CENP-E is a putative kinetochore motor that accumulates just before mitosis

Cytokinesis and midzone microtubule organization in Caenorhabditis elegans require the kinesin-like protein ZEN-4

Members of the MKLP1 subfamily of kinesin motor proteins localize to the equatorial region of the spindle midzone and are capable of bundling antiparallel microtubules in vitro. Despite these intriguing characteristics, it is unclear what role these kinesins play in dividing cells, particularly within the context of a developing embryo. Here, we report the identification of a null allele of zen-4, an MKLP1 homologue in the nematode Caenorhabditis elegans, and demonstrate that ZEN-4 is essential for cytokinesis. Embryos deprived of ZEN-4 form multinucleate single-celled embryos as they continue to cycle through mitosis but fail to complete cell division. Initiation of the cytokinetic furrow occurs at the normal time and place, but furrow propagation halts prematurely. Time-lapse recordings and microtubule staining reveal that the cytokinesis defect is preceded by the dissociation of the midzone microtubules. We show that ZEN-4 protein localizes to the spindle midzone during anaphase and persists at the midbody region throughout cytokinesis. We propose that ZEN-4 directly cross-links the midzone microtubules and suggest that these microtubules are required for the completion of cytokinesis.

Chromosome movement: kinetochores motor along

The equal division of chromosomes among daughter cells at mitosis involves a complex series of kinetochore-dependent chromosome movements. The kinetochore-associated CENP-E motor protein is critical for the sustained movement of chromosomes towards the metaphase plate during chromosome congression.

Anaphase A chromosome movement and poleward spindle microtubule flux occur At similar rates in Xenopus extract spindles

We have used local fluorescence photoactivation to mark the lattice of spindle microtubules during anaphase A in Xenopus extract spindles. We find that both poleward spindle microtubule flux and anaphase A chromosome movement occur at similar rates ( approximately 2 microm/min). This result suggests that poleward microtubule flux, coupled to microtubule depolymerization near the spindle poles, is the predominant mechanism for anaphase A in Xenopus egg extracts. In contrast, in vertebrate somatic cells a "Pacman" kinetochore mechanism, coupled to microtubule depolymerization near the kinetochore, predominates during anaphase A. Consistent with the conclusion from fluorescence photoactivation analysis, both anaphase A chromosome movement and poleward spindle microtubule flux respond similarly to pharmacological perturbations in Xenopus extracts. Furthermore, the pharmacological profile of anaphase A in Xenopus extracts differs from the previously established profile for anaphase A in vertebrate somatic cells. The difference between these profiles is consistent with poleward microtubule flux playing the predominant role in anaphase chromosome movement in Xenopus extracts, but not in vertebrate somatic cells. We discuss the possible biological implications of the existence of two distinct anaphase A mechanisms and their differential contributions to poleward chromosome movement in different cell types.

Mechanisms of chromosome segregation in metazoan cells

Despite over 100 year of research, the mechanisms that cells use to ensure the proper segregation of chromosomes during mitosis are still surprisingly obscure. However, recent high resolution video light microscopic studies of dividing cells are telling us new and important information about chromosome behavior. Molecular genetics is enabling us to build a more complete list of the components involved in chromosome segregation. And in vitro assays for chromosome segregation are providing information about the signals that control the equipartitioning of sister chromatids during cell division.

Centromere protein B null mice are mitotically and meiotically normal but have lower body and testis weights

CENP-B is a constitutive centromere DNA-binding protein that is conserved in a number of mammalian species and in yeast. Despite this conservation, earlier cytological and indirect experimental studies have provided conflicting evidence concerning the role of this protein in mitosis. The requirement of this protein in meiosis has also not previously been described. To resolve these uncertainties, we used targeted disruption of the Cenpb gene in mouse to study the functional significance of this protein in mitosis and meiosis. Male and female Cenpb null mice have normal body weights at birth and at weaning, but these subsequently lag behind those of the heterozygous and wild-type animals. The weight and sperm content of the testes of Cenpb null mice are also significantly decreased. Otherwise, the animals appear developmentally and reproductively normal. Cytogenetic fluorescence-activated cell sorting and histological analyses of somatic and germline tissues revealed no abnormality. These results indicate that Cenpb is not essential for mitosis or meiosis, although the observed weight reduction raises the possibility that Cenpb deficiency may subtly affect some aspects of centromere assembly and function, and result in reduced rate of cell cycle progression, efficiency of microtubule capture, and/or chromosome movement. A model for a functional redundancy of this protein is presented.

PUMA1: a novel protein that associates with the centrosomes, spindle and centromeres in the nematode Parascaris

We have identified a 227 kDa spindle- and centromere-associated protein in Parascaris, designated PUMA1 (Parascaris univalens mitotic apparatus), using a monoclonal antibody (mAb403) generated against Parascaris embryonic extracts. PUMA1 distribution was studied by immunofluorescence microscopy in mitotic and meiotic Parascaris cells, where centromere organization differs greatly. In mitosis, PUMA1 associates throughout cell division with the centrosomes and kinetochore-microtubules, and it concentrates at the continuous centromere region of the holocentric chromosomes. PUMA1 also localizes to the spindle mid-zone region during anaphase and at the midbody during telophase. In meiosis, PUMA1 associates with the centrosomes and with the discrete centromeric regions lacking kinetochore structures. The analysis of colchicine-treated embryos indicated that the association of PUMA1 with the centromeric region depends on microtubule integrity. mAb403 also recognizes spindle components in Drosophila. A series of overlapping cDNAs encoding the gene were isolated from a Parascaris embryonic expression library. Analysis of the nucleotide sequence identified an open reading frame capable of encoding a protein of 227 kDa. Analysis of the protein sequence indicated that PUMA1 is predicted to be a coiled-coil protein containing a large central alpha-helical domain flanked by nonhelical terminal domains. The structural features and cellular distribution of PUMA1 suggest that it may play a role in the organization of the spindle apparatus and in its interaction with the centromere in Parascaris.

