Cytoplasmic dynein is localized to kinetochores during mitosis

The role of cytoplasmic dynein in the human brain developmental disease lissencephaly

Lissencephaly is a brain developmental disorder characterized by disorganization of the cortical regions resulting from defects in neuronal migration. Recent evidence has implicated the human LIS-1 gene in Miller-Dieker lissencephaly and isolated lissencephaly sequence. LIS-1 is homologous to the fungal genes NudF and PAC1, which are involved in cytoplasmic dynein mediated nuclear transport, but it is also almost identical to a subunit of PAF acetylhydrolase, an enzyme which inactivates the lipid mediator platelet activating factor. Recent evidence from our laboratory has revealed that cytoplasmic dynein coimmunoprecipitates with LIS-1 in bovine brain cytosol, supporting a role in the dynein pathway in vertebrates. Overexpression of LIS-1 interferes with cell division, with noteworthy effects on chromosome attachment to the mitotic spindle and on the interaction of astral microtubules with the cell cortex. Other aspects of dynein function, such as the organization of the Golgi apparatus, are not affected. Together, these results suggest a role for LIS-1 in cytoplasmic dynein functions involving microtubule plus-ends. Furthermore, they suggest that mutations in LIS-1 may produce a lissencephalic phenotype either by interfering with the movement of neuronal nuclei within extending processes, or by interference with the division cycle of neuronal progenitor cells in the ventricular and subventricular zones of the developing nervous system.

Distinct cytoplasmic dynein complexes are transported by different mechanisms in axons

In neurons, cytoplasmic dynein is synthesized in the cell body, but its function is to move cargo from the axon back to the cell body. Dynein must therefore be delivered to the axon and its motor activity must be regulated during axonal transport. Cytoplasmic dynein is a large protein complex composed of a number of different subunits. The dynein heavy chains contain the motor domains and the intermediate chains are involved in binding the complex to cargo. Five different intermediate chain polypeptides, which are the result of the alternative splicing of the two intermediate chain genes, have been identified. We have characterized two distinct pools of dynein that are transported from the cell body along the axon by different mechanisms. One pool, which contains the ubiquitous intermediate chain, is associated with the membranous organelles transported by kinesin in the fast transport component. The other pool, which contains the other developmentally regulated intermediate chains, is transported in slow component b. The mechanism of dynein regulation will therefore depend on which pool of dynein is recruited to function as the retrograde motor. In addition, the properties of the large pool of dynein associated with actin in slow component b are consistent with the hypothesis that this dynein may be the motor for microtubule transport in the axon.

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

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

Actin-related proteins

Actin-related proteins (Arps) participate in a diverse array of cellular processes. They modulate assembly of conventional actin, contribute to microtubule-based motility catalyzed by dynein, and serve as integral components of large protein complexes required for gene expression. We highlight here recent work aimed at understanding the roles played by Arps in each of these processes.

Dynamic association of cytoplasmic dynein heavy chain 1a with the Golgi apparatus and intermediate compartment

Microtubule motors, such as the minus end-directed motor, cytoplasmic dynein, play an important role in maintaining the integrity, intracellular location, and function of the Golgi apparatus, as well as in the translocation of membrane between the endoplasmic reticulum and Golgi apparatus. We have immunolocalised conventional cytoplasmic dynein heavy chain to the Golgi apparatus in cultured vertebrate cells. In addition, we present evidence that cytoplasmic dynein heavy chain cycles constitutively between the endoplasmic reticulum and Golgi apparatus: it colocalises partially with the intermediate compartment, it is found on nocodazole-induced peripheral Golgi elements and, most strikingly, on Brefeldin A-induced tubules that are moving towards microtubule plus ends. The direction of movement of membrane between the endoplasmic reticulum and Golgi apparatus is therefore unlikely to be regulated by controlling motor-membrane interactions: rather, the motors probably remain bound throughout the whole cycle, with their activity being modulated instead. We also report that the overexpression of p50/dynamitin results in the loss of cytoplasmic dynein heavy chain from the membrane of peripheral Golgi elements. These results explain how dynamitin overexpression causes the inhibition of endoplasmic reticulum-to-Golgi transport complex movement towards the centrosomal region, and support the general model that an intact dynactin complex is required for cytoplasmic dynein binding to all cargoes.

