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γ-Tubulins are highly conserved members of the tubulin superfamily essential for microtubule nucleation. Humans possess 2 γ-tubulin genes. It is thought that γ-tubulin-1 represents a ubiquitous isotype, whereas γ-tubulin-2 is found predominantly in the brain, where it may be endowed with divergent functions beyond microtubule nucleation. The molecular basis of the purported functional differences between γ-tubulins is unknown. We report discrimination of human γ-tubulins according to their electrophoretic and immunochemical properties. In vitro mutagenesis revealed that the differences in electrophoretic mobility originate in the C-terminal regions of the γ-tubulins. Using epitope mapping, we discovered mouse monoclonal antibodies that can discriminate between human γ-tubulin isotypes. Real time quantitative RT-PCR and 2-dimensional-PAGE showed that γ-tubulin-1 is the dominant isotype in fetal neurons. Although γ-tubulin-2 accumulates in the adult brain, γ-tubulin-1 remains the major isotype in various brain regions. Localization of γ-tubulin-1 in mature neurons was confirmed by immunohistochemistry and immunofluorescence microscopy on clinical samples and tissue microarrays. Differentiation of SH-SY5Y human neuroblastoma cells by all-trans retinoic acid, or oxidative stress induced by mitochondrial inhibitors, resulted in upregulation of γ-tubulin-2, whereas the expression of γ-tubulin-1 was unchanged. Fractionation experiments and immunoelectron microscopy revealed an association of γ-tubulins with mitochondrial membranes. These data indicate that in the face of predominant γ-tubulin-1 expression, the accumulation of γ-tubulin-2 in mature neurons and neuroblastoma cells during oxidative stress may denote a prosurvival role of γ-tubulin-2 in neurons.-Draberova, E., Sulimenko, V., Vinopal, S., Sulimenko, T., Sladkova, V., D'Agostino, L., Sobol, M., Hozak, P., Křen, L., Katsetos, C. D., Draber, P. Differential expression of human γ-tubulin isotypes during neuronal development and oxidative stress points to γ-tubulin-2 prosurvival function.

The microtubule cytoskeleton is critically important for spatio-temporal organization of eukaryotic cells. The nucleation of new microtubules is typically restricted to microtubule organizing centers (MTOCs) and requires γ-tubulin that assembles into multisubunit complexes of various sizes. γ-Tubulin ring complexes (TuRCs) are efficient microtubule nucleators and are associated with large number of targeting, activating and modulating proteins. γ-Tubulin-dependent nucleation of microtubules occurs both from canonical MTOCs, such as spindle pole bodies and centrosomes, and additional sites such as Golgi apparatus, nuclear envelope, plasma membrane-associated sites, chromatin and surface of pre-existing microtubules. Despite many advances in structure of γ-tubulin complexes and characterization of γTuRC interacting factors, regulatory mechanisms of microtubule nucleation are not fully understood. Here, we review recent work on the factors and regulatory mechanisms that are involved in centrosomal and non-centrosomal microtubule nucleation.

Profilin is a well-known regulator of actin filament formation. It indirectly associates with microtubules and influences their growth rate. Formins are the linker molecules, and the turnover of the actin microfilament system balances profilin association with the microtubules., Profilin controls actin nucleation and assembly processes in eukaryotic cells. Actin nucleation and elongation promoting factors (NEPFs) such as Ena/VASP, formins, and WASP-family proteins recruit profilin:actin for filament formation. Some of these are found to be microtubule associated, making actin polymerization from microtubule-associated platforms possible. Microtubules are implicated in focal adhesion turnover, cell polarity establishment, and migration, illustrating the coupling between actin and microtubule systems. Here we demonstrate that profilin is functionally linked to microtubules with formins and point to formins as major mediators of this association. To reach this conclusion, we combined different fluorescence microscopy techniques, including superresolution microscopy, with siRNA modulation of profilin expression and drug treatments to interfere with actin dynamics. Our studies show that profilin dynamically associates with microtubules and this fraction of profilin contributes to balance actin assembly during homeostatic cell growth and affects micro­tubule dynamics. Hence profilin functions as a regulator of microtubule (+)-end turnover in addition to being an actin control element.

