Gamma-tubulin is present in Drosophila melanogaster and Homo sapiens and is associated with the centrosome

Gamma-tubulin in Chlamydomonas: characterization of the gene and localization of the gene product in cells

In addition to their role in nucleating the assembly of axonemal microtubules, basal bodies often are associated with a microtubule organizing center (MTOC) for cytoplasmic microtubules. In an effort to define molecular components of the basal body apparatus in Chlamydomonas reinhardtii, genomic and cDNA clones encoding gamma-tubulin were isolated and sequenced. The gene, present in a single copy in the Chlamydomonas genome, encodes a protein with a predicted molecular mass of 52,161 D and 73% and 65% conservation with gamma-tubulin from higher plants and humans, respectively. To examine the distribution of gamma-tubulin in cells, a polyclonal antibody was raised against two peptides contained within the protein. Immunoblots of Chlamydomonas proteins show a major cross-reaction with a protein of Mr 53,000. In Chlamydomonas cells, the antibody stains the basal body apparatus as two or four spots at the base of the flagella and proximal to the microtubule rootlets. During cell division, two groups of fluorescent dots separate and localize to opposite ends of the mitotic apparatus. They then migrate during cleavage to positions known to be occupied by basal bodies. Changes in gamma-tubulin localization during the cell cycle are consistent with a role for this protein in the nucleation of microtubules of both the interphase cytoplasmic array and the mitotic spindle. Immunogold labeling of cell sections showed that gamma-tubulin is closely associated with the basal bodies. The flagellar transition region was also labeled, possibly indicating a role for gamma-tubulin in assembly of the central pair microtubules of the axoneme.

Protein complexes containing gamma-tubulin are present in mammalian brain microtubule protein preparations

The presence of gamma-tubulin in microtubule preparations, obtained by disassembly/ assembly cycles at 0degreesC/37degreesC from the brain of several mammals, is demonstrated by immunoblotting with specific antibodies directed against three distinct regions of the protein. In contrast gamma-tubulin was absent from pure tubulin obtained by chromatography on phosphocellulose, but was retained on the column with the other microtubule-associated proteins. A large part of the gamma-tubulin was present in cold stable material remaining after microtubule disassembly at OdegreesC and was partially solubilized using high salt, thus preventing its purification by the usual assembly/disassembly procedure used for alpha/beta-tubulin heterodimers. Brain gamma-tubulin was purified by affinity chromatography with gamma-tubulin antibodies raised against its carboxyl terminal region. Purified gamma-tubulin consisted of at least two polypeptides present in equal quantities and exhibiting a pI of 6.5 and 6.6, respectively. It was associated with the alpha/beta-tubulin heterodimer and with at least five other polypeptides of 75, 105, 130, 195, and 250 kDa. With the exception of the 250 kDa polypeptide, all of these proteins seem to be present in gamma-tubulin complexes isolated from Xenopus eggs. But, in contrast with Xenopus egg complexes, brain complexes exhibited a considerable heterogeneity of their apparent masses and composition in sucrose gradient centrifugation, in agreement with the absence of an homogeneous structure in electron microscopy. Despite this heterogeneity, gamma-tubulin complexes bind quantitatively to microtubule extremities. The possibility to further use mammalian brain gamma-tubulin and some of its associated proteins in biochemical and pharmacological experiments is of interest since brain microtubule protein preparations have been extensively used for studying both microtubule dynamics and the activity of microtubule poisons.

A synergy of technologies: combining laser microsurgery with green fluorescent protein tagging

When focused through an objective lens with a high numerical aperture, nanosecond pulses of high-intensity green (532-nm) laser light can be used to selectively destroy any cellular component whose boundaries can be defined by light microscopy. These components include, for example, chromosomes, spindle fibers, bundles of keratin, or actin filaments, mitochondria, vacuoles, and so forth. In addition, the definition of poorly resolved components can be enhanced for selective destruction by tagging one or more of their constituent proteins with green fluorescence protein (GFP). As a example we show that the centrosome in living PtK1 cells can be clearly defined, and then destroyed by green laser light, after transforming the cells with gamma-tubulin/GFP fusion protein. In some transformed cells it is even possible to target and selectively destroy just one of the centrioles.

Characterization of gamma-tubulin in Artemia: isoform composition and spatial distribution in polarized cells of the larval epidermis

Microtubule arrangement is influenced by gamma-tubulin, a soluble protein of the eukaryotic cell cytosol and a component of microtubule-organizing centers. In this study, affinity purified antibodies to gamma-tubulin were prepared and their specificity demonstrated by immunostaining of Western blots and in competitive ELISAs. When employed to label mouse fibroblasts, one or two brightly stained dots appeared in each cell, a pattern characteristic of centrosomes. Antibody 9, raised to a conserved amino-terminal peptide of gamma-tubulin, was used with TU-30 (from P. Dráber) to characterize gamma-tubulin in the crustacean, Artemia franciscana. Cell-free protein extracts from Artemia contained gamma-tubulin and it purified with alpha/beta-tubulin through several preparative steps. Probing of Western blots prepared from two-dimensional gels yielded a single isoform of gamma-tubulin in Artemia with a pI of about 5.6. Immunostaining with TAT, a general antibody to alpha-tubulin, demonstrated that Artemia possess two morphological types of immune blood cells (hemocytes) with distinctive microtubule arrays. Both the compact spherical hemocytes and the flatter, spreading cells exhibited fluorescent dots, often in pairs, when labelled with antibodies to gamma-tubulin. Microtubules in polarized cells of the epidermis were also brightly stained with antibody to alpha-tubulin, revealing interphase arrangements, anastral mitotic spindles and midbodies. Antibody 9 and TU-30 gave punctate staining patterns in interphase epidermal cell layers and they occasionally labelled midbodies. Unexpectedly, gamma-tubulin was seen only rarely at both poles of mitotic spindles in epidermal cells. The complete absence of asters and the apparent lack of gamma-tubulin at all but a small number of poles indicate that formation and structure of the mitotic spindle in epidermal cells of Artemia are unusual.

The tyrosine kinase PYK-2/RAFTK regulates natural killer (NK) cell cytotoxic response, and is translocated and activated upon specific target cell recognition and killing

The compartmentalization of plasma membrane proteins has a key role in regulation of lymphocyte activation and development of immunity. We found that the proline-rich tyrosine kinase-2 (PYK-2/RAFTK) colocalized with the microtubule-organizing center (MTOC) at the trailing edge of migrating natural killer (NK) cells. When polyclonal NK cells bound to K562 targets, PYK-2 translocated to the area of NK-target cell interaction. The specificity of this process was assessed with NK cell clones bearing activatory or inhibitory forms of CD94/NKG2. The translocation of PYK-2, MTOC, and paxillin to the area of NK-target cell contact was regulated upon specific recognition of target cells through NK cell receptors, controlling target cell killing. Furthermore, parallel in vitro kinase assays showed that PYK-2 was activated in response to signals that specifically triggered its translocation and NK cell mediated cytotoxicity. The overexpression of both the wt and a dominant-negative mutant of PYK-2, but not ZAP-70 wt, prevented the specific translocation of the MTOC and paxillin, and blocked the cytotoxic response of NK cells. Our data indicate that subcellular compartmentalization of PYK-2 correlates with effective signal transduction. Furthermore, they also suggest an important role for PYK-2 on the assembly of the signaling complexes that regulate the cytotoxic response.