The rough deal protein is a new kinetochore component required for accurate chromosome segregation in Drosophila

Mutations in the rough deal (rod) gene of Drosophila greatly increase the missegregation of sister chromatids during mitosis, suggesting a role for this gene product in spindle or kinetochore function. The activity provided by rod also appears to be necessary for the recruitment of two known kinetochore components, Zw10 and cytoplasmic dynein. In this paper we describe the cloning of rough deal and an initial cytological characterization of its product. The Rod protein shares no identifiable structural motif with other known proteins, although apparent homologs exist in the genomes of nematode and man. By immunocytochemistry we show that Rod displays a dynamic intracellular staining pattern, localizing first to kinetochores in prometaphase, but moving to kinetochore microtubules at metaphase. Early in anaphase the protein is once again restricted to the kinetochores, where it persists until the end of telophase. This behavior is in all respects similar to that described for Zw10, and suggests that the proteins function together.

Direct interaction of pericentrin with cytoplasmic dynein light intermediate chain contributes to mitotic spindle organization

Pericentrin is a conserved protein of the centrosome involved in microtubule organization. To better understand pericentrin function, we overexpressed the protein in somatic cells and assayed for changes in the composition and function of mitotic spindles and spindle poles. Spindles in pericentrin-overexpressing cells were disorganized and mispositioned, and chromosomes were misaligned and missegregated during cell division, giving rise to aneuploid cells. We unexpectedly found that levels of the molecular motor cytoplasmic dynein were dramatically reduced at spindle poles. Cytoplasmic dynein was diminished at kinetochores also, and the dynein-mediated organization of the Golgi complex was disrupted. Dynein coimmunoprecipitated with overexpressed pericentrin, suggesting that the motor was sequestered in the cytoplasm and was prevented from associating with its cellular targets. Immunoprecipitation of endogenous pericentrin also pulled down cytoplasmic dynein in untransfected cells. To define the basis for this interaction, pericentrin was coexpressed with cytoplasmic dynein heavy (DHCs), intermediate (DICs), and light intermediate (LICs) chains, and the dynamitin and p150(Glued) subunits of dynactin. Only the LICs coimmunoprecipitated with pericentrin. These results provide the first physiological role for LIC, and they suggest that a pericentrin-dynein interaction in vivo contributes to the assembly, organization, and function of centrosomes and mitotic spindles.

Cytoplasmic dynein is required for distinct aspects of MTOC positioning, including centrosome separation, in the one cell stage Caenorhabditis elegans embryo

We have investigated the role of cytoplasmic dynein in microtubule organizing center (MTOC) positioning using RNA-mediated interference (RNAi) in Caenorhabditis elegans to deplete the product of the dynein heavy chain gene dhc-1. Analysis with time-lapse differential interference contrast microscopy and indirect immunofluorescence revealed that pronuclear migration and centrosome separation failed in one cell stage dhc-1 (RNAi) embryos. These phenotypes were also observed when the dynactin components p50/dynamitin or p150(Glued) were depleted with RNAi. Moreover, in 15% of dhc-1 (RNAi) embryos, centrosomes failed to remain in proximity of the male pronucleus. When dynein heavy chain function was diminished only partially with RNAi, centrosome separation took place, but orientation of the mitotic spindle was defective. Therefore, cytoplasmic dynein is required for multiple aspects of MTOC positioning in the one cell stage C. elegans embryo. In conjunction with our observation of cytoplasmic dynein distribution at the periphery of nuclei, these results lead us to propose a mechanism in which cytoplasmic dynein anchored on the nucleus drives centrosome separation.

The kinetochore of higher eucaryotes: a molecular view

This review summarizes results concerning the molecular nature of the higher eucaryotic kinetochore. The first major section of this review includes kinetochore proteins whose general functions remain to be determined, precluding their entry into a discrete functional category. Many of the proteins in this section, however, are likely to be involved in kinetochore formation or structure. The second major section is concerned with how microtubule motor proteins function to cause chromosome movement. The microtubule motors dynein, CENP-E, and MCAK have all been observed at the kinetochore. While their precise functions are not well understood, all three are implicated in chromosome movement during mitosis. Finally, the last section deals with kinetochore components that play a role in the spindle checkpoint; a checkpoint that delays mitosis until all kinetochores have attached to the mitotic spindle. Brief reviews of kinetochore morphology and of an important technical breakthrough that enabled the molecular dissection of the kinetochore are also included.

Cytoplasmic dynein is required for the nuclear attachment and migration of centrosomes during mitosis in Drosophila

Cytoplasmic dynein is a multisubunit minus-end-directed microtubule motor that serves multiple cellular functions. Genetic studies in Drosophila and mouse have demonstrated that dynein function is essential in metazoan organisms. However, whether the essential function of dynein reflects a mitotic requirement, and what specific mitotic tasks require dynein remains controversial. Drosophila is an excellent genetic system in which to analyze dynein function in mitosis, providing excellent cytology in embryonic and somatic cells. We have used previously characterized recessive lethal mutations in the dynein heavy chain gene, Dhc64C, to reveal the contributions of the dynein motor to mitotic centrosome behavior in the syncytial embryo. Embryos lacking wild-type cytoplasmic dynein heavy chain were analyzed by in vivo analysis of rhodamine-labeled microtubules, as well as by immunofluorescence in situ methods. Comparisons between wild-type and Dhc64C mutant embryos reveal that dynein function is required for the attachment and migration of centrosomes along the nuclear envelope during interphase/prophase, and to maintain the attachment of centrosomes to mitotic spindle poles. The disruption of these centrosome attachments in mutant embryos reveals a critical role for dynein function and centrosome positioning in the spatial organization of the syncytial cytoplasm of the developing embryo.