A dominant mutant of inner centromere protein (INCENP), a chromosomal protein, disrupts prometaphase congression and cytokinesis

INCENP is a tightly bound chromosomal protein that transfers to the spindle midzone at the metaphase/anaphase transition. Here, we show that an INCENP truncation mutant (INCENP382-839) associates with microtubules but does not bind to chromosomes, and coats the entire spindle throughout mitosis. Furthermore, an INCENP truncation mutant (INCENP43-839) previously shown not to transfer to the spindle at anaphase (Mackay, A.M., D.M. Eckley, C. Chue, and W.C. Earnshaw. 1993. J. Cell Biol. 123:373-385), is shown here to bind chromosomes, but is unable to target to the centromere. Thus, association with the chromosomes, and specifically with centromeres, appears to be essential for INCENP targeting to the correct spindle subdomain at anaphase. An INCENP truncation mutant (INCENP1-405) that targets to centromeres but lacks the microtubule association region acquires strong dominant-negative characteristics. INCENP1-405 interferes with both prometaphase chromosome alignment and the completion of cytokinesis. INCENP1-405 apparently exerts its effect by displacing the endogenous protein from centromeres. These experiments provide evidence of an unexpected link between this chromosomal protein and cytokinesis, and suggest that one function of INCENP may be to integrate the chromosomal and cytoskeletal events of mitosis.

Immunolocalization of alpha-tubulin, gamma-tubulin, and CENP-E in male rat and male mouse meiotic divisions: pathway of meiosis I spindle formation in mammalian spermatocytes

Recent findings on cell division suggest that differences exist in spindle organization not only between mitotic and meiotic systems, but also between female and male meiosis. In mammals, this has been difficult to demonstrate due to lack of appropriate methods. By taking advantage of the strict organization and ordered kinetics of mammalian spermatogenesis, we harvested highly enriched populations of dividing mouse and rat spermatocytes using transillumination-assisted micro-dissection of seminiferous tubules. In the spermatocytes, we examined the localization and distribution of microtubules, centrosomes, and kinetochores at different phases of the first meiotic division using immunohistochemistry with antibodies against alpha-tubulin, gamma-tubulin, and CENP-E, respectively. Fluorescence and confocal microscope analysis of dividing spermatocytes provides evidence that the formation of the male mammalian meiosis I spindle differs from that of female meiosis and mitosis. A short (1-2 microns) bipolar aggregate of microtubules is nucleated by two adjacent centrosomes located next to the nucleus. After nuclear envelope breakdown, adjacent centrosomes and the short spindle become surrounded by the mass of paired meiotic chromosomes. At prometaphase the distance between the centrosomes increases resulting in elongation of the microtubule arrays and eventually formation of a full-length metaphase spindle (12-14 microns). Based on these results we suggest a model for spindle morphogenesis in mammalian spermatocytes.

Targeted disruption of mouse centromere protein C gene leads to mitotic disarray and early embryo death

Centromere protein C (CENPC) is a key protein that has been localized to the inner kinetochore plate of active mammalian centromeres. Using gene targeting techniques, we have disrupted the mouse Cenpc gene and shown that the gene is essential for normal mouse embryonic development. Heterozygous mice carrying one functional copy of the gene are healthy and fertile, whereas homozygous embryos fail to thrive. In these embryos, mitotic arrest and gross morphological degeneration become apparent as early as the morula stage of development. The degenerating embryos demonstrate highly irregular cell and nuclear morphologies, including the presence of a large number of micronuclei. Mitotic chromosomes of these embryos display a scattered and often highly condensed configuration and do not segregate in an ordered fashion. These results describing the phenotype of the mutant mouse embryos indicate that CENPC has a direct role in the mitotic progression from metaphase to anaphase.

Cooperative interactions between the central spindle and the contractile ring during Drosophila cytokinesis

We analyzed male meiosis in mutants of the chickadee (chic) locus, a Drosophila melanogaster gene that encodes profilin, a low molecular weight actin-binding protein that modulates F-actin polymerization. These mutants are severely defective in meiotic cytokinesis. During ana-telophase of both meiotic divisions, they exhibit a central spindle less dense than wild type; certain chic allelic combinations cause almost complete disappearance of the central spindle. Moreover, chic mutant spermatocytes fail to form an actomyosin contractile ring. To further investigate the relationships between the central spindle and the contractile ring, we examined meiosis in the cytokinesis-defective mutants KLP3A and diaphanous and in testes treated with cytochalasin B. In all cases, we found that the central spindle and the contractile ring in meiotic ana-telophases were simultaneously absent. Together, these results suggest a cooperative interaction between elements of the actin-based contractile ring and the central spindle microtubules: When one of these structures is disrupted, the proper assembly of the other is also affected. In addition to effects on the central spindle and the cytokinetic apparatus, we observed another consequence of chic mutations: A large fraction of chic spermatocytes exhibit abnormal positioning and delayed migration of asters to the cell poles. A similar phenotype was seen in testes treated with cytochalasin B and has been noted previously in mutants at the twinstar locus, a gene that encodes a Drosophila member of the cofilin/ADF family of actin-severing proteins. These observations all indicate that proper actin assembly is necessary for centrosome separation and migration.

The design plan of kinesin motors

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

CENP-E function at kinetochores is essential for chromosome alignment

CENP-E is a kinesin-like protein that binds to kinetochores and may provide functions that are critical for normal chromosome motility during mitosis. To directly test the in vivo function of CENP-E, we microinjected affinity-purified antibodies to block the assembly of CENP-E onto kinetochores and then examined the behavior of these chromosomes. Chromosomes lacking CENP-E at their kinetochores consistently exhibited two types of defects that blocked their alignment at the spindle equator. Chromosomes positioned near a pole remained mono-oriented as they were unable to establish bipolar microtubule connections with the opposite pole. Chromosomes within the spindle established bipolar connections that supported oscillations and normal velocities of kinetochore movement between the poles, but these bipolar connections were defective because they failed to align the chromosomes into a metaphase plate. Overexpression of a mutant that lacked the amino-terminal 803 amino acids of CENP-E was found to saturate limiting binding sites on kinetochores and competitively blocked endogenous CENP-E from assembling onto kinetochores. Chromosomes saturated with the truncated CENP-E mutant were never found to be aligned but accumulated at the poles or were strewn within the spindle as was the case when cells were microinjected with CENP-E antibodies. As the motor domain was contained within the portion of CENP-E that was deleted, the chromosomal defect is likely attributed to the loss of motor function. The combined data show that CENP-E provides kinetochore functions that are essential for monopolar chromosomes to establish bipolar connections and for chromosomes with connections to both spindle poles to align at the spindle equator. Both of these events rely on activities that are provided by CENP-E's motor domain.

