Microtubule polyglutamylation in Drosophila melanogaster brain and testis.

The presence of glutamylated tubulin, a widespread posttranslational modification of alpha- and beta-tubulin, has been investigated in Drosophila melanogaster using the specific monoclonal antibody GT335. We show here that this modification is strongly detected in brain and testis whereas other tissues analyzed did not appear to contain any glutamylated isoforms. Neuronal microtubules are glutamylated on alpha-tubulin only whereas sperm flagella showed a strong modification of both alpha- and beta-tubulin. These results argue for an essential role for glutamylation in differentiation processes that require microtubule stabilization.     

Mutations of tubulin glycylation sites reveal cross-talk between the C termini of alpha- and beta-tubulin and affect the ciliary matrix in Tetrahymena

Two types of polymeric post-translational modifications of alpha/beta-tubulin, glycylation and glutamylation, occur widely in cilia and flagella. Their respective cellular functions are poorly understood. Mass spectrometry and immunoblotting showed that two closely related species, the ciliates Tetrahymena and Paramecium, have dramatically different compositions of tubulin post-translational modifications in structurally identical axonemes. Whereas the axonemal tubulin of Paramecium is highly glycylated and has a very low glutamylation content, the axonemal tubulin of Tetrahymena is glycylated and extensively glutamylated. In addition, only the alpha-tubulin of Tetrahymena undergoes detyrosination. Mutations of the known glycylation sites in Tetrahymena tubulin affected the level of each polymeric modification type in both the mutated and nonmutated subunits, revealing cross-talk between alpha- and beta-tubulin. Ultrastructural analyses of glycylation site mutants uncovered defects in the doublet B-subfiber of axonemes and revealed an accumulation of dense material in the ciliary matrix, reminiscent of intraflagellar transport particles seen by others in Chlamydomonas. We propose that polyglycylation and/or polyglutamylation stabilize the B-subfiber of outer doublets and regulate the intraflagellar transport.     

Distribution of glutamylated alpha and beta-tubulin in mouse tissues using a specific monoclonal antibody, GT335.

A monoclonal antibody (GT335) directed against polyglutamylated tubulin was obtained by immunization with a synthetic peptide which mimics the structure of the polyglutamylated site of alpha-tubulin. This peptide corresponds to the C-terminal sequence Glu441-Gly448 and was chemically modified by the addition of two glutamyl units at Glu445. The specificity of GT335 was assayed by direct and competitive enzyme-linked immunosorbent assay (ELISA) against tubulin and several synthetic peptides differing either by the structure of the added polyglutamyl chain or by their amino acid sequence. Further characterization was carried out by immunoblotting detection after one- or two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The epitope appears to be formed by at least two constituents: a basic motif of monoglutamylation which is retained in the polyglutamylated forms independent of their degree of glutamylation, and some elements of the polypeptide chain close to the site of glutamylation. Given the specificity of GT335 and the delineation of its epitope, our results indicate that, in addition to alpha and beta' (class III)-tubulin, other beta-tubulin isotypes are also glutamylated. This antibody has been used to analyze the cell and tissue distributions of glutamylated tubulin. In mouse brain extracts, GT335 reacts strongly with alpha-tubulin and, to a lesser extent, with beta' (class III) and beta-tubulin. The same reactivity is also observed with cultured neurons whereas astroglial cells exhibit only low levels of glutamylated tubulin. In non-nervous mouse tissues such as spleen, lung or testis, glutamylation was shown to involve only beta-tubulin, but at far lower levels than in brain.     