Mast cells play an effector role in innate immunity, allergy, and inflammation. Antigen-mediated activation of mast cells initiates signaling events leading to Ca2+ response and the release of inflammatory and allergic mediators from granules. Diseases associated with deregulated mast cell functions are hard to treat and there is an increasing demand for new therapeutic strategies. Miltefosine (hexadecylphosphocholine) is a new candidate for treatment of mast cell-driven diseases as it inhibits activation of mast cells. It has been proposed that miltefosine acts as a lipid raft modulator through its interference with the structural organization of surface receptors in the cell membrane. However, molecular mechanisms of its action are not fully understood. Here, we report that in antigen-activated bone marrow-derived mast cells (BMMCs), miltefosine inhibits degranulation, reorganization of microtubules, as well as antigen-induced chemotaxis. While aggregation and tyrosine phosphorylation of IgE receptors were suppressed in activated cells pre-treated with miltefosine, overall tyrosine phosphorylation levels of Lyn and Syk kinases, and Ca2+ influx were not inhibited. In contrast, lipid raft disruptor methyl-β-cyclodextrin attenuated the Ca2+ influx. Tagged-miltefosine rapidly localized into the cell interior, and live-cell imaging of BMMCs with labeled intracellular granules disclosed that miltefosine inhibited movement of some granules. Immunoprecipitation and in vitro kinase assays revealed that miltefosine inhibited Ca2+- and diacylglycerol-regulated conventional protein kinase C (cPKC) isoforms that are important for mast cell degranulation. Inhibition of cPKCs by specific inhibitor {"type":"entrez-nucleotide","attrs":{"text":"Ly333531","term_id":"1257370768","term_text":"LY333531"}}Ly333531 affected activation of BMMCs in the same way as miltefosine. Collectively, our data suggest that miltefosine modulates mast cells both at the plasma membrane and in the cytosol by inhibition of cPKCs. This alters intracellular signaling pathway(s) directed to microtubules, degranulation, and migration.

γ-Tubulin is essential for microtubule nucleation and also plays less understood roles in nuclear and cell-cycle-related functions. High abundancy of γ-tubulin in acentrosomal Arabidopsis cells facilitated purification and biochemical characterization of large molecular species of γ-tubulin. TEM, fluorescence, and atomic force microscopy of purified high molecular γ-tubulin forms revealed the presence of linear filaments with a double protofilament substructure, filament bundles and aggregates. Filament formation from highly purified γ-tubulin free of γ-tubulin complex proteins (GCPs) was demonstrated for both plant and human γ-tubulin. Moreover, γ-tubulin associated with porcine brain microtubules formed oligomers. Experimental evidence on the intrinsic ability of γ-tubulin to oligomerize/polymerize was supported by conservation of α- and β-tubulin interfaces for longitudinal and lateral interactions for γ-tubulins. STED (stimulated emission depletion) microscopy of Arabidopsis cells revealed fine, short γ-tubulin fibrillar structures enriched on mitotic microtubular arrays that accumulated at polar regions of acentrosomal spindles and the outer nuclear envelope before mitosis, and were also present in nuclei. Fine fibrillar structures of γ-tubulin representing assemblies of higher order were localized in cell-cycle-dependent manner at sites of dispersed γ-tubulin location in acentrosomal plant cells as well as at sites of local γ-tubulin enrichment after drug treatment. Our findings that γ-tubulin preserves the capability of prokaryotic tubulins to self-organize into filaments assembling by lateral interaction into bundles/clusters help understanding of the relationship between structure and multiple cellular functions of this protein species and suggest that besides microtubule nucleation and organization, γ-tubulin may also have scaffolding or sequestration functions.

