular hepatocytes, and was followed by liver cell proliferation. These data suggest that the marked increase in MF in MutaMouse liver after treatment in vivo with DMDBC at 90 mg/kg may be explained by the induction of replicative DNA synthesis due to a cytotoxic effect, allowing the fixation of persistent DNA adducts into mutations.

Treatment of Vicia faba root tip cells with specific inhibitors to cyclin-dependent kinases leads to abnormal spindle formation

Many events during cell division are triggered by an evolutionary conserved regulator, the cyclin-dependent kinase (Cdk). Here we used two novel drugs, the purine analogues bohemine and roscovitine, to study the role of Cdks in cell cycle progression and microtubule organisation in Vicia faba root tip cells. Both drugs inhibited the activity of immunopurified Vicia faba and alfalfa Cdc2-kinase. The transcript levels of an A- and B-type cyclin, as well as of the cdc2 genes, declined in treated root tips, while the mRNA level of a D-type cyclin gene was not affected. An observed transient arrest at the G1/S and G2/M regulatory points indicated that inhibition of the Cdc2-kinase had an effect on both transitions. In contrast to the regular bipolar spindle in untreated cell, in drug-treated metaphase cells abnormally short and dense kinetochore microtubule fibres were observed. These microtubules were randomly arranged in the vicinity of the kinetochores and connected the chromosomes. Thus, the chromosomes were not aligned on the metaphase plate but were arranged in a circle, with kinetochores pointing inwards and chromosome arms pointing outwards. gamma-Tubulin, which plays a role in microtubule nucleation, also localised to the centre of the monopolar spindle. The observed abnormalities in mitosis, after inhibition of Cdc2-kinase by specific drugs, suggest a role for this enzyme in regulating some of the steps leading to a bipolar spindle structure.

Localization of the kinesin-like protein Xklp2 to spindle poles requires a leucine zipper, a microtubule-associated protein, and dynein

Xklp2 is a plus end-directed Xenopus kinesin-like protein localized at spindle poles and required for centrosome separation during spindle assembly in Xenopus egg extracts. A glutathione-S-transferase fusion protein containing the COOH-terminal domain of Xklp2 (GST-Xklp2-Tail) was previously found to localize to spindle poles (Boleti, H., E. Karsenti, and I. Vernos. 1996. Cell. 84:49-59). Now, we have examined the mechanism of localization of GST-Xklp2-Tail. Immunofluorescence and electron microscopy showed that Xklp2 and GST-Xklp2-Tail localize specifically to the minus ends of spindle pole and aster microtubules in mitotic, but not in interphase, Xenopus egg extracts. We found that dimerization and a COOH-terminal leucine zipper are required for this localization: a single point mutation in the leucine zipper prevented targeting. The mechanism of localization is complex and two additional factors in mitotic egg extracts are required for the targeting of GST-Xklp2-Tail to microtubule minus ends: (a) a novel 100-kD microtubule-associated protein that we named TPX2 (Targeting protein for Xklp2) that mediates the binding of GST-Xklp2-Tail to microtubules and (b) the dynein-dynactin complex that is required for the accumulation of GST-Xklp2-Tail at microtubule minus ends. We propose two molecular mechanisms that could account for the localization of Xklp2 to microtubule minus ends.

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.

Expanding the role of HsEg5 within the mitotic and post-mitotic phases of the cell cycle

The BimC family of kinesin like proteins are involved in spindle dynamics in a wide variety of organisms. The human member of this family, HsEg5, has been implicated in centrosome separation during prophase/prometaphase and in the organization of in vitro mitotic asters. HsEg5 displays a complex distribution during mitosis, associating with the centrosomes, spindle microtubules, specific regions of the intracellular bridge and a microtubule bundle that forms in association with the post-mitotic migration of the centrosome. In an effort to determine the function of HsEg5 during late mitotic events and refine its proposed function during early mitotic centrosome separation, we microinjected antibodies specific to HsEg5 into HeLa cells during various stages of mitosis. In the presence of HsEg5 antibodies we find that the microtubule arrays responsible for both pre- and post-mitotic centrosome movement never form. Similarly, the microtubule bundle within the intracellular bridge becomes prematurely altered following karyokinesis resulting in the loss of the microtubule array at either end of the bridge. In addition, some peri-centrosomal material at the spindle poles becomes fragmented and the distribution of the spindle protein NuMA becomes more concentrated at the minus ends of the spindle microtubules. Our study also provides direct evidence that there is a link between post-mitotic centrosome migration and Golgi complex positioning and reformation following mitosis. We conclude that HsEg5 plays a recurrent role in establishing and/or determining the stability of specific microtubule arrays that form during cell division and that this role may encompass the ability of HsEg5 to influence the distribution of other protein components associated with cell division