Review: postchaperonin tubulin folding cofactors and their role in microtubule dynamics

The microtubule cytoskeleton consists of a highly organized network of microtubule polymers bound to their accessory proteins: microtubule-associated proteins, molecular motors, and microtubule-organizing proteins. The microtubule subunits are heterodimers composed of one alpha-tubulin polypeptide and one beta-tubulin polypeptide that should undergo a complex folding processing before they achieve a quaternary structure that will allow their incorporation into the polymer. Due to the extremely high protein concentration that exists at the cell cytoplasm, there are alpha- and beta-tubulin interacting proteins that prevent the unwanted interaction of these polypeptides with the surrounding protein pool during folding, thus allowing microtubule dynamics. Several years ago, the development of a nondenaturing electrophoretic technique made it possible to identify different tubulin intermediate complexes during tubulin biogenesis in vitro. By these means, the cytosolic chaperonin containing TCP-1 (CCT or TriC) and prefoldin have been demonstrated to intervene through tubulin and actin folding. Various other cofactors also identified along the alpha- and beta-tubulin postchaperonin folding route are now known to have additional roles in tubulin biogenesis such as participating in the synthesis, transport, and storage of alpha- and beta-tubulin. The future characterization of the tubulin-binding sites to these proteins, and perhaps other still unknown proteins, will help in the development of chemicals that could interfere with tubulin folding and thus modulating microtubule dynamics. In this paper, current knowledge of the above postchaperonin folding cofactors, which are in fact chaperones involved in tubulin heterodimer quaternary structure achievement, will be reviewed.     

Posttranslational modification of brain tubulins from the Antarctic fish Notothenia coriiceps: reduced C-terminal glutamylation correlates with efficient microtubule assembly at low temperature

We have shown previously that the tubulins of Antarctic fish assemble into microtubules efficiently at low temperatures (-2 to +2 degrees C) due to adaptations intrinsic to the tubulin subunits. To determine whether changes in posttranslational glutamylation of the fish tubulins may contribute to cold adaptation of microtubule assembly, we have characterized C-terminal peptides from alpha- and beta-tubulin chains from brains of adult specimens of the Antarctic rockcod Notothenia coriiceps by MALDI-TOF mass spectrometry and by Edman degradation amino acid sequencing. Of the four fish beta-tubulin isotypes, nonglutamylated isoforms were more abundant than glutamylated isoforms. In addition, maximal glutamyl side-chain length was shorter than that observed for mammalian brain beta tubulins. For the nine fish alpha-tubulin isotypes, nonglutamylated isoforms were also generally more abundant than glutamylated isoforms. When glutamylated, however, the maximal side-chain lengths of the fish alpha tubulins were generally longer than those of adult rat brain alpha chains. Thus, Antarctic fish adult brain tubulins are glutamylated differently than mammalian brain tubulins, resulting in a more heterogeneous population of alpha isoforms and a reduction in the number of beta isoforms. By contrast, neonatal rat brain tubulin possesses low levels of glutamylation that are similar to that of the adult fish brain tubulins. We suggest that unique residue substitutions in the primary structures of Antarctic fish tubulin isotypes and quantitative changes in isoform glutamylation act synergistically to adapt microtubule assembly to low temperatures.     

Identification by mass spectrometry of a new alpha-tubulin isotype expressed in human breast and lung carcinoma cell lines

The extensive C-terminal molecular heterogeneity of alpha- and beta-tubulin is a consequence of multiple isotypes, the products of distinct genes, that undergo several posttranslational modifications. These include polyglutamylation and polyglycylation of both subunits, reversible tyrosination and removal of the penultimate glutamate from alpha-tubulin, and phosphorylation of the beta III isotype. A mass spectrometry-based method has been developed for the analysis of the C-terminal diversity of tubulin from human cell lines. Total cell extracts are resolved by SDS--PAGE and transferred to nitrocellulose, and the region of the blot corresponding to tubulin (approximately 50 kDa) was excised and digested with CNBr to release the highly divergent C-terminal tubulin fragments. The masses of the human alpha- and beta-tubulin CNBr-derived C-terminal peptides are all in the 1500--4000 Da mass range and can be analyzed directly by MALDI-TOF mass spectrometry in the negative ion mode without significant interference from other released peptides. In this study, the tubulin isotype diversity in MDA-MB-231, a human breast carcinoma cell line, and A549, a human non-small lung cancer cell line, is reported. The major tubulin isotypes present in both cell lines are k-alpha 1 and beta 1. Importantly, we report a previously unknown alpha isotype present at significant levels in both cell lines. Moreover, the degree of posttranslational modifications to all isotypes was limited. Glu-tubulin, in which the C-terminal tyrosine of alpha-tubulin is removed, was not detected. In contrast to mammalian neuronal tubulin which exhibits extensive polyglutamylation, only low-level monoglutamylation of the k-alpha 1 and beta 1 isotypes was observed in these two human cell lines.     