Localization of gamma-tubulin in interphase and mitotic cells of a unicellular eukaryote, Giardia intestinalis

Giardia intestinalis, a bi-nucleated amitochondrial flagellate, possesses a complex cytoskeleton based on several microtubular systems (flagella, adhesive disk, median body, funis, mitotic spindles). MTOCs of the individual systems have not been fully defined. By using monoclonal antibodies against a conserved synthetic peptide from the C-terminus of human gamma-tubulin we investigated occurrence and distribution of gamma-tubulin in interphase and mitotic Giardia cells. On the immunoblots of Giardia cytoskeletal extracts the antibodies bound to a single polypeptide of approximately 50 kDa. Immunostaining of the interphase cell demonstrated gamma-tubulin as four bright spots at the basis of four out of eight flagella. Gamma-tubulin label was associated with perikinetosomal areas of the ventral and posterolateral pairs of flagella which are formed de novo during cell division. Basal body regions of the anterolateral and caudal pairs of flagella which persist during the division and are integrated into the flagellar systems of the daughter cells did not show gamma-tubulin staining. At early mitosis, gamma-tubulin spots disappeared reappearing again at late mitosis in accord with reorientation of parent flagella and reorganization of flagellar apparatus during cell division. The antibody-detectable gamma-tubulin epitope was absent at the poles of both mitotic spindles. Albendazole-treated Giardia, in which spindle assembly was completely inhibited, showed the same gamma-tubulin staining pattern thus confirming that the fluorescent label is exclusively located in the basal body regions. Our results point to a role of gamma-tubulin in nucleation of microtubules of newly formed flagella and indicate unusual mitotic spindle assembly. Moreover, the demonstration of gamma-tubulin in Giardia shows ubiquity of this protein through the evolutionary history of eukaryotes.

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

Recurring views on the structure and function of the cytoskeleton: a 300-year epic

Some unnoticed or seldom remembered precedents of current views on biological motion and its structural bases are briefly outlined, followed by a concise recapitulation of how the present theory has been constructed in the last few decades. It is shown that the evolution of the concept of fibers as main constituents of living matter led to hypothesizing microscopic structures closely resembling microtubules in the 18th century. At the beginning of this period, fibers sliding over each other and driven by interposed moving elements were envisioned as the cause of muscle contraction. In the following century, an account of the mechanism of myofibril contraction visualized longitudinal displacements of myosin-containing submicroscopic rodlets. The existence of fibrils in the protoplasm of non-muscle cells, a subject of long debate in the second half of the 19th century, was virtually discarded as irrelevant or fallacious 100 years ago. The issue resurfaced in the early 1930s as a theoretical notion--the cytosquelette--nearly two decades before intracellular filamentous structures were first observed with electron microscopy. The role originally assumed for such fibrils as signal conductors is nowadays being reappraised, although under new interpretations with a much wider significance including modulation of gene expression, morphogenesis, and even consciousness. Since all of the above ancestral conceptions were eventually abandoned, the corresponding current views are, to a certain extent, recurrent.

Two domains of p80 katanin regulate microtubule severing and spindle pole targeting by p60 katanin

The assembly and function of the mitotic spindle requires the activity of a number of microtubule-binding proteins. Some microtubule-binding proteins bind microtubules in vitro but do not co-localize with microtubules in interphase cells. Instead these proteins associate with specific subregions of the mitotic spindle. Katanin, a heterodimeric microtubule-severing ATPase, is found localized at mitotic spindle poles. In this paper we demonstrate that human p60 katanin and the C-terminal domain of human p80 katanin both bind microtubules in vitro. Association of these two proteins results in an increased microtubule affinity and increased microtubule-severing activity in vitro. Association of these subunits in transfected HeLa cells increases microtubule disassembly activity and targeting to spindle poles. The N-terminal WD40 domain of p80 katanin acts as a negative regulator of microtubule disassembly activity and is also required for spindle pole localization, possibly through interactions with another spindle-pole protein. These results support a model in which katanin is targeted to spindle poles through a combination of direct microtubule binding by the p60 subunit and through interactions between the WD40 domain and an unknown protein. We propose that both domains of p80 are essential in precisely regulating katanin's activity in vivo.

Nek2B, a novel maternal form of Nek2 kinase, is essential for the assembly or maintenance of centrosomes in early Xenopus embryos

Nek2, a NIMA-related kinase, has been postulated to play a role in both the meiotic and mitotic cell cycles in vertebrates. Xenopus has two Nek2 splice variants, Nek2A and Nek2B, which are zygotic and maternal forms, respectively. Here we have examined the role of Nek2B in oocyte meiosis and early embryonic mitosis. Specific inhibition of Nek2B function does not interfere with the oscillation of Cdc2 activity in either the meiotic or mitotic cell cycles; however, it does cause abortive cleavage of early embryos, in which bipolar spindle formation is severely impaired due to fragmentation or dispersal of the centrosomes, to which endogenous Nek2B protein localizes. In contrast, inhibition of Nek2B function does not affect meiotic spindle formation in oocytes, in which functional centrosomes are absent. Thus, strikingly, Nek2B is specifically required for centrosome assembly and/or maintenance (and hence for normal bipolar spindle formation and cleavage) in early Xenopus embryos. Finally, (ectopic) Nek2A but not Nek2B is very labile in cleaving embryos, suggesting that Nek2A cannot replace the centrosomal function of Nek2B in early embryos.

Mechanisms of action and resistance to tubulin-binding agents

Tubulin binding agents constitute an important class of antimitotics and are widely used for the treatment of solid tumours an haematopoietic malignancies. These compounds, currently represented by the vinca alkaloids and the taxanes, differ from most of the other clinically useful antimitotics in that their target is not nucleic acids, but the mitotic spindle, which is an essential component of the mitotic machinery. Recent data on the mechanisms of action of and mechanisms of resistance to tubulin binding agents are presented. The importance of microtubule dynamics is emphasised, in particular in relationship to the usefulness of drug combinations. Concerning the reported resistance mechanisms, an emerging body of data show that altered microtubule structure may be involved in reduced sensitivity to these compounds. Promising new molecules, including those derived from marine organisms are described.

The Drosophila wispy gene is required for RNA localization and other microtubule-based events of meiosis and early embryogenesis

RNAs are localized by microtubule-based pathways to both the anterior and posterior poles of the developing Drosophila oocyte. We describe a new gene, wispy, required for localization of mRNAs to both poles of the egg. Embryos from wispy mothers arrest development after abnormal oocyte meiosis and failure of pronuclei to fuse. Our analysis of spindle and chromosome movements during meiosis reveals defects in spindle structures correlated with very high frequencies of chromosome nondisjunction and loss. Spindle defects include abnormally shaped spindles, spindle spurs, and ectopic spindles associated with lost chromosomes, as well as mispositioning of the meiosis II spindles. The polar body nuclei do not associate with their normal monastral arrays of microtubules, the sperm aster is reduced in size, and the centrosomes often dissociate from a mitotic spindle that forms in association with the male pronucleus. We show that wispy is required to recruit or maintain known centrosomal proteins with two types of microtubule organizing centers (MTOCs): (1) the central MTOC that forms between the meiosis II tandem spindles and (2) the centrosomes of the mitotic spindle. We propose that the wispy gene product functions directly in several microtubule-based events in meiosis and early embryogenesis and speculate about its possible mode of action.