M phase phosphorylation of cytoplasmic dynein intermediate chain and p150(Glued)

To understand how the dramatic cell biological changes of oocyte maturation are brought about, we have begun to identify proteins whose phosphorylation state changes during Xenopus oocyte maturation. Here we have focused on one such protein, p83. We partially purified p83, obtained peptide sequence, and identified it as the intermediate chain of cytoplasmic dynein. During oocyte maturation, dynein intermediate chain became hyperphosphorylated at the time of germinal vesicle breakdown and remained hyperphosphorylated throughout the rest of meiosis and early embryogenesis. p150(Glued), a subunit of dynactin that has been shown to bind to dynein intermediate chain, underwent similar changes in its phosphorylation. Both dynein intermediate chain and p150(Glued) also became hyperphosphorylated during M phase in XTC-2 cells and HeLa cells. Thus, two components of the dynein-dynactin complex undergo coordinated phosphorylation changes at two G2/M transitions (maturation in oocytes and mitosis in cells in culture) but remain constitutively in their M phase forms during early embryogenesis. Dynein intermediate chain and p150(Glued) phosphorylation may positively regulate mitotic processes, such as spindle assembly or orientation, or negatively regulate interphase processes such as minus-end-directed organelle trafficking.

Dynein-dynactin function and sensory axon growth during Drosophila metamorphosis: A role for retrograde motors

Mutations in the genes for components of the dynein-dynactin complex disrupt axon path finding and synaptogenesis during metamorphosis in the Drosophila central nervous system. In order to better understand the functions of this retrograde motor in nervous system assembly, we analyzed the path finding and arborization of sensory axons during metamorphosis in wild-type and mutant backgrounds. In wild-type specimens the sensory axons first reach the CNS 6-12 h after puparium formation and elaborate their terminal arborizations over the next 48 h. In Glued1 and Cytoplasmic dynein light chain mutants, proprioceptive and tactile axons arrive at the CNS on time but exhibit defects in terminal arborizations that increase in severity up to 48 h after puparium formation. The results show that axon growth occurs on schedule in these mutants but the final process of terminal branching, synaptogenesis, and stabilization of these sensory axons requires the dynein-dynactin complex. Since this complex functions as a retrograde motor, we suggest that a retrograde signal needs to be transported to the nucleus for the proper termination of some sensory neurons.

Dynein is required for spindle assembly in cytoplasmic extracts of Spisula solidissima oocytes

Meiosis I spindle assembly is induced in lysate-extract mixtures prepared from clam (Spisula solidissima) oocytes. Unactivated lysate prepared from unactivated oocytes contain nuclei (germinal vesicles, GVs) which house condensed chromosomes. Treatment of unactivated lysate with clarified activated extract prepared from oocytes induced to complete meiosis by treatment with KCl induces GV breakdown (GVBD) and assembly of monopolar, bipolar, and multipolar aster-chromosome complexes. The process of in vitro meiosis I spindle assembly involves the assembly of microtubule asters and the association of these asters with the surfaces of the GVs, followed by GVBD and spindle assembly. Monoclonal antibody m74-1, known to react specifically with the N terminus of the intermediate chain of cytoplasmic dynein, recognizes Spisula oocyte dynein and inhibits in vitro meiosis I spindle assembly. Control antibody has no affect on spindle assembly. A similar inhibitory effect on spindle assembly was observed in the presence of orthovanadate, a known inhibitor of dynein ATPase activity. Neither m74-1 nor orthovanadate has any obvious affect on GVBD or aster formation. We propose that dynein function is required for the association of chromosomes with astral microtubules during in vitro meiosis I spindle assembly in these lysate-extract mixtures. However, we conclude that dynein function is not required for centrosome assembly and maturation or for centrosome-dependent aster formation.