Differential binding regulation of microtubule-associated proteins MAP1A, MAP1B, and MAP2 by tubulin polyglutamylation

The major neuronal post-translational modification of tubulin, polyglutamylation, can act as a molecular potentiometer to modulate microtubule-associated proteins (MAPs) binding as a function of the polyglutamyl chain length. The relative affinity of Tau, MAP2, and kinesin has been shown to be optimal for tubulin modified by approximately 3 glutamyl units. Using blot overlay assays, we have tested the ability of polyglutamylation to modulate the interaction of two other structural MAPs, MAP1A and MAP1B, with tubulin. MAP1A and MAP2 display distinct behavior in terms of tubulin binding; they do not compete with each other, even when the polyglutamyl chains of tubulin are removed, indicating that they have distinct binding sites on tubulin. Binding of MAP1A and MAP1B to tubulin is also controlled by polyglutamylation and, although the modulation of MAP1B binding resembles that of MAP2, we found that polyglutamylation can exert a different mode of regulation toward MAP1A. Interestingly, although the affinity of the other MAPs tested so far decreases sharply for tubulins carrying long polyglutamyl chains, the affinity of MAP1A for these tubulins is maintained at a significant level. This differential regulation exerted by polyglutamylation toward different MAPs might facilitate their selective recruitment into distinct microtubule populations, hence modulating their functional properties.     

Monoclonal antibody ID5: epitope characterization and minimal requirements for the recognition of polyglutamylated alpha- and beta-tubulin

A monoclonal antibody (ID5) raised against the synthetic tetradecapeptide corresponding to the C-terminal region of detyrosinated alpha-tubulin showed an unexpected cross-reactivity with beta-tubulin from pig brain tissue. The specificity and the minimal epitope requirements of ID5 were characterized by competitive enzyme-linked immunosorbent assay (ELISA) and spot blots using a series of synthetic peptides and the natural peptides of beta-tubulin and detyrosinated alpha-tubulin from brain. The epitope of ID5 is comprised of the carboxyterminal sequence -XEE carrying the terminal alpha-carboxylate group with X being a variable residue. All linkages in the epitope involve alpha-peptide bonds. This epitope is provided by the detyrosinated alpha-tubulin main chain and the polyglutamyl side chains of both brain alpha- and beta-tubulins. Affinity purification of beta-tubulin peptides and mass spectrometric characterization reveal that peptides carrying three to nine glutamyl residues in the side chain are recognized by ID5. These results show that except for the first gamma-peptide linkage the alpha-peptide bond is the preferred linkage type in the tubulin polyglutamyl side chains.     

Dinitroaniline herbicide-resistant transgenic tobacco plants generated by co-overexpression of a mutant alpha-tubulin and a beta-tubulin.

Dinitroaniline herbicides are used for the selective control of weeds in arable crops. Dinitroaniline herbicide resistance in the invasive weed goosegrass was previously shown to stem from a spontaneous mutation in an alpha-tubulin gene. We transformed and regenerated tobacco plants with an alpha/beta-tubulin double gene construct containing the mutant alpha-tubulin gene and showed that expression of this construct confers a stably inherited dinitroaniline-resistant phenotype in tobacco. In all transformed lines, the transgene alpha- and beta-tubulins increased the cytoplasmic pool of tubulin approximately 1.5-fold while repressing endogenous alpha- and beta-tubulin synthesis by up to 45% in some tissues. Transgene alpha- and beta-tubulin were overexpressed in every plant tissue analyzed and comprised approximately 66% of the total tubulin in these tissues. Immunolocalization studies revealed that transgene alpha- and beta-tubulins were incorporated into all four microtubule arrays, indicating that they are functional. The majority of the alpha/beta-tubulin pools are encoded by the transgenes, which implies that the mutant alpha-tubulin and the beta-tubulin can perform the majority, if not all, of the roles of microtubules in both juvenile and adult tobacco plants.     