Synergistic induction of centrosome hyperamplification by loss of p53 and cyclin E overexpression

Centrosome hyperamplification and the consequential mitotic defects contribute to chromosome instability in cancers. Loss or mutational inactivation of p53 has been shown to induce chromosome instability through centrosome hyperamplification. It has recently been found that Cdk2-cyclin E is involved in the initiation of centrosome duplication, and that constitutive activation of Cdk2-cyclin E results in the uncoupling of the centrosome duplication cycle and the DNA replication cycle. Cyclin E overexpression and p53 mutations occur frequently in tumors. Here, we show that cyclin E overexpression and loss of p53 synergistically increase the frequency of centrosome hyperamplification in cultured cells as well as in tumors developed in p53-null, heterozygous, and wildtype mice. Through examination of cells derived from Waf1-null mice, we further found that Waf1, a potent inhibitor of Cdk2-cyclin E and a major target of p53's transactivation function, is involved in coordinating the initiation of centrosome duplication and DNA replication, suggesting that Waf1 may act as a molecular link between p53 and Cdk2-cyclin E in the control of the centrosome duplication cycle.

Hsp90 is a core centrosomal component and is required at different stages of the centrosome cycle in Drosophila and vertebrates

To determine the molecular composition of the centrosome of a higher eukaryote, we carried out a systematic nano-electrospray tandem or MALDI mass spectrometry analysis of the polypeptides present in highly enriched preparations of immunoisolated Drosophila centrosomes. One of the proteins identified is Hsp83, a member of the highly conserved Hsp90 family including chaperones known to maintain the activity of many proteins but suspected to have other essential, unidentified functions. We have found that a fraction of the total Hsp90 pool is localized at the centrosome throughout the cell cycle at different stages of development in Drosophila and vertebrates. This association between Hsp90 and the centrosome can be observed in purified centrosomes and after treatment with microtubule depolymerizing drugs, two criteria normally used to define core centrosomal components. Disruption of Hsp90 function by mutations in the Drosophila gene or treatment of mammalian cells with the Hsp90 inhibitor geldanamycin, results in abnormal centrosome separation and maturation, aberrant spindles and impaired chromosome segregation. This suggests that another role of Hsp90 might be to ensure proper centrosome function.

Reconstitution of microtubule nucleation potential in centrosomes isolated from Spisula solidissima oocytes

Treatment of isolated Spisula solidissima centrosomes with KI removes (gamma)-tubulin, 25 nm rings, and their microtubule nucleation potential, revealing the presence of a filamentous lattice, the ‘centromatrix’. Treatment of this centromatrix with Spisula oocyte extract results in the binding of (gamma)-tubulin and 25 nm rings, and the recovery of microtubule nucleation potential. Fractionation of this extract resulted in the separation of elements that are required for the recovery of microtubule nucleation potential. We show that some, but not all, of the elements needed cosediment with microtubules. Further, extracts prepared from activated (meiotic) and non-activated (interphase) Spisula oocytes, CHO cells blocked in S phase, Drosophila embryos and Xenopus oocytes all support the recovery of microtubule nucleation potential by the Spisula centromatrix. These results demonstrate that components necessary for centrosome-dependent microtubule nucleation are functionally conserved and abundant in both interphase and meiotic/mitotic cytoplasm.

DNA-replication/DNA-damage-dependent centrosome inactivation in Drosophila embryos

During early embryogenesis of Drosophila melanogaster, mutations in the DNA-replication checkpoint lead to chromosome-segregation failures. Here we show that these segregation failures are associated with the assembly of an anastral microtubule spindle, a mitosis-specific loss of centrosome function, and dissociation of several components of the gamma-tubulin ring complex from a core centrosomal structure. The DNA-replication inhibitor aphidicolin and DNA-damaging agents trigger identical mitotic defects in wild-type embryos, indicating that centrosome inactivation is a checkpoint-independent and mitosis-specific response to damaged or incompletely replicated DNA. We propose that centrosome inactivation is part of a damage-control system that blocks chromosome segregation when replication/damage checkpoint control fails.

Delta-tubulin and epsilon-tubulin: two new human centrosomal tubulins reveal new aspects of centrosome structure and function

The centrosome organizes microtubules, which are made up of alpha-tubulin and beta-tubulin, and contains centrosome-bound gamma-tubulin, which is involved in microtubule nucleation. Here we identify two new human tubulins and show that they are associated with the centrosome. One is a homologue of the Chlamydomonas delta-tubulin Uni3, and the other is a new tubulin, which we have named epsilon-tubulin. Localization of delta-tubulin and epsilon-tubulin to the centrosome is independent of microtubules, and the patterns of localization are distinct from each other and from that of gamma-tubulin. Delta-tubulin is found in association with the centrioles, whereas epsilon-tubulin localizes to the pericentriolar material. epsilon-Tubulin exhibits a cell-cycle-specific pattern of localization, first associating with only the older of the centrosomes in a newly duplicated pair and later associating with both centrosomes. epsilon-Tubulin thus distinguishes the old centrosome from the new at the level of the pericentriolar material, indicating that there may be a centrosomal maturation event that is marked by the recruitment of epsilon-tubulin.

Cell cycle dependent localization of the telomeric PARP, tankyrase, to nuclear pore complexes and centrosomes

Tankyrase is a human poly(ADP-ribose) polymerase that was initially identified through its interaction with the telomeric protein TRF1, a negative regulator of telomere length. In vitro poly(ADP-ribosyl)ation by tankyrase inhibits TRF1 binding to telomeric DNA suggesting a role for tankyrase in telomere function. We previously demonstrated that tankyrase co-localizes with TRF1 at the ends of human chromosomes in metaphase. Here we show that tankyrase localizes to additional subcellular sites in a cell cycle dependent manner. In interphase, tankyrase co-localized with TRF1 to telomeres, but in addition was found to reside at nuclear pore complexes, as evidenced by indirect immunofluorescence, subcellular fractionation and immunoelectron microscopy. At mitosis, concomitant with nuclear envelope breakdown and nuclear pore complex disassembly, tankyrase was found to relocate around the pericentriolar matrix of mitotic centrosomes. This complex staining pattern along with the observation that tankyrase did not contain a nuclear localization signal suggested that its telomeric localization might be regulated, perhaps by TRF1. Indeed, localization of exogenously-expressed tankyrase to telomeres was dependent upon co-transfection with TRF1. These data indicate that the subcellular localization of tankyrase can be regulated by both the cell cycle and TRF1.

