gamma-Tubulin in mammalian cells: the centrosomal and the cytosolic forms

Paxillin localizes to the lymphocyte microtubule organizing center and associates with the microtubule cytoskeleton

Paxillin is a focal adhesion-associated protein that functions as a multi-domain adapter protein, binding several structural and signaling molecules. alpha-Tubulin was identified as an interacting protein in a two-hybrid screen using the paxillin C-terminal LIM domain as a bait. In vitro binding assays with glutathione S-transferase-paxillin demonstrated an interaction of alpha-tubulin with the C terminus of paxillin. Another member of the tubulin family, gamma-tubulin, bound to both the N and the C terminus of paxillin. The interaction between paxillin and both alpha- and gamma-tubulin in vivo was confirmed by co-immunoprecipitation from human T lymphoblasts. Immunofluorescence studies revealed that, in adherent T cells, paxillin localized to sites of cell-matrix interaction as well as to a large perinuclear region. Confocal microscopy revealed that this region corresponds to the lymphocyte microtubule organizing center, where paxillin colocalizes with alpha- and gamma-tubulin. The localization of paxillin to this area was observed in cells in suspension as well as during adhesion to integrin ligands. These data constitute the first characterization of the interaction of paxillin with the microtubule cytoskeleton, and suggest that paxillin, in addition to its well established role at focal adhesions, could also be associated with the lymphocyte microtubule network.

Characterization of gamma-tubulin complexes in Aspergillus nidulans and detection of putative gamma-tubulin interacting proteins

gamma-Tubulin is central to the nucleation of microtubule assembly in vivo. Although it is most obviously located at microtubule organizing centers, it is also found in soluble cytoplasmic complexes. Characterizing these complexes and identifying proteins that interact with gamma-tubulin in vivo will be necessary if gamma-tubulin function is to be understood fully. We have begun to investigate soluble complexes of gamma-tubulin in Aspergillus nidulans, the organism in which gamma-tubulin was discovered and in which a great deal of genetic and molecular genetic analysis of gamma-tubulin has been carried out. We find that approximately 32% of the gamma-tubulin in A. nidulans is soluble. Sucrose density gradients revealed that the soluble gamma-tubulin is in 8-20S complexes with little or no monomeric gamma-tubulin present. In the presence of 0.5 M KCl the average size of the complexes decreased and a peak was present between 4S and 11S. Cross-linking experiments with a zero-length cross-linker suggest that gamma-tubulin in isolated nuclei and in intact hyphae interacts physically with three proteins with molecular weights of approximately 105, 95, and 80 kDa.

Expression of glycylated tubulin during the differentiation of spermatozoa in mammals

Using quantitative immunogold analyses of tubulin isoforms we previously demonstrated a unique differential expression of glutamylated tubulin in the flagellum of mouse and man spermatozoa [Fouquet et al., 1997: Tissue Cell 29:573-583]. We have performed similar analyses for glycylated tubulin using two monoclonal antibodies, TAP 952 and AXO 49, directed to mono- and polyglycylated tubulin respectively. Glycylated tubulin was not found in centrioles and cytoplasmic microtubules (manchette) of germ cells. In mouse and man, axonemal tubulin was first monoglycylated and uniformly distributed in all doublets at all levels of the flagellum in elongating spermatids. In human mature spermatozoa axonemal microtubules were enriched in monoglycylated tubulin from the base to the tip of the flagellum. In mouse sperm flagellum a similar gradient of monoglycylated tubulin was also observed in addition to an opposite gradient of polyglycylated tubulin. In both species, monoglycylated tubulin labeling predominated in doublets 3-8 whereas glutamylated tubulin labeling [Fouquet et al., 1997] predominated in doublets 1-5-6. These differential labelings were suppressed after motility inhibition of mouse spermatozoa by sodium azide treatment and in non-motile human spermatozoa lacking dynein arms. The unique distribution of these tubulin isoforms and the known inhibition of motility induced by their specific antibodies are consistent with a complementary role of tubulin glycylation and glutamylation in the regulation of flagellar beating in mammalian spermatozoa.

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.