Characterization of the p22 subunit of dynactin reveals the localization of cytoplasmic dynein and dynactin to the midbody of dividing cells

Dynactin, a multisubunit complex that binds to the microtubule motor cytoplasmic dynein, may provide a link between dynein and its cargo. Many subunits of dynactin have been characterized, elucidating the multifunctional nature of this complex. Using a dynein affinity column, p22, the smallest dynactin subunit, was isolated and microsequenced. The peptide sequences were used to clone a full-length human cDNA. Database searches with the predicted amino acid sequence of p22 indicate that this polypeptide is novel. We have characterized p22 as an integral component of dynactin by biochemical and immunocytochemical methods. Affinity chromatography experiments indicate that p22 binds directly to the p150(Glued) subunit of dynactin. Immunocytochemistry with antibodies to p22 demonstrates that this polypeptide localizes to punctate cytoplasmic structures and to the centrosome during interphase, and to kinetochores and to spindle poles throughout mitosis. Antibodies to p22, as well as to other dynactin subunits, also revealed a novel localization for dynactin to the cleavage furrow and to the midbodies of dividing cells; cytoplasmic dynein was also localized to these structures. We therefore propose that dynein/dynactin complexes may have a novel function during cytokinesis.

C-Nap1, a novel centrosomal coiled-coil protein and candidate substrate of the cell cycle-regulated protein kinase Nek2

Nek2 (for NIMA-related kinase 2) is a mammalian cell cycle-regulated kinase structurally related to the mitotic regulator NIMA of Aspergillus nidulans. In human cells, Nek2 associates with centrosomes, and overexpression of active Nek2 has drastic consequences for centrosome structure. Here, we describe the molecular characterization of a novel human centrosomal protein, C-Nap1 (for centrosomal Nek2-associated protein 1), first identified as a Nek2-interacting protein in a yeast two-hybrid screen. Antibodies raised against recombinant C-Nap1 produced strong labeling of centrosomes by immunofluorescence, and immunoelectron microscopy revealed that C-Nap1 is associated specifically with the proximal ends of both mother and daughter centrioles. On Western blots, anti-C-Nap1 antibodies recognized a large protein (>250 kD) that was highly enriched in centrosome preparations. Sequencing of overlapping cDNAs showed that C-Nap1 has a calculated molecular mass of 281 kD and comprises extended domains of predicted coiled-coil structure. Whereas C-Nap1 was concentrated at centrosomes in all interphase cells, immunoreactivity at mitotic spindle poles was strongly diminished. Finally, the COOH-terminal domain of C-Nap1 could readily be phosphorylated by Nek2 in vitro, as well as after coexpression of the two proteins in vivo. Based on these findings, we propose a model implicating both Nek2 and C-Nap1 in the regulation of centriole-centriole cohesion during the cell cycle.

Characterization of microtubule binding domains in the Arabidopsis kinesin-like calmodulin binding protein

The kinesin-like calmodulin binding protein (KCBP) is a new member of the kinesin superfamily that appears to be present only in plants. The KCBP is unique in its ability to interact with calmodulin in a Ca2+-dependent manner. To study the interaction of the KCBP with microtubules, we expressed different regions of the Arabidopsis KCBP and used the purified proteins in cosedimentation assays with microtubules. The motor domain with or without the calmodulin binding domain bound to microtubules. The binding of the motor domain containing the calmodulin binding region to microtubules was inhibited by Ca2+-calmodulin. This Ca2+-calmodulin regulation of motor domain interactions with microtubules was abolished in the presence of antibodies specific to the calmodulin binding region. In addition, the binding of the motor domain lacking the calmodulin binding region to microtubules was not inhibited in the presence of Ca2+-calmodulin, suggesting an essential role for the calmodulin binding region in Ca2+-calmodulin modulation. Results of the cosedimentation assays with the N-terminal tail suggest the presence of a second microtubule binding site on the KCBP. However, the interaction of the N-terminal tail region of the KCBP with microtubules was insensitive to ATP. These data on the interaction of the KCBP with microtubules provide new insights into the functioning of the KCBP in plants.

Organisation and functional regulation of the centrosome in animal cells

Molecular characterisation of centrosomal components is slowly progressing. Recent results indicate that the major aspects of centrosome-mediated microtubule nucleation may soon be understood at the molecular level. In contrast, centrosome reproduction, which is an important aspect of animal cell division, remains terra incognita. The most challenging issue for the future is to understand the molecular mechanisms which control centrosome biogenesis. There is a urgent need to identify with certainty proteins implicated in this process. Comparison between organisms with structurally different centrosomes might be critical for a better understanding of centrosome duplication if a general mechanism has been conserved throughout evolution.

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.