Microtubules in Xenopus oocytes are oriented with their minus-ends towards the cortex

Despite lacking centrosomes, stage VI Xenopus oocytes contain extensive networks of cytoplasmic microtubules (MTs). To gain additional insight into the factors regulating MT organization during oogenesis, we have used electron microscopy and "hook decoration" to examine the distribution and orientation of MTs in Xenopus oocytes. A limited survey of two "undecorated" stage VI oocytes revealed 218 MTs in images covering approximately 2,500 microm(2), and indicated that the MT number density of the animal cytoplasm was greater than that of the vegetal cytoplasm. Examination of five "decorated" stage VI oocytes (three animal and five vegetal hemispheres) revealed 653 MTs. Of these, 76% could be scored as having exclusively counterclockwise (CCW) or clockwise (CW) hooks. In the animal hemispheres, 93% of the scored MTs exhibited CCW hooks when viewed from the direction of the cortex, indicating that most MTs were oriented with their minus-ends out. MT orientation appeared relatively uniform throughout the animal cytoplasm: more than 90% of the scored MTs in the cortical (90%), subcortical (96%), or perinuclear (98%) cytoplasm were oriented with their minus-ends out. In the vegetal hemispheres, approximately 80% of the scored MTs exhibited CCW hooks, and thus were oriented with their minus-ends out; 96% of the scored MTs in stage III oocytes were oriented minus-end out. These observations support a model in which the cortex plays a significant role in MT nucleation and organization in Xenopus oocytes, and have significant implications for the MT-dependent transport and localization of cytoplasmic organelles and RNAs during oogenesis.

Components of an SCF ubiquitin ligase localize to the centrosome and regulate the centrosome duplication cycle

Centrosomes organize the mitotic spindle to ensure accurate segregation of the chromosomes in mitosis. The mechanism that ensures accurate duplication and separation of the centrosomes underlies the fidelity of chromosome segregation, but remains unknown. In Saccharomyces cerevisiae, entry into S phase and separation of spindle pole bodies each require CDC4 and CDC34, which encode components of an SCF (Skp1-cullin-F-box) ubiquitin ligase, but a direct (SCF) connection to the spindle pole body is unknown. Using immunofluorescence microscopy, we show that in mammalian cells the Skp1 protein and the cullin Cul1 are localized to interphase and mitotic centrosomes and to the cytoplasm and nucleus. Deconvolution and immunoelectron microscopy suggest that Skp1 forms an extended pericentriolar structure that may function to organize the centrosome. Purified centrosomes also contain Skp1, and Cul1 modified by the ubiquitin-like molecule NEDD8, suggesting a role for NEDD8 in targeting. Using an in vitro assay for centriole separation in Xenopus extracts, antibodies to Skp1 or Cul1 block separation. Proteasome inhibitors block both centriole separation in vitro and centrosome duplication in Xenopus embryos. We identify candidate centrosomal F-box proteins, suggesting that distinct SCF complexes may direct proteolysis of factors mediating multiple steps in the centrosome cycle.

Gamma-tubulin complexes: size does matter

gamma-Tubulin is a conserved component of all microtubule-organizing centres and is required for these organelles to nucleate microtubule polymerization. However, the mechanism of nucleation is not known. In addition to its localization to organizing centres, a large pool of gamma-tubulin exists in the cytoplasm in a complex with other proteins. The size of the gamma-tubulin complex and number of associated proteins vary among organisms, and the functional significance of these differences is unknown. Recently, the nature of these gamma-tubulin complexes has been explored in different organisms, and this has led us closer to a molecular understanding of microtubule nucleation.

Biparental inheritance of gamma-tubulin during human fertilization: molecular reconstitution of functional zygotic centrosomes in inseminated human oocytes and in cell-free extracts nucleated by human sperm

Human sperm centrosome reconstitution and the parental contributions to the zygotic centrosome are examined in mammalian zygotes and after exposure of spermatozoa to Xenopus laevis cell-free extracts. The presence and inheritance of the conserved centrosomal constituents gamma-tubulin, centrin, and MPM-2 (which detects phosphorylated epitopes) are traced, as is the sperm microtubule-nucleating capability on reconstituted centrosomes. gamma-Tubulin is biparentally inherited in humans (maternal >> than paternal): Western blots detect the presence of paternal gamma-tubulin. Recruitment of maternal gamma-tubulin to the sperm centrosome occurs after sperm incorporation in vivo or exposure to cell-free extract, especially after sperm "priming" induced by disulfide bond reduction. Centrin is found in the proximal sperm centrosomal region, demonstrates expected calcium sensitivity, but appears absent from the zygotic centrosome after sperm incorporation or exposure to extracts. Sperm centrosome phosphorylation is detected after exposure of primed sperm to egg extracts as well as during the early stages of sperm incorporation after fertilization. Finally, centrosome reconstitution in cell-free extracts permits sperm aster microtubule assembly in vitro. Collectively, these results support a model of a blended zygotic centrosome composed of maternal constituents attracted to an introduced paternal template after insemination.

Distribution of gamma-tubulin in multipolar spindles and multinucleated cells induced by dimethylarsinic acid, a methylated derivative of inorganic arsenics, in Chinese hamster V79 cells

Localization of gamma-tubulin, a well-characterized component of microtubule-organizing centers (MTOCs), was investigated because of interest in the mechanism of the induction of aberrant mitotic spindles in Chinese hamster V79 cells exposed to dimethylarsinic acid (DMAA). In control cultures, gamma-tubulin in interphase cells was located as a perinuclear dot on which the microtubules were nucleated. In metaphase cells, the location of gamma-tubulin coincided with that of the mitotic spindle poles. DMAA caused mitotic delay and aberrant spindles, such as tripolar- and quadripolar spindles, in the mitotic cells. Gamma-tubulin was co-localized with the aberrant spindles induced by DMAA. The incidence of gamma-tubulin in the mitotic cells coincided with that of the aberrant spindles and rose with an increasing concentration of DMAA. By contrast, DMAA did not influence the number and location of gamma-tubulin signals in interphase cells. These results suggest that multiple microtubule nucleation sites were induced by DMAA during transition from interphase to mitotic phase. DMAA-induced multiple signals of gamma-tubulin were integrated into one signal at the center of multinucleated cells, surrounded by multiple nuclei as the cell cycle progressed to the next interphase, suggesting the presence of a self-integration mechanism of centrosomal MTOCs during the cell cycle.

The sudden recruitment of gamma-tubulin to the centrosome at the onset of mitosis and its dynamic exchange throughout the cell cycle, do not require microtubules

gamma-Tubulin is a centrosomal component involved in microtubule nucleation. To determine how this molecule behaves during the cell cycle, we have established several vertebrate somatic cell lines that constitutively express a gamma-tubulin/green fluorescent protein fusion protein. Near simultaneous fluorescence and DIC light microscopy reveals that the amount of gamma-tubulin associated with the centrosome remains relatively constant throughout interphase, suddenly increases during prophase, and then decreases to interphase levels as the cell exits mitosis. This mitosis-specific recruitment of gamma-tubulin does not require microtubules. Fluorescence recovery after photobleaching (FRAP) studies reveal that the centrosome possesses two populations of gamma-tubulin: one that turns over rapidly and another that is more tightly bound. The dynamic exchange of centrosome-associated gamma-tubulin occurs throughout the cell cycle, including mitosis, and it does not require microtubules. These data are the first to characterize the dynamics of centrosome-associated gamma-tubulin in vertebrate cells in vivo and to demonstrate the microtubule-independent nature of these dynamics. They reveal that the additional gamma-tubulin required for spindle formation does not accumulate progressively at the centrosome during interphase. Rather, at the onset of mitosis, the centrosome suddenly gains the ability to bind greater than three times the amount of gamma-tubulin than during interphase.

