Mitotic motors

Measurements of forces produced by the mitotic spindle using optical tweezers

An optical trap is used to stop chromosome movement in spermatocytes from an insect and a flatworm and to stop pole movement in PtK cells. The forces required are much smaller than previously believed.

We used a trapping laser to stop chromosome movements in Mesostoma and crane-fly spermatocytes and inward movements of spindle poles after laser cuts across Potorous tridactylus (rat kangaroo) kidney (PtK2) cell half-spindles. Mesostoma spermatocyte kinetochores execute oscillatory movements to and away from the spindle pole for 1–2 h, so we could trap kinetochores multiple times in the same spermatocyte. The trap was focused to a single point using a 63× oil immersion objective. Trap powers of 15–23 mW caused kinetochore oscillations to stop or decrease. Kinetochore oscillations resumed when the trap was released. In crane-fly spermatocytes trap powers of 56–85 mW stopped or slowed poleward chromosome movement. In PtK2 cells 8-mW trap power stopped the spindle pole from moving toward the equator. Forces in the traps were calculated using the equation F = Q′P/c, where P is the laser power and c is the speed of light. Use of appropriate Q′ coefficients gave the forces for stopping pole movements as 0.3–2.3 pN and for stopping chromosome movements in Mesostoma spermatocytes and crane-fly spermatocytes as 2–3 and 6–10 pN, respectively. These forces are close to theoretical calculations of forces causing chromosome movements but 100 times lower than the 700 pN measured previously in grasshopper spermatocytes.

One-step purification of assembly-competent tubulin from diverse eukaryotic sources

A method is presented that allows rapid and efficient purification of native, active tubulin from a variety of species and tissue sources by affinity chromatography. It eliminates the need to use heterologous systems for the study of microtubule-associated proteins and motor proteins, which has been a major issue in microtubule-related research.

We have developed a protocol that allows rapid and efficient purification of native, active tubulin from a variety of species and tissue sources by affinity chromatography. The affinity matrix comprises a bacterially expressed, recombinant protein, the TOG1/2 domains from Saccharomyces cerevisiae Stu2, covalently coupled to a Sepharose support. The resin has a high capacity to specifically bind tubulin from clarified crude cell extracts, and, after washing, highly purified tubulin can be eluted under mild conditions. The eluted tubulin is fully functional and can be efficiently assembled into microtubules. The method eliminates the need to use heterologous systems for the study of microtubule-associated proteins and motor proteins, which has been a major issue in microtubule-related research.

Polymer motors: pushing out the front and pulling up the back

Mechanical work in cells is performed by specialized motor proteins that operate in a continuous mechanochemical cycle. Less complex, but still efficient, 'one-shot' motors evolved based on the assembly and disassembly of polymers. We review the mechanisms of pushing and pulling by actin and microtubule filaments and the organizational principles of actin networks. We show how these polymer force generators are used for the propulsion of intracellular pathogens, protrusion of lamellipodia and mitotic movements. We discuss several examples of cellular forces generated by the assembly and disassembly of polymer gels.

Chromosome-microtubule interactions during mitosis

Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.

Mitosis and motor proteins in the filamentous ascomycete, Nectria haematococca, and some related fungi

Among filamentous fungi, mitosis has been studied in-depth in just a few species. The mitotic apparatuses in the ascomycetous Fusarium spp. are the most clearly and readily visualized in vivo within this group; fluorescent labeling is unnecessary. This superior cytological tractability has enabled detailed studies and revealing experiments that have led the way toward a more complete understanding of fungal mitosis. Some of the most important discoveries include the role of half-spindles in development of the bipolar spindle, the existence of true kinetochores in fungi, the unorthodox chromosome configurations and movements comprising metaphase and anaphase A, the attachment of astral microtubules to the plasmalemma, the role of the astral pulling force in elongating the spindle, an inwardly directed force within the spindle, and microtubule cross-bridging in both spindle and asters. Recent research has focused on the roles of microtubuleassociated motor proteins in Fusarium solani f. sp. pisi (anamorph of Nectria haematococca). Cytoplasmic dynein was shown to be involved in the development and/or maintenance of mitotic asters and necessary for motility and functionality of the interphase spindle pole body. The inwardly directed force within the anaphase spindle was shown to be produced by a kinesin-related protein, NhKRP1. Because of its superior cytological tractability, the considerable and unique knowledge we have of many aspects of its mitosis, and its genetic tractability, Fusarium solani f. sp. pisi is a good choice for further investigations of mitosis in filamentous fungi.

Clomiphene citrate-induced perturbations during meiotic maturation and cytogenetic abnormalities in mouse oocytes in vivo and in vitro

OBJECTIVE

To determine if clomiphene citrate induces temporal perturbations during meiotic maturation and aneuploidy in mouse oocytes.

DESIGN

A controlled dose study involving mouse oocytes in vivo and in vitro.

SETTING

Clinical and academic research setting in a university medical center.

INTERVENTION(S)

Oocytes were obtained after superovulation and from mature follicles.

