Two Saccharomyces cerevisiae kinesin-related gene products required for mitotic spindle assembly

Analysis of the distribution of the kinetochore protein Ndc10p in Saccharomyces cerevisiae using 3-D modeling of mitotic spindles

Ndc10p is one of the DNA-binding constituents of the kinetochore in Saccharomyces cerevisiae but light microscopy analysis suggests that Ndc10p is not limited to kinetochore regions. We examined the localization of Ndc10p using immunoelectron microscopy and showed that Ndc10p is associated with spindle microtubules from S-phase through anaphase. By serial section reconstruction of mitotic spindles combined with immunogold detection, we showed that Ndc10p interacts with microtubules laterally as well as terminally. About 50% of the gold label in serial section reconstructions of short mitotic spindles was associated with the walls of spindle microtubules. Interaction of kinetochore components with microtubule walls was also shown for kinetochore protein Ndc80p. Our data suggest that at least a subset of kinetochore-associated protein is dispersed throughout the mitotic spindle.

The molecular function of Ase1p: evidence for a MAP-dependent midzone-specific spindle matrix. Microtubule-associated proteins

The midzone is the domain of the mitotic spindle that maintains spindle bipolarity during anaphase and generates forces required for spindle elongation (anaphase B). Although there is a clear role for microtubule (MT) motor proteins at the spindle midzone, less is known about how microtubule-associated proteins (MAPs) contribute to midzone organization and function. Here, we report that budding yeast Ase1p is a member of a conserved family of midzone-specific MAPs. By size exclusion chromatography and velocity sedimentation, both Ase1p in extracts and purified Ase1p behaved as a homodimer. Ase1p bound and bundled MTs in vitro. By live cell microscopy, loss of Ase1p resulted in a specific defect: premature spindle disassembly in mid-anaphase. Furthermore, when overexpressed, Ase1p was sufficient to trigger spindle elongation in S phase-arrested cells. FRAP revealed that Ase1p has both a very slow rate of turnover within the midzone and limited lateral diffusion along spindle MTs. We propose that Ase1p functions as an MT cross-bridge that imparts matrix-like characteristics to the midzone. MT-dependent networks of spindle midzone MAPs may be one molecular basis for the postulated spindle matrix.

The budding yeast Ipl1/Aurora protein kinase regulates mitotic spindle disassembly

Ipl1p is the budding yeast member of the Aurora family of protein kinases, critical regulators of genomic stability that are required for chromosome segregation, the spindle checkpoint, and cytokinesis. Using time-lapse microscopy, we found that Ipl1p also has a function in mitotic spindle disassembly that is separable from its previously identified roles. Ipl1-GFP localizes to kinetochores from G1 to metaphase, transfers to the spindle after metaphase, and accumulates at the spindle midzone late in anaphase. Ipl1p kinase activity increases at anaphase, and ipl1 mutants can stabilize fragile spindles. As the spindle disassembles, Ipl1p follows the plus ends of the depolymerizing spindle microtubules. Many Ipl1p substrates colocalize with Ipl1p to the spindle midzone, identifying additional proteins that may regulate spindle disassembly. We propose that Ipl1p regulates both the kinetochore and interpolar microtubule plus ends to regulate its various mitotic functions.

A novel action of terpendole E on the motor activity of mitotic Kinesin Eg5

To reveal the mechanism of mitosis, the development of M phase-specific inhibitors is an important strategy. We have been screening microbial products to find specific M phase inhibitors that do not directly target tubulins, and rediscovered terpendole E (TerE) as a novel Eg5 inhibitor. TerE did not affect microtubule integrity in interphase, but induced formation of a monoastral spindle in M phase. TerE inhibited both motor and microtubule-stimulated ATPase activities of human Eg5, but did not affect conventional kinesin from either Drosophila or bovine brain. Although terpendoles have been reported as inhibitors of acyl-CoA:cholesterol O-acyltransferase (ACAT), the Eg5 inhibitory activity of TerE was independent of ACAT inhibition. Taken together, we demonstrate that TerE is a novel Eg5 inhibitor isolated from a fungal strain.

Microfilaments and microtubules: the news from yeast

New evidence that cortical actin patches and the endocytic machinery share components supports the idea that actin patches are in fact transient membrane coats at the initial stage of endocytosis. Recent studies of actin cables have identified formins as the core of a novel actin-filament-assembling machine. Meanwhile, microtubule-binding proteins have been found in the kinetochore, and factors affecting microtubule dynamic instability have been identified.