Tubulin self-assembly into microtubules is a fascinating natural phenomenon. Its importance is not just crucial for functional and structural biological processes, but it also serves as an inspiration for synthetic nanomaterial innovations. The modulation of the tubulin self-assembly process without introducing additional chemical inhibitors/promoters or stabilizers has remained an elusive process. This work reports a versatile and vigorous strategy for controlling tubulin self-assembly by nanosecond electropulses (nsEPs). The polymerization assessed by turbidimetry is dependent on nsEPs dosage. The kinetics of microtubules formation is tightly linked to the nsEPs effects on structural properties of tubulin, and tubulin-solvent interface, assessed by autofluorescence, and the zeta potential. Moreover, the overall size of tubulin assessed by dynamic light scattering is affected as well. Additionally, atomic force microscopy imaging reveals the formation of different assemblies reflecting applied nsEPs. It is suggested that changes in C-terminal modification states alter tubulin polymerization-competent conformations. Although the assembled tubulin preserve their integral structure, they might exhibit a broad range of new properties important for their functions. Thus, these transient conformation changes of tubulin and their collective properties can result in new applications.

Microtubules formed by αβ-tubulin dimers represent cellular structures that are indispensable for the maintenance of cell morphology and for cell motility generation. Microtubules in intact cells are in highly regulated equilibrium with cellular pools of soluble tubulin dimers. Sensitive, reproducible and rapid assays are necessary to monitor tubulin changes in cytosolic pools after treatment with anti-mitotic drugs, during the cell cycle or activation and differentiation events. Here we describe new assays for α-tubulin quantification. The assays are based on sandwich ELISA, and the signal is amplified with biotinyl-tyramide or immuno-PCR. Matching monoclonal antibody pair recognizes phylogenetically highly conserved epitopes localized outside the C-terminal isotype-defining region. This makes it possible to detect α-tubulin isotypes in different cell types of various species. Biotinyl-tyramide amplification and immuno-PCR amplification enable detection of tubulin at concentrations 2.5ng/ml and 0.086ng/ml, respectively. Immuno-PCR detection shows enhanced sensitivity and wider dynamic range when compared to ELISA with biotinyl-tyramide detection. Our results on taxol-treated and activated bone marrow-derived mast cells demonstrate, that the assays allow sensitive quantification of tubulin in complex biological fluids.

Microtubules represent cytoplasmic structures that are indispensable for the maintenance of cell morphology and motility generation. Due to their regular structural organization, microtubules have become of great interest for preparation of in vitro nanotransport systems. However, tubulin, the major building protein of microtubules, is a thermolabile protein and is usually stored at −80 °C to preserve its conformation and polymerization properties. Here we describe a novel method for freeze-drying of assembly-competent tubulin in the presence of a nonreducing sugar trehalose. Even after prolonged storage at ambient temperature, rehydrated tubulin is capable of binding antimitotic drugs and assembling to microtubules that bind microtubule-associated proteins in the usual way. Electron microscopy confirmed that rehydrated tubulin assembles into normal microtubules that are able to generate motility by interaction with the motor protein kinesin in a cell-free environment. Freeze-drying also preserved preformed microtubules. Rehydrated tubulin and microtubules can be used for preparation of diverse in vitro and in vivo assays as well as for preparation of bionanodevices.

Phosphorylation of alphabeta-tubulins dimers by protein tyrosine kinases plays an important role in the regulation of cellular growth and differentiation in animal cells. In plants, however, the role of tubulin tyrosine phosphorylation is unknown and data on this tubulin modification are limited. In this study, we used an immunochemical approach to demonstrate that tubulin isolated by both immunoprecipitation and DEAE-chromatography is phosphorylated on tyrosine residues in cultured cells of Nicotiana tabacum. This opens up the possibility that tyrosine phosphorylation of tubulin could be involved in modulating the properties of plant microtubules.

