Location of the regions recognized by five commercial antibodies on the tubulin molecule

A putative beta-tubulin phosphate-binding motif is involved in lateral microtubule protofilament interactions

We have investigated the role of a putative GTP-binding beta-tubulin motif in microtubule polymerization. A peptide containing residues 126-142 of the beta-tubulin subunit (peptide G) was synthesised and an antibody against it raised. Peptide G prevents the binding of GTP to tubulin and also microtubule polymerization but not the formation of vinblastine-induced tubulin spirals, suggesting that it may prevent lateral but not longitudinal tubulin-tubulin interactions. The antibody to peptide G shows little reaction with the interphase microtubule network, mitotic spindles or midbody of cultured cells, whereas it clearly reacts with vinblastine-induced paracrystals. These results suggest that this putative phosphate-binding site present in beta-tubulin could be involved in the lateral tubulin-tubulin interactions along the microtubule structure.

Glutamylated tubulin probed in ciliates with the monoclonal antibody GT335

Microtubular networks are extensively developed in many ciliate species. In several of them, we investigate the occurrence of the post-translational glutamylation of tubulin [Eddé et al., 1990: Science 247:82-85; Eddé et al., 1991: J. Cell. Biochem. 46:134-142] using as a probe for such modified tubulin, the monoclonal antibody GT335 [Wolff et al., 1992: Eur. J. Cell Biol. 59:425-432]. Results obtained in Paramecium strongly suggest that both axonemal and cytoplasmic tubulin are glutamylated. As in the vertebrate brain tubulin so far tested, the GT335 epitope is located at the carboxy-terminal fragment of cytoplasmic tubulin removed by subtilisin treatment. Immunoblotting and immunofluorescence experiments reveal that, unlike tubulin acetylation, glutamylation is not restricted to cold-resistant microtubules. In addition, immunofluorescence studies performed on dividing cells show that glutamylation takes place soon after the polymerization of microtubules. Finally, glutamylated tubulin is also detected in the ciliate species Euplotes, Tetrahymena, and Paraurostyla. Together with results obtained on flagellate species, this suggests that tubulin glutamylation came out early in the course of eukaryotic evolution and has been widely exploited in various cellular strategies.

Glycolytic enzyme-tubulin interactions: role of tubulin carboxy terminals

Tubulin and microtubules were modified with the protease, subtilisin. The modification reduced the length of alpha- or beta-tubulin by cleaving a peptide fragment from the C-terminals. Generation of alpha'beta'-tubulin, which is cleaved at both the alpha- and beta-subunit terminals, and alpha beta'-tubulin, which is cleaved at the beta-subunit C-terminal, have already been reported. In this work an isotype, alpha'beta-tubulin, was produced. The three modified tubulin isotypes were compared for their ability to interact with glycolytic enzymes. Cleavage of alpha led to a poorer interaction when tested via affinity chromatography. Tubulin also inhibits the activity of aldolase and glyceraldehyde 3-phosphate dehydrogenase. When the alpha-subunit C-terminal was intact, inhibition was greatest. These results imply that the C-terminal of the tubulin alpha-subunit is responsible for interactions with glycolytic enzymes.

Characterization and biological activities of anti-Brugia pahangi tubulin monoclonal antibodies

Three monoclonal antibodies (mAb) specific to beta-tubulin were used to investigate the heterogeneity of tubulins from nematodes and mammals. Western blot analysis of one-dimensional SDS-PAGE showed that anti-Brugia pahangi tubulin mAb 1B6 and P3D react with epitope(s) specific to nematode beta-tubulin and recognize tubulin from adults and microfilariae of B. pahangi, adult B. malayi and Dirofilaria immitis, eggs of Haemonchus contortus and adult Ascaris suum. However, the same mAb did not recognize tubulin from trophozoites of Giardia lamblia, pig brain or 3T3 mouse fibroblast cells. In two-dimensional SDS-PAGE, mAb 1B6 recognized one isoform of beta-tubulin and mAb P3D recognized two beta-tubulin isoforms. Limited proteolysis showed that mAb 1B6 reacted with the amino-terminal fragments of beta-tubulin. In contrast, mAb P3D recognized the carboxy-terminal fragments of beta-tubulin. In ELISA, mAb P3D reacted with an 18 amino acid peptide corresponding to residues 430-448 of B. pahangi beta-tubulin. These observations confirm that the epitope of mAb P3D is located on the extreme carboxy-terminal region. Immunogold labelling of adult B. pahangi sections with mAb P3D revealed the presence of beta-tubulin isoforms in the cuticle, hypodermal layer and somatic muscle blocks of B. pahangi. Under in vitro conditions, mAb P3D caused 80% reduction in worm viability, during exposure over 48 h.