Neocentromere-mediated chromosome movement in maize

Neocentromere activity is a classic example of nonkinetochore chromosome movement. In maize, neocentromeres are induced by a gene or genes on Abnormal chromosome 10 (Ab10) which causes heterochromatic knobs to move poleward at meiotic anaphase. Here we describe experiments that test how neocentromere activity affects the function of linked centromere/kinetochores (kinetochores) and whether neocentromeres and kinetochores are mobilized on the spindle by the same mechanism. Using a newly developed system for observing meiotic chromosome congression and segregation in living maize cells, we show that neocentromeres are active from prometaphase through anaphase. During mid-anaphase, normal chromosomes move on the spindle at an average rate of 0.79 micron/min. The presence of Ab10 does not affect the rate of normal chromosome movement but propels neocentromeres poleward at rates as high as 1.4 micron/min. Kinetochore-mediated chromosome movement is only marginally affected by the activity of a linked neocentromere. Combined in situ hybridization/immunocytochemistry is used to demonstrate that unlike kinetochores, neocentromeres associate laterally with microtubules and that neocentromere movement is correlated with knob size. These data suggest that microtubule depolymerization is not required for neocentromere motility. We argue that neocentromeres are mobilized on microtubules by the activity of minus end-directed motor proteins that interact either directly or indirectly with knob DNA sequences.

A method that allows the assembly of kinetochore components onto chromosomes condensed in clarified Xenopus egg extracts

Kinetochores are complex macromolecular structures that link mitotic chromosomes to spindle microtubules. Although a small number of kinetochore components have been identified, including the kinesins CENP-E and XKCM1 as well as cytoplasmic dynein, neither how these and other proteins are organized to produce a kinetochore nor their exact functions within this structure are understood. For this reason, we have developed an assay that allows kinetochore components to assemble onto discrete foci on in vitro-condensed chromosomes. The source of the kinetochore components is a clarified cell extract from Xenopus eggs that can be fractionated or immunodepleted of individual proteins. Kinetochore assembly in these clarified extracts requires preincubating the substrate sperm nuclei in an extract under low ATP conditions. Immunodepletion of XKCM1 from the extracts prevents the localization of kinetochore-associated XKCM1 without affecting the targeting of CENP-E and cytoplasmic dynein or the binding of monomeric tubulin to the kinetochore. Extension of this assay for the analysis of other components should help to dissect the protein-protein interactions involved in kinetochore assembly and function.

Immunolocalization of CENP-A suggests a distinct nucleosome structure at the inner kinetochore plate of active centromeres

The trilaminar kinetochore directs the segregation of chromosomes in mitosis and meiosis. Despite its importance, the molecular architecture of this structure remains poorly understood [1]. The best known component of the kinetochore plates is CENP-C, a protein that is required for kinetochore assembly [2], but whose molecular role in kinetochore structure and function is unknown. Here we have raised for the first time monospecific antisera to CENP-A [3], a 17 kD centromere-specific histone variant that is 62% identical to the carboxy-terminal domain of histone H3 [4,5] and that resembles the yeast centromeric component CSE4 [6]. We have found by simultaneous immunofluorescence with centromere antigens of known ultrastructural location that CENP-A is concentrated in the region of the inner kinetochore plate at active centromeres. Because CENP-A was previously shown to co-purify with nucleosomes [7], our data suggest a specific nucleosomal substructure for the kinetochore. In human cells, these kinetochore-specific nucleosomes are enriched in alpha-satellite DNA [8]. However, the association of CENP-A with neocentromeres lacking detectable alpha-satellite DNA, and the lack of CENP-A association with alpha-satellite-rich inactive centromeres of dicentric chromosomes together suggest that CENP-A association with kinetochores is unlikely to be determined solely by DNA sequence recognition. We speculate that CENP-A binding could be a consequence of epigenetic tagging of mammalian centromeres.

CENP-E is a plus end-directed kinetochore motor required for metaphase chromosome alignment

Mitosis requires dynamic attachment of chromosomes to spindle microtubules. This interaction is mediated largely by kinetochores. During prometaphase, forces exerted at kinetochores, in combination with polar ejection forces, drive congression of chromosomes to the metaphase plate. A major question has been whether kinetochore-associated microtubule motors play an important role in congression. Using immunodepletion from and antibody addition to Xenopus egg extracts, we show that the kinetochore-associated kinesin-like motor protein CENP-E is essential for positioning chromosomes at the metaphase plate. We further demonstrate that CENP-E powers movement toward microtubule plus ends in vitro. These findings support a model in which CENP-E functions in congression to tether kinetochores to dynamic microtubule plus ends.

The microtubule-dependent motor centromere-associated protein E (CENP-E) is an integral component of kinetochore corona fibers that link centromeres to spindle microtubules

Centromere-associated protein E (CENP-E) is a kinesin-related microtubule motor protein that is essential for chromosome congression during mitosis. Using immunoelectron microscopy, CENP-E is shown to be an integral component of the kinetochore corona fibers that tether centromeres to the spindle. Immediately upon nuclear envelope fragmentation, an associated plus end motor trafficks cytoplasmic CENP-E toward chromosomes along astral microtubules that enter the nuclear volume. Before or concurrently with initial lateral attachment of spindle microtubules, CENP-E targets to the outermost region of the developing kinetochores. After stable attachment, throughout chromosome congression, at metaphase, and throughout anaphase A, CENP-E is a constituent of the corona fibers, extending at least 50 nm away from the kinetochore outer plate and intertwining with spindle microtubules. In congressing chromosomes, CENP-E is preferentially associated with (or accessible at) the stretched, leading kinetochore known to provide the primary power for chromosome movement. Taken together, this evidence strongly supports a model in which CENP-E functions in congression to tether kinetochores to the disassembling microtubule plus ends.

Conservation of the centromere/kinetochore protein ZW10

Mutations in the essential Drosophila melanogaster gene zw10 disrupt chromosome segregation, producing chromosomes that lag at the metaphase plate during anaphase of mitosis and both meiotic divisions. Recent evidence suggests that the product of this gene, DmZW10, acts at the kinetochore as part of a tension-sensing checkpoint at anaphase onset. DmZW10 displays an intriguing cell cycle-dependent intracellular distribution, apparently moving from the centromere/kinetochore at prometaphase to kinetochore microtubules at metaphase, and back to the centromere/kinetochore at anaphase (Williams, B.C., M. Gatti, and M.L. Goldberg. 1996. J. Cell Biol. 134:1127-1140). We have identified ZW10-related proteins from widely diverse species with divergent centromere structures, including several Drosophilids, Caenorhabditis elegans, Arabidopsis thaliana, Mus musculus, and humans. Antibodies against the human ZW10 protein display a cell cycle-dependent staining pattern in HeLa cells strikingly similar to that previously observed for DmZW10 in dividing Drosophila cells. Injections of C. elegans ZW10 antisense RNA phenocopies important aspects of the mutant phenotype in Drosophila: these include a strong decrease in brood size, suggesting defects in meiosis or germline mitosis, a high percentage of lethality among the embryos that are produced, and the appearance of chromatin bridges at anaphase. These results indicate that at least some aspects of the functional role of the ZW10 protein in ensuring proper chromosome segregation are conserved across large evolutionary distances.