Microtubule motors in spindle and chromosome motility

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

A nonerythroid isoform of protein 4.1R interacts with the nuclear mitotic apparatus (NuMA) protein

Red blood cell protein 4.1 (4.1R) is an 80- kD erythrocyte phosphoprotein that stabilizes the spectrin/actin cytoskeleton. In nonerythroid cells, multiple 4.1R isoforms arise from a single gene by alternative splicing and predominantly code for a 135-kD isoform. This isoform contains a 209 amino acid extension at its NH2 terminus (head piece; HP). Immunoreactive epitopes specific for HP have been detected within the cell nucleus, nuclear matrix, centrosomes, and parts of the mitotic apparatus in dividing cells. Using a yeast two-hybrid system, in vitro binding assays, coimmunolocalization, and coimmunoprecipitation studies, we show that a 135-kD 4.1R isoform specifically interacts with the nuclear mitotic apparatus (NuMA) protein. NuMA and 4.1R partially colocalize in the interphase nucleus of MDCK cells and redistribute to the spindle poles early in mitosis. Protein 4.1R associates with NuMA in the interphase nucleus and forms a complex with spindle pole organizing proteins, NuMA, dynein, and dynactin during cell division. Overexpression of a 135-kD isoform of 4.1R alters the normal distribution of NuMA in the interphase nucleus. The minimal sequence sufficient for this interaction has been mapped to the amino acids encoded by exons 20 and 21 of 4.1R and residues 1788-1810 of NuMA. Our results not only suggest that 4.1R could, possibly, play an important role in organizing the nuclear architecture, mitotic spindle, and spindle poles, but also could define a novel role for its 22-24-kD domain.

Centromere proteins and chromosome inheritance: a complex affair

Centromeres and the associated kinetochores are involved in essential aspects of chromosome transmission. Recent advances have included the identification and understanding of proteins that have a pivotal role in centromere structure, kinetochore formation, and the coordination of chromosome inheritance with the cell cycle in several organisms. A picture is beginning to emerge of the centromere-kinetechore as a complex and dynamic structure with conservation of function at the protein level across diverse species.

Gene knockouts reveal separate functions for two cytoplasmic dyneins in Tetrahymena thermophila

In many organisms, there are multiple isoforms of cytoplasmic dynein heavy chains, and division of labor among the isoforms would provide a mechanism to regulate dynein function. The targeted disruption of somatic genes in Tetrahymena thermophila presents the opportunity to determine the contributions of individual dynein isoforms in a single cell that expresses multiple dynein heavy chain genes. Substantial portions of two Tetrahymena cytoplasmic dynein heavy chain genes were cloned, and their motor domains were sequenced. Tetrahymena DYH1 encodes the ubiquitous cytoplasmic dynein Dyh1, and DYH2 encodes a second cytoplasmic dynein isoform, Dyh2. The disruption of DYH1, but not DYH2, resulted in cells with two detectable defects: 1) phagocytic activity was inhibited, and 2) the cells failed to distribute their chromosomes correctly during micronuclear mitosis. In contrast, the disruption of DYH2 resulted in a loss of regulation of cell size and cell shape and in the apparent inability of the cells to repair their cortical cytoskeletons. We conclude that the two dyneins perform separate tasks in Tetrahymena.

Cytoplasmic dynein and dynactin in cell division and intracellular transport

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

Mammalian centromeres: DNA sequence, protein composition, and role in cell cycle progression

The centromere is a specialized region of the eukaryotic chromosome that is responsible for directing chromosome movements in mitosis and for coordinating the progression of mitotic events at the crucial transition between metaphase and anaphase. In this review, we will focus on recent advances in the understanding of centromere composition at the protein and DNA level and of the role of centromeres in sister-chromatid cohesion and mitotic checkpoint control.

Characterization of the kinetochore binding domain of CENP-E reveals interactions with the kinetochore proteins CENP-F and hBUBR1

We have identified a 350-amino acid domain in the kinetochore motor CENP-E that specifies kinetochore binding in mitosis but not during interphase. The kinetochore binding domain was used in a yeast two-hybrid screen to isolate interacting proteins that included the kinetochore proteins CENP-E, CENP-F, and hBUBR1, a BUB1-related kinase that was found to be mutated in some colorectal carcinomas (Cahill, D.P., C. Lengauer, J. Yu, G.J. Riggins, J.K. Wilson, S.D. Markowitz, K.W. Kinzler, and B. Vogelstein. 1998. Nature. 392:300-303). CENP-F, hBUBR1, and CENP-E assembled onto kinetochores in sequential order during late stages of the cell cycle. These proteins therefore define discrete steps along the kinetochore assembly pathway. Kinetochores of unaligned chromosome exhibited stronger hBUBR1 and CENP-E staining than those of aligned chromosomes. CENP-E and hBUBR1 remain colocalized at kinetochores until mid-anaphase when hBUBR1 localized to portions of the spindle midzone that did not overlap with CENP-E. As CENP-E and hBUBR1 can coimmunoprecipitate with each other from HeLa cells, they may function as a motor-kinase complex at kinetochores. However, the complex distribution pattern of hBUBR1 suggests that it may regulate multiple functions that include the kinetochore and the spindle midzone.