The expression, cellular distribution, and subcellular sorting of the microtubule (MT)-nucleating γ-tubulin small complex (γTuSC) proteins, GCP2 and GCP3, were studied in human glioblastoma cell lines and in clinical tissue samples representing all histologic grades of adult diffuse astrocytic gliomas (n = 54). Quantitative real-time polymerase chain reaction revealed a significant increase in the expression of GCP2 and GCP3 transcripts in glioblastoma cells versus normal human astrocytes; these were associated with higher amounts of both γTuSC proteins. GCP2 and GCP3 were concentrated in the centrosomes in interphase glioblastoma cells, but punctate and diffuse localizations were also detected in the cytosol and nuclei/nucleoli. Nucleolar localization was fixation dependent. GCP2 and GCP3 formed complexes with γ-tubulin in the nucleoli as confirmed by reciprocal immunoprecipitation experiments and immunoelectron microscopy. GCP2 and GCP3 depletion caused accumulation of cells in G2/M and mitotic delay but did not affect nucleolar integrity. Overexpression of GCP2 antagonized the inhibitory effect of the CDK5 regulatory subunit-associated tumor suppressor protein 3 (C53) on DNA damage G2/M checkpoint activity. Tumor cell GCP2 and GCP3 immunoreactivity was significantly increased over that in normal brains in glioblastoma samples; it was also associated with microvascular proliferation. These findings suggest that γTuSC protein dysregulation in glioblastomas may be linked to altered transcriptional checkpoint activity or interaction with signaling pathways associated with a malignant phenotype.

Many of the highly organized microtubular arrangements in ciliates are located in the cortical area containing membrane vesicles and vacuoles. In Tetrahymena thermophila and Paramecium caudatum, immunofluorescence microscopy with the monoclonal antibody TU-06, directed against beta-tubulin, revealed distinct staining of this cortical region alone, while the cilia and other microtubular structures were unstained. The specificity of the antibody was confirmed by immunoblotting and by preabsorption of the antibody with purified tubulin. Double-label immunofluorescence with antibodies against gamma-tubulin, detyrosinated alpha-tubulin, and centrin showed that the TU-06 epitope is localized outside the basal body region. This was also confirmed by immunogold electron microscopy of thin sections. Proteolytic digestion of porcine brain beta-tubulin combined with a peptide scan of immobilized, overlapping peptides disclosed that the epitope was in the beta-tubulin region beta81-95, a region which is phylogenetically highly conserved. As known posttranslational modifications of beta-tubulin are located outside this area, the observed staining pattern cannot be interpreted as evidence of subcellular sequestration of modified tubulin. The limited distribution of the epitope could rather reflect the dependence of TU-06 epitope exposition on conformations of tubulin molecules in microtubule arrangements or on differential masking by interacting proteins.

γ-Tubulin is assumed to participate in microtubule nucleation in acentrosomal plant cells, but the underlying molecular mechanisms are still unknown. Here, we show that γ-tubulin is present in protein complexes of various sizes and different subcellular locations in Arabidopsis and fava bean. Immunoprecipitation experiments revealed an association of γ-tubulin with αβ-tubulin dimers. γ-Tubulin cosedimented with microtubules polymerized in vitro and localized along their whole length. Large γ-tubulin complexes resistant to salt treatment were found to be associated with a high-speed microsomal fraction. Blue native electrophoresis of detergent-solubilized microsomes showed that the molecular mass of the complexes was >1 MD. Large γ-tubulin complexes were active in microtubule nucleation, but nucleation activity was not observed for the smaller complexes. Punctate γ-tubulin staining was associated with microtubule arrays, accumulated with short kinetochore microtubules interacting in polar regions with membranes, and localized in the vicinity of nuclei and in the area of cell plate formation. Our results indicate that the association of γ-tubulin complexes with dynamic membranes might ensure the flexibility of noncentrosomal microtubule nucleation. Moreover, the presence of other molecular forms of γ-tubulin suggests additional roles for this protein species in microtubule organization.