MAIN OUTCOME MEASURE(S)

Cytogenetic analysis of oocytes for aneuploidy, premature centromere separation, premature anaphase, and single chromatids, and the frequencies of metaphase I and diploid oocytes.

RESULT(S)

Clomiphene citrate resulted in a decrease in the number of ovulated oocytes and a significant (P<.05) increase in hyperploidy at 100 mg/kg in vivo. In vitro, 5.0 microg/mL of clomiphene citrate significantly (P<.05) increased hyperploidy and reduced the proportion of metaphase I oocytes.

CONCLUSION(S)

These findings suggest that clomiphene citrate has the potential for inducing aneuploidy in mouse oocytes both in vivo and in vitro and that the rate of oocyte maturation is altered after clomiphene exposure in vitro. Additional data are needed to support the results of this study.

Microtubule-organizing centers in the mitotic melanophores of Xenopus laevis larvae in vivo: ultrastructural study

Mitotic melanophores of Xenopus laevis larvae at 51-53 stages of development were morphologically studied using light and electron microscopy, with special reference to their microtubule-organizing centers. These melanophores represented a highly branched cell shape in mitosis, each cell process is distributed with melanosomes without exhibiting any responsiveness to hormonal (melatonin) stimulation, and upon completion of mitosis, recovered the ability to translocate these granules in response to such a stimulus. At the metaphase, these cells contained bipolar or multipolar spindles, whose poles were composed of three zones: the centrosome with centrioles; the centrosphere; and an outlying radial arrangement of microtubules and their associated inclusions. In these mitotic melanophores, a number of microtubules are distributed within the radially stretching cell processes, whereas an abundance of microtubules reside in the spindles. Possible origins of the microtubules observed in these cytoplasmic processes are discussed in relation to the loss of the ability of pigment translocation during mitosis.

Time-lapse microscopy reveals unique roles for kinesins during anaphase in budding yeast

The mitotic spindle is a complex and dynamic structure. Genetic analysis in budding yeast has identified two sets of kinesin-like motors, Cin8p and Kip1p, and Kar3p and Kip3p, that have overlapping functions in mitosis. We have studied the role of three of these motors by video microscopy of motor mutants whose microtubules and centromeres were marked with green fluorescent protein. Despite their functional overlap, each motor mutant has a specific defect in mitosis: cin8Delta mutants lack the rapid phase of anaphase B, kip1Delta mutants show defects in the slow phase of anaphase B, and kip3Delta mutants prolong the duration of anaphase to the point at which the spindle becomes longer than the cell. The kip3Delta and kip1Delta mutants affect the duration of anaphase, but cin8Delta does not.

Mitosis and checkpoints that control progression through mitosis in vertebrate somatic cells

During mitosis in vertebrates the sister kinetochores on each replicated chromosome interact with two separating arrays of astral microtubules to form a bipolar spindle that produces and/or directs the forces for chromosome motion. In order to ensure faithful chromosome segregation cells have evolved mechanisms that delay progress into and out of mitosis until certain events are completed. At least two of these mitotic "checkpoint controls" can be identified in vertebrates. The first prevents nuclear envelope breakdown, and thus spindle formation, when the integrity of some nuclear component(s) is compromised. The second prevents chromosome disjunction and exit from mitosis until all of the kinetochores are attached to the spindle.

The yeast motor protein, Kar3p, is essential for meiosis I

The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown. We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type. These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

Kinetochore fiber maturation in PtK1 cells and its implications for the mechanisms of chromosome congression and anaphase onset

Kinetochore microtubules (kMts) are a subset of spindle microtubules that bind directly to the kinetochore to form the kinetochore fiber (K-fiber). The K-fiber in turn interacts with the kinetochore to produce chromosome motion toward the attached spindle pole. We have examined K-fiber maturation in PtK1 cells using same-cell video light microscopy/serial section EM. During congression, the kinetochore moving away from its spindle pole (i.e., the trailing kinetochore) and its leading, poleward moving sister both have variable numbers of kMts, but the trailing kinetochore always has at least twice as many kMts as the leading kinetochore. A comparison of Mt numbers on sister kinetochores of congressing chromosomes with their direction of motion, as well as distance from their associated spindle poles, reveals that the direction of motion is not determined by kMt number or total kMt length. The same result was observed for oscillating metaphase chromosomes. These data demonstrate that the tendency of a kinetochore to move poleward is not positively correlated with the kMt number. At late prometaphase, the average number of Mts on fully congressed kinetochores is 19.7 +/- 6.7 (n = 94), at late metaphase 24.3 +/- 4.9 (n = 62), and at early anaphase 27.8 +/- 6.3 (n = 65). Differences between these distributions are statistically significant. The increased kMt number during early anaphase, relative to late metaphase, reflects the increased kMt stability at anaphase onset. Treatment of late metaphase cells with 1 microM taxol inhibits anaphase onset, but produces the same kMt distribution as in early anaphase: 28.7 +/- 7. 4 (n = 54). Thus, a full complement of kMts is not sufficient to induce anaphase onset. We also measured the time course for kMt acquisition and determined an initial rate of 1.9 kMts/min. This rate accelerates up to 10-fold during the course of K-fiber maturation, suggesting an increased concentration of Mt plus ends in the vicinity of the kinetochore at late metaphase and/or cooperativity for kMt acquisition.