A phosphorylation epitope on MAP 1B that is transiently expressed in growing axons in the developing rat nervous system

We have isolated a monoclonal antibody (150) that recognizes a phosphorylation epitope on the microtubule-associated protein (MAP) 1B. Immunoblot analysis of the developing rat central nervous system shows that monoclonal antibody 150 is directed against a protein of approximately 325 kDa (MAP 1B) that copolymerizes with microtubules through successive cycles of temperature-dependent assembly and disassembly. Furthermore, immunoprecipitated MAP 1B contains the epitope recognized by monoclonal antibody 150. Removal of phosphate from blotted proteins using alkaline phosphatase abolishes the binding of monoclonal antibody 150 to MAP 1B, indicating that the epitope is phosphorylated. In the developing rat nervous system, immunohistochemistry with monoclonal antibody 150 shows that the phosphorylation epitope on MAP 1B is transiently expressed in growing axons but not in dendrites. For instance, in the neonatal rat cerebellum, the parallel fibres of granule cells are stained only during elongation and not after synaptogenesis. The monoclonal antibody 150 epitope is also transiently expressed in radial glial fibres and in certain cell nuclei. All immunostaining of sections with monoclonal antibody 150 was completely abolished by alkaline phosphatase treatment. These observations and previous ones made by us in cell culture (Mansfield et al., J. Neurocytol., 20, 654-666, 1991) suggest that the phosphorylation epitope on MAP 1B recognized by monoclonal antibody 150, which has not been previously detected in vivo, may be important in axonogenesis.

A functionally specialized alpha-tubulin is required for oocyte meiosis and cleavage mitoses in Drosophila

Three alpha-tubulin proteins contribute to microtubules during oogenesis and early embryogenesis in Drosophila melanogaster: alpha TUB84B, alpha TUB84D, and alpha TUB67C. alpha TUB67C is unique in two respects. It is a structurally divergent alpha-tubulin, sharing only 67% amino acid identity with the generic isotypes alpha TUB84B and alpha TUB84D, and its expression is exclusively maternal. The genetic analysis of the alpha Tub67C gene described here demonstrates that alpha TUB67C is required for nuclear division in the oocyte and early embryo. Both meiosis and cleavage-stage mitoses are severely affected by mutations that result in a substantial decrease in the ratio of alpha TUB67C/alpha TUB84B+alpha TUB84D. A large increase in this ratio, achieved by increasing the gene dosage of alpha Tub67C, has little or no effect on meiosis, but severely disrupts mitotic spindle function. Thus, both classes of alpha-tubulin isotype present in the mature oocyte, alpha TUB67C and alpha TUB84B/84D, are essential for normal spindle function in early Drosophila development. These alpha-tubulins provide the first example of tubulin isotypes known to be coexpressed in wild-type animals whose encoded variation is required for the normal function of a microtubule array.

Microtubule assembly protects the region 28-38 of the beta-tubulin subunit

Polyclonal antibodies have been raised against the peptide 28-38 of the beta-subunit of the tubulin heterodimer in order to study the accessibility of this region in the tubulin heterodimer and in various tubulin assemblies. These antibodies were specific for all beta-tubulin subunits, except for beta'-tubulin isotypes, and did not recognize the alpha-tubulin subunit. The 28-38 region does not play a role in the interaction between the alpha- and beta-subunits since it was accessible to the antibodies on the native heterodimer. The accessibility of the antibodies was not modified by several microtubular poisons. In contrast, in all tubulin assemblies obtained in the presence of microtubular associated proteins, the region 28-38 was not available to the antibodies. These antibodies did not react with microtubules or tubulin spirals assembled either from microtubule proteins or from pure tubulin when these tubulin assemblies were probed in the absence of free tubulin after centrifugation on glass coverslips. In addition, antibodies failed to interact with the microtubule cytoskeleton in cultured Ptk2 cells indicating that the 28-38 region of beta-tubulin is also protected in cellular structures. These observations suggest that the 28-38 region of the beta-tubulin subunit is either located in a zone of interaction between two successive tubulin dimers within a protofilament or hidden by an allosteric conformational change which occurs during tubulin assembly.