Purification and characterization of basal apparatuses from a flagellate green alga

Basal apparatuses consisting of two basal bodies and several attached fibers were isolated from the naked green flagellate Spermatozopsis similis by detergent extraction and mechanical disintegration. Sucrose density centrifugation yielded highly enriched basal apparatuses as shown by electron microscopy. SDS-PAGE revealed the absence of histones, indicating the removal of nuclear contaminations from the isolated basal apparatuses. A mass spectrometric analysis of the carboxyterminal peptides of alpha tubulin documented detyrosination and glutamylation as posttranslational modifications and showed that some 5% of the alpha tubulin carries a polyglutamyl side chain which can reach at least 17 residues in length. Monoclonal antibodies raised against the purified basal apparatuses were used to characterize novel components in the basal apparatus. A 210-kD component identified by mAB BAS (basal apparatus of Spermatozopsis) 1.4 was localized in the flagellar transitional region by immunogold electron microscopy. Antibody BAS 16.4 reacted with two high molecular weight bands (approximately 265 and 240 kD) in Western blotting and decorated a fiber attached to the proximal end of the basal bodies. Immunofluorescence staining of isolated cytoskeletons with these mABs demonstrated that the antigens are also present in the basal apparatuses of Chlamydomonas reinhardtii and Dunahella bioculata. These antibodies are useful tools for the molecular cloning of components from the basal apparatus.

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.

Sequence analysis and immunofluorescence study of alpha- and beta-tubulins in Reticulomyxa filosa: implications of the high degree of beta2-tubulin divergence

We have cloned and sequenced 2 alpha- and 2 beta-tubulin isoforms from the giant freshwater amoeba Reticulomyxa filosa. The microtubules of this organism exhibit some unusual properties, including the highest rates of assembly and disassembly known and the inability to be stabilized by taxol. The cloned alpha-tubulins show a high degree of identity when compared to an alpha-tubulin consensus sequence. The beta-tubulins, however, are more divergent, the beta2-tubulin being the most unusual beta-tubulin found so far. The deduced amino acid sequence of beta2 shows 55% identity to a beta-tubulin consensus sequence. It also features 51 unique exchanges which cluster in the C-terminal half of the molecule. Several unique exchanges and two insertions occur in regions adjacent to, or directly implicated in, conserved beta-tubulin functions. A phylogenetic analysis places the beta-tubulins of R. filosa in the vicinity of beta-tubulins from fungi and slime molds. Monoclonal and polyclonal antibodies raised against R. filosa tubulins show that the electrophoretic mobility of alpha- and beta-tubulins is reversed with respect to tubulins from most other sources. Immunofluorescence experiments reveal a ubiquitous distribution of both beta-tubulins in the amoebal network. Our observations suggest possible links between the aberrant primary structure of the beta2-tubulin and the unusual properties of R. filosa microtubules.

A new function for the gamma-tubulin ring complex as a microtubule minus-end cap

Microtubule nucleation from centrosomes involves a lockwasher-shaped protein complex containing gamma-tubulin, named the gamma-tubulin ring complex (gammaTuRC). Here we investigate the mechanism by which the gammaTuRC nucleates microtubules, using a direct labelling method to visualize the behaviour of individual gammaTuRCs. A fluorescently-labelled version of the gammaTuRC binds to the minus ends of microtubules nucleated in vitro. Both gammaTuRC-mediated nucleation and binding of the gammaTuRC to preformed microtubules block further minus-end growth and prevent microtubule depolymerization. The gammaTuRC therefore acts as a minus-end-capping protein, as confirmed by electron-microscopic examination of gold-labelled gammaTuRCs. These data support a nucleation model for gammaTuRC function that involves capping of microtubules.

The respective contributions of the mother and daughter centrioles to centrosome activity and behavior in vertebrate cells

We have generated several stable cell lines expressing GFP-labeled centrin. This fusion protein becomes concentrated in the lumen of both centrioles, making them clearly visible in the living cell. Time-lapse fluorescence microscopy reveals that the centriole pair inherited after mitosis splits during or just after telophase. At this time the mother centriole remains near the cell center while the daughter migrates extensively throughout the cytoplasm. This differential behavior is not related to the presence of a nucleus because it is also observed in enucleated cells. The characteristic motions of the daughter centriole persist in the absence of microtubules (Mts). or actin, but are arrested when both Mts and actin filaments are disrupted. As the centrioles replicate at the G1/S transition the movements exhibited by the original daughter become progressively attenuated, and by the onset of mitosis its behavior is indistinguishable from that of the mother centriole. While both centrioles possess associated gamma-tubulin, and nucleate similar number of Mts in Mt repolymerization experiments. during G1 and S only the mother centriole is located at the focus of the Mt array. A model, based on differences in Mt anchoring and release by the mother and daughter centrioles, is proposed to explain these results.