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.

Anaphase in vitro

Images

Fission yeast pkl1 is a kinesin-related protein involved in mitotic spindle function

We have used anti-peptide antibodies raised against highly conserved regions of the kinesin motor domain to identify kinesin-related proteins in the fission yeast Schizosaccharomyces pombe. Here we report the identification of a new kinesin-related protein, which we have named pkl1. Sequence homology and domain organization place pkl1 in the Kar3/ncd subfamily of kinesin-related proteins. Bacterially expressed pkl1 fusion proteins display microtubule-stimulated ATPase activity, nucleotide-sensitive binding, and bundling of microtubules. Immunofluorescence studies with affinity-purified antibodies indicate that the pkl1 protein localizes to the nucleus and the mitotic spindle. Pkl1 null mutants are viable but have increased sensitivity to microtubule-disrupting drugs. Disruption of pkl1+ suppresses mutations in another kinesin-related protein, cut7, which is known to act in the spindle. Overexpression of pkl1 to very high levels causes a similar phenotype to that seen in cut7 mutants: V-shaped and star-shaped microtubule structures are observed, which we interpret to be spindles with unseparated spindle poles. These observations suggest that pkl1 and cut7 provide opposing forces in the spindle. We propose that pkl1 functions as a microtubule-dependent motor that is involved in microtubule organization in the mitotic spindle.

Going mobile: microtubule motors and chromosome segregation

Proper chromosome segregation in eukaryotes depends upon the mitotic and meiotic spindles, which assemble at the time of cell division and then disassemble upon its completion. These spindles are composed in large part of microtubules, which either generate force by controlled polymerization and depolymerization or transduce force generated by molecular microtubule motors. In this review, we discuss recent insights into chromosome segregation mechanisms gained from the analyses of force generation during meiosis and mitosis. These analyses have demonstrated that members of the kinesin superfamily and the dynein family are essential in all organisms for proper chromosome and spindle behavior. It is also apparent that forces generated by microtubule polymerization and depolymerization are capable of generating forces sufficient for chromosome movement in vitro; whether they do so in vivo is as yet unclear. An important realization that has emerged is that some spindle activities can be accomplished by more than one motor so that functional redundancy is evident. In addition, some meiotic or mitotic movements apparently occur through the cooperative action of independent semiredundant processes. Finally, the molecular characterization of kinesin-related proteins has revealed that variations both in primary sequence and in associations with other proteins can produce motor complexes that may use a variety of mechanisms to transduce force in association with microtubules. Much remains to be learned about the regulation of these activities and the coordination of opposing and cooperative events involved in chromosome segregation; this set of problems represents one of the most important future frontiers of research.

The force for poleward chromosome motion in Haemanthus cells acts along the length of the chromosome during metaphase but only at the kinetochore during anaphase

The force for poleward chromosome motion during mitosis is thought to act, in all higher organisms, exclusively through the kinetochore. We have used time-lapse. video-enhanced, differential interference contrast light microscopy to determine the behavior of kinetochore-free "acentric" chromosome fragments and "monocentric" chromosomes containing one kinetochore, created at various stages of mitosis in living higher plant (Haemanthus) cells by laser microsurgery. Acentric fragments and monocentric chromosomes generated during spindle formation and metaphase both moved towards the closest spindle pole at a rate (approximately 1.0 microm/min) similar to the poleward motion of anaphase chromosomes. This poleward transport of chromosome fragments ceased near the onset of anaphase and was replaced. near midanaphase, by another force that now transported the fragments to the spindle equator at 1.5-2.0 microm/min. These fragments then remained near the spindle midzone until phragmoplast development, at which time they were again transported randomly poleward but now at approximately 3 microm/min. This behavior of acentric chromosome fragments on anastral plant spindles differs from that reported for the astral spindles of vertebrate cells, and demonstrates that in forming plant spindles, a force for poleward chromosome motion is generated independent of the kinetochore. The data further suggest that the three stages of non-kinetochore chromosome transport we observed are all mediated by the spindle microtubules. Finally, our findings reveal that there are fundamental differences between the transport properties of forming mitotic spindles in plants and vertebrates.

Xklp2, a novel Xenopus centrosomal kinesin-like protein required for centrosome separation during mitosis

We describe a novel Xenopus plus end-directed kinesin-like protein (KLP), Xklp2, localized on centrosomes throughout the cell cycle and on spindle pole microtubules during metaphase. Using mitotic spindles assembled in Xenopus egg extracts and different recombinant GST-Xklp2 mutants, we show that this motor is targeted to spindle poles through its C-terminal domain. Xklp2-truncated polypeptides lacking the motor domain block centrosome separation and disrupt preassembled metaphase spindles. Antibodies directed against the tail of Xklp2 have a similar effect. These results show that Xklp2 protein is required for centrosome separation and maintenance of spindle bipolarity. This study is an example of the application of the dominant negative mutant effect on spindle assembly in Xenopus egg extracts, demonstrating the usefulness of this approach in probing the function of proteins in this system.