The mammalian interphase centrosome: two independent units maintained together by the dynamics of the microtubule cytoskeleton

In mammalian cells the centrosome or diplosome is defined by the two parental centrioles observed in electron microscopy and by the pericentriolar material immunostained with several antibodies directed against various centrosomal proteins (gamma-tubulin, pericentrin, centrin and centractin). Partial destabilization of the microtubule cytoskeleton by microtubule-disassembling substances induced a splitting and a slow migration of the two diplosome units to opposite nuclear sides during most of the interphase in several mammalian cell lines. These units relocated close together following drug removal, while microtubule stabilization by nM taxol concentrations inhibited this process. Cytochalasin slowed down diplosome splitting but did not affect its relocation after colcemid washing. These results account for the apparently opposite effects induced by microtubule poisons on centriole separation. Moreover, they provide new information concerning the centrosome cycle and stability. First, the centrosome is formed by two units, distinguished only by the number of attached stable microtubules, but not by pericentrin, gamma-tubulin, centrin and centractin and their potency to nucleate microtubules. Second, the centrosomal units are independent during most of the interphase. Third, according to the cell type, these centrosomal units are localized in close proximity because they are either linked or maintained close together by the normal dynamics of the microtubule cytoskeleton. Finally, the relocalization of the centrosomal units with their centrioles in cells possessing one or two centrosomes suggests that their relative position results from the overall tensional forces involving at least partially the microtubule arrays nucleated by each of these entities.

The centrosomin protein is required for centrosome assembly and function during cleavage in Drosophila

Centrosomin is a 150 kDa centrosomal protein of Drosophila melanogaster. To study the function of Centrosomin in the centrosome, we have recovered mutations that are viable but male and female sterile (cnnmfs). We have shown that these alleles (1, 2, 3, 7, 8 and hk21) induce a maternal effect on early embryogenesis and result in the accumulation of low or undetectable levels of Centrosomin in the centrosomes of cleavage stage embryos. Hemizygous cnn females produce embryos that show dramatic defects in chromosome segregation and spindle organization during the syncytial cleavage divisions. In these embryos the syncytial divisions proceed as far as the twelfth cycle, and embryos fail to cellularize. Aberrant divisions and nuclear fusions occur in the early cycles of the nuclear divisions, and become more prominent at later stages. Giant nuclei are seen in late stage embryos. The spindles that form in mutant embryos exhibit multiple anomalies. There is a high occurrence of apparently linked spindles that share poles, indicating that Centrosomin is required for the proper spacing and separation of mitotic spindles within the syncytium. Spindle poles in the mutants contain little or no detectable amounts of the centrosomal proteins CP60, CP190 and (gamma)-tubulin and late stage embryos often do not have astral microtubules at their spindle poles. Spindle morphology and centrosomal composition suggest that the primary cause of these division defects in mutant embryos is centrosomal malfunction. These results suggest that Centrosomin is required for the assembly and function of centrosomes during the syncytial cleavage divisions.

Mitosis in filamentous fungi: how we got where we are

This review traces the principal advances in the study of mitosis in filamentous fungi from its beginnings near the end of the 19(th) century to the present day. Meiosis and mitosis had been accurately described and illustrated by the second decade of the present century and were known to closely resemble nuclear divisions in higher eukaryotes. This information was effectively lost in the mid-1950s, and the essential features of mitosis were then rediscovered from about the mid-1960s to the mid-1970s. Interest in the forces that separate chromatids and spindle poles during fungal mitosis followed closely on the heels of detailed descriptions of the mitotic apparatus in vivo and ultrastructurally during this and the following decade. About the same time, fundamental studies of the structure of fungal chromatin and biochemical characterization of fungal tubulin were being carried out. These cytological and biochemical studies set the stage for a surge of renewed interest in fungal mitosis that was issued in by the age of molecular biology. Filamentous fungi have provided model studies of the cytology and genetics of mitosis, including important advances in the study of mitotic forces, microtubule-associated motor proteins, and mitotic regulatory mechanisms.

Molecular characteristics of the centrosome

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

Computational prediction of the three-dimensional structures for the Caenorhabditis elegans tubulin family

In this article we characterize, from a structural point of view, all 16 members of the tubulin gene family of Caenorhabditis elegans (9 alpha-tubulins, 6 beta-tubulins, and 1 gamma-tubulin). We obtained their tertiary structures by computationally modifying the X-ray crystal structure of the pig brain alpha/beta-tubulin dimer published by Nogales et al. [Nature (London) 1998;391:199-203]. Our computational protocol involves changing the amino acids (with MIDAS; Jarvis et al., UCSF MIDAS. University of California, San Francisco, 1986) in the 3D structure of pig brain alpha/beta-tubulin dimer followed by geometry optimization with the AMBER force field (Perlman et al., AMBER 4. University of California, San Francisco, 1990). We subsequently analyze and compare the resulting structures in terms of the differences in their secondary and tertiary structures. In addition, we compare the pattern of hydrogen bonds and hydrophobic contacts in the guanosine triphosphate (GTP)-binding site for all members of the tubulin family. Our computational results show that, except for gamma-tubulin, all members of the C. elegans tubulin family have similar secondary and 3D structures and that the change in the pattern of hydrogen bonds in the GTP-binding site may be used to assess the relative stability of different alpha/beta-tubulin dimers formed by monomers of the tubulin family.

Gamma-tubulin complexes and their interaction with microtubule-organizing centers

Gamma-tubulin is as ubiquitous in eukaryotes as alpha- and beta-tubulin. Rather than forming part of the microtubule wall, however, gamma-tubulin is involved in microtubule nucleation. Although gamma-tubulin concentrates at microtubule-organizing centers, it also exists in a cytoplasmic complex whose size and complexity depends on the organism and cell type. In the past year, progress in understanding the functions of gamma-tubulin was made on two fronts: identifying the proteins that interact with gamma-tubulin and identifying the proteins that interact with the gamma-tubulin complex to tether it to the microtubule-organizing center.

Centrosome hyperamplification in human cancer: chromosome instability induced by p53 mutation and/or Mdm2 overexpression

We have previously reported that loss of p53 tumor suppressor protein results in centrosome hyperamplification, which leads to aberrant mitosis and chromosome instability. Since p53 is either deleted or mutated in human cancers at a high frequency, we investigated whether human cancers showed centrosome hyperamplification. Screening of advanced stage breast ductal carcinomas and squamous cell carcinomas of the head and neck (SCCHN) revealed that centrosome hyperamplification is frequent in both tumor types. Moreover, through the analyses of p53 in SCCHN samples by direct sequencing and by loss-of-heterozygosity test, we found that p53 mutations correlated with occurrence of centrosome hyperamplification. However, in some cases, we observed centrosome hyperamplification in tumors that retained wild-type p53. These tumors contained high levels of Mdm2. Since Mdm2 can inactivate p53 through physical association, we investigated whether Mdm2 overexpression induced centrosome hyperamplification. We found that Mdm2 overexpression, like loss of p53, induced centrosome hyperamplification and chromosome instability in cultured cells.