An apparently acentric marker chromosome originating from 9p with a functional centromere without detectable alpha and beta satellite sequences

Recently, we studied a patient with minor abnormalities and an apparently acentric marker chromosome who carried a deleted chromosome 9 and a marker chromosome in addition to a normal chromosome 9. The marker was stable in mitosis but lacked a primary constriction. The origin of the marker was established by fluorescent in situ hybridization (FISH) using a chromosome 9 painting probe. Hybridization of unique sequence 9p probes localized the breakpoint proximal to 9p13. Additional FISH studies with all-human centromere alpha satellite, chromosome 9 classical satellite, and beta satellite probes showed no visible evidence of these sequences on the marker [Curtis et al.: Am J Hum Genet 57:A111, 1995]. Studies using centromere proteins (CENP-B, CENP-C, and CENP-E) were performed and demonstrated the presence of centromere proteins. These studies and the patient's clinical findings are reported here.

CENP-E is an essential kinetochore motor in maturing oocytes and is masked during mos-dependent, cell cycle arrest at metaphase II

CENP-E, a kinesin-like protein that is known to associate with kinetochores during all phases of mitotic chromosome movement, is shown here to be a component of meiotic kinetochores as well. CENP-E is detected at kinetochores during metaphase I in both mice and frogs, and, as in mitosis, is relocalized to the midbody during telophase. CENP-E function is essential for meiosis I because injection of an antibody to CENP-E into mouse oocytes in prophase completely prevented progression of those oocytes past metaphase I. Beyond this, CENP-E is modified or masked during the natural, Mos-dependent, cell cycle arrest that occurs at metaphase II, although it is readily detectable at the kinetochores in metaphase II oocytes derived from mos-deficient (MOS-/-) mice that fail to arrest at metaphase II. This must reflect a masking of some CENP-E epitopes, not the absence of CENP-E, in meiosis II because a different polyclonal antibody raised to the tail of CENP-E detects CENP-E at kinetochores of metaphase II-arrested eggs and because CENP-E reappears in telophase of mouse oocytes activated in the absence of protein synthesis.

The centromere enhancer mediates centromere activation in Schizosaccharomyces pombe

The centromere enhancer is a functionally important DNA region within the Schizosaccharomyces pombe centromeric K-type repeat. We have previously shown that addition of the enhancer and cen2 centromeric central core to a circular minichromosome is sufficient to impart appreciable centromere function. A more detailed analysis of the enhancer shows that it is dispensable for centromere function in a cen1-derived minichromosome containing the central core and the remainder of the K-type repeat, indicating that the critical centromeric K-type repeat, like the central core, is characterized by functional redundancy. The centromeric enhancer is required, however, for a central core-carrying minichromosome to exhibit immediate centromere activity when the circular DNA is introduced via transformation into S. pombe. This immediate activation is probably a consequence of a centromere-targeted epigenetic system that governs the chromatin architecture of the region. Moreover, our studies show that two entirely different DNA sequences, consisting of elements derived from two native centromeres, can display centromere function. An S. pombe CENP-B-like protein, Abp1p/Cbp1p, which is required for proper chromosome segregation in vivo, binds in vitro to sites within and adjacent to the modular centromere enhancer, as well as within the centromeric central cores. These results provide direct evidence in fission yeast of a model, similar to one proposed for mammalian systems, whereby no specific sequence is necessary for centromere function but certain classes of sequences are competent to build the appropriate chromatin foundation upon which the centromere/kinetochore can be formed and activated.

A novel myosin-like protein (myocilin) expressed in the connecting cilium of the photoreceptor: molecular cloning, tissue expression, and chromosomal mapping

We have isolated a human cDNA clone encoding a novel acidic protein of MW 55,000 that we designated "myocilin" since it has homology to myosin and is localized preferentially in the ciliary rootlet and basal body of the connecting cilium of photoreceptor cells. The deduced amino acid sequence of human myocilin showed significant homologies with nonmuscle myosin of Dictyostelium discoideum in the N-terminal region and also with olfactomedin of bullfrog in the C-terminal region. Myocilin contained a leucine zipper-like motif similar to that seen in kinectin and other cytoskeletal proteins. These findings suggest that myocilin is a novel cytoskeletal protein involved in the morphogenesis of ciliated neuroepithelium such as photoreceptor cells. The myocilin gene (MYOC) was mapped to human chromosome 1q23-q24 by fluorescence in situ hybridization.

Formation of de novo centromeres and construction of first-generation human artificial microchromosomes

We have combined long synthetic arrays of alpha satellite DNA with telomeric DNA and genomic DNA to generate artificial chromosomes in human HT1080 cells. The resulting linear microchromosomes contain exogenous alpha satellite DNA, are mitotically and cytogenetically stable in the absence of selection for up to six months in culture, bind centromere proteins specific for active centromeres, and are estimated to be 6-10 megabases in size, approximately one-fifth to one-tenth the size of endogenous human chromosomes. We conclude that this strategy results in the formation of de novo centromere activity and that the microchromosomes so generated contain all of the sequence elements required for stable mitotic chromosome segregation and maintenance. This first-generation system for the construction of human artificial chromosomes should be suitable for dissecting the sequence requirements of human centromeres, as well as developing constructs useful for therapeutic applications.