Phosphorylation by p34cdc2 regulates spindle association of human Eg5, a kinesin-related motor essential for bipolar spindle formation in vivo

We have isolated a human homolog of Xenopus Eg5, a kinesin-related motor protein implicated in the assembly and dynamics of the mitotic spindle. We report that microinjection of antibodies against human Eg5 (HsEg5) blocks centrosome migration and causes HeLa cells to arrest in mitosis with monoastral microtubule arrays. Furthermore, an evolutionarily conserved cdc2 phosphorylation site (Thr-927) in HsEg5 is phosphorylated specifically during mitosis in HeLa cells and by p34cdc2/cyclin B in vitro. Mutation of Thr-927 to nonphosphorylatable residues prevents HsEg5 from binding to centrosomes, indicating that phosphorylation controls the association of this motor with the spindle apparatus. These results indicate that HsEg5 is required for establishing a bipolar spindle and that p34cdc2 protein kinase directly regulates its localization.

Chromosomal control of meiotic cell division

Chromosomes have multiple roles both in controlling the cell assembly and structure of the spindle and in determining chromosomal position on the spindle in many meiotic cells and in some types of mitotic cells. Moreover, functionally significant chromosome-microtubule interactions are not limited to the kinetochore but are also mediated by proteins localized along the arms of chromosomes. Finally, chromosomes also play a crucial role in control of the cell cycle.

Force generation by microtubule assembly/disassembly in mitosis and related movements

In this article, we review the dynamic nature of the filaments (microtubules) that make up the labile fibers of the mitotic spindle and asters, we discuss the roles that assembly and disassembly of microtubules play in mitosis, and we consider how such assembling and disassembling polymer filaments can generate forces that are utilized by the living cell in mitosis and related movements.

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

The KLP61F gene product is essential for Drosophila development. Mutations in KLP61F display a mitotic arrest phenotype caused by a failure in the proper separation of duplicated centrosomes (Heck et al., 1993). Sequence analysis of KLP61F identified it as a member of the bimC family of kinesin-like microtubule motor proteins. Here we report that KLP61F is distinct from KRP130, a kinesin-like protein recently purified from Drosophila embryos and suggested to be the product of the KLP61F gene (Cole et al., 1994). We also characterized recombinant KLP61F and found that it possesses microtubule-stimulated ATPase and microtubule translocation activities in vitro. In addition, we have used an affinity-purified, KLP61F-specific antiserum to localize native KLP61F and an epitope-tagged KLP61F fusion protein during various stages of mitosis in Drosophila syncytial blastoderm embryos. From early prophase through anaphase, KLP61F is coincident with the distribution of tubulin. Together these results confirm the existence of multiple bimC-like kinesins in Drosophila and suggest that KLP61F function is intrinsic to the mitotic spindle.

A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes

Orientational movements and modes of segregation at anaphase I were analyzed in three different types of univalents in living spermatocytes of the grasshopper species Eyprepocnemis plorans, namely the sex univalent, three types of accessory chromosomes and spontaneous and induced autosomal univalents. When two or more univalents were present in the same spindle, their dynamics were directly compared. Chromosomes may show variable velocity and number of reorientations: the X and the most common B types (B1 and B2) are slow and rarely reorient, a more geographically restricted B (B5) is faster and reorients more often, and autosomal univalents are the fastest and show the highest frequency of reorientations. Nonetheless, the X and the accessories are rigorously reductional at anaphase I whereas autosomal univalents often fail to migrate or divide equationally. This indicates that orientational and segregational behavior are controlled mainly by chromosomal rather than cellular characteristics and that chromosomes may display a great variety of strategies to achieve regular segregation.

Mechanics of motility: distinct dynein binding domains on alpha- and beta-tubulin

Microtubules (MTs) interact with force-generating proteins to generate a variety of intracellular movements, including intracellular particle transport, ciliary-flagellar beating, and chromosome-spindle movements during mitosis-meiosis. Relatively little is known about the mechanics of these motor-MT interactions, in part because the motor binding domains of the MT and the corresponding MT binding domains of the motor have not been well characterized. Using a flagellar motility assay, we report that the MT subunits, alpha- and beta-tubulin, each contain a dynein binding domain located near the C-termini of their respective tubulin subunits. Blocking either alpha- or beta-tubulin binding domains of dynein attenuates motility in demembranated sea urchin sperm up to 50%. Interestingly, blocking both alpha- and beta-tubulin binding domains on dynein produces much greater decreases in motility. These data suggest that flagellar dynein binds to both subunits of the MT polymer, alpha- and beta-tublin. In addition, the two subunits appear to contribute equivalent, but functionally separate, roles to flagellar motility.