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.

gamma-tubulin complexes: binding to the centrosome, regulation and microtubule nucleation

Microtubule assembly is initiated in vivo by gamma-tubulin complexes. Cytoplasmic gamma-tubulin complexes are targeted to centrosomes or to other microtubule organizing centers (MTOCs) via a set of so called gamma-tubulin complex binding proteins (GTBPs) that probably interact with the conserved Spc97p/Spc98p protein family of gamma-tubulin complexes. In other cell types, gamma-tubulin complexes may initiate microtubule formation near chromosomes in a MTOC-independent manner. Recently, major advances have been achieved through the finding that gamma-tubulin, Spc97p and Spc98p form a conserved core that is probably responsible for microtubule nucleation, and by the discovery that a yeast GTBP is regulated in a cell-cycle-dependent manner and in response to an external signal. Furthermore, it was found that the small GTPase Ran in its GDP-bound state may promote spindle assembly. In addition, an essential function of gamma-tubulin in basal body duplication has been demonstrated in Paramecium.

Retinoic acid differentiation of HL-60 cells promotes cytoskeletal polarization

Retinoic acid (RA) treatment of HL-60 cells in vitro induces granulocytic differentiation, involving reorganization of the nucleus and cytoplasm, development of chemoattractant-directed migration, and eventual apoptosis. The present studies with HL-60/S4 cells document that major elements of the cytoskeleton are changed: actin increases by 50%; vimentin decreases by more than 95%. The cellular content of alpha-tubulin does not significantly change; but the centrosomal-microtubule (MT) array moves away from the lobulating nucleus. Cytoskeletal-modifying chemicals modulate this polarized reorganization: Taxol and cytochalasin D enhance centrosome movement; nocodazole reverses it. Cytoskeletal-modifying chemicals do not appear to affect nuclear lobulation or the integrity of envelope-limited chromatin sheets (ELCS). Employing bcl-2-overexpressing HL-60 cells permitted demonstration of nuclear lobulation, ELCS formation, and centrosome-MT movement concomitantly during RA-induced differentiation, implying independence between the cellular reorganization and apoptotic programs. RA appears to promote an inherent potential in HL-60 cells for cytoskeletal polarization, likely to be important for chemoattractant-directed cell migration, an established characteristic of mature granulocytes.

Activity and regulation of the centrosome-associated proteasome

Regulated proteolysis is important for maintaining appropriate cellular levels of many proteins. The bulk of intracellular protein degradation is catalyzed by the proteasome. Recently, the centrosome was identified as a novel site for concentration of the proteasome and associated regulatory proteins (Wigley, W. C., Fabunmi, R. P., Lee, M. G., Marino, C. R., Muallem, S., DeMartino, G. N., and Thomas, P. J. (1999) J. Cell Biol. 145, 481-490). Here we provide evidence that centrosomes contain the active 26 S proteasome that degrades ubiquitinated-protein and proteasome-specific peptide substrates. Moreover, the centrosomes contain an ubiquitin isopeptidase activity. The proteolytic activity is ATP-dependent and is inhibited by proteasome inhibitors. Notably, treatment of cells with inhibitors of proteasome activity promotes redistribution of the proteasome and associated regulatory proteins to the centrosome independent of an intact microtubule system. These data provide biochemical evidence for active proteasomal complexes at the centrosome, highlighting a novel function for this organizing structure.

XYbp, a novel RING-finger protein, is a component of the XY body of spermatocytes and centrosomes

RING-finger proteins participate in developmental processes, including gametogenesis. A fetal oocyte cDNA library was used to select genes expressed during male germ-cell differentiation. A novel RING-finger protein, XYbp (XY body protein), participating in mouse spermatogenesis has been identified. This novel gene generates a ubiquitously expressed transcript of 4.2 kb and a testis-specific one of 2.8 kb, processed by an alternative polyadenylation mechanism from a non-canonical polyadenylation signal. Transcription of XYbp is regulated during spermatocyte differentiation. The antiserum raised against the XYbp peptide demonstrated that XYbp is localised mainly in the XY bivalent of spermatocytes (XY body) and in the centrosomes of somatic and germ cells in all phases of the cell cycle. These studies indicate that we have identified a new member of the RING-finger family of proteins associated with the XY meiotic bivalent during spermatogenesis development and with the centrosomes of all cells.