Localization of Saccharomyces cerevisiae protein phosphatase 2A subunits throughout mitotic cell cycle

Protein phosphatase 2A (PP2A) regulates a broad spectrum of cellular processes. This enzyme is a collection of varied heterotrimeric complexes, each composed of a catalytic (C) and regulatory (B) subunit bound together by a structural (A) subunit. To understand the cell cycle dynamics of this enzyme population, we carried out quantitative and qualitative analyses of the PP2A subunits of Saccharomyces cerevisiae. We found the following: the level of each subunit remained constant throughout the cell cycle; there is at least 10 times more of one of the regulatory subunits (Rts1p) than the other (Cdc55p); Tpd3p, the structural subunit, is limiting for both catalytic and regulatory subunit binding. Using green fluorescent protein-tagged forms of each subunit, we monitored the sites of significant accumulation of each protein throughout the cell cycle. The two regulatory subunits displayed distinctly different dynamic localization patterns that overlap with the A and C subunits at the bud tip, kinetochore, bud neck, and nucleus. Using strains null for single subunit genes, we confirmed the hypothesis that regulatory subunits determine sites of PP2A accumulation. Although Rts1p and Tpd3p required heterotrimer formation to achieve normal localization, Cdc55p achieved its normal localization in the absence of either an A or C subunit.

On the role of aurora-A in centrosome function

Mammalian aurora-A belongs to a multigenic family of mitotic serine/threonine kinases comprising two other members: aurora-B and aurora-C. In this review we will focus on aurora-A that starts to localize to centrosomes only in S phase as soon as centrioles have been duplicated, the protein is then degraded in early G1. Works in various organisms have revealed that the kinase is involved in centrosome separation, duplication and maturation as well as in bipolar spindle assembly and stability. Aurora kinases are found in all organisms in which their function has been conserved throughout evolution, namely the control of chromosome segregation. In human, aurora-A has focused a lot of attention, since its overexpression has been found to be correlated with the grade of various solid tumours. Ectopic kinase overexpression in any culture cell line leads to polyploidy and centrosome amplification. However, overexpression of aurora-A in particular cell lines such as NIH3T3 is sufficient to induce growth on soft agar. Those transformed cells form tumours when implanted in immunodeficient mice, indicating that the kinase is an oncogene.

Cell division: MAST sails through mitosis

Successful mitosis requires coordinated activities of microtubules and numerous associated proteins. A recent study implicates the microtubule-associated protein MAST/Orbit in a surprisingly wide array of mitotic activities, ranging from maintaining mitotic spindle bipolarity to tethering chromosomes to the ends of microtubules.

Curcumin disrupts mitotic spindle structure and induces micronucleation in MCF-7 breast cancer cells

The dietary phytochemical curcumin possesses anti-inflammatory, -oxidant, and cytostatic properties, and exhibits significant potential as a chemopreventative agent in humans. Although many cell types are arrested in the G2/M-phase of the cell cycle after curcumin treatment, the mechanisms by which this occurs are not well understood. The purpose of this study was to examine the effects of curcumin on the cell cycle of MCF-7 breast cancer cells to determine whether growth arrest is associated with structural changes in cellular organization during mitosis. For this purpose, MCF-7 breast cancer cells were treated with 10-20 microM curcumin, and the effects on cell proliferation and mitosis studied. Structural changes were monitored by immunolabeling cells with antibodies to a number of cytoplasmic and nuclear proteins, including beta-tubulin, NuMA, lamins A/C and B1, lamin B receptor, and centromere antigens. At the concentrations used, a single dose of curcumin does not induce significant apoptosis, but is highly effective in inhibiting cell proliferation for over 6 days. During the first 24-48 h of treatment, many cells are arrested in M-phase, and DNA synthesis is almost completely inhibited. Remarkably, arrested mitotic cells exhibit monopolar spindles, and chromosomes do not undergo normal anaphase movements. After 48 h, most cells eventually leave M-phase, and many form multiple micronuclei instead of individual daughter nuclei. These observations indicate that the curcumin-induced G2/M arrest previously described for MCF-7 cells is due to the assembly of aberrant, monopolar mitotic spindles that are impaired in their ability to segregate chromosomes. The production of cells with extensive micronucleation after curcumin treatment suggests that at least some of the cytostatic effects of this phytochemical are due to its ability to disrupt normal mitosis, and raises the possibility that curcumin may promote genetic instability under some circumstances.

Stu1p is physically associated with beta-tubulin and is required for structural integrity of the mitotic spindle

Formation of the bipolar mitotic spindle relies on a balance of forces acting on the spindle poles. The primary outward force is generated by the kinesin-related proteins of the BimC family that cross-link antiparallel interpolar microtubules and slide them past each other. Here, we provide evidence that Stu1p is also required for the production of this outward force in the yeast Saccharomyces cerevisiae. In the temperature-sensitive stu1-5 mutant, spindle pole separation is inhibited, and preanaphase spindles collapse, with their previously separated poles being drawn together. The temperature sensitivity of stu1-5 can be suppressed by doubling the dosage of Cin8p, a yeast BimC kinesin-related protein. Stu1p was observed to be a component of the mitotic spindle localizing to the midregion of anaphase spindles. It also binds to microtubules in vitro, and we have examined the nature of this interaction. We show that Stu1p interacts specifically with beta-tubulin and identify the domains required for this interaction on both Stu1p and beta-tubulin. Taken together, these findings suggest that Stu1p binds to interpolar microtubules of the mitotic spindle and plays an essential role in their ability to provide an outward force on the spindle poles.