Domains required for CENP-C assembly at the kinetochore

Chromosomes segregate at mitosis along microtubules attached to the kinetochore, an organelle that assembles at the centromere. Despite major advances in defining molecular components of the yeast segregation apparatus, including discrete centromere sequences and proteins of the kinetochore, relatively little is known of corresponding elements in more complex eukaryotes. We show here that human CENP-C, a human autoantigen previously localized to the kinetochore, assembles at centromeres of divergent species, and that the specificity of this targeting is maintained by an inherent destruction mechanism that prevents the accumulation of CENP-C and toxicity of mistargeted CENP-C. The N-terminus of CENP-C is not only required for CENP-C destruction but renders unstable proteins that otherwise possess long half-lives. The conserved targeting of CENP-C is underscored by the discovery of significant homology between regions of CENP-C and Mif2, a protein of Saccharomyces cerevisiae required for the correct segregation of chromosomes. Mutations in the Mif2 homology domain of CENP-C impair the ability of CENP-C to assemble at the kinetochore. Together, these data indicate that essential elements of the chromosome segregation apparatus are conserved in eukaryotes.

Characterization of a minus end-directed kinesin-like motor protein from cultured mammalian cells

Using the CHO2 monoclonal antibody raised against CHO spindles (Sellitto, C., M. Kimble, and R. Kuriyama. 1992. Cell Motil. Cytoskeleton. 22:7-24) we identified a 66-kD protein located at the interphase centrosome and mitotic spindle. Isolated cDNAs for the antigen encode a 622-amino acid polypeptide. Sequence analysis revealed the presence of 340-amino acid residues in the COOH terminus, which is homologous to the motor domain conserved among other members of the kinesin superfamily. The protein is composed of a central alpha-helical portion with globular domains at both NH2 and COOH termini, and the epitope to the monoclonal antibody resides in the central alpha-helical stalk. A series of deletion constructs were created for in vitro analysis of microtubule interactions. While the microtubule binding and bundling activities require both the presence of the COOH terminus and the alpha-helical domain, the NH2-terminal half of the antigen lacked the ability to interact with microtubules. The full-length as well as deleted proteins consisting of the COOH-terminal motor and the central alpha-helical stalk supported microtubule gliding, with velocity ranging from 1.0 to 8.4 microns/minute. The speed of microtubule movement decreased with decreasing lengths of the central stalk attached to the COOH-terminal motor. The microtubules moved with their plus end leading, indicating that the antigen is a minus end-directed motor. The CHO2 sequence shows 86% identify to HSET, a gene located at the centromeric end of the human MHC region in chromosome 6 (Ando, A., Y. Y. Kikuti, H. Kawata, N. Okamoto, T. Imai, T. Eki, K. Yokoyama, E. Soeda, T. Ikemura, K. Abe, and H. Inoko. 1994. Immunogenetics. 39:194-200), indicating that HSET might represent a human homologue of the CHO2 antigen.

The Drosophila kinesin-like protein KLP3A is a midbody component required for central spindle assembly and initiation of cytokinesis

We describe here a new member of the kinesin superfamily in Drosophila, KLP3A (Kinesin-Like-Protein-at-3A). The KLP3A protein localizes to the equator of the central spindle during late anaphase and telophase of male meiosis. Mutations in the KLP3A gene disrupt the interdigitation of microtubules in spermatocyte central spindles. Despite this defect, anaphase B spindle elongation is not obviously aberrant. However, cytokinesis frequently fails after both meiotic divisions in mutant testes. Together, these findings strongly suggest that the KLP3A presumptive motor protein is a critical component in the establishment or stabilization of the central spindle. Furthermore, these results imply that the central spindle is the source of signals that initiate the cleavage furrow in higher cells.

Chromokinesin: a DNA-binding, kinesin-like nuclear protein

Microtubule-associated mechanoenzymes have been proposed to play a fundamental role in chromosome movement. We have cloned and characterized the cDNA for a novel protein, named Chromokinesin, that fulfills several of the criteria expected of a mitotic motor. Chromokinesin contains both a kinesin motor-like domain and an unusual basic-leucine zipper DNA-binding domain. Its mRNA is readily detectable in proliferating cells, but not in postmitotic cells. Immunocytochemical analysis with antibodies directed against the nonconserved COOH-terminal region of Chromokinesin indicates that the protein is localized in the nucleus, and primarily associated with chromosome arms in mitotic cells. These data suggest that Chromokinesin is likely to function as a microtubule-based mitotic motor with DNA as its cargo.