gamma-Tubulin is necessary for nucleation and polar orientation of microtubules in vivo. The molecular mechanism of microtubule nucleation by gamma-tubulin and the regulation of this process are not fully understood. Here we show that there are two gamma-tubulin forms in the brain that are present in complexes of various sizes. Large complexes tend to dissociate in the presence of a high salt concentration. Both gamma-tubulins co-polymerized with tubulin dimers, and multiple gamma-tubulin bands were identified in microtubule protein preparations under conditions of non-denaturing electrophoresis. Immunoprecipitation experiments with monoclonal antibodies against gamma-tubulin and alpha-tubulin revealed interactions of both gamma-tubulin forms with tubulin dimers, irrespective of the size of complexes. We suggest that, besides small and large gamma-tubulin complexes, other molecular gamma-tubulin form(s) exist in brain extracts. Two-dimensional electrophoresis revealed multiple charge variants of gamma-tubulin in both brain extracts and microtubule protein preparations. Post-translational modification(s) of gamma-tubulins might therefore have an important role in the regulation of microtubule nucleation in neuronal cells.

The mechanism of marginal band (MB) formation in differentiating erythroid cells is not fully understood, and the proteins involved in nucleation of MB microtubules are largely unknown. To gain insights into the function of γ-tubulin in MB formation, we have followed its distribution in developing chicken erythrocytes and characterized soluble forms of the protein. In early stages of erythroid cells differentiation, γ-tubulin was present in microtubule-organizing centers, mitotic spindles, as well as on MB. Its subcellular localization changed in the course of differentiation, and in postnatal peripheral erythrocytes γ-tubulin was found only in soluble forms. After cold-induced depolymerization γ-tubulin in erythroid cells formed large clusters that were not observed in matured cells, and re-growth experiments demonstrated that γ-tubulin was not present in distinct nucleation structures at the cell periphery. Soluble γ-tubulin formed complexes of various size and large complexes were prone to dissociation in the presence of high salt concentration. Interaction of γ-tubulin with tubulin dimers was revealed by precipitation experiments. γ-Tubulin occurred in multiple charge variants whose number increased in the course of erythrocyte differentiation and corresponded with decreased binding to MB. The presented data demonstrate for the first time that γ-tubulin is a substrate for developmentally regulated posttranslational modifications and that the binding properties of γ-tubulin or its complexes change during differentiation events. © 2002 Wiley-Liss, Inc.

Microtubules, polymers of the heterodimeric protein αβ-tubulin, are indispensable for many cellular activities such as maintenance of cell shape, division, migration, and ordered vesicle transport. In vitro assays to study microtubule functions and their regulation by associated proteins require the availability of assembly-competent purified tubulin. However, tubulin is a thermolabile protein that rapidly converts into non-polymerizing state. For this reason it is usually stored at -80 °C to preserve its conformation and polymerization properties. In this chapter we describe a method for freeze-drying of assembly-competent tubulin in the presence of nonreducing sugar trehalose and methods enabling evaluation of tubulin functions in rehydrated samples.

The exposure of tubulin epitopes was studied in ejaculated boar spermatozoa using a panel of four monoclonal antibodies specific to the N-terminal or C-terminal structural domains of tubulin and three monoclonal antibodies against class III β-tubulin. The specificity of the antibodies was confirmed by immunoblotting. Immunocytochemical staining showed that antibodies discriminated between various parts of a spermatozoon, and that epitopes of class III β-tubulin were present in the flagellum. A tubulin epitope from the C-terminal domain of β-tubulin was detected in the triangular segment of the postacrosomal part of the sperm head. Its distribution changed after an A23187 ionophore-induced acrosome reaction, indicating that tubulin participates in the early stages of fertilization. Three monoclonal antibodies, TU-20, SDL.3D10, and TUJ1 directed against epitopes on the C-terminal end of neuron-specific class III β-tubulin that is widely used as a neuronal marker, stained the flagella. The reactivit...