Aging and the neurocytoskeleton

It has been often demonstrated that during senescence some neurons undergo atrophic changes while others add new processes and terminals. Because microtubules form a substantial component of the dendritic and axonal cytoskeleton, we have studied the amount of tubulin and acetylated alpha-tubulin in three young (6 months) and three old (24 months) rats (Fischer 344). We have used sodium dodecyl sulfate (SDS) extracts of brain homogenates and Triton solubilized fractionated brain homogenates. With the first method we did not detect any age-related differences in total brain protein, total tubulin, or in relative amounts of acetylated alpha-tubulin. With the second method, we have observed a small systematic increase in relative amount of acetylated alpha-tubulin in the Ca2+/cold insoluble fraction. These results are similar to those reported in the literature, and they indicate a possible alteration in the cytoskeletal dynamics.

The CDC20 gene product of Saccharomyces cerevisiae, a beta-transducin homolog, is required for a subset of microtubule-dependent cellular processes

Previous analysis of cdc20 mutants of the yeast Saccharomyces cerevisiae suggests that the CDC20 gene product (Cdc20p) is required for two microtubule-dependent processes, nuclear movements prior to anaphase and chromosome separation. Here we report that cdc20 mutants are defective for a third microtubule-mediated event, nuclear fusion during mating of G1 cells, but appear normal for a fourth microtubule-dependent process, nuclear migration after DNA replication. Therefore, Cdc20p is required for a subset of microtubule-dependent processes and functions at multiple stages in the life cycle. Consistent with this interpretation, we find that cdc20 cells arrested by alpha-factor or at the restrictive temperature accumulate anomalous microtubule structures, as detected by indirect immunofluorescence. The anomalous microtubule staining patterns are due to cdc20 because intragenic revertants that revert the temperature sensitivity have normal microtubule morphologies. cdc20 mutants have a sevenfold increase in the intensity of antitubulin fluorescence in intranuclear spindles compared with spindles from wild-type cells, yet the total amount of tubulin is indistinguishable by Western immunoblot analysis. This result suggests that Cdc20p modulates microtubule structure in wild-type cells either by promoting microtubule disassembly or by altering the surface of the microtubules. Finally, we cloned and sequenced CDC20 and show that it encodes a member of a family of proteins that share homology to the beta subunit of transducin.

The CDC20 gene product of Saccharomyces cerevisiae, a beta-transducin homolog, is required for a subset of microtubule-dependent cellular processes

Previous analysis of cdc20 mutants of the yeast Saccharomyces cerevisiae suggests that the CDC20 gene product (Cdc20p) is required for two microtubule-dependent processes, nuclear movements prior to anaphase and chromosome separation. Here we report that cdc20 mutants are defective for a third microtubule-mediated event, nuclear fusion during mating of G1 cells, but appear normal for a fourth microtubule-dependent process, nuclear migration after DNA replication. Therefore, Cdc20p is required for a subset of microtubule-dependent processes and functions at multiple stages in the life cycle. Consistent with this interpretation, we find that cdc20 cells arrested by alpha-factor or at the restrictive temperature accumulate anomalous microtubule structures, as detected by indirect immunofluorescence. The anomalous microtubule staining patterns are due to cdc20 because intragenic revertants that revert the temperature sensitivity have normal microtubule morphologies. cdc20 mutants have a sevenfold increase in the intensity of antitubulin fluorescence in intranuclear spindles compared with spindles from wild-type cells, yet the total amount of tubulin is indistinguishable by Western immunoblot analysis. This result suggests that Cdc20p modulates microtubule structure in wild-type cells either by promoting microtubule disassembly or by altering the surface of the microtubules. Finally, we cloned and sequenced CDC20 and show that it encodes a member of a family of proteins that share homology to the beta subunit of transducin.

Distribution of acetylated alpha-tubulin in brain. In situ localization and biochemical characterization

We studied the solubility properties of brain acetylated alpha-tubulin, as well as the localization of this tubulin in brain tissue. Endogenous unpolymerized tubulin and cytoskeletal tubulin were fractionated after brain Triton-solubilization. Using the immunoblotting technique, we found that acetylated alpha-tubulin was recovered in the cytoskeletal fraction, and that most (92%) of the acetylated microtubules of this fraction were depolymerized by cold/Ca2+ treatment. In another set of experiments, axonal and soma-dendritic preparations were found to have equivalent amounts of acetylated alpha-tubulin. By immunogold electron microscopy, we established that acetylated microtubules are widely distributed in dendrites of the central nervous system.