Polyglycylation of tubulin is essential and affects cell motility and division in Tetrahymena thermophila

We analyzed the role of tubulin polyglycylation in Tetrahymena thermophila using in vivo mutagenesis and immunochemical analysis with modification-specific antibodies. Three and five polyglycylation sites were identified at glutamic acids near the COOH termini of alpha- and beta-tubulin, respectively. Mutants lacking all polyglycylation sites on alpha-tubulin have normal phenotype, whereas similar sites on beta-tubulin are essential. A viable mutant with three mutated sites in beta-tubulin showed reduced tubulin glycylation, slow growth and motility, and defects in cytokinesis. Cells in which all five polyglycylation sites on beta-tubulin were mutated were viable if they were cotransformed with an alpha-tubulin gene whose COOH terminus was replaced by the wild-type COOH terminus of beta-tubulin. In this double mutant, beta-tubulin lacked detectable polyglycylation, while the alpha-beta tubulin chimera was hyperglycylated compared with alpha-tubulin in wild-type cells. Thus, the essential function of polyglycylation of the COOH terminus of beta-tubulin can be transferred to alpha-tubulin, indicating it is the total amount of polyglycylation on both alpha- and beta-tubulin that is essential for survival.     

Tubulin expression in trypanosomes

Microtubules in trypanosomes are the main component of the flagellar axoneme and of the subpellicular microtubule corset, whose relative positions determine the morphology of each cell stage of the life cycle of these parasites. Microtubules are polymers of tubulin, a protein dimer of two 55-kDa subunits, alpha- and beta-tubulin; in Trypanosoma brucei, the tubulin-coding sequences are clustered in a 40-kb fragment of tandemly repeated alpha- and beta-tubulin genes separated by a 170-bp intergenic zone. This cluster is transcribed in a unique RNA which is rapidly processed into mature mRNAs carrying the 5' 35-nucleotide leader sequence found in all trypanosome mRNAs. Although no heterogeneity has been found at the gene level, tubulin can be post-translationally modified in 2 ways: the C-terminal tyrosine of alpha-tubulin can be selectively cleaved and added again with 2 enzymes, tubulin carboxypeptidase and tubulin-tyrosine ligase; alpha-tubulin can also be acetylated on a lysine residue. Some molecular domains of tubulin are restricted to subpopulations of microtubules; for instance, the beta-tubulin form defined by the monoclonal antibody 1B41 is sequestered into a part of the subpellicular cytoskeleton limited to the flagellar adhesion zone, which might correspond to the group of 4 microtubules associated with a cisterna of the endoplasmic reticulum, forming the so-called "subpellicular microtubule quartet" (SFMQ). The early assembly of this zone in each daughter cell during the cell division of T. brucei, together with the alterations undergone by the domain defined by the monoclonal antitubulin 24E3 during the differentiation of Trypanosoma cruzi, suggest that specific tubulin forms are responsible for dynamic properties of SFMQ possibly involved in trypanosome morphogenesis.     

Coordination of posttranslational modifications of bovine brain alpha-tubulin. Polyglycylation of delta2 tubulin