Higher plants represent an important group of eukaryotes in which discrete sites for microtubule nucleation are absent in all mitotic and meiotic cells, and a concept of dispersed microtubule-nucleation sites has been generally accepted. Current models of mitotic spindle formation in the absence of centrosomes are based on chromatin-mediated microtubule organization. Moreover, even in cells equipped with centrosomes, microtubules can nucleate on cytoplasmic factors, which are so far poorly characterized (Vorobjev et al., 1997). To understand how microtubules are nucleated and organized without the centrosome, the first step is to determine the molecular composition of the dispersed cytoplasmic or chromatin-associated microtubulenucleation sites. -Tubulin, a distantly related member of the tubulin superfamily, plays a pivotal role in microtubule nucleation. It is organized into the large w2000 kDa open-ring complexes ( -TuRC), located at the centrosomes in animal cells and at the spindle pole bodies in yeast (Moritz et al., 1995). The complete genome of Arabidopsis contains two expressed genes encoding -tubulins. Using affinity purified polyclonal and monoclonal antibodies, directed against different antigenic determinants on the -tubulin molecule, we studied the subcellular distribution of -tubulin in six plant species (Arabidopsis thaliana, Vicia faba, Pisum sativum, Medicago sativa, Hordeum vulgare and Zea mays). Biochemical and immunolocalization studies revealed that -tubulin is highly abundant in different cellular compartments. -Tubulin was localized along all microtubular arrays, discrete staining was found in some interphase nuclei. The size of nuclear spots gradually increased until late G2 when they often occurred as double spots. A similar labelling pattern for -tubulin was found in prekinetochore region of isolated G2 nuclei. -Tubulin was also localized on the nuclear surface, decorating perinuclear microtubules focused to the poles. At the onset of mitosis, when nuclear envelope broke down, -tubulin was found on short kinetochore microtubule fibres organized from kinetochores. A strong signal for -tubulin was associated with kinetochore microtubules along their whole length including the close vicinity of the kinetochores. In cells recovering from anti-microtubular drugs, -tubulin was localized with the re-growing kinetochore microtubules nucleated or captured by kinetochore/centromeric regions. Similarly, on isolated chromosomes, -tubulin co-localized with -tubulin in the kinetochore/centromeric region. Flow cytometry was employed to sort isolated plant nuclei in the G1 and G2 stages of the cell cycle. They were thereafter subjected to quantitative immunoblot analysis. An increase of about 50% of -tubulin in G2 compared to G1 nuclei was found in V. faba. In contrast, when supernatants of centrifuged cell extracts from synchronized cells were analyzed by immunoblotting, there were no differences in the amount of -tubulin between cells in the G1 and G2 stages of the cell cycle (Binarova et al., 2000). Circular configurations of chromosomes with the kinetochore region in the centre and chromosome arms pointing outwards were induced transiently by treatment of root meristems of V. faba, M. sativa and A. thaliana with specific cyclin-dependent protein kinase inhibitors (Binarova et al., 1998a). Kinetochore microtubules, * Corresponding author. Tel. +42-2-4106-2130; fax: +42-4-4106-2384. Cell Biology International 27 (2003) 167–169 Cell Biology International

There is an error in the Funding. Grant P302/10/1759(ED) is incorrect. The correct number is P302/10/1701(ED).

Mast cell activation mediated by the high affinity receptor for IgE (FcεRI) is a key event in allergic response and inflammation. Other receptors on mast cells, as c-Kit for stem cell factor and G protein-coupled receptors (GPCRs) synergistically enhance the FcεRI-mediated release of inflammatory mediators. Activation of various signaling pathways in mast cells results in changes in cell morphology, adhesion to substrate, exocytosis, and migration. Reorganization of cytoskeleton is pivotal in all these processes. Cytoskeletal proteins also play an important role in initial stages of FcεRI and other surface receptors induced triggering. Highly dynamic microtubules formed by αβ-tubulin dimers as well as microfilaments build up from polymerized actin are affected in activated cells by kinases/phosphatases, Rho GTPases and changes in concentration of cytosolic Ca(2+). Also important are nucleation proteins; the γ-tubulin complexes in case of microtubules or Arp 2/3 complex with its nucleation promoting factors and formins in case of microfilaments. The dynamic nature of microtubules and microfilaments in activated cells depends on many associated/regulatory proteins. Changes in rigidity of activated mast cells reflect changes in intermediate filaments build up from vimentin. This review offers a critical appraisal of current knowledge on the role of cytoskeleton in mast cells signaling.