Human 76p: A new member of the gamma-tubulin-associated protein family

The role of the centrosomes in microtubule nucleation remains largely unknown at the molecular level. gamma-Tubulin and the two associated proteins h103p (hGCP2) and h104p (hGCP3) are essential. These proteins are also present in soluble complexes containing additional polypeptides. Partial sequencing of a 76- kD polypeptide band from these complexes allowed the isolation of a cDNA encoding for a new protein (h76p = hGCP4) expressed ubiquitously in mammalian tissues. Orthologues of h76p have been characterized in Drosophila and in the higher plant Medicago. Several pieces of evidence indicate that h76p is involved in microtubule nucleation. (1) h76p is localized at the centrosome as demonstrated by immunofluorescence. (2) h76p and gamma-tubulin are associated in the gamma-tubulin complexes. (3) gamma-tubulin complexes containing h76p bind to microtubules. (4) h76p is recruited to the spindle poles and to Xenopus sperm basal bodies. (5) h76p is necessary for aster nucleation by sperm basal bodies and recombinant h76p partially replaces endogenous 76p in oocyte extracts. Surprisingly, h76p shares partial sequence identity with human centrosomal proteins h103p and h104p, suggesting a common protein core. Hence, human gamma-tubulin appears associated with at least three evolutionary related centrosomal proteins, raising new questions about their functions at the molecular level.

Protein tyrosine kinase p53/p56(lyn) forms complexes with gamma-tubulin in rat basophilic leukemia cells

The aggregation of receptors with high affinity for IgE (FcepsilonRI) on the surface of mast cells and basophils initiates a chain of biochemical events culminating in the release of allergy mediators. Although microtubules have been implicated in the activation process, the molecular mechanism of their interactions with signal transduction molecules is poorly understood. Here we show that in rat basophilic leukemia cells large amounts of alphabeta-tubulin dimers ( approximately 70%) and gamma-tubulin ( approximately 85%) are found in a soluble pool which was released from the cells after permeabilization with saponin, or extraction with non-ionic detergents. Soluble tubulins were found in large complexes with other molecules. Complexes of soluble gamma-tubulin released from activated cells contained tyrosine-phosphorylated proteins of relative mol. wt approximately 25, 50, 53, 56, 60, 75, 80, 97, 115 and 200 kDa. Increased tyrosine phosphorylation of proteins associated with the cytoskeleton, i.e. around centrosomes, was detected by immunofluorescence microscopy. In vitro kinase assays revealed increased tyrosine phosphorylation of proteins in gamma-tubulin complexes isolated from activated cells. Two of the tyrosine phosphorylated proteins in these complexes were identified as the p53/56(lyn) kinase. Furthermore, gamma-tubulin bound to the N-terminal fragment of recombinant Lyn kinase and its binding was slightly enhanced in activated cells. Pretreatment of the cells with Src family-selective tyrosine kinase inhibitor, PP1, decreased the amount of tyrosine phosphorylated proteins in gamma-tubulin complexes, as well as the amount of gamma-tubulin in Lyn kinase immunocomplexes. The combined data suggest that gamma-tubulin is involved in early stages of mast cell activation.

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.

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.

Activity of the human centrosomal kinase, Nek2, depends on an unusual leucine zipper dimerization motif

Nek2 is a human cell cycle-regulated kinase that is structurally related to the mitotic regulator, NIMA, of Aspergillus nidulans. Localization studies have shown that Nek2 is a core component of the centrosome, the microtubule organizing center of the cell, and functional approaches suggest a possible role for Nek2 in centrosome separation at the G2/M transition. Here, we have investigated the importance of an unusual leucine zipper coiled-coil motif present in the C-terminal noncatalytic domain of the Nek2 kinase. Glycerol gradient centrifugation indicated that endogenous Nek2 is present in HeLa cells as a salt-resistant 6 S complex, the predicted size of a Nek2 homodimer. Recombinant Nek2 overexpressed in insect cells also formed a 6 S complex, whereas a Nek2 mutant specifically lacking the leucine zipper motif was monomeric. Using yeast two-hybrid interaction analyses and coprecipitation assays, we found that Nek2 can indeed form homodimers both in vivo and in vitro and that this dimerization specifically required the leucine zipper motif. Moreover, deletion of the leucine zipper prevented a trans-autophosphorylation reaction on the C-terminal domain of Nek2 and strongly reduced Nek2 kinase activity on exogenous substrates. Finally, we emphasize that the Nek2 leucine zipper described here differs from classical leucine zippers in that it exhibits a radically different arrangement of hydrophobic and charged amino acids. Thus, this study reveals not only an important mechanism for the regulation of the Nek2 kinase but, furthermore, highlights an unusual organization of a leucine zipper dimerization motif.