A cytoplasmic dynein required for mitotic aster formation in vivo

An astral pulling force helps to elongate the mitotic spindle in the filamentous ascomycete, Nectria haematococca. Evidence is mounting that dynein is required for the formation of mitotic spindles and asters. Obviously, this would be an important mitotic function of dynein, since it would be a prerequisite for astral force to be applied to a spindle pole. Missing from the evidence for such a role of dynein in aster formation, however, has been a dynein mutant lacking mitotic asters. To determine whether or not cytoplasmic dynein is involved in mitotic aster formation in N. haematococca, a dynein-deficient mutant was made. Immunocytochemistry visualized few or no mitotic astral microtubules in the mutant cells, and studies of living cells confirmed the veracity of this result by revealing the absence of mitotic aster functions in vivo: intra-astral motility of membranous organelles was not apparent; the rate and extent of spindle elongation during anaphase B were reduced; and spindle pole body separation almost stopped when the anaphase B spindle in the mutant was cut by a laser microbeam, demonstrating unequivocally that no astral pulling force was present. These unique results not only provide a demonstration that cytoplasmic dynein is required for the formation of mitotic asters in N. haematococca; they also represent the first report of mitotic phenotypes in a dynein mutant of any filamentous fungus and the first cytoplasmic dynein mutant of any organism whose mitotic phenotypes demonstrate the requirement of cytoplasmic dynein for aster formation in vivo.

ZW10 helps recruit dynactin and dynein to the kinetochore

Mutations in the Drosophila melanogaster zw10 gene, which encodes a conserved, essential kinetochore component, abolish the ability of dynein to localize to kinetochores. Several similarities between the behavior of ZW10 protein and dynein further support a role for ZW10 in the recruitment of dynein to the kinetochore: (a) in response to bipolar tension across the chromosomes, both proteins mostly leave the kinetochore at metaphase, when their association with the spindle becomes apparent; (b) ZW10 and dynein both bind to functional neocentromeres of structurally acentric minichromosomes; and (c) the localization of both ZW10 and dynein to the kinetochore is abolished in cells mutant for the gene rough deal. ZW10's role in the recruitment of dynein to the kinetochore is likely to be reasonably direct, because dynamitin, the p50 subunit of the dynactin complex, interacts with ZW10 in a yeast two-hybrid screen. Since in zw10 mutants no defects in chromosome behavior are observed before anaphase onset, our results suggest that dynein at the kinetochore is essential for neither microtubule capture nor congression to the metaphase plate. Instead, dynein's role at the kinetochore is more likely to be involved in the coordination of chromosome separation and/or poleward movement at anaphase onset.

In vitro assembly of the CENP-B/alpha-satellite DNA/core histone complex: CENP-B causes nucleosome positioning

BACKGROUND

We have studied the nucleosome structure formed from alpha-satellite DNA bound with CENP-B and core histones, in order to develop a previous proposal that the CENP-B dimer may play a critical role in the assembly of higher order structures of the human centromere by juxtaposing CENP-B boxes in long alpha-satellite arrays.

RESULTS

The dimeric structure of CENP-B was sufficiently stable to bundle together two 3.5 kbp DNA fragments when each DNA contained a CENP-B box. When the same length of DNA included two CENP-B boxes, the intra-molecular interaction with the CENP-B dimer predominated, resulting in the formation of loop structures. The in vitro assembly of CENP-B/alpha-satellite DNA/core histone complexes with the aid of nucleosome assembly protein-1 (NAP-1) permitted an investigation into the nucleosome arrangement in alpha-satellite DNA with CENP-B bound to CENP-B boxes. Footprint analyses with micrococcal nuclease (MNase) revealed that CENP-B causes nucleosome positioning between pairs of CENP-B boxes with unique hypersensitive sites created on both sides.

CONCLUSION

We propose that CENP-B functions as a structural factor in the centromere region in order to establish a unique, centromere specific pattern of nucleosome positioning.