Cytochalasin J affects chromosome congression and spindle microtubule organization in PtK1 cells

PtK1 cells were treated with 10 micrograms/ml cytochalasin J (CJ) for 15 min at various stages of mitosis. When applied at nuclear envelope breakdown (NEB) chromosome congression was blocked or substantially slowed, and chromosomes failed to show organization patterns typical of prometaphase. Spindle microtubule (MT) numbers appeared unaffected as judged by the pattern of birefringent retardation. However, ultrastructural analysis showed MTs to be reorganized within the spindle domain with some exhibiting fragmentation and others failing to interact with poorly defined kinetochore laminae. The spindle domain took on a curved, almost banana-like shape, as related to the position of the centrosomes and lack of orientation of chromosomes. Serial section analysis of kinetochore regions showed reduced contour length and maturation of the kinetochore plate with few MTs associated with this structure. Cells similarly treated with 10 micrograms/ml CJ at NEB for 15 min and then released into conditioned medium for 15 min showed the most chromosomes resumed congression to the metaphase plate. Ultrastructural analysis revealed a more normal organization of spindle MTs, but kinetochore structure remained affected. CJ treatment of cells in prometaphase slightly affected chromosomes congression with most chromosomes aligning at the metaphase plate after 10-15 min of treatment. Ultrastructural analysis showed that astral MTs were disrupted and spindle MTs were fragmented; few MTs coursed from kinetochore to pole. Kinetochore structure was also affected with only small numbers of short MTs seen associated with kinetochores. Application of CJ at anaphase onset had little effect on anaphase A and B, but cytokinesis failed to occur. Anti-tubulin staining of a monolayer of cells treated with 10 micrograms/ml CJ for 15 min showed that over 60% of mitotic figures exhibited changes in MT organization. Cells showing the greatest effect of treatment had several foci of bundles of MTs, as if the spindle were multipolar. Chromosomes were arranged near the periphery of the spindle which could be a result of abnormalities of kinetochore structure. Improper association of spindle MTs with kinetochores and, thus, changes in kinetochore position could account for these changes in spindle architecture.

Heterogeneity and microtubule interaction of the CHO1 antigen, a mitosis-specific kinesin-like protein. Analysis of subdomains expressed in insect sf9 cells

The CHO1 antigen is a mitosis-specific kinesin-like motor located at the interzonal region of the spindle. The human cDNA coding for the antigen contains a domain with sequence similarity to the motor domain of kinesin-like protein (Nislow et al., Nature 359, 543, 1992). Here we cloned cDNAs encoding the CHO1 antigen by immunoscreening of a CHO Uni-Zap expression library, the same species in which the original monoclonal antibody was raised. cDNAs of CHO cells encode a 953 amino acid polypeptide with a calculated molecular mass of 109 kDa. The N-terminal 73% of the antigen was 87% identical to the human clone, whereas the remaining 27% of the coding region showed only 48% homology. Insect Sf9 cells infected with baculovirus containing the full-length insert produced 105 and 95 kDa polypeptides, the same doublet identified as the original antigen in CHO cells. Truncated polypeptides corresponding to the N-terminal motor and C-terminal tail produced a 56 and 54 kDa polypeptide in Sf9 cells, respectively. Full and N-terminal proteins co-sedimented with, and caused bundling of, brain microtubules in vitro, whereas the C-terminal polypeptide did not. Cells expressing the N terminus formed one or more cytoplasmic processes. Immunofluorescence as well as electron microscopic observations revealed the presence of thick bundles of microtubules, which were closely packed, forming a marginal ring just beneath the cell membrane and a core in the processes. The diffusion coefficient and sedimentation coefficient were determined for the native CHO1 antigen by gel filtration and sucrose density gradient centrifugation, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors required for the binding of reassembled yeast kinetochores to microtubules in vitro

Kinetochores are structures that assemble on centromeric DNA and mediate the attachment of chromosomes to the microtubules of the mitotic spindle. The protein components of kinetochores are poorly understood, but the simplicity of the S. cerevisiae kinetochore makes it an attractive candidate for molecular dissection. Mutations in genes encoding CBF1 and CBF3, proteins that bind to yeast centromeres, interfere with chromosome segregation in vivo. To determine the roles played by these factors and by various regions of centromeric DNA in kinetochore function, we have developed a method to partially reassemble kinetochores on exogenous centromeric templates in vitro and to visualize the attachment of these reassembled kinetochore complexes to microtubules. In this assay, single reassembled complexes appear to mediate microtubule binding. We find that CBF3 is absolutely essential for this attachment but, contrary to previous reports (Hyman, A. A., K. Middleton, M. Centola, T.J. Mitchison, and J. Carbon. 1992. Microtubule-motor activity of a yeast centromere-binding protein complex. Nature (Lond.). 359:533-536) is not sufficient. Additional cellular factors interact with CBF3 to form active microtubule-binding complexes. This is mediated primarily by the CDEIII region of centromeric DNA but CDEII plays an essential modulatory role. Thus, the attachment of kinetochores to microtubules appears to involve a hierarchy of interactions by factors that assemble on a core complex consisting of DNA-bound CBF3.

Molecular motors and cell motility in the brain

The advent of video computer-enhanced microscopy has provided a new vision of cell migrations, growth cones, and fast axonal transport in the nervous system. In images obtained in studies of fast transport in isolated axoplasm from the squid giant axon, a virtual torrent of membrane traffic could be seen moving in both directions. Similarly, examination of growth cones and cell migrations in vitro and in vivo revealed properties of cell motility that were previously unsuspected. Evidence has accumulated that many of these activities are driven by a variety of microtubule and microfilament based motors.