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.

GMAP-210, A cis-Golgi network-associated protein, is a minus end microtubule-binding protein

We report that a peripheral Golgi protein with a molecular mass of 210 kD localized at the cis-Golgi network (Rios, R.M., A.M. Tassin, C. Celati, C. Antony, M.C. Boissier, J.C. Homberg, and M. Bornens. 1994. J. Cell Biol. 125:997-1013) is a microtubule-binding protein that associates in situ with a subpopulation of stable microtubules. Interaction of this protein, now called GMAP-210, for Golgi microtubule-associated protein 210, with microtubules in vitro is direct, tight and nucleotide-independent. Biochemical analysis further suggests that GMAP-210 specifically binds to microtubule ends. The full-length cDNA encoding GMAP-210 predicts a protein of 1, 979 amino acids with a very long central coiled-coil domain. Deletion analyses in vitro show that the COOH terminus of GMAP-210 binds to microtubules whereas the NH2 terminus binds to Golgi membranes. Overexpression of GMAP-210-encoding cDNA induced a dramatic enlargement of the Golgi apparatus and perturbations in the microtubule network. These effects did not occur when a mutant lacking the COOH-terminal domain was expressed. When transfected in fusion with the green fluorescent protein, the NH2-terminal domain associated with the cis-Golgi network whereas the COOH-terminal microtubule-binding domain localized at the centrosome. Altogether these data support the view that GMAP-210 serves to link the cis-Golgi network to the minus ends of centrosome-nucleated microtubules. In addition, this interaction appears essential for ensuring the proper morphology and size of the Golgi apparatus.

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.

Microtubule-dependent plus- and minus end-directed motilities are competing processes for nuclear targeting of adenovirus

Adenovirus (Ad) enters target cells by receptor-mediated endocytosis, escapes to the cytosol, and then delivers its DNA genome into the nucleus. Here we analyzed the trafficking of fluorophore-tagged viruses in HeLa and TC7 cells by time-lapse microscopy. Our results show that native or taxol-stabilized microtubules (MTs) support alternating minus- and plus end-directed movements of cytosolic virus with elementary speeds up to 2.6 micrometer/s. No directed movement was observed in nocodazole-treated cells. Switching between plus- and minus end-directed elementary speeds at frequencies up to 1 Hz was observed in the periphery and near the MT organizing center (MTOC) after recovery from nocodazole treatment. MT-dependent motilities allowed virus accumulation near the MTOC at population speeds of 1-10 micrometer/min, depending on the cell type. Overexpression of p50/dynamitin, which is known to affect dynein-dependent minus end-directed vesicular transport, significantly reduced the extent and the frequency of minus end-directed migration of cytosolic virus, and increased the frequency, but not the extent of plus end-directed motility. The data imply that a single cytosolic Ad particle engages with two types of MT-dependent motor activities, the minus end- directed cytoplasmic dynein and an unknown plus end- directed activity.

Basal body duplication in Paramecium requires gamma-tubulin

First discovered in the fungus Aspergillus nidulans[1], gamma-tubulin is a ubiquitous component of microtubule organizing centres [2]. In centrosomes, gamma-tubulin has been immunolocalized at the pericentriolar material, suggesting a role in cytoplasmic microtubule nucleation [3], as well as within the centriole core itself [4]. Although its function in the nucleation of the mitotic spindle and of cytoplasmic interphasic microtubules has been demonstrated in vitro [5] [6] and in vivo[7] [8] [9], the hypothesis that gamma-tubulin could intervene in centriole assembly has never been experimentally addressed because the mitotic arrest caused by the inactivation of gamma-tubulin in vivo precludes any further phenotypic analysis of putative centriole defects. The issue can be addressed in the ciliate Paramecium, which is characterized by numerous basal bodies that are similar to centrioles but the biogenesis of which is not tightly coupled to the nuclear division cycle. We demonstrate that the inactivation of the Paramecium gamma-tubulin genes leads to inhibition of basal body duplication.