The Arabidopsis ATK1 gene is required for spindle morphogenesis in male meiosis

The spindle plays a central role in chromosome segregation during mitosis and meiosis. In particular, various kinesins are thought to play crucial roles in spindle structure and function in both mitosis and meiosis of fungi and animals. A group of putative kinesins has been previously identified in Arabidopsis, called ATK1-ATK4 (previously known as KATA-KATD), but their in vivo functions have not been tested with genetic studies. We report here the isolation and characterization of a mutant, atk1-1, which has a defective ATK1 gene. The atk1-1 mutant was identified in a collection of Ds transposon insertion lines by its reduced fertility. Reciprocal crosses between the atk1-1 mutant and wild type showed that only male fertility was reduced, not female fertility. Molecular analyses, including revertant studies, indicated that the Ds insertion in the ATK1 gene was responsible for the fertility defect. Light microscopy revealed that, in the atk1-1 mutant, male meiosis was defective, producing an abnormal number of microspores of variable sizes. Further cytological studies indicated that meiotic chromosome segregation and spindle organization were both abnormal in the mutant. Specifically, the atk1-1 mutant male meiotic cells had spindles that were broad, unfocused and multi-axial at the poles at metaphase I, unlike the typical fusiform bipolar spindle found in the wild-type metaphase I cells. Therefore, the ATK1 gene plays a crucial role in spindle morphogenesis in male Arabidopsis meiosis.

The carboxy terminus of simian virus 40 large T antigen is required to disrupt the yeast cell cycle

Wild-type and J domain mutant simian virus 40 large T antigens alter the cell cycle and bud morphology of Saccharomyces cerevisiae. In contrast, yeast cells expressing mutant T antigen lacking the carboxy-terminal 150 aa exhibit normal morphology, indicating that this region of T antigen is required for cell cycle disruption.

The mitotic spindle is required for loading of the DASH complex onto the kinetochore

A role for the mitotic spindle in the maturation of the kinetochore has not been defined previously. Here we describe the isolation of a novel and conserved essential gene, ASK1, from Saccharomyces cerevisiae involved in this process. ask1 mutants display either G(2)/M arrest or segregation of DNA masses without the separation of sister chromatids, resulting in massive nondisjunction and broken spindles. Ask1 localizes along mitotic spindles and to kinetochores, and cross-links to centromeric DNA. Microtubules are required for Ask1 binding to kinetochores, and are partially required to maintain its association. We found Ask1 is part of a multisubunit complex, DASH, that contains approximately 10 components, including several proteins essential for mitosis including Dam1, Duo1, Spc34, Spc19, and Hsk1. The Ipl1 kinase controls the phosphorylation of Dam1 in the DASH complex and may regulate its function. We propose that DASH is a microtubule-binding complex that is transferred to the kinetochore prior to mitosis, thereby defining a new step in kinetochore maturation.

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

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

Degradation of the kinesin Kip1p at anaphase onset is mediated by the anaphase-promoting complex and Cdc20p

Kip1p of Saccharomyces cerevisiae is a bipolar kinesin in the conserved bimC kinesin subfamily that mediates mitotic spindle-pole separation. Here, we show that Kip1p is regulated immediately after anaphase initiation by its rapid degradation. Degradation required the ubiquitin protein ligase called the anaphase-promoting complex, the anaphase-promoting complex activating protein Cdc20, and a unique 43-aa sequence in Kip1p. Degradation also required import of Kip1p into the nucleus, but occurred independently of spindle association. A mutation that stabilized Kip1p impaired anaphase progression. The timing of degradation suggests that Kip1p functions primarily during spindle assembly and metaphase, and that Kip1p degradation facilitates structural changes in the mitotic spindle as anaphase progresses.

Gamma-tubulin and the C-terminal motor domain kinesin-like protein, KLPA, function in the establishment of spindle bipolarity in Aspergillus nidulans

Previous research has found that a gamma-tubulin mutation in Schizosaccharomyces pombe is synthetically lethal with a deletion of the C-terminal motor domain kinesin-like protein gene pkl1, but the lethality of the double mutant prevents a phenotypic analysis of the synthetic interaction. We have investigated interactions between klpA1, a deletion of an Aspergillus nidulans homolog of pkl1, and mutations in the mipA, gamma-tubulin gene. We find that klpA1 dramatically increases the cold sensitivity and slightly reduces the growth rate at all temperatures, of three mipA alleles. In synchronized cells we find that klpA1 causes a substantial but transient inhibition of the establishment of spindle bipolarity. At a restrictive temperature, mipAD123 causes a slight, transient inhibition of spindle bipolarity and a more significant inhibition of anaphase A. In the mipAD123/klpA1 strain, formation of bipolar spindles is more strongly inhibited than in the klpA1 single mutant and many spindles apparently never become bipolar. These results indicate, surprisingly, that gamma-tubulin and the klpA kinesin have overlapping roles in the establishment of spindle bipolarity. We propose a model to account for these data.