Cytoplasmic dynein is involved in nuclear migration in Aspergillus nidulans

Nuclear migration plays an important role in the growth and development of many organisms including the multinuclear fungus Aspergillus nidulans. We have identified four genes, nudA, nudC, nudF, and nudG, in which temperature-sensitive mutations affect nuclear distribution. In this report, we describe the cloning of the nudA gene by complementation of the mutant phenotype by using a chromosome VIII-specific cosmid library. A genomic fragment of nudA hybridized to an mRNA of approximately 14 kb. Sequencing analysis of nudA revealed four ATP-binding sites that are characteristic of the cytoplasmic dynein heavy chain. The amino acid sequence of the nudA gene product shows 52% overall identity with the rat brain cytoplasmic dynein heavy chain. Our study provides in vivo evidence that dynein, a microtubule motor molecule, plays a role in the nuclear migration process.

Motile kinetochores and polar ejection forces dictate chromosome position on the vertebrate mitotic spindle

We argue that hypotheses for how chromosomes achieve a metaphase alignment, that are based solely on a tug-of-war between poleward pulling forces produced along the length of opposing kinetochore fibers, are no longer tenable for vertebrates. Instead, kinetochores move themselves and their attached chromosomes, poleward and away from the pole, on the ends of relatively stationary but shortening/elongating kinetochore fiber microtubules. Kinetochores are also "smart" in that they switch between persistent constant-velocity phases of poleward and away from the pole motion, both autonomously and in response to information within the spindle. Several molecular mechanisms may contribute to this directional instability including kinetochore-associated microtubule motors and kinetochore microtubule dynamic instability. The control of kinetochore directional instability, to allow for congression and anaphase, is likely mediated by a vectorial mechanism whose magnitude and orientation depend on the density and orientation or growth of polar microtubules. Polar microtubule arrays have been shown to resist chromosome poleward motion and to push chromosomes away from the pole. These "polar ejection forces" appear to play a key role in regulating kinetochore directional instability, and hence, positions achieved by chromosomes on the spindle.

Microtubule assembly and kinetochore directional instability in vertebrate monopolar spindles: implications for the mechanism of chromosome congression

We have proposed previously a kinetochore motor-polar ejection model for chromosome congression to the metaphase plate where forces generated at the kinetochore are antagonized by away-from-the pole forces generated within each half-spindle on the chromosome arms. This model was based in large part on observations of the behavior of chromosomes on monopolar spindles. In these cells chromosomes typically become attached to the pole by only one kinetochore fiber. These mono-oriented chromosomes move to positions away from the pole even though they are pulled poleward at their kinetochores. Their arms are also ejected away from the pole when severed from the centromere. Here we have characterized further the properties of monopolar spindles in newt lung epithelial cells to determine the similarities between monopolar and bipolar spindles. We found no significant differences between monopolar and bipolar spindles over the parameters examined, which included: microtubule dynamics as measured by fluorescence redistribution after photobleaching; the ability of polar microtubule arrays to push chromosome arms away from the pole; the dependence of chromosome position relative to the pole on microtubule assembly; the number of kinetochore microtubules per kinetochore; and the directional instability of kinetochore motion during chromosome oscillations poleward and away-from-the-pole. As in bipolar spindles, kinetochore directional instability is characterized by abrupt switching between constant velocity phases of poleward and away-from-the-pole motion. From these data we conclude that the mechanism(s) responsible for chromosome positioning in monopolar spindles are fundamentally the same as those in bipolar spindles; only the geometry of the two spindle forms and the interplay between sister kinetochore directional instabilities are different. We also found no correlation in the kinetochore-to-pole distance with kinetochore microtubule number in monopolar spindles, but a strong qualitative correlation with microtubule density. This finding indicates that oscillations of mono-oriented chromosomes in both monopolar and bipolar spindles occur because chromosomes persist in poleward motion until they reach a density of polar microtubules sufficiently high to promote switching to away-from-the-pole motion. As the kinetochore and chromosome arms move away-from-the-pole, microtubule density decreases and the kinetochore switches to poleward motion, pulling the chromosome arms back into regions of higher microtubule density. The mechanism regulating kinetochore switching between poleward and away-from-the-pole motion is poorly understood, but may depend on tension at the kinetochore generated by pushing forces on the chromosome arms produced by the polar microtubule arrays.