Unusual centrosome cycle in Dictyostelium: correlation of dynamic behavior and structural changes

Centrosome duplication and separation are of central importance for cell division. Here we provide a detailed account of this dynamic process in Dictyostelium. Centrosome behavior was monitored in living cells using a gamma-tubulin-green fluorescent protein construct and correlated with morphological changes at the ultrastructural level. All aspects of the duplication and separation process of this centrosome are unusual when compared with, e.g., vertebrate cells. In interphase the Dictyostelium centrosome is a box-shaped structure comprised of three major layers, surrounded by an amorphous corona from which microtubules emerge. Structural duplication takes place during prophase, as opposed to G1/S in vertebrate cells. The three layers of the box-shaped core structure increase in size. The surrounding corona is lost, an event accompanied by a decrease in signal intensity of gamma-tubulin-green fluorescent protein at the centrosome and the breakdown of the interphase microtubule system. At the prophase/prometaphase transition the separation into two mitotic centrosomes takes place via an intriguing lengthwise splitting process where the two outer layers of the prophase centrosome peel away from each other and become the mitotic centrosomes. Spindle microtubules are now nucleated from surfaces that previously were buried inside the interphase centrosome. Finally, at the end of telophase, the mitotic centrosomes fold in such a way that the microtubule-nucleating surface remains on the outside of the organelle. Thus in each cell cycle the centrosome undergoes an apparent inside-out/outside-in reversal of its layered structure.

Ciliated differentiation of rabbit tracheal epithelial cells in vitro

Primary cultures of rabbit tracheal epithelial (RbTE) cells have been performed in two different ways. Quantitative analysis of both proliferative capacities and ciliated differentiation process were carried out using epithelial cell cultures from tracheal explants and from dissociated tracheal epithelial cells in air-liquid interface conditions. We show that both alpha- and beta-tubulins from RbTE cells are polyglutamylated and that this posttranslational modification is restricted to cilia axonemes and centrioles of non-ciliated cells. A monoclonal antibody raised against polyglutamylated tubulins was used to quantify the proportion of ciliated cells. Even though epithelial cells from outgrowths obtained by the explant technique highly proliferated during the first days of culture, no ciliated differentiation occurred. On the other hand, using air-liquid interface conditions after proliferation of dissociated cells, we could observe and quantify a ciliated cell differentiation in vitro by both Western blot and flow cytometric analysis. The specific detection and quantification of ciliated cells open the way for the biochemical and molecular characterization of centriolar components during ciliated differentiation.

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.

Isolation of nucleation-competent centrosomes from Dictyostelium discoideum

The centrosome of Dictyostelium discoideum is a box-shaped, layered core structure surrounded by a corona which is made up of dense nodules embedded in amorphous material. It is also known as nucleus-associated body. Because of its tight association with the nucleus the centrosome has resisted so far all attempts for isolation in sufficient purity and quantity for biochemical analysis. Here we report on the large-scale isolation of D. discoideum centrosomes after treatment of nucleus-centrosome complexes with a buffer containing sodium pyrophosphate. Following heparin treatment and a filtration step, centrosomes were further purified by density gradient centrifugation. Immunofluorescence analysis of the isolated centrosomes revealed the presence of the D. discoideum 350-kDa antigen, a centrosomal marker protein, gamma-tubulin, and the D. discoideum homologues of pericentrin, Spc110p, and Cdc31p. The structural integrity of the isolated centrosomes was demonstrated by confocal laser microscopy and electron microscopy. Microtubule nucleation assays with purified pig brain tubulin showed that the isolation procedure did not only preserve the structure but also the functionality of the isolated centrosomes. D. discoideum centrosomes should now become an attractive new model system in addition to, and for comparison with, centriolar centrosomes and yeast spindle pole bodies.

C-Nap1, a novel centrosomal coiled-coil protein and candidate substrate of the cell cycle-regulated protein kinase Nek2

Nek2 (for NIMA-related kinase 2) is a mammalian cell cycle-regulated kinase structurally related to the mitotic regulator NIMA of Aspergillus nidulans. In human cells, Nek2 associates with centrosomes, and overexpression of active Nek2 has drastic consequences for centrosome structure. Here, we describe the molecular characterization of a novel human centrosomal protein, C-Nap1 (for centrosomal Nek2-associated protein 1), first identified as a Nek2-interacting protein in a yeast two-hybrid screen. Antibodies raised against recombinant C-Nap1 produced strong labeling of centrosomes by immunofluorescence, and immunoelectron microscopy revealed that C-Nap1 is associated specifically with the proximal ends of both mother and daughter centrioles. On Western blots, anti-C-Nap1 antibodies recognized a large protein (>250 kD) that was highly enriched in centrosome preparations. Sequencing of overlapping cDNAs showed that C-Nap1 has a calculated molecular mass of 281 kD and comprises extended domains of predicted coiled-coil structure. Whereas C-Nap1 was concentrated at centrosomes in all interphase cells, immunoreactivity at mitotic spindle poles was strongly diminished. Finally, the COOH-terminal domain of C-Nap1 could readily be phosphorylated by Nek2 in vitro, as well as after coexpression of the two proteins in vivo. Based on these findings, we propose a model implicating both Nek2 and C-Nap1 in the regulation of centriole-centriole cohesion during the cell cycle.