Transposon mutagenesis reinforces the correlation between Mycoplasma pneumoniae cytoskeletal protein HMW2 and cytadherence

A new genetic locus associated with Mycoplasma pneumoniae cytadherence was previously identified by transposon mutagenesis with Tn4001. This locus maps approximately 160 kbp from the genes encoding cytadherence-associated proteins HMW1 and HMW3, and yet insertions therein result in loss of these proteins and a hemadsorption-negative (HA-) phenotype, prompting the designation cytadherence-regulatory locus (crl). In the current study, passage of transformants in the absence of antibiotic selection resulted in loss of the transposon, a wild-type protein profile, and a HA+ phenotype, underscoring the correlation between crl and M. pneumoniae cytadherence. Nucleotide sequence analysis of crl revealed open reading frames (ORFs) orfp65, orfp216, orfp41, and orfp24, arranged in tandem and flanked by a promoter-like and a terminator-like sequence, suggesting a single transcriptional unit, the P65 operon. The 5' end of orfp65 mRNA was mapped by primer extension, and a likely promoter was identified just upstream. The product of each ORF was identified by using antisera prepared against fusion proteins. The previously characterized surface protein P65 is encoded by orfp65, while the 190,000 Mr cytadherence-associated protein HMW2 is a product of orfp216. Proteins with sizes of 47,000 and 41,000 Mr and unknown function were identified for orfp41 and orfp24, respectively. Structural analyses of HMW2 predict a periodicity highly characteristic of a coiled-coil conformation and five leucine zipper motifs, indicating that HMW2 probably forms dimers in vivo, which is consistent with a structural role in cytadherence. Each transposon insertion mapped to orfp216 but affected the levels of all products of the P65 operon. HMW2 is thought to form a disulfide-linked dimer, formerly designated HMW5, and examination of an hmw2 deletion mutant confirms that HMW5 is a product of the hmw2 gene.

Molecular characterization of p62, a mitotic apparatus protein required for mitotic progression

A 62-kDa (p62) mitotic apparatus-associated protein is important for the proper progression of mitosis in sea urchin embryos (Dinsmore, J. H., and Sloboda, R. D. (1989) Cell 53, 769-780). We have isolated and characterized a full-length p62 cDNA of 3374 base pairs which encodes an extremely acidic polypeptide of 411 amino acids having a calculated Mr of 46,388 and a pI of 4.01; p62 is a unique protein with no significant identity to any known proteins. Southern and Northern blot analyses demonstrate that the gene for p62 is present once in the sea urchin genome and the corresponding mRNA is present in unfertilized eggs and in early embryos through and up to the gastrula stage. Sequence analysis suggests certain regions may participate in chromatin association and microtubule binding, an observation that is consistent with previous immunological data (Ye, X., and Sloboda, R. D. (1995) Cell Motil. Cytoskeleton 30, 310-323) as well as data reported herein. Confocal microscopy reveals that during interphase the protein binds to chromatin in the nuclei of sea urchin eggs. In the germinal vesicles of clam oocytes at prophase of meiosis I, p62 binds to the condensed chromosomes. Currently, truncated clones of p62 are being used to identify the tubulin and chromatin binding domains.

Analysis of a 103 kbp cluster homology region from the left end of Saccharomyces cerevisiae chromosome I

The DNA sequence and preliminary functional analysis of a 103-kbp section of the left arm of yeast chromosome I is presented. This region, from the left telomere to the LTE1 gene, can be divided into two distinct portions. One portion, the telomeric 29 kbp, has a very low gene density (only five potential genes and 21 kbp of noncoding sequence), does not encode any "functionally important" genes, and is rich in sequences repeated several times within the yeast genome. The other portion, with 37 genes and only 14.5 kbp of noncoding sequence, is gene rich and codes for at least 16 "functionally important" genes. The entire gene-rich portion is apparently duplicated on chromosome XV as an extensive region of partial gene synteney called a cluster homology region. A function can be assigned with varying degrees of precision to 23 of the 42 potential genes in this region; however, the precise function is know for only eight genes. Nineteen genes encode products presently novel to yeast, although five of these have homologs elsewhere in the yeast genome.

Cloning and characterization of novel gene, DCRR1, expressed from Down's syndrome critical region of human chromosome 21q22.2

The new gene, DCRR1, from the proximal part of the Down's syndrome critical region (DCR) was identified by the GRAIL analysis of the 97-kb nucleotide sequence of two P1 DNAs and the cDNA for DCRR1 gene was cloned. A 7.36-kb cDNA encodes the imcompleted open reading frame composed of 1941 amino acid residues (220.2 kDa). The deduced amino acid sequence contains the conserved domain for protein phosphatases at the N-terminus. The domain encoding the rod-like tail of a myosin heavy chain was also found near the C-terminal region besides the signature for an actin binding protein, profilin, suggesting its possible role as a microtuble-associated protein. Two different sizes (7.9 and 9.0 kb) of mRNAs were detected in the poly(A)+ RNA from abundant tissues by the Northern analysis. The smaller transcript was only transcribed at a high level in the testis. The imbalance of the DCRR1 gene dosage may contibute to the pathogenesis of Down's syndrome.

Dynamic elastic behavior of alpha-satellite DNA domains visualized in situ in living human cells

We have constructed a fluorescent alpha-satellite DNA-binding protein to explore the motile and mechanical properties of human centromeres. A fusion protein consisting of human CENP-B coupled to the green fluorescent protein (GFP) of A. victoria specifically targets to centromeres when expressed in human cells. Morphometric analysis revealed that the alpha-satellite DNA domain bound by CENPB-GFP becomes elongated in mitosis in a microtubule-dependent fashion. Time lapse confocal microscopy in live mitotic cells revealed apparent elastic deformations of the central domain of the centromere that occurred during metaphase chromosome oscillations. These observations demonstrate that the interior region of the centromere behaves as an elastic element that could play a role in the mechanoregulatory mechanisms recently identified at centromeres. Fluorescent labeling of centromeres revealed that they disperse throughout the nucleus in a nearly isometric expansion during chromosome decondensation in telophase and early G1. During interphase, centromeres were primarily stationary, although motility of individual or small groups of centromeres was occasionally observed at very slow rates of 7-10 microns/h.