Localization of Tctex-1, a cytoplasmic dynein light chain, to the Golgi apparatus and evidence for dynein complex heterogeneity

To date, much attention has been focused on the heavy and intermediate chains of the multisubunit cytoplasmic dynein complex; however, little is known about the localization or function of dynein light chains. In this study, we find that Tctex-1, a light chain of cytoplasmic dynein, localizes predominantly to the Golgi apparatus in interphase fibroblasts. Immunofluorescent staining reveals striking juxtanuclear staining characteristic of the Golgi apparatus as well as nuclear envelope and punctate cytoplasmic staining that often decorates microtubules. Tctex-1 colocalization with Golgi compartment markers, its distribution upon treatment with various pharmacological agents, and the cofractionation of Tctex-1-associated membranes with Golgi membranes are all consistent with a Golgi localization. The distribution of Tctex-1 in interphase cells only partially overlaps with the dynein intermediate chain and p150(Glued) upon immunofluorescence, but most of Tctex-1 is redistributed onto mitotic spindles along with other dynein/dynactin subunits. Using sequential immunoprecipitations, we demonstrate that there is a subset of Tctex-1 not associated with the intermediate chain at steady state; the converse also appears to be true. Distinct populations of dynein complexes are likely to exist, and such diversity may occur in part at the level of their light chain compositions.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration

Cytoplasmic dynein is a microtubule-associated motor protein with several putative subcellular functions. Sequencing of the gene (DHC1) for cytoplasmic dynein heavy chain of the filamentous ascomycete, Nectria haematococca, revealed a 4,349-codon open reading frame (interrupted by two introns) with four highly conserved P-loop motifs, typical of cytoplasmic dynein heavy chains. The predicted amino acid sequence is 78.0% identical to the cytoplasmic dynein heavy chain of Neurospora crassa, 70.2% identical to that of Aspergillus nidulans and 24.8% identical to that of Saccharomyces cerevisiae. The genomic copy of DHC1 in N. haematococca wild-type strain T213 was disrupted by inserting a selectable marker into the central motor domain. Mutants grew at 33% of the wild-type rate, forming dense compact colonies composed of spiral and highly branched hyphae. Major cytological phenotypes included (1) absence of aster-like arrays of cytoplasmic microtubules focused at the spindle pole bodies of post-mitotic and interphase nuclei, (2) limited post-mitotic nuclear migration, (3) lack of spindle pole body motility at interphase, (4) failure of spindle pole bodies to anchor interphase nuclei, (5) nonuniform distribution of interphase nuclei and (6) small or ephemeral Spitzenkorper at the apices of hyphal tip cells. Microtubule distribution in the apical region of tip cells of the mutant was essentially normal. The nonuniform distribution of nuclei in hyphae resulted primarily from a lack of both post-mitotic nuclear migration and anchoring of interphase nuclei by the spindle pole bodies. The results support the hypothesis that DHC1 is required for the motility and functions of spindle pole bodies, normal secretory vesicle transport to the hyphal apex and normal hyphal tip cell morphogenesis.

Focusing on spindle poles

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

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.

Evidence for a role of CLIP-170 in the establishment of metaphase chromosome alignment

CLIPs (cytoplasmic linker proteins) are a class of proteins believed to mediate the initial, static interaction of organelles with microtubules. CLIP-170, the CLIP best characterized to date, is required for in vitro binding of endocytic transport vesicles to microtubules. We report here that CLIP-170 transiently associates with prometaphase chromosome kinetochores and codistributes with dynein and dynactin at kinetochores, but not polar regions, during mitosis. Like dynein and dynactin, a fraction of the total CLIP-170 pool can be detected on kinetochores of unattached chromosomes but not on those that have become aligned at the metaphase plate. The COOH-terminal domain of CLIP-170, when transiently overexpressed, localizes to kinetochores and causes endogenous full-length CLIP-170 to be lost from the kinetochores, resulting in a delay in prometaphase. Overexpression of the dynactin subunit, dynamitin, strongly reduces the amount of CLIP-170 at kinetochores suggesting that CLIP-170 targeting may involve the dynein/dynactin complex. Thus, CLIP-170 may be a linker for cargo in mitosis as well as interphase. However, dynein and dynactin staining at kinetochores are unaffected by this treatment and further overexpression studies indicate that neither CLIP-170 nor dynein and dynactin are required for the formation of kinetochore fibers. Nevertheless, these results strongly suggest that CLIP-170 contributes in some way to kinetochore function in vivo.