Characterization of a beta-tubulin gene and a beta-tubulin gene products of Brugia pahangi

A genomic clone containing a beta-tubulin gene from the parasitic nematode Brugia pahangi was isolated. This gene was sequenced to determine its size, structural organization, and corresponding primary amino acid sequence. The coding sequence of the beta-tubulin gene spans 3.8 kb, is organized into 9 exons and expresses an mNRA of 1.8 kb which codes for a protein of 448 amino acids. The predicted beta-tubulin amino acid sequence is 89%, 94%, 90% and 88% identical to the chicken beta 2, and the Caenorhabditis elegans ben-1, tub-1 and mec-7 gene products, respectively. Southern hybridization analyses demonstrated that there is only one copy of this gene isotype but that other distinct beta-tubulin genes may exist in the Brugia pahangi genome. A nematode specific antipeptide rabbit antiserum raised against the predicted amino acid sequence of the extreme carboxy-terminal region of the B. pahangi beta-tubulin was used to identify beta-tubulin isoforms in adult nematodes and microfilariae. Isoforms detected by this nematode-specific antipeptide antiserum were identical in both adult worms and microfilariae and did not differ from the isoform patterns detected by a monoclonal antibody recognizing a conserved beta-tubulin epitope. This suggests that this carboxy-terminal peptide is highly represented in the beta-tubulin isoforms of B. pahangi.

Alpha-, beta-, and gamma-tubulins: sequence comparisons and structural constraints

Comparison of congruent to 160 alpha-, beta-, and gamma-tubulins, and excluding the highly divergent C-terminal peptide, indicates that the three subclasses have similar tertiary structures. Conserved sequences within or between the subclasses have been identified, together with the locations of known epitopes, chemical modifications, and mutations. Evidence is also reviewed concerning the identity of the GTP-binding sites, about which residues are exposed in the assembled microtubule and at subunit:subunit interfaces. These characteristics constrain the possible tertiary structure of the tubulin subunit.

Determination of polyprotein processing sites by amino terminal sequencing of nonstructural proteins encoded by plum pox potyvirus

Nonstructural proteins of plum pox potyvirus were partially purified following a procedure described for the isolation of tobacco etch virus nuclear inclusion proteins. Plum pox virus proteins with electrophoretic mobilities corresponding to 49, 59 and 68 kDa reacted with antibodies against the 49 kDa and 54 kDa components of the nuclear inclusions and the 70 kDa component of the cylindrical inclusions of tobacco etch virus, respectively. Further purification by size exclusion high performance liquid chromatography or SDS-polyacrylamide gel electrophoresis, and amino terminal amino acid sequencing permitted the location in the plum pox virus polyprotein of the cleavage sites from which the 49 kDa (NIa-type, protease), 59 kDa (NIb-type, putative RNA replicase), and 68 kDa (CI-type) proteins originate. A 110 kDa protein which copurified with the plum pox virus inclusion proteins reacted with both anti-NIa and anti-NIb sera and had the same amino terminus as the plum pox virus 49 kDa protein, indicating that it is a non-processed 49-59 kDa polypeptide.

Direct visualisation of the soluble pool of tubulin in the neuronal growth cone: immunofluorescence studies following taxol polymerisation

Previously we have shown that the predominant form of soluble alpha-tubulin in the growth cone is C-terminally tyrosinated (Gordon-Weeks and Lang. Dev. Brain Res., 42 (1988) 156-160). Here we show that when growth cones are incubated in taxol, this soluble pool of alpha-tubulin is polymerised onto the ends of the neurite microtubules that enter the proximal part of the growth cone indicating that it is assembly-competent.