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.

Organisation and functional regulation of the centrosome in animal cells

Molecular characterisation of centrosomal components is slowly progressing. Recent results indicate that the major aspects of centrosome-mediated microtubule nucleation may soon be understood at the molecular level. In contrast, centrosome reproduction, which is an important aspect of animal cell division, remains terra incognita. The most challenging issue for the future is to understand the molecular mechanisms which control centrosome biogenesis. There is a urgent need to identify with certainty proteins implicated in this process. Comparison between organisms with structurally different centrosomes might be critical for a better understanding of centrosome duplication if a general mechanism has been conserved throughout evolution.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

Spc98p directs the yeast gamma-tubulin complex into the nucleus and is subject to cell cycle-dependent phosphorylation on the nuclear side of the spindle pole body

In the yeast Saccharomyces cerevisiae, microtubules are organized by the spindle pole body (SPB), which is embedded in the nuclear envelope. Microtubule organization requires the gamma-tubulin complex containing the gamma-tubulin Tub4p, Spc98p, and Spc97p. The Tub4p complex is associated with cytoplasmic and nuclear substructures of the SPB, which organize the cytoplasmic and nuclear microtubules. Here we present evidence that the Tub4p complex assembles in the cytoplasm and then either binds to the cytoplasmic side of the SPB or is imported into the nucleus followed by binding to the nuclear side of the SPB. Nuclear import of the Tub4p complex is mediated by the essential nuclear localization sequence of Spc98p. Our studies also indicate that Spc98p in the Tub4p complex is phosphorylated at the nuclear, but not at the cytoplasmic, side of the SPB. This phosphorylation is cell cycle dependent and occurs after SPB duplication and nucleation of microtubules by the new SPB and therefore may have a role in mitotic spindle function. In addition, activation of the mitotic checkpoint stimulates Spc98p phosphorylation. The kinase Mps1p, which functions in SPB duplication and mitotic checkpoint control, seems to be involved in Spc98p phosphorylation. Our results also suggest that the nuclear and cytoplasmic Tub4p complexes are regulated differently.

Dictyostelium gamma-tubulin: molecular characterization and ultrastructural localization

The centrosome of Dictyostelium discoideum is a nucleus-associated body consisting of an electron-dense, three-layered core surrounded by an amorphous matrix, the corona. To elucidate the molecular and supramolecular architecture of this unique microtubule-organizing center, we have isolated and sequenced the gene encoding gamma-tubulin and have studied its localization in the Dictyostelium centrosome using immunofluorescence and postembedding immunoelectron microscopy. D. discoideum possesses a single copy of a gamma-tubulin gene that is related to, but more divergent from, other gamma-tubulins. The low-abundance gene product is localized to the centrosome in an intriguing pattern: it is highly concentrated in the corona in regularly spaced clusters whose distribution correlates with the patterning of dense nodules that are a prominent feature of the corona. These observations lend support to the notion that the corona is the functional homologue of the pericentriolar matrix of ‘higher’ eukaryotic centrosomes, and that nodules are the functional equivalent of gamma-tubulin ring complexes that serve as nucleation sites for microtubules in animal centrosomes.

Expression of amino- and carboxyl-terminal gamma- and alpha-tubulin mutants in cultured epithelial cells

Three distinct tubulin proteins are essential for microtubule function: alpha-, beta-, and gamma-tubulin. After translation, alpha- and beta-tubulin proteins combine into a soluble, 7 S heterodimer that is multimerized to form the microtubule filament. Conversely, gamma-tubulin combines with several proteins into a soluble, 25 S multi-protein particle, the gammasome that is essential for nucleating microtubule filaments at the centrosome. The proteins that assist tubulins in executing their specific functions are largely unknown. As an initial approach to address this issue, we first decided to identify domains of mammalian alpha- and gamma-tubulin necessary for their function by creating mutant mammalian alpha- and gamma-tubulin (both deletion and hybrid mutants) and assaying their behavior in stably transfected Chinese hamster ovary epithelial cells. First, we demonstrated that addition of a carboxyl-terminal epitope tag had no effect on the subcellular localization of either alpha- and gamma-tubulin. Second, we found that both the amino and carboxyl termini of gamma-tubulin were essential for its incorporation into the gammasome. Third, we found that the amino and carboxyl termini of alpha-tubulin were necessary for incorporation of the alpha-beta-tubulin heterodimer into the microtubule filament network. In general, alpha-tubulin sequences could not replace those of gamma-tubulin and vice versa. Taken together, these results suggest that the amino and carboxyl termini of alpha- and gamma-tubulin and perhaps regions throughout these proteins were necessary for their specific functions.