Microtubules participate in a large number of intracellular events including cell division, intracellular transport and secretion, axonal transport, and maintenance of cell morphology. They are composed of tubulin, a heterodimeric protein, consisting of two similar polypeptides alpha and beta. In mammalian cells, both alpha- and beta-tubulin occur as seven to eight different genetic variants, which also undergo numerous posttranslational modifications that include tyrosination-detyrosination and deglutamylation, phosphorylation, acetylation, polyglutamylation, and polyglycylation. Tyrosination-detyrosination is one of the major posttranslational modifications in which the C-terminal tyrosine residue in alpha-tubulin is added or removed reversibly. Although this modification does not alter the assembly activity of tubulin in vitro, these two forms of tubulin have been found to be distributed differently in vivo and are also correlated with microtubule stability (Gunderson, G. G., Kalnoski, M. H., and Bulinski, J. C. (1984) Cell 38, 779-789). Thus, the question arises as to whether these two forms of tubulin differ in any other modifications. In an effort to answer this question, the tyrosinated and the nontyrosinated forms of the alpha1/2 isoform have been purified from brain tubulin by immunoaffinity chromatography. matrix-assisted laser desorption/ionization-time of flight mass spectrometric analysis of the C-terminal peptide revealed that the tyrosinated form is polyglutamylated with one to four Glu residues, while the Delta2 tubulin is polyglycylated with one to three Gly residues. These results indicate that posttranslational modifications of tubulin are correlated with each other and that polyglutamylation and polyglycylation of tubulin may have important roles in regulating microtubule assembly, stability, and function in vivo.     

Tubulin is phosphorylated at tyrosine by pp60c-src in nerve growth cone membranes

We show here that tubulin is the major in vivo substrate of the tyrosine-specific protein kinase pp60c-src in nerve growth cone membranes. Phosphotyrosine antibodies were used to demonstrate phosphotyrosyl residues in a subpopulation of alpha- and beta-tubulin that was highly enriched in a subcellular fraction of growth cone membranes from fetal rat brain. The presence of phosphotyrosine- modified isoforms of alpha- and beta-tubulin in vivo was confirmed by 32p labeling of rat cortical neurons in culture. Tubulin in growth cone membranes was phosphorylated at tyrosine in endogenous membrane phosphorylation reactions (0.068 mol phosphotyrosine/mol alpha-tubulin and 0.045 mol phosphotyrosine/mol beta-tubulin), and phosphorylation was specifically inhibited by antibodies directed against pp60c-src, which is localized in the growth cone membranes. pp60c-src was capable of directly phosphorylating tubulin as shown in immune complex kinase assays with purified brain tubulin. Phosphopeptide mapping revealed a limited number of sites of tyrosine phosphorylation in alpha- and beta- tubulin, with similar phosphopeptides observed in vivo and in vitro. These results reveal a novel posttranslational modification of tubulin that could regulate microtubule dynamics at the growth cone.     

A novel step in beta-tubulin folding is important for heterodimer formation in Saccharomyces cerevisiae

Undimerized beta-tubulin is toxic in the yeast S. cerevisiae. It can arise if levels of beta-tubulin and alpha-tubulin are unbalanced or if the tubulin heterodimer dissociates. We are using the toxicity of beta-tubulin to understand early steps in microtubule morphogenesis. We find that deletion of PLP1 suppresses toxic beta-tubulin formed by disparate levels of alpha- and beta-tubulin. That suppression occurs either when alpha-tubulin is modestly underexpressed relative to beta-tubulin or when beta-tubulin is inducibly and strongly overexpressed. Plp1p does not affect tubulin expression. Instead, a significant proportion of the undimerized beta-tubulin in plp1Delta cells is less toxic than that in wild-type cells. It is also less able to combine with alpha-tubulin to form a heterodimer. As a result, plp1Delta cells have lower levels of heterodimer. Importantly, plp1Delta cells that also lack Pac10, a component of the GimC/PFD complex, are even less affected by free beta-tubulin. Our results suggest that Plp1p defines a novel early step in beta-tubulin folding.     

Localization and Characterization of Tubulin-Like Proteins Associated with Brain Mitochondria: The Presence of a Membrane-Specific Isoform

A mitochondrial fraction, purified from pig brain, was found to contain associated polypeptides with the same electrophoretic migration and isoelectric points as the alpha- and beta-tubulin subunits present in brain microtubules. When analyzed by Western blotting these polypeptides reacted specifically with purified tubulin antibodies. The tubulin-like proteins were then visualized in mitochondrial membranes by protein A-gold complexes after the incubation of purified mitochondria with tubulin antibodies. When membrane and microtubule proteins were compared by isoelectric focussing and two-dimensional gel electrophoresis, differences were observed in the patterns of tubulin isoforms. An additional polypeptide, with the electrophoretic migration of beta-tubulin but the isoelectric point of alpha-tubulin, was found to be enriched in the mitochondrial fraction. This peptide had several Staphylococcus aureus V8 protease peptides in common with alpha-tubulin and may result from a posttranslational modification of that subunit.     