Kinesin-like molecules involved in spindle formation

To study the possible involvement of kinesin-like molecules in mitosis a polyclonal antibody against the head domain of Drosophila kinesin heavy chain (HD antibody) was microinjected into PtK1 cells at the prophase-prometaphase transition. Progress of the cell through mitosis was recorded for subsequent detailed analysis. Cells injected with pre-immune IgG progressed through mitosis at rates similar to those for noninjected cells. After HD antibody injections, chromosomes failed to congress to an equatorial plane and cells failed to form a bipolar spindle. Rather, the spindle poles came together, resulting in a monopolar-like configuration with chromosomes arranged about the poles in a rosette. Sometimes the monopolar array moved to the margin of the cell in a way similar to anaphase B movement in normal cells. Antibody-injected cells progressed into the next cell cycle as evidenced by chromosome decondensation and nuclear envelope reformation. Anti-tubulin immunofluorescence confirmed the presence of a radial monopolar array of microtubules in injected cells. HD antibody stained in a punctate pattern in interphase and the spindle region in mitotic PtK1 cells. The antibody also reacted with spindle fibers of isolated mitotic CHO spindles and with kinetochores of isolated CHO chromosomes. Immunoblotting indicated that the major component recognized by the antibody is the 120 kDa kinesin heavy chain. At higher protein loads the antibody recognized also a 34 kDa polypeptide in PtK1 cell extracts, a 135 kDa polypeptide in a preparation of CHO spindles and a 300 kDa polypeptide in a preparation of CHO mitotic chromosomes. We conclude that a kinesin-like molecule is important for the formation and/or maintenance of the structure of mitotic spindle.

Autoantibodies to a novel cell cycle-regulated protein that accumulates in the nuclear matrix during S phase and is localized in the kinetochores and spindle midzone during mitosis

We have employed human autoantibodies to characterize a novel cell cycle-regulated nuclear protein, provisionally designated p330d (doublet polypeptide of 330 kDa). The expression and intracellular distribution of this protein was followed throughout the cell cycle using immunofluorescence microscopy, laser confocal microscopy, immunoelectron microscopy and flow cytometry. p330d was expressed only in proliferating cells and began accumulating in the nucleus during early S phase. The protein reached maximum expression levels during G2/M. In situ extractions with detergent, salt and nucleases failed to abolish the nuclear staining of interphase cells, suggesting a tight binding of p330d to the nuclear matrix during interphase. p330d was concentrated in the kinetochores during prophase but was relocated to the spindle midzone at the onset of anaphase. By late telophase, it was localized predominantly in the intercellular bridge regions flanking the midbody and disappeared gradually as the daughter cells separated. Immunoblotting analysis showed that the autoimmune sera recognized a doublet of 330 kDa, and affinity-purified antibodies from this doublet reproduced the fluorescence staining pattern of the whole serum. We propose that p330d is a novel member of the class of ‘chromosomal passenger’ proteins, which are associated transiently with centromeres during early mitosis and are then redistributed to other sites of the mitotic apparatus after the metaphase/anaphase transition. Possible in vivo functions for p330d and related proteins might include roles in centromere/kinetochore maturation and assembly, chromosome segregation, central spindle stabilization and cytokinesis.

Cytoplasmic dynein plays a role in mammalian mitotic spindle formation

The formation and functioning of a mitotic spindle depends not only on the assembly/disassembly of microtubules but also on the action of motor enzymes. Cytoplasmic dynein has been localized to spindles, but whether or how it functions in mitotic processes is not yet known. We have cloned and expressed DNA fragments that encode the putative ATP-hydrolytic sites of the cytoplasmic dynein heavy chain from HeLa cells and from Dictyostelium. Monospecific antibodies have been raised to the resulting polypeptides, and these inhibit dynein motor activity in vitro. Their injection into mitotic mammalian cells blocks the formation of spindles in prophase or during recovery from nocodazole treatment at later stages of mitosis. Cells become arrested with unseparated centrosomes and form monopolar spindles. The injected antibodies have no detectable effect on chromosome attachment to a bipolar spindle or on motions during anaphase. These data suggest that cytoplasmic dynein plays a unique and important role in the initial events of bipolar spindle formation, while any later roles that it may play are redundant. Possible mechanisms of dynein's involvement in mitosis are discussed.

Roles of kinesin and kinesin-like proteins in sea urchin embryonic cell division: evaluation using antibody microinjection

Previous studies suggest that kinesin heavy chain (KHC) is associated with ER-derived membranes that accumulate in the mitotic apparatus in cells of early sea urchin embryos (Wright, B. D., J. H. Henson, K. P. Wedaman, P. J. Willy, J. N. Morand, and J. M. Scholey. 1991. J. Cell Biol. 113:817-833). Here, we report that the microinjection of KHC-specific antibodies into these cells has no effect on mitosis or ER membrane organization, even though one such antibody, SUK4, blocks kinesin-driven motility in vitro and in mammalian cells. Microinjected SUK4 was localized to early mitotic figures, suggesting that it is able to access kinesin in spindles. In contrast to KHC-specific antibodies, two antibodies that react with kinesin-like proteins (KLPs), namely CHO1 and HD, disrupted mitosis and prevented subsequent cell division. CHO1 is thought to exert this effect by blocking the activity of a 110-kD KLP. The relevant target of HD, which was raised against the KHC motor domain, is unknown; HD may disrupt mitosis by interfering with an essential spindle KLP but not with KHC itself, as preabsorption of HD with KHC did not alter its ability to block mitosis. These data indicate that some KLPs have essential mitotic functions in early sea urchin embryos but KHC itself does not.