γ-Tubulin is the key protein for microtubule nucleation. Duplication of the γ-tubulin gene occurred several times during evolution, and in mammals γ-tubulin genes encode proteins which share ∼97% sequence identity. Previous analysis of Tubg1 and Tubg2 knock-out mice has suggested that γ-tubulins are not functionally equivalent. Tubg1 knock-out mice died at the blastocyst stage, whereas Tubg2 knock-out mice developed normally and were fertile. It was proposed that γ-tubulin 1 represents ubiquitous γ-tubulin, while γ-tubulin 2 may have some specific functions and cannot substitute for γ-tubulin 1 deficiency in blastocysts. The molecular basis of the suggested functional difference between γ-tubulins remains unknown. Here we show that exogenous γ-tubulin 2 is targeted to centrosomes and interacts with γ-tubulin complex proteins 2 and 4. Depletion of γ-tubulin 1 by RNAi in U2OS cells causes impaired microtubule nucleation and metaphase arrest. Wild-type phenotype in γ-tubulin 1-depleted cells is restored by expression of exogenous mouse or human γ-tubulin 2. Further, we show at both mRNA and protein levels using RT-qPCR and 2D-PAGE, respectively, that in contrast to Tubg1, the Tubg2 expression is dramatically reduced in mouse blastocysts. This indicates that γ-tubulin 2 cannot rescue γ-tubulin 1 deficiency in knock-out blastocysts, owing to its very low amount. The combined data suggest that γ-tubulin 2 is able to nucleate microtubules and substitute for γ-tubulin 1. We propose that mammalian γ-tubulins are functionally redundant with respect to the nucleation activity.

γ-Tubulin is assumed to be a typical cytosolic protein necessary for nucleation of microtubules from microtubule organizing centers. Using immunolocalization and cell fractionation techniques in combination with siRNAi and expression of FLAG-tagged constructs, we have obtained evidence that γ-tubulin is also present in nucleoli of mammalian interphase cells of diverse cellular origins. Immunoelectron microscopy has revealed γ-tubulin localization outside fibrillar centers where transcription of ribosomal DNA takes place. γ-Tubulin was associated with nucleolar remnants after nuclear envelope breakdown and could be translocated to nucleoli during mitosis. Pretreatment of cells with leptomycin B did not affect the distribution of nuclear γ-tubulin, making it unlikely that rapid active transport via nuclear pores participates in the transport of γ-tubulin into the nucleus. This finding was confirmed by heterokaryon assay and time-lapse imaging of photoconvertible protein Dendra2 tagged to γ-tubulin. Immunoprecipitation from nuclear extracts combined with mass spectrometry revealed an association of γ-tubulin with tumor suppressor protein C53 located at multiple subcellular compartments including nucleoli. The notion of an interaction between γ-tubulin and C53 was corroborated by pull-down and co-immunoprecipitation experiments. Overexpression of γ-tubulin antagonized the inhibitory effect of C53 on DNA damage G2/M checkpoint activation. The combined results indicate that aside from its known role in microtubule nucleation, γ-tubulin may also have nuclear-specific function(s). J. Cell. Physiol. 227: 367–382, 2012. © 2011 Wiley Periodicals, Inc.

Nonreceptor protein tyrosine kinases of the Src family have been shown to play an important role in signal transduction as well as in regulation of microtubule protein interactions. Here we show that gamma-tubulin (gamma-Tb) in P19 embryonal carcinoma cells undergoing neuronal differentiation is phosphorylated and forms complexes with protein tyrosine kinases of the Src family, Src and Fyn. Elevated expression of both kinases during differentiation corresponded with increased level of proteins phosphorylated on tyrosine. Immunoprecipitation experiments with antibodies against Src, Fyn, gamma-tubulin, and with anti-phosphotyrosine antibody revealed that gamma-tubulin appeared in complexes with these kinases. In vitro kinase assays showed tyrosine phosphorylation of proteins in gamma-tubulin complexes isolated from differentiated cells. Pretreatment of cells with Src family selective tyrosine kinase inhibitor PP2 reduced the amount of phosphorylated gamma-tubulin in the complexes. Binding experiments with recombinant SH2 and SH3 domains of Src and Fyn kinases revealed that protein complexes containing gamma-tubulin bound to SH2 domains and that these interactions were of SH2-phosphotyrosine type. The combined data suggest that Src family kinases might have an important role in the regulation of gamma-tubulin interaction with tubulin dimers or other proteins during neurogenesis.