The role of microtubules in platelet secretory release

The role of microtubules in platelet aggregation and secretion has been analyzed using platelets permeabilized with digitonin and monoclonal antibodies to alpha (DM1A) and beta (DM1B) subunits of tubulin. Permeabilized platelets were able to undergo aggregation and secretory release. However, threshold doses of agonists capable of eliciting a second wave of aggregation and the platelet release reaction were higher than in control platelets exposed to dimethyl sulfoxide, the solvent for digitonin. Both antibodies to alpha and beta tubulin caused a further increase in the threshold concentration of agonists and inhibited the secretory release of permeabilized platelets, but were ineffective using intact platelets. Neither monoclonal antibody inhibited polymerization or depolymerization of platelet tubulin in vitro. Antibodies to platelet actin and myosin also exhibited an inhibitory activity on platelet aggregation albeit less severe than that observed with the antibodies to alpha and beta tubulin. There was evidence of an interaction between DM1A and DM1B and the antibodies to actin and myosin. The interaction of platelet tubulin and myosin was investigated by two different methods. (1) Coprecipitation of the proteins at low ionic strength at which tubulin by itself did not precipitate and (2) affinity chromatography on columns of immobilized myosin. Tubulin freed of its associated proteins (MAPs) by phosphocellulose chromatography bound to myosin in a molar ratio which approached 2. Platelet actin competed with tubulin for 1 binding site on the myosin molecule. MAPs also reduced the binding stoichiometry of tubulin/myosin. Treatment of microtubule protein with p-chloromercuribenzoate or colchicine did not influence its binding to myosin. DM1A and DM1B inhibited the interaction of tubulin and myosin. This effect could also be demonstrated by reaction of electrophoretic transblots of extracted platelet tubulin with the respective proteins. We interpret these results as evidence for an interference of the two monoclonal antibodies to the tubulin subunits (DM1A and DM1B) with the translocation of microtubule protein from its submembranous site to a more central one during the activation process.

Anti-tubulin antibodies in rabbits before and after immunization with pig tubulin

Sera from rabbits before and after repeated injections of pig tubulin in complete Freund's adjuvant were examined for antibody activity against pig and rabbit tubulins and against a panel of antigens: actin, myosin, DNA, TNP/BSA. Antibody activity against all the antigens of the panel (PAg) increased moderately after the first but not after subsequent injections. Antibody activity against pig and rabbit tubulins strongly increased after the second immunization when the maximum was reached. Isolation of anti-tubulin antibodies from normal or immune sera on tubulin-immunoadsorbent demonstrated the presence of three different antibody populations: (1) polyspecific IgM reacting with the PAg and the tubulins, present in substantial amounts in normal sera and moderately increased in immune sera; (2) small amounts of polyspecific IgG detected only in immune sera; (3) high amounts of specific IgG reacting with pig and rabbit tubulins, present in immune but not normal sera. Western blot analysis of the specific IgG population showed that it contained antibodies reacting with both native pig and rabbit tubulins, as well as antibodies recognizing only the 30,000 proteolytic fragment of pig, but not that of rabbit tubulin. The results indicate that the immunization of rabbits with heterologous tubulin induced specific IgG anti-tubulin antibodies which recognize the self and non-self antigens differently.

The alpha-tubulin of the growth cone is predominantly in the tyrosinated form

Growth cone cytoskeletons were prepared by detergent extraction of growth cones isolated from neonatal rat forebrain by the method of Gordon-Weeks and Lockerbie (Neuroscience, 13 (1984) 119-136). SDS-PAGE analysis of growth cone cytoskeletons revealed the presence of several major bands, identified by their mobility as actin (43 kDa Mr), myosin heavy chain (195 kDa Mr), spectrin (235 and 240 kDa Mr), and tubulin (51-54 kDa Mr). The identity of these proteins was confirmed by immunoblot analysis using specific antibodies to these proteins which further revealed that the predominant form of alpha-tubulin in the growth cone cytoskeleton and in the soluble pool of tubulin is tyrosinated at the C-terminal.

Effect of specific proteolytic cleavages on tubulin polymer formation

The capacity for self-polymerization and shape of the tubulin polymers assembled after digestion with trypsin, Pronase, chymotrypsin, subtilisin, Staphylococcus aureus proteinase V8 and proteinase K were investigated. Digestion with trypsin, Pronase or chymotrypsin resulted in a decrease in the ability of tubulin for self-assembly, whereas limited proteolysis with subtilisin, S. aureus proteinase V8 or proteinase K resulted in an increase in such ability. The shape of the assembled polymers varied from typical microtubules (after the treatment with trypsin or Pronase) to sheets (after the treatment with chymotrypsin) and from hooked microtubules with a constant polarity (after the treatment with subtilisin) to the disappearance of a defined polarity of such polymers (after the treatment with S. aureus V8 proteinase or proteinase K). These results indicate that the tubulin C-terminal regions are involved in the regulation of microtubule polymerization, shape, directional growth and lateral interactions between tubulin protofilaments.