A screen for dynein synthetic lethals in Aspergillus nidulans identifies spindle assembly checkpoint genes and other genes involved in mitosis

Cytoplasmic dynein is a ubiquitously expressed microtubule motor involved in vesicle transport, mitosis, nuclear migration, and spindle orientation. In the filamentous fungus Aspergillus nidulans, inactivation of cytoplasmic dynein, although not lethal, severely impairs nuclear migration. The role of dynein in mitosis and vesicle transport in this organism is unclear. To investigate the complete range of dynein function in A. nidulans, we searched for synthetic lethal mutations that significantly reduced growth in the absence of dynein but had little effect on their own. We isolated 19 sld (synthetic lethality without dynein) mutations in nine different genes. Mutations in two genes exacerbate the nuclear migration defect seen in the absence of dynein. Mutations in six other genes, including sldA and sldB, show a strong synthetic lethal interaction with a mutation in the mitotic kinesin bimC and, thus, are likely to play a role in mitosis. Mutations in sldA and sldB also confer hypersensitivity to the microtubule-destabilizing drug benomyl. sldA and sldB were cloned by complementation of their mutant phenotypes using an A. nidulans autonomously replicating vector. Sequencing revealed homology to the spindle assembly checkpoint genes BUB1 and BUB3 from Saccharomyces cerevisiae. Genetic interaction between dynein and spindle assembly checkpoint genes, as well as other mitotic genes, indicates that A. nidulans dynein plays a role in mitosis. We suggest a model for dynein motor action in A. nidulans that can explain dynein involvement in both mitosis and nuclear distribution.

Cell cycle regulation of organelle transport

Microtubule- and actin-based motors play a wide range of vital roles in the organisation and function of cells during both interphase and mitosis, all of which are likely to be under strict control. Here, we describe how one of these roles--the movement of membranes--is regulated through the cell cycle. Organelle movement in many species is greatly reduced in mitosis as compared to interphase, and this change occurs concomitantly with an inhibition of most membrane traffic functions. Data from in vitro studies is shedding light on how microtubule motor regulation may be achieved.

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.

Golgi vesiculation and lysosome dispersion in cells lacking cytoplasmic dynein

Cytoplasmic dynein, a minus end-directed, microtubule-based motor protein, is thought to drive the movement of membranous organelles and chromosomes. It is a massive complex that consists of multiple polypeptides. Among these polypeptides, the cytoplasmic dynein heavy chain (cDHC) constitutes the major part of this complex. To elucidate the function of cytoplasmic dynein, we have produced mice lacking cDHC by gene targeting. cDHC-/- embryos were indistinguishable from cDHC+/-or cDHC+/+ littermates at the blastocyst stage. However, no cDHC-/- embryos were found at 8.5 d postcoitum. When cDHC-/- blastocysts were cultured in vitro, they showed interesting phenotypes. First, the Golgi complex became highly vesiculated and distributed throughout the cytoplasm. Second, endosomes and lysosomes were not concentrated near the nucleus but were distributed evenly throughout the cytoplasm. Interestingly, the Golgi "fragments" and lysosomes were still found to be attached to microtubules. These results show that cDHC is essential for the formation and positioning of the Golgi complex. Moreover, cDHC is required for cell proliferation and proper distribution of endosomes and lysosomes. However, molecules other than cDHC might mediate attachment of the Golgi complex and endosomes/lysosomes to microtubules.

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.

Interaction of huntingtin-associated protein with dynactin P150Glued

Huntingtin is the protein product of the gene for Huntington's disease (HD) and carries a polyglutamine repeat that is expanded in HD (>36 units). Huntingtin-associated protein (HAP1) is a neuronal protein and binds to huntingtin in association with the polyglutamine repeat. Like huntingtin, HAP1 has been found to be a cytoplasmic protein associated with membranous organelles, suggesting the existence of a protein complex including HAP1, huntingtin, and other proteins. Using the yeast two-hybrid system, we found that HAP1 also binds to dynactin P150(Glued) (P150), an accessory protein for cytoplasmic dynein that participates in microtubule-dependent retrograde transport of membranous organelles. An in vitro binding assay showed that both huntingtin and P150 selectively bound to a glutathione transferase (GST)-HAP1 fusion protein. An immunoprecipitation assay demonstrated that P150 and huntingtin coprecipitated with HAP1 from rat brain cytosol. Western blot analysis revealed that HAP1 was enriched in rat brain microtubules and comigrated with P150 and huntingtin in sucrose gradients. Immunofluorescence showed that transfected HAP1 colocalized with P150 and huntingtin in human embryonic kidney (HEK) 293 cells. We propose that HAP1, P150, and huntingtin are present in a protein complex that may participate in dynein-dynactin-associated intracellular transport.