Genetics of the Tubulin Gene Families of Physarum

The organization of the alpha- and beta-tubulin gene families in Physarum was investigated by Mendelian analysis. Restriction endonuclease-generated DNA fragments homologous to alpha- and beta-tubulin show length polymorphisms that can be used as markers for genetic mapping. Analysis of meiotic assortment among progeny of heterozygotes allowed alpha- and beta-tubulin sequence loci to be defined. There are four unlinked alpha-tubulin sequence loci (altA, altB, altC and altD) and at least three unlinked beta-tubulin sequence loci (betA, betB and betC). The alpha-tubulin loci are not linked to the beta-tubulin loci. --Segregation of tubulin sequence loci with respect to ben mutations that confer resistance to antitubulin benzimidazole drugs was used to investigate whether any members of the alpha- or beta-tubulin gene families are allelic to ben loci. The beta-tubulin sequence locus betB is allelic to the resistance locus benD, the betA locus is probably allelic to benA and the alpha-tubulin sequence locus altC may be allelic to benC. The molecular implications of benzimidazole resistance phenotypes when only one of the expressed beta-tubulin gene family members mutates to drug resistance are discussed in relation to tubulin function.     

Complex regulation and functional versatility of mammalian alpha- and beta-tubulin isotypes during the differentiation of testis and muscle cells

In the accompanying paper (Gu, W., S. A. Lewis, and N. J. Cowan. 1988. J. Cell Biol. 106: 2011-2022), we report the generation of three antisera, each of which uniquely recognizes a different mammalian alpha-tubulin isotype, plus a fourth antibody that distinguishes between microtubules containing the tyrosinated and nontyrosinated form of the only known mammalian alpha-tubulin gene product that lacks an encoded carboxy-terminal tyrosine residue. These sera, together with five sera we raised that distinguish among the known mammalian beta-tubulin isotypes, have been used to study patterns of tubulin isotype-specific expression in muscle and testis, two tissues in which characteristic developmental changes are accompanied by dramatic rearrangements in microtubule structures. As in the case of cells in culture, there is no evidence to suggest that there is subcellular sorting of different tubulin isotypes among different kinds of microtubule, even in a cell type (the developing spermatid) that simultaneously contains such functionally distinct structures as the manchette and the flagellum. On the other hand, the patterns of expression of the various tubulin isotypes show marked and distinctive differences in different cell types and, in at least one case, evidence is presented for regulation at the translational or posttranslational level. The significance of these observations is discussed in terms of the existence of the mammalian alpha- and beta-tubulin multigene families.     

Structural and functional implications of an unusual foraminiferal beta-tubulin

We have obtained sequence data for beta-tubulin genes from eight species of Foraminifera (forams) and alpha-tubulin sequences from four species, sampling major taxonomic groups from a wide range of environments. Analysis of the beta-tubulin sequences demonstrates that foram beta-tubulins possess the highest degree of divergence of any tubulin gene sequenced to date and represent a novel form of the protein. In contrast, foram alpha-tubulin genes resemble the conventional alpha-tubulins seen in other organisms. Partition homogeneity analysis shows that the foraminiferal beta-tubulin gene has followed an evolutionary path that is distinct from that of all other organisms. Our findings indicate that positive selective pressure occurred on the beta-tubulin subunit in ancestral forams prior to their diversification. The specific substitutions observed have implications for microtubule (MT) assembly dynamics. The regions most strongly affected are implicated in lateral contacts between protofilaments and in taxol binding. We predict that these changes strengthen lateral contacts between adjacent dimers in a manner similar to that induced by taxol binding, thus allowing the formation of the tubulin "helical filaments" observed in forams by electron microscopy. Our results also indicate that substantial changes to these portions of the beta-tubulin molecule can be made without sacrificing essential MT functions.