A panel of six anti-peptide antibodies recognizing epitopes in different regions of the gamma-tubulin molecule was used for the characterization and localization of gamma-tubulin during cell cycle in Leishmania promastigotes. Immunofluorescence microscopy revealed the presence of gamma-tubulin in the basal bodies, posterior pole of the cell, and in the flagellum. Furthermore, the antibodies showed punctuate staining in the subpellicular microtubule. This complex localization pattern was observed in both interphase and dividing cells, where staining of posterior poles and the subpellicular corset was more prominent. In posterior poles, gamma-tubulin co-distributed with the 210-kDa microtubule-interacting protein and the 57-kDa protein immunodetected with anti-vimentin antibody. Immunogold electron microscopy on thin sections of isolated flagella showed that gamma-tubulin was associated with the paraflagellar rod (PFR) that runs adjacent to the axonemal microtubules. Under different extraction conditions, gamma-tubulin in Leishmania was found only in insoluble cytoskeletal fractions, in contrast to tubulin dimers that were both in soluble and cytoskeletal pool. Two-dimensional electrophoresis revealed multiple charge variants of gamma-tubulin. Posttranslational modifications of Leishmania gamma-tubulin might therefore have an important role in the regulation of microtubule nucleation and interaction with other proteins. The complex pattern of gamma-tubulin localization and its properties indicate that gamma-tubulin in Leishmania might have other function(s) besides microtubule nucleation.

The MA-01 antigen, a thermolabile 210 kDa microtubule-interacting protein, is present in P19 embryonal carcinoma cells on microtubular structures as well as in cytosol. After aggregation of the cells and subsequent incubation with all-trans-retinoic acid (RA), the level of MA-01 expression increased approximately 10 times during 15 days. The increase started after 2 days of incubation with RA and preceded the appearance of neuron-specific tubulin βIII, MAP2C and neurofilaments. Such elevated expression of MA-01 antigen was not detected in P19 cells treated with dimethylsulfoxide. These data indicate that enhanced expression of MA-01 antigen is one of the earliest events occurring in P19 cells during neuronal differentiation.

BackgroundThe function of the cortical microtubules, composed of αβ-tubulin heterodimers, is linked to their organizational state which is subject to spatial and temporal modulation by environmental cues. The role of tubulin posttranslational modifications in these processes is largely unknown. Although antibodies against small tubulin regions represent useful tool for studying molecular configuration of microtubules, data on the exposure of tubulin epitopes on plant microtubules are still limited.ResultsUsing homology modeling we have generated anArabidopsis thalianamicrotubule protofilament model that served for the prediction of surface exposure of five β-tubulin epitopes as well as tyrosine residues. Peptide scans newly disclosed the position of epitopes detected by antibodies 18D6 (β1-10), TUB2.1 (β426-435) and TU-14 (β436-445). Experimental verification of the results by immunofluorescence microscopy revealed that the exposure of epitopes depended on the mode of fixation. Moreover, homology modeling showed that only tyrosines in the C-terminal region of β-tubulins (behind β425) were exposed on the microtubule external side. Immunofluorescence microscopy revealed tyrosine phosphorylation of microtubules in plant cells, implying that β-tubulins could be one of the targets for tyrosine kinases.ConclusionsWe predicted surface exposure of five β-tubulin epitopes, as well as tyrosine residues, on the surface ofA. thalianamicrotubule protofilament model, and validated the obtained results by immunofluorescence microscopy on cortical microtubules in cells.The results suggest that prediction of epitope exposure on microtubules by means of homology modeling combined with site-directed antibodies can contribute to a better understanding of the interactions of plant microtubules with associated proteins.