Confocal microscopy and 3-D reconstruction of the cytoskeleton of Xenopus oocytes

Xenopus oocytes contain a complex cytoskeleton composed of three filament systems: (1) microtubules, composed of tubulin and at least three different microtubule-associated proteins (XMAPs); (2) microfilaments composed of actin and associated proteins; and (3) intermediate filaments, composed of keratins. For the past several years, we have used confocal immunofluorescence microscopy to characterize the organization of the oocyte cytoskeleton throughout the course of oogenesis. Together with computer-assisted reconstruction of the oocyte in three dimensions, confocal microscopy gives an unprecedented view of the assembly and reorganization of the cytoskeleton during oocyte growth and differentiation. Results of these studies, combined with the effects of cytoskeletal inhibitors, suggest that organization of the cytoskeleton in Xenopus oocytes is dependent upon a hierarchy of interactions between microtubules, microfilaments, and keratin filaments. This article presents a gallery of confocal images and 3-D reconstructions depicting the assembly and organization of the oocyte cytoskeleton during stages 0-VI of oogenesis, a discussion of the mechanisms that might regulate cytoskeletal organization during oogenesis, and speculates on the potential roles of the oocyte cytoskeleton during oogenesis and axis formation.

Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death

PURPOSE

To analyze the available data concerning mechanisms of action of and mechanisms of resistance to the antitubulin agents, vinca alkaloids and taxanes, and more recently described compounds.

DESIGN

We conducted a review of the literature on classic and recent antitubulin agents, focusing particularly on the relationships between antitubulin agents and their intracellular target, the soluble tubulin/microtubule complex.

RESULTS AND CONCLUSION

Although it is widely accepted that antitubulin agents block cell division by inhibition of the mitotic spindle, the mechanism of action of antitubulin agents on microtubules remains to be determined. The classic approach is that vinca alkaloids depolymerize microtubules, thereby increasing the soluble tubulin pool, whereas taxanes stabilize microtubules and increase the microtubular mass. More recent data suggest that both classes of agents have a similar mechanism of action, involving the inhibition of microtubule dynamics. These data suggest that vinca alkaloids and taxanes may act synergistically as antitumor agents and may be administered as combination chemotherapy in the clinic. However, enhanced myeloid and neurologic toxicity, as well as a strong dependence on the sequence of administration, presently exclude these combinations outside the context of clinical trials. Although the multidrug resistance phenotype mediated by Pgp appears to be an important mechanism of resistance to these agents, alterations of microtubule structure resulting in altered microtubule dynamics and/or altered binding of antitubulin agents may constitute a significant mechanism of drug resistance.

Cytological characterisation of the mutant phenotypes produced during early embryogenesis by null and loss-of-function alleles of the gammaTub37C gene in Drosophila

We have studied the mutant phenotypes brought about during early embryogenesis by mutation in the gammaTub37C gene, one of the two isoforms of gamma-tubulin that have been identified in Drosophila. We have focused our attention on fs(2)TW1(1) and fs(2)TW1(RU34), a null and a hypomorph allele of this gene, whose sequences we report in this work. We have found that the abnormal meiotic figures observed in mutant stage 14 oocytes are not observed in laid oocytes or fertilised embryos, suggesting that these abnormal meiotic figures are not terminally arrested. We have also concluded that both null and hypomorph alleles lead to a total arrest of nuclear proliferation during early embryogenesis. This is in contrast to their effect on female meiosis-I where hypomorph alleles display a much weaker phenotype. Finally, we have observed that null and hypomorph alleles lead to some distinct phenotypes. Unfertilised laid oocytes and fertilised embryos deficient for gammaTub37C do not contain polar bodies and have a few bipolar microtubule arrays. In contrast, oocytes and embryos from weaker alleles do not have these microtubule arrays, but do contain polar bodies, or polar-body-like structures. These results indicate that gammaTub37C is essential for nuclear proliferation in the early Drosophila embryo.

Tobacco BY-2 cell-free extracts induce the recovery of microtubule nucleating activity of inactivated mammalian centrosomes

The structure and the molecular composition of the microtubule-organizing centers in acentriolar higher plant cells remain unknown. We developed an in vitro complementation assay where tobacco BY-2 extracts can restore the microtubule-nucleating activity of urea-inactivated mammalian centrosomes. Our results provide first evidence that soluble microtubule-nucleating factors are present in the plant cytosolic fraction. The implication for microtubule nucleation in higher plants is discussed.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Centrosomal control of microtubule dynamics

In many animal cells, minus ends of microtubules (MTs) are thought to be capped by the centrosome whereas plus ends are free and display dynamic instability. We tested the role of the centrosome by examining MT behavior in cytoplasts from which the centrosome was removed. Cells were injected with Cy3-tubulin to fluorescently label MTs and were enucleated by using a centrifugation procedure. Enucleation resulted in a mixture of cytoplasts containing or lacking the centrosome. Fibroblast (CHO-K1) and epithelial (BSC-1) cells were investigated. In fibroblast cytoplasts containing the centrosome, MTs showed dynamic instability indistinguishable from that in intact cells. In contrast, in cytoplasts lacking the centrosome, MTs treadmilled-shortened at the minus end at about 12 micrometers/min while growing at the plus end at the same rate. The change in behavior of the plus end from dynamic instability to persistent growth correlated with an elevated level of free tubulin subunits (78% in centrosome-free cytoplasts vs. 44% in intact cells) generated by minus-end depolymerization. In contrast to fibroblast cells, in centrosome-free cytoplasts prepared from epithelial cells, MTs displayed dynamic instability at plus ends and relative stability at minus ends presumably because of specific minus-end stability factors distributed throughout the cytoplasm. We suggest that, in fibroblast cells, a minus-end depolymerization mechanism functions to eliminate errors in MT organization and that dynamic instability of MT plus ends is a result of capping of minus ends by the centrosome.

GAMMA.-Tubulin at Ten. Progress and Prospects

The existence of gamma-tubulin was first reported approximately ten years ago, and it is appropriate to review the progress that has been made in gamma-tubulin research and to discuss some of the unanswered questions about gamma-tubulin function. gamma-Tubulin is ubiquitous in eukaryotes and is generally quite conserved. Two highly divergent gamma-tubulins have been discovered, however, one in Saccharomyces cerevisiae and one in Caenorhabditis elegans. Several organisms have two gamma-tubulin genes. In Drosophila melanogaster, the two gamma-tubulins differ significantly in sequence and expression pattern. In other organisms the two gamma-tubulins are almost identical and expression patterns have not been determined. gamma-Tubulin is located at microtubule organizing centers in many organisms, and it is also frequently associated with the mitotic spindle. gamma-Tubulin is essential for the formation of functional mitotic spindles in all organisms that have been examined to date. In animal cells, complexes containing gamma-tubulin are located at microtubule organizing centers where they nucleate the assembly of microtubules. In spite of the considerable progress that has been made in gamma-tubulin research important questions remain to be answered. The exact mechanisms of microtubule nucleation by gamma-tubulin complexes remain to be resolved as do the mechanisms by which microtubule nucleation from gamma-tubulin complexes is regulated. Finally, there is evidence that gamma-tubulin has important functions in addition to microtubule nucleation, and these functions are just beginning to be investigated.