Centrosome and spindle function of the Drosophila Ncd microtubule motor visualized in live embryos using Ncd-GFP fusion proteins

The Ncd microtubule motor protein is required for meiotic and early mitotic chromosome distribution in Drosophila. Null mutant females expressing the Ncd motor fused to the Aequorea victoria green fluorescent protein (GFP), regulated by the wild-type ncd promoter, are rescued for chromosome segregation and embryo viability. Analysis of mitosis in live embryos shows cell cycle-dependent localization of Ncd-GFP to centrosomes and spindles. The distribution of Ncd-GFP in spindles during metaphase differs strikingly from that of tubulin: the tubulin staining is excluded by the chromosomes at the metaphase plate; in contrast, Ncd-GFP forms filaments along the spindle microtubules that extend across the chromosomes. The existence of Ncd-GFP fibers that cross the metaphase plate suggests that Ncd interacts functionally with chromosomes in metaphase. Differences are no longer observed in anaphase when the chromosomes have moved off the metaphase plate. A mutant form of Ncd fused to GFP also localizes to spindles in live embryos. Mutant embryos show frequent centrosome and spindle abnormalities, including free centrosomes that dissociate from interphase nuclei, precociously split centrosomes, and spindles with microtubule spurs or bridges to nearby spindles. The precociously split and free centrosomes indicate that the Ncd motor acts in cleavage stage embryos to maintain centrosome integrity and attachment to nuclei. The frequent spindle spurs of mutant embryos are associated with mis-segregating chromosomes that partially detach from the spindle in metaphase, but can be recaptured in early anaphase. This implies that the Ncd motor functions to prevent chromosome loss by maintaining chromosome attachment to the spindle in metaphase, consistent with the Ncd-GFP fibers that across the metaphase plate.

Cellular expression of human centromere protein C demonstrates a cyclic behavior with highest abundance in the G1 phase

Centromere proteins are localized within the centromere-kinetochore complex, which can be proven by means of immunofluorescence microscopy and immunoelectron microscopy. In consequence, their putative functions seem to be related exclusively to mitosis, namely to the interaction of the chromosomal kinetochores with spindle microtubules. However, electron microscopy using immune sera enriched with specific antibodies against human centromere protein C (CENP-C) showed that it occurs not only in mitosis but during the whole cell cycle. Therefore, we investigated the cell cycle-specific expression of CENP-C systematically on protein and mRNA levels applying HeLa cells synchronized in all cell cycle phases. Immunoblotting confirmed protein expression during the whole cell cycle and revealed an increase of CENP-C from the S phase through the G2 phase and mitosis to highest abundance in the G1 phase. Since this was rather surprising, we verified it by quantifying phase-specific mRNA levels of CENP-C, paralleled by the amplification of suitable internal standards, using the polymerase chain reaction. The results were in excellent agreement with abundant protein amounts and confirmed the cyclic behavior of CENP-C during the cell cycle. In consequence, we postulate that in addition to its role in mitosis, CENP-C has a further role in the G1 phase that may be related to cell cycle control.

Interaction of kinesin motor domains with alpha- and beta-tubulin subunits at a tau-independent binding site. Regulation by polyglutamylation

Interaction of rat kinesin and Drosophila nonclaret disjunctional motor domains with tubulin was studied by a blot overlay assay. Either plus-end or minus-end-directed motor domain binds at the same extent to both alpha- and beta-tubulin subunits, suggesting that kinesin binding is an intrinsic property of each tubulin subunit and that motor directionality cannot be related to a preferential interaction with a given tubulin subunit. Binding features of dimeric versus monomeric rat kinesin heads suggest that dimerization could drive conformational changes to enhance binding to tubulin. Competition experiments have indicated that kinesin interacts with tubulin at a Tau-independent binding site. Complementary experiments have shown that kinesin does not interact with the same efficiency with the different tubulin isoforms. Masking the polyglutamyl chains with a specific monoclonal antibody leads to a complete inhibition of kinesin binding. These results are consistent with a model in which polyglutamylation of tubulin regulates kinesin binding through progressive conformational changes of the whole carboxyl-terminal domain of tubulin as a function of the polyglutamyl chain length, thus modulating the affinity of tubulin for kinesin and Tau as well. These results indicate that microtubules, through tubulin polymorphism, do have the ability to control microtubule-associated protein binding.

Topoisomerase II alpha is associated with the mammalian centromere in a cell cycle- and species-specific manner and is required for proper centromere/kinetochore structure

A study of the distribution of Topoisomerase II alpha (Topo II) in cells of six tissue culture cell lines, human (HeLa), mouse (L929), rat, Indian muntjac, rat kangaroo (PTK-2), and wallaby revealed the following features: (1) There is a cell cycle association of a specific population of Topo II with the centromere. (2) The centromere is distinguished from the remainder of the chromosome by the intensity of its Topo II reactivity. (3) The first appearance of a detectable population of Topo II at the centromere varies between species but is correlated with the onset of centromeric heterochromatin condensation. (4) Detectable centromeric Topo II declines at the completion of cell division. (5) The distribution pattern of Topo II within the centromere is species- and stage-specific and is conserved only within the kinetochore domain. In addition, we report that the Topo II inhibitor ICRF-193 can prevent the normal accumulation of Topo II at the centromere. This results in the disruption of chromatin condensation sub-adjacent to the kinetochore as well as the perturbation of kinetochore structure. Taken together, our studies indicate that the distribution of Topo II at the centromere is unlike that reported for the remainder of the chromosome and is essential for proper formation of centromere/kinetochore structure.

Bipolar spindle attachments affect redistributions of ZW10, a Drosophila centromere/kinetochore component required for accurate chromosome segregation

Previous efforts have shown that mutations in the Drosophila ZW10 gene cause massive chromosome missegregation during mitotic divisions in several tissues. Here we demonstrate that mutations in ZW10 also disrupt chromosome behavior in male meiosis I and meiosis II, indicating that ZW10 function is common to both equational and reductional divisions. Divisions are apparently normal before anaphase onset, but ZW10 mutants exhibit lagging chromosomes and irregular chromosome segregation at anaphase. Chromosome missegregation during meiosis I of these mutants is not caused by precocious separation of sister chromatids, but rather the nondisjunction of homologs. ZW10 is first visible during prometaphase, where it localizes to the kinetochores of the bivalent chromosomes (during meiosis I) or to the sister kinetochores of dyads (during meiosis II). During metaphase of both divisions, ZW10 appears to move from the kinetochores and to spread toward the poles along what appear to be kinetochore microtubules. Redistributions of ZW10 at metaphase require bipolar attachments of individual chromosomes or paired bivalents to the spindle. At the onset of anaphase I or anaphase II, ZW10 rapidly relocalizes to the kinetochore regions of the separating chromosomes. In other mutant backgrounds in which chromosomes lag during anaphase, the presence or absence of ZW10 at a particular kinetochore predicts whether or not the chromosome moves appropriately to the spindle poles. We propose that ZW10 acts as part of, or immediately downstream of, a tension-sensing mechanism that regulates chromosome separation or movement at anaphase onset.

Are proteasomes involved in the formation of the kinetochore?