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

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

Molecular analysis of kinetochore-microtubule attachment in budding yeast

The complex series of movements that mediates chromosome segregation during mitosis is dependent on the attachment of microtubules to kinetochores, DNA-protein complexes that assemble on centromeric DNA. We describe the use of live-cell imaging and chromatin immunoprecipitation in S. cerevisiae to identify ten kinetochore subunits, among which are yeast homologs of microtubule binding proteins in animal cells. By analyzing conditional mutations in several of these proteins, we show that they are required for the imposition of tension on paired sister kinetochores and for correct chromosome movement. The proteins include both molecular motors and microtubule associated proteins (MAPs), implying that motors and MAPs function together in binding chromosomes to spindle microtubules.

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

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

All kinesin superfamily protein, KIF, genes in mouse and human

Intracellular transport is essential for morphogenesis and functioning of the cell. The kinesin superfamily proteins (KIFs) have been shown to transport membranous organelles and protein complexes in a microtubule- and ATP-dependent manner. More than 30 KIFs have been reported in mice. However, the nomenclature of KIFs has not been clearly established, resulting in various designations and redundant names for a single KIF. Here, we report the identification and classification of all KIFs in mouse and human genome transcripts. Previously unidentified murine KIFs were found by a PCR-based search. The identification of all KIFs was confirmed by a database search of the total human genome. As a result, there are a total of 45 KIFs. The nomenclature of all KIFs is presented. To understand the function of KIFs in intracellular transport in a single tissue, we focused on the brain. The expression of 38 KIFs was detected in brain tissue by Northern blotting or PCR using cDNA. The brain, mainly composed of highly differentiated and polarized cells such as neurons and glia, requires a highly complex intracellular transport system as indicated by the increased number of KIFs for their sophisticated functions. It is becoming increasingly clear that the cell uses a number of KIFs and tightly controls the direction, destination, and velocity of transportation of various important functional molecules, including mRNA. This report will set the foundation of KIF and intracellular transport research.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

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

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

Microtubule motors in mitosis

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

Probing spindle assembly mechanisms with monastrol, a small molecule inhibitor of the mitotic kinesin, Eg5

Monastrol, a cell-permeable small molecule inhibitor of the mitotic kinesin, Eg5, arrests cells in mitosis with monoastral spindles. Here, we use monastrol to probe mitotic mechanisms. We find that monastrol does not inhibit progression through S and G2 phases of the cell cycle or centrosome duplication. The mitotic arrest due to monastrol is also rapidly reversible. Chromosomes in monastrol-treated cells frequently have both sister kinetochores attached to microtubules extending to the center of the monoaster (syntelic orientation). Mitotic arrest-deficient protein 2 (Mad2) localizes to a subset of kinetochores, suggesting the activation of the spindle assembly checkpoint in these cells. Mad2 localizes to some kinetochores that have attached microtubules in monastrol-treated cells, indicating that kinetochore microtubule attachment alone may not satisfy the spindle assembly checkpoint. Monastrol also inhibits bipolar spindle formation in Xenopus egg extracts. However, it does not prevent the targeting of Eg5 to the monoastral spindles that form. Imaging bipolar spindles disassembling in the presence of monastrol allowed direct observations of outward directed forces in the spindle, orthogonal to the pole-to-pole axis. Monastrol is thus a useful tool to study mitotic processes, detection and correction of chromosome malorientation, and contributions of Eg5 to spindle assembly and maintenance.

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

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

The Saccharomyces cerevisiae centromere protein Slk19p is required for two successive divisions during meiosis

Meiotic cell division includes two separate and distinct types of chromosome segregation. In the first segregational event the sister chromatids remain attached at the centromere; in the second the chromatids are separated. The factors that control the order of chromosome segregation during meiosis have not yet been identified but are thought to be confined to the centromere region. We showed that the centromere protein Slk19p is required for the proper execution of meiosis in Saccharomyces cerevisiae. In its absence diploid cells skip meiosis I and execute meiosis II division. Inhibiting recombination does not correct this phenotype. Surprisingly, the initiation of recombination is apparently required for meiosis II division. Thus Slk19p appears to be part of the mechanism by which the centromere controls the order of meiotic divisions.

Roles of motor proteins in building microtubule-based structures: a basic principle of cellular design

Eukaryotic cells must build a complex infrastructure of microtubules (MTs) and associated proteins to carry out a variety of functions. A growing body of evidence indicates that a major function of MT-associated motor proteins is to assemble and maintain this infrastructure. In this context, we examine the mechanisms utilized by motors to construct the arrays of MTs and associated proteins contained within the mitotic spindle, neuronal processes, and ciliary axonemes. We focus on the capacity of motors to drive the 'sliding filament mechanism' that is involved in the construction and maintenance of spindles, axons and dendrites, and on a type of particle transport called 'intraflagellar transport' which contributes to the assembly and maintenance of axonemes.

Mitotic motors in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae provides a unique opportunity for study of the microtubule-based motor proteins that participate in mitotic spindle function. The genome of Saccharomyces encodes a relatively small and genetically tractable set of microtubule-based motor proteins. The single cytoplasmic dynein and five of the six kinesin-related proteins encoded have been implicated in mitotic spindle function. Each motor protein is unique in amino acid sequence. On account of functional overlap, no single motor is uniquely required for cell viability, however. The ability to create and analyze multiple mutants has allowed experimental dissection of the roles performed by each mitotic motor. Some of the motors operate within the nucleus to assemble and elongate the bipolar spindle (kinesin-related Cin8p, Kip1p, Kip3p and Kar3p). Others operate on the cytoplasmic microtubules to effect spindle and nuclear positioning within the cell (dynein and kinesin-related Kip2p, Kip3p and Kar3p). The six motors apparently contribute three fundamental activities to spindle function: motility, microtubule cross-linking and regulation of microtubule dynamics.