Loss of Hsp70-Hsp40 chaperone activity causes abnormal nuclear distribution and aberrant microtubule formation in M-phase of Saccharomyces cerevisiae

The 70-kDa heat shock proteins, hsp70, are highly conserved among both prokaryotes and eukaryotes, and function as chaperones in diverse cellular processes. To elucidate the function of the yeast cytosolic hsp70 Ssa1p in vivo, we characterized a Saccharomyces cerevisiae ssa1 temperature-sensitive mutant (ssa1-134). After shifting to the restrictive temperature (37 degreesC), ssa1-134 mutant cells showed abnormal distribution of nuclei and accumulated as large-budded cells with a 2 N DNA content. We observed more prominent mutant phenotypes using nocodazole-synchronized cells: when cells were incubated at the restrictive temperature following nocodazole treatment, viability was rapidly lost and abnormal arrays of bent microtubules were formed. Chemical cross-linking and immunoprecipitation analyses revealed that the interaction of mutant Ssa1p with Ydj1p (cytosolic DnaJ homologue in yeast) was much weaker compared with wild-type Ssa1p. These results suggest that Ssa1p and Ydj1p chaperone activities play important roles in the regulation of microtubule formation in M phase. In support of this idea, a ydj1 null mutant at the restrictive temperature was found to exhibit more prominent phenotypes than ssa1-134. Furthermore, both ssa1-134 and ydj1 null mutant cells exhibited greater sensitivity to anti-microtubule drugs. Finally, the observation that SSA1 and YDJ1 interact genetically with a gamma-tubulin, TUB4, supports the idea that they play a role in the regulation of microtubule formation.

BRCA1 is associated with the centrosome during mitosis

Centrosomes and their associated microtubules direct events during mitosis and control the organization of animal cell structures and movement during interphase. The centrosome replicates during the cell cycle, directs the assembly of bipolar mitotic spindles, and plays an important role in maintaining the fidelity of cell division. Recently, tumor suppressors such as p53 and retinoblastoma protein pRB have been localized to the centrosome in a cell cycle-dependent manner. Immunofluorescence microscopy and analysis of isolated centrosomes now provide evidence that BRCA1 protein, a suppressor of tumorigenesis in breast and ovary, also is associated with centrosomes during mitosis. Our results indicate that BRCA1 localizes with the centrosome during mitosis and coimmunoprecipitates with gamma-tubulin, a centrosomal component essential for nucleation of microtubules. Furthermore, gamma-tubulin associates preferentially with a hypophosphorylated form of BRCA1.

The Drosophila POLO kinase localises to multiple compartments of the mitotic apparatus and is required for the phosphorylation of MPM2 reactive epitopes

The MPM2 antibody is a valuable tool for studying the regulation of mitotic events since it specifically recognises a subset of mitosis-specific phosphoproteins. Some MPM2 epitopes have been shown to be phosphorylated by p34(cdc2). However, recent results suggest that the newly emerging family of polo-like kinases (Plks) may also act as MPM2 kinases. In this study, we present evidence suggesting that the Drosophila POLO protein is required for the phosphorylation of MPM2 reactive epitopes. POLO displays a dynamic localisation pattern during mitosis, which parallels that of the MPM2 phosphoepitopes, since it is found in the centrosome and centromere from early prophase until late anaphase, the microtubule-overlapping region during anaphase, and the region on either side of the midbody during telophase. Centromere localisation is not dependent upon microtubules since it is retained in colchicine-arrested cells and is present in isolated chromosomes. Furthermore, the level of MPM2 immunoreactivity is directly correlated to the severity of the polo mutant alleles. In cells carrying a hypomorphic allele, the centrosomes of abnormal cells are small and fail to efficiently recruit MPM2 epitopes. In neuroblasts homozygous for a severe loss-of-function allele, the mitotic index is low and the MPM2 labelling is severely reduced or absent. Finally, rephosphorylation of MPM2 epitopes in detergent-extracted Schneider cells requires either POLO stably bound to the cytoskeletons or POLO present in soluble extracts. These results suggest that POLO is required for the phosphorylation of MPM2 epitopes in Drosophila, at the centrosomes, centromeres and the mitotic spindle, and thus might be involved in co-ordinating the mitotic changes of cellular architecture with the activity of the maturation promoting factor.

Recruitment of the gamma-tubulin ring complex to Drosophila salt-stripped centrosome scaffolds

Extracting isolated Drosophila centrosomes with 2 M KI generates salt-resistant scaffolds that lack the centrosomal proteins CP190, CP60, centrosomin, and gamma-tubulin. To clarify the role of these proteins in microtubule nucleation by centrosomes and to identify additional centrosome components required for nucleation, we have developed an in vitro complementation assay for centrosome function. Centrosome aster formation is reconstituted when these inactive, salt-stripped centrosome scaffolds are supplemented with a soluble fraction of a Drosophila embryo extract. The CP60 and CP190 can be removed from this extract without effect, whereas removing the gamma-tubulin destroys the complementing activity. Consistent with these results, we find no evidence that these three proteins form a complex together. Instead, gamma-tubulin is found in two distinct protein complexes of 240,000 and approximately 3,000,000 D. The larger complex, which is analogous to the Xenopus gamma-tubulin ring complex (gammaTuRC) (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578-583), is necessary but not sufficient for complementation. An additional factor found in the extract is required. These results provide the first evidence that the gammaTuRC is required for microtubule nucleation at the centrosome.

Spindle self-organization and cytokinesis during male meiosis in asterless mutants of Drosophila melanogaster

While Drosophila female meiosis is anastral, both meiotic divisions in Drosophila males exhibit prominent asters. We have identified a gene we call asterless (asl) that is required for aster formation during male meiosis. Ultrastructural analysis showed that asl mutants have morphologically normal centrioles. However, immunostaining with antibodies directed either to gamma tubulin or centrosomin revealed that these proteins do not accumulate in the centrosomes, as occurs in wild-type. Thus, asl appears to specify a function required for the assembly of centrosomal material around the centrioles. Despite the absence of asters, meiotic cells of asl mutants manage to develop an anastral spindle. Microtubules grow from multiple sites around the chromosomes, and then focus into a peculiar bipolar spindle that mediates chromosome segregation, although in a highly irregular way. Surprisingly, asl spermatocytes eventually form a morphologically normal ana-telophase central spindle that has full ability to stimulate cytokinesis. These findings challenge the classical view on central spindle assembly, arguing for a self-organization of this structure from either preexisting or newly formed microtubules. In addition, these findings strongly suggest that the asters are not required for signaling cytokinesis.