Evidence for four cytoplasmic dynein heavy chain isoforms in rat testis

Recent studies have revealed the expression of multiple putative cytoplasmic dynein heavy chain (DHC) genes in several organisms, with each gene encoding a separate protein isoform. This finding is consistent with the hypothesis that different isoforms do different things, as is the case for the axonemal dyneins. Furthermore, the large number of tasks ascribed to cytoplasmic dynein suggests that there may be additional isoforms not yet identified. Two of the mammalian cytoplasmic dynein heavy chains are DHC1a and DHC1b. DHC1a is conventional cytoplasmic dynein and is found in all organisms examined. DHC1b is expressed in organisms that have multiple dyneins, and has been implicated in the intracellular trafficking of molecules in unciliated and ciliated cells. In the present study, we examined the DHC1b protein from rat testis. Testis cytoplasmic dynein contains a large amount of dynein heavy chain reactive with an antibody raised against a peptide sequence of rat DHC1b. The testis anti-DHC1b immunoreactive protein is slightly smaller than testis DHC1a, as assessed by SDS-PAGE. In Northern blots, the DHC1b mRNA is smaller than the DHC1a mRNA. In sucrose gradients made in low ionic strength, DHC1a sedimented at approximately 20S, and the anti-1b immunoreactive heavy chains sedimented in a broad band centered at approximately 14S. The V1-photolysis reaction of individual sucrose gradient fractions revealed three distinct patterns of photolysis, suggesting that there are at least three separate 1b-like heavy chain isoforms in testis. Using a high-stringency Western blotting protocol, the anti-1b antibody and the anti-DHC2 antibody recognized the same heavy chain and specifically bound to one of the three 1b-like heavy chains. We conclude that rat testis contains three 1b-like dynein heavy chains, and one of these is the product of the DHC1b/DHC2 gene previously identified.

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.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

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.

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.

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

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

Displacement of gold marker in immunoelectron microscopy of human respiratory cilia

Preembedding immunogold electron microscopy was performed to evaluate the position of outer arm dynein heavy chains in normal human respiratory cilia. Anti-dynein antibody (AD2), which is specific for sea urchin sperm flagellar dynein heavy chains, was used as primary antibody. Direct cross-sections of cilia were selected, and the distance between the center of a cilium and the center of a colloidal gold particle attached to the cilium (X) was measured. The distance between the center of a cilium and the farthest edge of an outer dynein arm of the cilium was measured by ordinary electron microscopy (Yo) and by immunoelectron microscopy (Yi). X was significantly longer than Yo and Yi. If it is assumed that the structure of respiratory cilia is dense and that antibodies are located at the outer side of the actual position of the heavy chains, then the average distance difference of approximately 90-120 A may represent the length of two conjugated antibodies. This length should be kept in mind when performing immunoelectron microscopy. The data suggest that AD2 recognizes the outer arm dynein heavy chains of normal human respiratory cilia.

Spindle assembly in Xenopus egg extracts: respective roles of centrosomes and microtubule self-organization

In Xenopus egg extracts, spindles assembled around sperm nuclei contain a centrosome at each pole, while those assembled around chromatin beads do not. Poles can also form in the absence of chromatin, after addition of a microtubule stabilizing agent to extracts. Using this system, we have asked (a) how are spindle poles formed, and (b) how does the nucleation and organization of microtubules by centrosomes influence spindle assembly? We have found that poles are morphologically similar regardless of their origin. In all cases, microtubule organization into poles requires minus end-directed translocation of microtubules by cytoplasmic dynein, which tethers centrosomes to spindle poles. However, in the absence of pole formation, microtubules are still sorted into an antiparallel array around mitotic chromatin. Therefore, other activities in addition to dynein must contribute to the polarized orientation of microtubules in spindles. When centrosomes are present, they provide dominant sites for pole formation. Thus, in Xenopus egg extracts, centrosomes are not necessarily required for spindle assembly but can regulate the organization of microtubules into a bipolar array.

Phosphorylation by p34cdc2 protein kinase regulates binding of the kinesin-related motor HsEg5 to the dynactin subunit p150

The kinesin-related motor HsEg5 is essential for centrosome separation, and its association with centrosomes appears to be regulated by phosphorylation of tail residue threonine 927 by the p34(cdc2) protein kinase. To identify proteins able to interact with the tail of HsEg5, we performed a yeast two-hybrid screen with a HsEg5 stalk-tail construct as bait. We isolated a cDNA coding for the central, alpha-helical region of human p150(Glued), a prominent component of the dynactin complex. The interaction between HsEg5 and p150(Glued) was enhanced upon activation of p34(CDC28), the budding yeast homolog of p34(cdc2), provided that HsEg5 had a phosphorylatable residue at position 927. Phosphorylation also enhanced the specific binding of p150(Glued) to the tail domain of HsEg5 in vitro, indicating that the two proteins are able to interact directly. Immunofluorescence microscopy revealed co-localization of HsEg5 and p150(Glued) during mitosis but not during interphase, consistent with a cell cycle-dependent association between the two proteins. Taken together, these results suggest that HsEg5 and p150(Glued) may interact in mammalian cells in vivo and that p34(cdc2) may regulate this interaction. Furthermore, they imply that the dynactin complex may functionally interact not only with dynein but also with kinesin-related motors.