Proteasomes catalyse the degradation of proteins responsible for the regulation of mitosis enabling the cell to complete cell division. We have studied the effect of an inhibitor of the chymotrypsin-like activity of the proteasome on the trilaminar structure of the kinetochore in HeLa cells. Whereas a role for the proteasome in the degeneration of the kinetochore was predicted, we found instead that the inhibitor strongly regarded kinetochore development. We observed different 'developmental' stages of the kinetochore from the fibrous ball of a 'prekinetochore' to the 'mature' kinetochore in one cell. The data presented here support the proposition that proteasomes are involved in kinetochore formation.

Specific interaction between human kinetochore protein CENP-C and a nucleolar transcriptional regulator

CENP-C is a human kinetochore protein that was originally identified as a chromosomal autoantigen in patients with scleroderma spectrum disease. To begin to establish a comprehensive protein map of the human centromere, affinity chromatography was used to identify nuclear proteins that specifically interact with CENP-C. Whereas a number of polypeptides appeared to interact with the full-length CENP-C protein, only a pair of similarly sized proteins of approximately 100 kDa specifically interacted with the isolated carboxyl-terminal third of the CENP-C protein. Neither protein of the doublet bound to control affinity columns. Affinity purification and microsequence analysis of the proteins in the doublet identified them as the two highly related nucleolar transcription factors, UBF1 and UBF2 (also known as the nucleolar autoantigen NOR-90). Immunoblot analysis confirmed that both proteins also interacted with the full-length CENP-C polypeptide with similar affinities. Double indirect immunofluorescence using monospecific antibodies demonstrated that a subset of CENP-C and UBF/NOR-90 is colocalized at nucleoli of interphase HeLa cells, suggesting that the in vitro interaction detected by affinity chromatography may reflect an interaction that occurs in vivo. We discuss the implications of these findings in terms of the properties of interphase centromeres and the role of the nucleolus in scleroderma autoimmunity.

The centromere kinesin-like protein, CENP-E. An autoantigen in systemic sclerosis

OBJECTIVE

Autoantibodies directed against centromere proteins (CENPs) are a serologic feature in some patients with systemic sclerosis (SSc). Previous studies have focused on autoantibodies to CENPs A, B, and C. CENP-E is a recently described 312-kd protein that also localizes to the centromere. Therefore, we studied the presence of autoantibodies to recombinant CENP-E in patients with SSc.

METHODS

Sixty sera from patients with the SSc spectrum of diseases were screened for the presence of autoantibodies against CENP-E, by indirect immunofluorescence and immunoblotting using recombinant CENP-E protein. HLA class II alleles were determined by DNA oligotyping.

RESULTS

Among the SSc sera, 15 of 60 (25%) demonstrated antibody reactivity with recombinant CENP-E, and 14 of these 15 sera (93%) had antibodies directed against another CENP. Anti-CENP-E was seen in 13 of 30 sera with anti-CENP (43%). All patients with anti-CENP-E had a limited form of SSc, known as the CREST variant (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasias). When patients with anti-CENPs A, B, or C were compared with patients with anti-CENP-E, no unique clinical features in the anti-CENP-E positive group were identified. Ninety-three percent of the patients with anti-CENP-E had HLA-DQB1 alleles that had polar amino acids at position 26 (primarily DQB1*05), similar to patients with other CENP autoantibodies.

CONCLUSION

Antibodies to CENP-E are common in patients with SSc, and are seen in higher frequency in sera from patients with a limited form, or CREST variant, of the disease.

Identification of overlapping DNA-binding and centromere-targeting domains in the human kinetochore protein CENP-C

The kinetochore in eukaryotes serves as the chromosomal site of attachment for microtubules of the mitotic spindle and directs the movements necessary for proper chromosome segregation. In mammalian cells, the kinetochore is a highly differentiated trilaminar structure situated at the surface of the centromeric heterochromatin. CENP-C is a basic, DNA-binding protein that localizes to the inner kinetochore plate, the region that abuts the heterochromatin. Microinjection experiments using antibodies specific for CENP-C have demonstrated that this protein is required for the assembly and/or stability of the kinetochore as well as for a timely transition through mitosis. From these observations, it has been suggested that CENP-C is a structural protein that is involved in the organization or the kinetochore. In this report, we wished to identify and map the functional domains of CENP-C. Analysis of CENP-C truncation mutants expressed in vivo demonstrated that CENP-C possesses an autonomous centromere-targeting domain situated at the central region of the CENP-C polypeptide. Similarly, in vitro assays revealed that a region of CENP-C with the ability to bind DNA is also located at the center of the CENP-C molecule, where it overlaps the centromere-targeting domain.

Molecular cloning of an intronless gene for the hamster centromere antigen CENP-B

Centromere protein B (CENP-B) is a DNA-binding protein present at both active and inactive centromeres. It was first localized at the kinetochore region by human autoimmune sera from CREST patients. Using a previously identified human cDNA we have isolated a genomic clone containing the complete hamster CENP-b intronless coding sequence. At the nucleotide level it was found to possess a high degree of homology with the human and mouse CENP-B genes, being 75% and 90% respectively. This codes for 606 amino acid residues, which represent seven more than the human and mouse centromeric proteins. Hamster CENP-B protein analysis revealed at the N-terminal region a 133 amino acid fragment of 100% homology to the DNA binding motif identified previously for the human autoantigen. Expression of hamster CENP-B during the cell cycle was analyzed by using a specific antiCENP-B serum generated against the C-terminal conserved region. These data indicate that CENP-B is highly conserved and it represents a universal component of the centromere structure and function in mammals.

Kinetochore function: molecular motors, switches and gates

Kinetochores are essential for accurate chromosome segregation. Recent studies reveal that vertebrate kinetochores are sophisticated propulsion systems composed of not only force generators but also "navigation' and "fail-safe' mechanisms. Advances toward the understanding of the biochemical composition and activities of the components of the kinetochore have come from the molecular characterization of key proteins of the kinetochore complex.