Tubulin post-translational modifications--enzymes and their mechanisms of action

This review describes the enzymes responsible for the post-translational modifications of tubulin, including detyrosination/tyrosination, acetylation/deacetylation, phosphorylation, polyglutamylation, polyglycylation and the generation of non-tyrosinatable alpha-tubulin. Tubulin tyrosine-ligase, which reattaches tyrosine to detyrosinated tubulin, has been extensively characterized and its gene sequenced. Enzymes such as tubulin-specific carboxypeptidase and alpha-tubulin acetyltransferase, required, respectively, for detyrosination and acetylation of tubulin, have yet to be purified to homogeneity and examined in defined systems. This has produced some conflicting results, especially for the carboxypeptidase. The phosphorylation of tubulin by several different types of kinases has been studied in detail but drawing conclusions is difficult because many of these enzymes modify proteins other than their actual substrates, an especially pertinent consideration for in vitro experiments. Tubulin phosphorylation in cultured neuronal cells has proven to be the best model for evaluation of kinase effects on tubulin/microtubule function. There is little information on the enzymes required for polyglutamylation, polyglycylation, and production of non-tyrosinatable tubulin, but the available data permit interesting speculation of a mechanistic nature. Clearly, to achieve a full appreciation of tubulin post-translational changes the responsible enzymes must be characterized. Knowing when the enzymes are active in cells, if soluble or polymerized tubulin is the preferred substrate and the amino acid residues modified by each enzyme are all important. Moreover, acquisition of purified enzymes will lead to cloning and sequencing of their genes. With this information, one can manipulate cell genomes in order to either modify key enzymes or change their relative amounts, and perhaps reveal the physiological significance of tubulin post-translational modifications.

Centrosome-microtubule nucleation

In many cell types the formation of microtubules from tubulin subunits is initiated at defined nucleation sites at the centrosome. These sites contain the conserved gamma-tubulin which is in association with additional not very will characterised proteins, identified as components of a gamma-tubulin ring complex from Xenopus egg extracts or from suppressor screens in the yeast Saccharomyces cerevisiae. In this review we discuss two recently proposed models of how the gamma-tubulin complex assists in the assembly of tubulin to form microtubules. These models propose different roles for gamma-tubulin and the other proteins in the complex in tubulin assembly. While the structure and composition of a microtubule nucleation site is becoming clearer, it is still unknown how the cell-cycle dependent regulation of microtubule nucleation sites is achieved and whether they disassemble after microtubule formation in order to allow microtubule fluxes towards the centrosome which have been observed in mitotic cells.

gamma-tubulin in trypanosomes: molecular characterisation and localisation to multiple and diverse microtubule organising centres

A genomic clone from Trypanosoma brucei, which contains a full length gamma-tubulin gene, was isolated using degenerate oligonucleotide primers. The sequence of this clone predicts a protein of 447 amino acids having a high degree of homology with gamma-tubulins from human and Xenopus laevis (67.2% amino acid identity) and only 57.7% identity with the Plasmodium falciparum gamma-tubulin. Northern blot analysis of poly(A)+ selected RNA from a procyclic culture detects a major transcript of approximately 2.2 kb plus a minor transcript of approximately 3.6 kb. A fusion protein comprising almost the full length gamma-tubulin gene product (amino acids 8–447) plus an amino-terminal histidine tag has been expressed and purified from Escherichia coli and used to raise a polyclonal antibody. Immunofluorescence, using this antibody, shows classical centrosomal localisation in mammalian cells. In T. brucei gamma-tubulin is present in the basal bodies which subtend the flagellum and also at the anterior tip of the cell body where many minus ends of microtubules are located. Furthermore the antibody reveals a small subset of the sub-pellicular microtubules and a discrete dot within the nucleus which alters form with progression through the mitotic cycle. Evidence is also presented for discrete punctate staining within the microtubules of the cell body which may represent the presence of gamma-tubulin on the ends of individual microtubules. Our results indicate that gamma-tubulin is associated with diverse microtubule organising centres and structures in trypanosomes.