Functional coordination of three mitotic motors in Drosophila embryos

It is well established that multiple microtubule-based motors contribute to the formation and function of the mitotic spindle, but how the activities of these motors interrelate remains unclear. Here we visualize spindle formation in living Drosophila embryos to show that spindle pole movements are directed by a temporally coordinated balance of forces generated by three mitotic motors, cytoplasmic dynein, KLP61F, and Ncd. Specifically, our findings suggest that dynein acts to move the poles apart throughout mitosis and that this activity is augmented by KLP61F after the fenestration of the nuclear envelope, a process analogous to nuclear envelope breakdown, which occurs at the onset of prometaphase. Conversely, we find that Ncd generates forces that pull the poles together between interphase and metaphase, antagonizing the activity of both dynein and KLP61F and serving as a brake for spindle assembly. During anaphase, however, Ncd appears to have no effect on spindle pole movements, suggesting that its activity is down-regulated at this time, allowing dynein and KLP61F to drive spindle elongation during anaphase B.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

Dr. Dolittle and the making of the mitotic spindle

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

Bni1p regulates microtubule-dependent nuclear migration through the actin cytoskeleton in Saccharomyces cerevisiae

The RHO1 gene encodes a yeast homolog of the mammalian RhoA protein. Rho1p is localized to the growth sites and is required for bud formation. We have recently shown that Bni1p is one of the potential downstream target molecules of Rho1p. The BNI1 gene is implicated in cytokinesis and the establishment of cell polarity in Saccharomyces cerevisiae but is not essential for cell viability. In this study, we screened for mutations that were synthetically lethal in combination with a bni1 mutation and isolated two genes. They were the previously identified PAC1 and NIP100 genes, both of which are implicated in nuclear migration in S. cerevisiae. Pac1p is a homolog of human LIS1, which is required for brain development, whereas Nip100p is a homolog of rat p150(Glued), a component of the dynein-activated dynactin complex. Disruption of BNI1 in either the pac1 or nip100 mutant resulted in an enhanced defect in nuclear migration, leading to the formation of binucleate mother cells. The arp1 bni1 mutant showed a synthetic lethal phenotype while the cin8 bni1 mutant did not, suggesting that Bni1p functions in a kinesin pathway but not in the dynein pathway. Cells of the pac1 bni1 and nip100 bni1 mutants exhibited a random distribution of cortical actin patches. Cells of the pac1 act1-4 mutant showed temperature-sensitive growth and a nuclear migration defect. These results indicate that Bni1p regulates microtubule-dependent nuclear migration through the actin cytoskeleton. Bni1p lacking the Rho-binding region did not suppress the pac1 bni1 growth defect, suggesting a requirement for the Rho1p-Bni1p interaction in microtubule function.

Small molecule inhibitor of mitotic spindle bipolarity identified in a phenotype-based screen

Small molecules that perturb specific protein functions are valuable tools for dissecting complex processes in mammalian cells. A combination of two phenotype-based screens, one based on a specific posttranslational modification, the other visualizing microtubules and chromatin, was used to identify compounds that affect mitosis. One compound, here named monastrol, arrested mammalian cells in mitosis with monopolar spindles. In vitro, monastrol specifically inhibited the motility of the mitotic kinesin Eg5, a motor protein required for spindle bipolarity. All previously known small molecules that specifically affect the mitotic machinery target tubulin. Monastrol will therefore be a particularly useful tool for studying mitotic mechanisms.

Novel roles for saccharomyces cerevisiae mitotic spindle motors

The single cytoplasmic dynein and five of the six kinesin-related proteins encoded by Saccharomyces cerevisiae participate in mitotic spindle function. Some of the motors operate within the nucleus to assemble and elongate the bipolar spindle. Others operate on the cytoplasmic microtubules to effect spindle and nuclear positioning within the cell. This study reveals that kinesin-related Kar3p and Kip3p are unique in that they perform roles both inside and outside the nucleus. Kar3p, like Kip3p, was found to be required for spindle positioning in the absence of dynein. The spindle positioning role of Kar3p is performed in concert with the Cik1p accessory factor, but not the homologous Vik1p. Kar3p and Kip3p were also found to overlap for a function essential for the structural integrity of the bipolar spindle. The cytoplasmic and nuclear roles of both these motors could be partially substituted for by the microtubule-destabilizing agent benomyl, suggesting that these motors perform an essential microtubule-destabilizing function. In addition, we found that yeast cell viability could be supported by as few as two microtubule-based motors: the BimC-type kinesin Cin8p, required for spindle structure, paired with either Kar3p or Kip3p, required for both spindle structure and positioning.