Disruption of CENP antigen function perturbs dynein anchoring to the mitotic kinetochore

Injection of purified autoantibodies against human centromeric proteins into HeLa cells during interphase disrupts the organization of the kinetochore and interferes with chromosomal movements during the subsequent mitosis even though the chromosomes retain the ability to bind microtubules. We have investigated the hypothesis that this phenotype arises from effects on cytoplasmic dynein, the microtubule motor protein. In previous experiments we found that introduction of anticentromere antibodies into cell nuclei during the G1- or S-phases causes a prometaphase-like arrest, while injections during G2-phase cause a metaphase arrest. We show here that, in both cases, the level of detectable cytoplasmic dynein at kinetochores is significantly decreased. In contrast, when injected cells were permitted to enter mitosis in the absence of microtubules (conditions where trilaminar kinetochores could be detected by electron microscopy), the intensity of dynein labeling on the kinetochores was identical to that seen in uninjected control cells exposed to colcemid. Therefore, the loss of dynein label on mitotic kinetochores was correlated both with the injection of anticentromere antibodies and with the presence of intact spindle microtubules. We suggest that the injection of anticentromere antibodies somehow weakens the association of dynein with the kinetochore, so that when microtubules are present, these motor molecules are pulled away from the kinetochores as they generate force. This model offers an explanation for the failure of chromosomes of injected cells to move normally in mitosis even though they have attached microtubules.

The kinesin-like protein CENP-E is kinetochore-associated throughout poleward chromosome segregation during anaphase-A

The kinesin-like protein CENP-E transiently associates with kinetochores following nuclear envelope breakdown in late prophase, remains bound throughout metaphase, but sometime after anaphase onset it releases and by telophase becomes bound to interzonal microtubules of the mitotic spindle. Inhibition of poleward chromosome movement in vitro by CENP-E antibodies and association of CENP-E with minus-end directed microtubule motility in vitro have combined to suggest a key role for CENP-E as an anaphase chromosome motor. For this to be plausible in vivo depends on whether CENP-E remains kinetochore associated during anaphase. Using Indian muntjac cells whose seven chromosomes have large, easily tracked kinetochores, we now show that CENP-E is kinetochore-associated throughout the entirety of anaphase-A (poleward chromosome movement), relocating gradually during spindle elongation (anaphase-B) to the interzonal microtubules. These observations support roles for CENP-E not only in the initial alignment of chromosomes at metaphase and in spindle elongation in anaphase-B, but also in poleward chromosome movement in anaphase-A.

Pds1p is required for faithful execution of anaphase in the yeast, Saccharomyces cerevisiae

To identify mutations that cause defects in mitosis, a collection of mutants in Saccharomyces cerevisiae was screened by a rapid visual assay for abnormal chromosome segregation. From this screen we identified one mutation, pds1-1 that was independently identified in an alternative screen for mutants that exhibit inviability after transient exposure to nocodazole and precocious disassociation of sister chromatids (Guacci, V., A. Yamamoto, A. Strunnikov, J. Kingsbury, E. Hogan, P. Meluh, and D. Koshland. 1993. CSH Symp. Quant. Biol. 58:677-685; Yamamoto, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nature (Lond.). 359:536-539). At 23 degrees C pds1-1 mutants exhibit frequent cell death and a 300-fold increase in chromosome loss compared to wild type. At 37 degrees C pds1-1 cells fail to elongate their spindles during anaphase. This spindle defect of pds1 mutants results from a temperature-sensitive step that occurs around the G1/S boundary about the time of spindle assembly. In the absence of spindle elongation pds1 mutants undergo cytokinesis, leading to the missegregation of both chromosomes and spindle pole bodies. After abnormal cell division pds1-1 mutants also initiate new rounds of DNA replication, spindle pole body duplication, and bud formation. Thus, in the pds1-1 mutant at 37 degrees C, cell cycle progression is uncoupled from the completion of anaphase. A pds1 deletion allele has similar phenotypes to the original allele. Taken together these results suggest that Pds1 protein plays an important role in chromosome segregation at 23 degrees C and an essential role for this process at 37 degrees C. The PDS1 gene encodes a novel 42-kD nuclear protein that has both basic and acidic domains. The level of PDS1 mRNA varies with the cell cycle with maximal accumulation around the G1/S boundary. The stability of Pds1 protein also appears to change during the cell cycle as overproduced Pds1p is stable in S and M but degraded in early G1. Therefore, expression of Pds1p is regulated apparently both transcriptionally and postranslationally during the cell cycle. The phenotypes of pds1 mutants and expression pattern of Pds1p are discussed in the context of other spindle-defective mutants and the knowledge that Pds1 protein is an inhibitor of anaphase (Yamamoto, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nature (Lond.). 359:536-539).

Sequencing and analysis of 51 kb on the right arm of chromosome XV from Saccharomyces cerevisiae reveals 30 open reading frames

We have sequenced a region of 51 kb of the right arm from chromosome XV of Saccharomyces cerevisiae. The sequence contains 30 open reading frames (ORFs) of more than 100 amino acid residues. Thirteen new genes have been identified. Thirteen ORFs correspond to known yeast genes. One delta element and one tRNA gene were identified. Upstream of the RPO31 gene, encoding the largest subunit of RNA polymerase III, lies a Abf1p binding site. The nucleotide sequence data reported in this paper are available in the EMBL, GenBank and DDBJ nucleotide sequence databases under the Accession Number X90518.

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.

Kinesin proteins: a phylum of motors for microtubule-based motility

The cellular processes of transport, division and, possibly, early development all involve microtubule-based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin-related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule-motor proteins that share a 340-amino-acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles.

Molecular characterization of the 50-kD subunit of dynactin reveals function for the complex in chromosome alignment and spindle organization during mitosis

Dynactin is a multi-subunit complex which has been implicated in cytoplasmic dynein function, though its mechanism of action is unknown. In this study, we have characterized the 50-kD subunit of dynactin, and analyzed the effects of its overexpression on mitosis in living cells. Rat and human cDNA clones revealed p50 to be novel and highly conserved, containing three predicted coiled-coil domains. Immunofluorescence staining of dynactin and cytoplasmic dynein components in cultured vertebrate cells showed that both complexes are recruited to kinetochores during prometaphase, and concentrate near spindle poles thereafter. Overexpression of p50 in COS-7 cells disrupted mitosis, causing cells to accumulate in a prometaphase-like state. Chromosomes were condensed but unaligned, and spindles, while still bipolar, were dramatically distorted. Sedimentation analysis revealed the dynactin complex to be dissociated in the transfected cultures. Furthermore, both dynactin and cytoplasmic dynein staining at prometaphase kinetochores was markedly diminished in cells expressing high levels of p50. These findings represent clear evidence for dynactin and cytoplasmic dynein codistribution within cells, and for the presence of dynactin at kinetochores. The data also provide direct in vivo evidence for a role for vertebrate dynactin in modulating cytoplasmic dynein binding to an organelle, and implicate both dynactin and dynein in chromosome alignment and spindle organization.