A genetic analysis of interactions with Spc110p reveals distinct functions of Spc97p and Spc98p, components of the yeast gamma-tubulin complex

The spindle pole body (SPB) in Saccharomyces cerevisiae functions as the microtubule-organizing center. Spc110p is an essential structural component of the SPB and spans between the central and inner plaques of this multilamellar organelle. The amino terminus of Spc110p faces the inner plaque, the substructure from which spindle microtubules radiate. We have undertaken a synthetic lethal screen to identify mutations that enhance the phenotype of the temperature-sensitive spc110-221 allele, which encodes mutations in the amino terminus. The screen identified mutations in SPC97 and SPC98, two genes encoding components of the Tub4p complex in yeast. The spc98-63 allele is synthetic lethal only with spc110 alleles that encode mutations in the N terminus of Spc110p. In contrast, the spc97 alleles are synthetic lethal with spc110 alleles that encode mutations in either the N terminus or the C terminus. Using the two-hybrid assay, we show that the interactions of Spc110p with Spc97p and Spc98p are not equivalent. The N terminus of Spc110p displays a robust interaction with Spc98p in two different two-hybrid assays, while the interaction between Spc97p and Spc110p is not detectable in one strain and gives a weak signal in the other. Extra copies of SPC98 enhance the interaction between Spc97p and Spc110p, while extra copies of SPC97 interfere with the interaction between Spc98p and Spc110p. By testing the interactions between mutant proteins, we show that the lethal phenotype in spc98-63 spc110-221 cells is caused by the failure of Spc98-63p to interact with Spc110-221p. In contrast, the lethal phenotype in spc97-62 spc110-221 cells can be attributed to a decreased interaction between Spc97-62p and Spc98p. Together, these studies provide evidence that Spc110p directly links the Tub4p complex to the SPB. Moreover, an interaction between Spc98p and the amino-terminal region of Spc110p is a critical component of the linkage, whereas the interaction between Spc97p and Spc110p is dependent on Spc98p.

Maternally expressed gamma Tub37CD in Drosophila is differentially required for female meiosis and embryonic mitosis

We report functional analysis of gamma Tub37CD, a maternally synthesized gamma-tubulin that is highly expressed during oogenesis and utilized at centrosomes in precellular embryos. Two gamma Tub37CD mutants contained missense mutations that altered residues conserved in all gamma-tubulins and alpha- and/or beta-tubulins. A third gamma Tub37CD missense mutant identified a conserved motif unique to gamma-tubulins. A fourth gamma Tub37CD mutant contained a nonsense mutation and the corresponding premature stop codon generated a protein null allele. Immunofluorescence analysis of laid eggs and activated oocytes derived from the mutants revealed microtubules and meiotic spindles that were close to normal even in the absence of gamma Tub37CD. Eggs lacking the maternal gamma-tubulin were arrested in meiosis, indicative of a deficiency in activation. Analysis of meiosis with in vitro activation techniques showed that the cortical microtubule cytoskeleton of mature wild-type eggs was reorganized upon activation and expressed as transient assembly of cortical asters, and this cortical reorganization was altered in gamma Tub37CD mutants. In precellular embryos of partial loss of function mutants, spindles were frequently abnormal and cell cycle progression was inhibited. Thus, gamma Tub37CD functions differentially in female meiosis and in the early embryo; while involved in oocyte activation, it is apparently not required or plays a subtle role in formation of the female meiotic spindle which is acentriolar, but is essential for assembly of a discrete bipolar mitotic spindle which is directed by centrosomes organized about centrioles.

Katanin is responsible for the M-phase microtubule-severing activity in Xenopus eggs

Microtubules are dynamic structures whose proper rearrangement during the cell cycle is essential for the positioning of membranes during interphase and for chromosome segregation during mitosis. The previous discovery of a cyclin B/cdc2-activated microtubule-severing activity in M-phase Xenopus egg extracts suggested that a microtubule-severing protein might play an important role in cell cycle-dependent changes in microtubule dynamics and organization. However, the isolation of three different microtubule-severing proteins, p56, EF1alpha, and katanin, has only confused the issue because none of these proteins is directly activated by cyclin B/cdc2. Here we use immunodepletion with antibodies specific for a vertebrate katanin homologue to demonstrate that katanin is responsible for the majority of M-phase severing activity in Xenopus eggs. This result suggests that katanin is responsible for changes in microtubules occurring at mitosis. Immunofluorescence analysis demonstrated that katanin is concentrated at a microtubule-dependent structure at mitotic spindle poles in Xenopus A6 cells and in human fibroblasts, suggesting a specific role in microtubule disassembly at spindle poles. Surprisingly, katanin was also found in adult mouse brain, indicating that katanin may have other functions distinct from its mitotic role.

The mammalian gamma-tubulin complex contains homologues of the yeast spindle pole body components spc97p and spc98p

gamma-Tubulin is a universal component of microtubule organizing centers where it is believed to play an important role in the nucleation of microtubule polymerization. gamma-Tubulin also exists as part of a cytoplasmic complex whose size and complexity varies in different organisms. To investigate the composition of the cytoplasmic gamma-tubulin complex in mammalian cells, cell lines stably expressing epitope-tagged versions of human gamma-tubulin were made. The epitope-tagged gamma-tubulins expressed in these cells localize to the centrosome and are incorporated into the cytoplasmic gamma-tubulin complex. Immunoprecipitation of this complex identifies at least seven proteins, with calculated molecular weights of 48, 71, 76, 100, 101, 128, and 211 kD. We have identified the 100- and 101-kD components of the gamma-tubulin complex as homologues of the yeast spindle pole body proteins Spc97p and Spc98p, and named the corresponding human proteins hGCP2 and hGCP3. Sequence analysis revealed that these proteins are not only related to their respective homologues, but are also related to each other. GCP2 and GCP3 colocalize with gamma-tubulin at the centrosome, cosediment with gamma-tubulin in sucrose gradients, and coimmunoprecipitate with gamma-tubulin, indicating that they are part of the gamma-tubulin complex. The conservation of a complex involving gamma-tubulin, GCP2, and GCP3 from yeast to mammals suggests that structurally diverse microtubule organizing centers such as the yeast spindle pole body and the animal centrosome share a common molecular mechanism for microtubule nucleation.

Characterization of the human homologue of the yeast spc98p and its association with gamma-tubulin

A trimeric complex formed by Tub4p, the budding yeast gamma-tubulin, and the two spindle pole body components, Spc98p and Spc97p, has recently been characterized in Saccharomyces cerevisiae. We reasoned that crucial functions, such as the control of microtubule nucleation, could be maintained among divergent species. SPC98-related sequences were searched in dbEST using the BLASTN program. Primers derived from the human expressed sequence tag matching SPC98 were used to clone the 5' and 3' cDNA ends by rapid amplification of cDNA ends (RACE)-PCR. The human Spc98 cDNA presents an alternative splicing at the 3' end. The deduced protein possesses 22% identity and 45% similarity with the yeast homologue. We further report that the human Spc98p, like gamma-tubulin, is concentrated at the centrosome, although a large fraction is found in cytosolic complexes. Sucrose gradient sedimentation of the cytosolic fraction and immunoprecipitation experiments demonstrate that both gamma-tubulin and HsSpc98p are in the same complex. Interestingly, Xenopus sperm centrosomes, which are incompetent for microtubule nucleation before their activation in the egg cytoplasm, were found to contain similar amounts of both Spc98p and gamma-tubulin to human somatic centrosomes, which are competent for microtubule nucleation. Finally, affinity-purified antibodies against Spc98p inhibit microtubule nucleation on isolated centrosomes, as well as in microinjected cells, suggesting that this novel protein is indeed required for the nucleation reaction.