The kinesin-related protein Kip1p of Saccharomyces cerevisiae is bipolar

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

Slk19p is a centromere protein that functions to stabilize mitotic spindles

We have identified a novel centromere-associated gene product from Saccharomyces cerevisiae that plays a role in spindle assembly and stability. Strains with a deletion of SLK19 (synthetic lethal Kar3p gene) exhibit abnormally short mitotic spindles, increased numbers of astral microtubules, and require the presence of the kinesin motor Kar3p for viability. When cells are deprived of both Slk19p and Kar3p, rapid spindle breakdown and mitotic arrest is observed. A functional fusion of Slk19p to green fluorescent protein (GFP) localizes to kinetochores and, during anaphase, to the spindle midzone, whereas Kar3p-GFP was found at the nuclear side of the spindle pole body. Thus, these proteins seem to play overlapping roles in stabilizing spindle structure while acting from opposite ends of the microtubules.

All-trans-retinoic acid-mediated growth inhibition involves inhibition of human kinesin-related protein HsEg5

In this study we used differential display reverse transcription-polymerase chain reaction to search for differentially expressed all-trans-retinoic acid (ATRA)-responsive genes in pancreatic carcinoma cells. We identified the kinesin-related protein HsEg5, which plays an essential role in spindle assembly and spindle function during mitosis, as a novel molecule involved in ATRA-mediated growth inhibition. Using Northern and Western blot analysis we demonstrated that ATRA significantly inhibits HsEg5 expression in various pancreatic carcinoma cell lines as well as in HaCat keratinocytes. Inhibition of HsEg5 expression by ATRA occurs at the posttranscriptional level. As a consequence, tumor cells synchronized in S-phase revealed a retarded progression through G2/M phase of the cell cycle indicating that HsEg5 inhibition results in a delayed progression through mitosis. Furthermore, a significant decrease of HsEg5 protein expression achieved by antisense transfection revealed a significant growth inhibition compared with control cells. Therefore, HsEg5 represents a novel molecule involved in ATRA-mediated growth inhibition, suggesting that vitamin A derivatives can interact with the bipolar spindle apparatus during mitosis.

Yeast Dam1p is required to maintain spindle integrity during mitosis and interacts with the Mps1p kinase

We have identified a mutant allele of the DAM1 gene in a screen for mutations that are lethal in combination with the mps1-1 mutation. MPS1 encodes an essential protein kinase that is required for duplication of the spindle pole body and for the spindle assembly checkpoint. Mutations in six different genes were found to be lethal in combination with mps1-1, of which only DAM1 was novel. The remaining genes encode a checkpoint protein, Bub1p, and four chaperone proteins, Sti1p, Hsc82p, Cdc37p, and Ydj1p. DAM1 is an essential gene that encodes a protein recently described as a member of a microtubule binding complex. We report here that cells harboring the dam1-1 mutation fail to maintain spindle integrity during anaphase at the restrictive temperature. Consistent with this phenotype, DAM1 displays genetic interactions with STU1, CIN8, and KAR3, genes encoding proteins involved in spindle function. We have observed that a Dam1p-Myc fusion protein expressed at endogenous levels and localized by immunofluorescence microscopy, appears to be evenly distributed along short mitotic spindles but is found at the spindle poles at later times in mitosis.

Sli15 associates with the ipl1 protein kinase to promote proper chromosome segregation in Saccharomyces cerevisiae

The conserved Ipl1 protein kinase is essential for proper chromosome segregation and thus cell viability in the budding yeast Saccharomyces cerevisiae. Its human homologue has been implicated in the tumorigenesis of diverse forms of cancer. We show here that sister chromatids that have separated from each other are not properly segregated to opposite poles of ipl1-2 cells. Failures in chromosome segregation are often associated with abnormal distribution of the spindle pole-associated Nuf2-GFP protein, thus suggesting a link between potential spindle pole defects and chromosome missegregation in ipl1 mutant cells. A small fraction of ipl1-2 cells also appears to be defective in nuclear migration or bipolar spindle formation. Ipl1 associates, probably directly, with the novel and essential Sli15 protein in vivo, and both proteins are localized to the mitotic spindle. Conditional sli15 mutant cells have cytological phenotypes very similar to those of ipl1 cells, and the ipl1-2 mutation exhibits synthetic lethal genetic interaction with sli15 mutations. sli15 mutant phenotype, like ipl1 mutant phenotype, is partially suppressed by perturbations that reduce protein phosphatase 1 function. These genetic and biochemical studies indicate that Sli15 associates with Ipl1 to promote its function in chromosome segregation.

Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae

The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint.

High-voltage electron tomography of spindle pole bodies and early mitotic spindles in the yeast Saccharomyces cerevisiae

The spindle pole body (SPB) is the major microtubule-organizing center of budding yeast and is the functional equivalent of the centrosome in higher eukaryotic cells. We used fast-frozen, freeze-substituted cells in conjunction with high-voltage electron tomography to study the fine structure of the SPB and the events of early spindle formation. Individual structures were imaged at 5-10 nm resolution in three dimensions, significantly better than can be achieved by serial section electron microscopy. The SPB is organized in distinct but coupled layers, two of which show ordered two-dimensional packing. The SPB central plaque is anchored in the nuclear envelope with hook-like structures. The minus ends of nuclear microtubules (MTs) are capped and are tethered to the SPB inner plaque, whereas the majority of MT plus ends show a distinct flaring. Unbudded cells containing a single SPB retain 16 MTs, enough to attach to each of the expected 16 chromosomes. Their median length is approximately 150 nm. MTs growing from duplicated but not separated SPBs have a median length of approximately 130 nm and interdigitate over the bridge that connects the SPBs. As a bipolar spindle is formed, the median MT length increases to approximately 300 nm and then decreases to approximately 30 nm in late anaphase. Three-dimensional models confirm that there is no conventional metaphase and that anaphase A occurs. These studies complement and extend what is known about the three-dimensional structure of the yeast mitotic spindle and further our understanding of the organization of the SPB in intact cells.

The Xenopus laevis aurora-related protein kinase pEg2 associates with and phosphorylates the kinesin-related protein XlEg5

We have previously reported on the cloning of XlEg5, a Xenopus laevis kinesin-related protein from the bimC family (Le Guellec, R., Paris, J., Couturier, A., Roghi, C., and Philippe, M. (1991) Mol. Cell. Biol. 11, 3395-3408) as well as pEg2, an Aurora-related serine/threonine kinase (Roghi, C., Giet, R., Uzbekov, R., Morin, N., Chartrain, I., Le Guellec, R., Couturier, A., Dorée, M., Philippe, M., and Prigent, C. (1998) J. Cell Sci. 111, 557-572). Inhibition of either XlEg5 or pEg2 activity during mitosis in Xenopus egg extract led to monopolar spindle formation. Here, we report that in Xenopus XL2 cells, pEg2 and XlEg5 are both confined to separated centrosomes in prophase, and then to the microtubule spindle poles. We also show that pEg2 co-immunoprecipitates with XlEg5 from egg extracts and XL2 cell lysates. Both proteins can directly interact in vitro, but also through the two-hybrid system. Furthermore immunoprecipitated pEg2 were found to remain active when bound to the beads and phosphorylate XlEg5 present in the precipitate. Two-dimensional mapping of XlEg5 tryptic peptides phosphorylated in vivo first confirmed that XlEg5 was phosphorylated by p34(cdc2) and next revealed that in vitro pEg2 kinase phosphorylated XlEg5 on the same stalk domain serine residue that was phosphorylated in metabolically labeled XL2 cells. The kinesin-related XlEg5 is to our knowledge the first in vivo substrate ever reported for an Aurora-related kinase.

Antagonistic microtubule-sliding motors position mitotic centrosomes in Drosophila early embryos

The positioning of centrosomes, or microtubule-organizing centres, within cells plays a critical part in animal development. Here we show that, in Drosophila embryos undergoing mitosis, the positioning of centrosomes within bipolar spindles and between daughter nuclei is determined by a balance of opposing forces generated by a bipolar kinesin motor, KLP61F, that is directed to microtubule plus ends, and a carboxy-terminal kinesin motor, Ncd, that is directed towards microtubule minus ends. This activity maintains the spacing between separated centrosomes during prometaphase and metaphase, and repositions centrosomes and daughter nuclei during late anaphase and telophase. Surprisingly, we do not observe a function for KLP61F in the initial separation of centrosomes during prophase. Our data indicate that KLP61F and Ncd may function by crosslinking and sliding antiparallel spindle microtubules in relation to one another, allowing KLP61F to push centrosomes apart and Ncd to pull them together.

Motile properties of the kinesin-related Cin8p spindle motor extracted from Saccharomyces cerevisiae cells

We have developed microtubule binding and motility assays for Cin8p, a kinesin-related mitotic spindle motor protein from Saccharomyces cerevisiae. The methods examine Cin8p rapidly purified from crude yeast cell extracts. We created a recombinant form of CIN8 that fused the biotin carrying polypeptide from yeast pyruvate carboxylase to the carboxyl terminus of Cin8p. This form was biotinated in yeast cells and provided Cin8p activity in vivo. Avidin-coated glass surfaces were used to specifically bind biotinated Cin8p from crude extracts. Microtubules bound to the Cin8p-coated surfaces and moved at 3.4 +/- 0.5 micrometer/min in the presence of ATP. Force production by Cin8p was directed toward the plus ends of microtubules. A mutation affecting the microtubule-binding site within the motor domain (cin8-F467A) decreased Cin8p's ability to bind microtubules to the glass surface by >10-fold, but reduced gliding velocity by only 35%. The cin8-3 mutant form, affecting the alpha2 helix of the motor domain, caused a moderate defect in microtubule binding, but motility was severely affected. cin8-F467A cells, but not cin8-3 cells, were greatly impaired in bipolar spindle forming ability. We conclude that microtubule binding by Cin8p is more important than motility for proper spindle formation.

Microtubule-based motor function in mitosis

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

Differential regulation of the Kar3p kinesin-related protein by two associated proteins, Cik1p and Vik1p

The mechanisms by which kinesin-related proteins interact with other proteins to carry out specific cellular processes is poorly understood. The kinesin-related protein, Kar3p, has been implicated in many microtubule functions in yeast. Some of these functions require interaction with the Cik1 protein (Page, B.D., L.L. Satterwhite, M.D. Rose, and M. Snyder. 1994. J. Cell Biol. 124:507-519). We have identified a Saccharomyces cerevisiae gene, named VIK1, encoding a protein with sequence and structural similarity to Cik1p. The Vik1 protein is detected in vegetatively growing cells but not in mating pheromone-treated cells. Vik1p physically associates with Kar3p in a complex separate from that of the Kar3p-Cik1p complex. Vik1p localizes to the spindle-pole body region in a Kar3p-dependent manner. Reciprocally, concentration of Kar3p at the spindle poles during vegetative growth requires the presence of Vik1p, but not Cik1p. Phenotypic analysis suggests that Cik1p and Vik1p are involved in different Kar3p functions. Disruption of VIK1 causes increased resistance to the microtubule depolymerizing drug benomyl and partially suppresses growth defects of cik1Delta mutants. The vik1Delta and kar3Delta mutations, but not cik1Delta, partially suppresses the temperature-sensitive growth defect of strains lacking the function of two other yeast kinesin-related proteins, Cin8p and Kip1p. Our results indicate that Kar3p forms functionally distinct complexes with Cik1p and Vik1p to participate in different microtubule-mediated events within the same cell.

The conserved protein kinase Ipl1 regulates microtubule binding to kinetochores in budding yeast

Chromosome segregation depends on kinetochores, the structures that mediate chromosome attachment to the mitotic spindle. We isolated mutants in IPL1, which encodes a protein kinase, in a screen for budding yeast mutants that have defects in sister chromatid separation and segregation. Cytological tests show that ipl1 mutants can separate sister chromatids but are defective in chromosome segregation. Kinetochores assembled in extracts from ipl1 mutants show altered binding to microtubules. Ipl1p phosphorylates the kinetochore component Ndc10p in vitro and we propose that Ipl1p regulates kinetochore function via Ndc10p phosphorylation. Ipl1p localizes to the mitotic spindle and its levels are regulated during the cell cycle. This pattern of localization and regulation is similar to that of Ipl1p homologs in higher eukaryotes, such as the human aurora2 protein. Because aurora2 has been implicated in oncogenesis, defects in kinetochore function may contribute to genetic instability in human tumors.

The bipolar kinesin, KLP61F, cross-links microtubules within interpolar microtubule bundles of Drosophila embryonic mitotic spindles

Previous genetic and biochemical studies have led to the hypothesis that the essential mitotic bipolar kinesin, KLP61F, cross-links and slides microtubules (MTs) during spindle assembly and function. Here, we have tested this hypothesis by immunofluorescence and immunoelectron microscopy (immunoEM). We show that Drosophila embryonic spindles at metaphase and anaphase contain abundant bundles of MTs running between the spindle poles. These interpolar MT bundles are parallel near the poles and antiparallel in the midzone. We have observed that KLP61F motors, phosphorylated at a cdk1/cyclin B consensus domain within the BimC box (BCB), localize along the length of these interpolar MT bundles, being concentrated in the midzone region. Nonphosphorylated KLP61F motors, in contrast, are excluded from the spindle and display a cytoplasmic localization. Immunoelectron microscopy further suggested that phospho-KLP61F motors form cross-links between MTs within interpolar MT bundles. These bipolar KLP61F MT-MT cross-links should be capable of organizing parallel MTs into bundles within half spindles and sliding antiparallel MTs apart in the spindle midzone. Thus we propose that bipolar kinesin motors and MTs interact by a "sliding filament mechanism" during the formation and function of the mitotic spindle.

The role of Saccharomyces cerevisiae coronin in the actin and microtubule cytoskeletons

Coronin was originally identified as a cortical protein associated with the actin cytoskeleton in Dictyostelium [1]. More recent studies have revealed that coronin is involved in actin-based motility, cytokinesis and phagocytosis [2,3]. Here, we describe the identification of a single homolog of coronin in Saccharomyces cerevisiae, which we show localizes to cortical actin patches in an actin-dependent manner. Unlike Dictyostelium mutants that lack coronin, yeast strains lacking coronin had no detectable defects in actin-based processes. This may reflect differences in the functions of the actin cytoskeleton in these two organisms. Previous studies have shown that cortical actin may mediate astral microtubule-based movements of the mitotic spindle in S. cerevisiae [4,5] and that, during mitosis in Dictyostelium, the regions of the cell cortex that overlap with astral microtubules become enriched in actin and coronin [6]. We therefore examined whether yeast lacking coronin had defects in the microtubule cytoskeleton. The mutant strains had increased sensitivity to the microtubule-destabilizing drug benomyl and an increased number of large-budded cells with short spindles. Further examination of microtubule-related processes, including spindle formation, migration of the mitotic spindle to the bud neck, spindle elongation, and translocation of the elongating spindle through the bud neck, failed to reveal any defects in the coronin mutant. Taken together, these results suggest that S. cerevisiae coronin is a component of the actin cytoskeleton that may interact with the microtubule cytoskeleton.