Analysis of beta-tubulin sequences reveals highly conserved, coordinated amino acid substitutions. Evidence that these 'hot spots' are directly involved in the conformational change required for dynamic instability

Understanding molecular mechanisms and predicting phenotypic effects of pathogenic tubulin mutations

Cells rely heavily on microtubules for several processes, including cell division and molecular trafficking. Mutations in the different tubulin-α and -β proteins that comprise microtubules have been associated with various diseases and are often dominant, sporadic and congenital. While the earliest reported tubulin mutations affect neurodevelopment, mutations are also associated with other disorders such as bleeding disorders and infertility. We performed a systematic survey of tubulin mutations across all isotypes in order to improve our understanding of how they cause disease, and increase our ability to predict their phenotypic effects. Both protein structural analyses and computational variant effect predictors were very limited in their utility for differentiating between pathogenic and benign mutations. This was even worse for those genes associated with non-neurodevelopmental disorders. We selected tubulin-α and -β disease mutations that were most poorly predicted for experimental characterisation. These mutants co-localise to the mitotic spindle in HeLa cells, suggesting they may exert dominant-negative effects by altering microtubule properties. Our results show that tubulin mutations represent a blind spot for current computational approaches, being much more poorly predicted than mutations in most human disease genes. We suggest that this is likely due to their strong association with dominant-negative and gain-of-function mechanisms.

Why is the microtubule lattice helical?

Microtubules polymerize from identical tubulin heterodimers, which form a helical lattice pattern that is the microtubule. This pattern always has left-handed chirality, but it is not known why. But as tubulin, similar to other proteins, evolved for a purpose, the question of the title of this artcile appears to be meaningful. In a computer simulation that explores the 'counterfactual biology' of microtubules without helicity, we demonstrate that these have the same mechanical properties as Nature's microtubules with helicity. Thus only a dynamical reason for helicity is left as potential explanation. We find that helicity solves 'the problem of the blind mason', i.e. how to correctly build a structure, guided only by the shape of the bricks. This answer in turn raises some new questions for researchers to address.

Caltubin, a novel molluscan tubulin-interacting protein, promotes axonal growth and attenuates axonal degeneration of rodent neurons

Axotomized central neurons of most invertebrate species demonstrate a strong regenerative capacity, and as such may provide valuable molecular insights and new tools to promote axonal regeneration in injured mammalian neurons. In this study, we identified a novel molluscan protein, caltubin, ubiquitously expressed in central neurons of Lymnaea stagnalis and locally synthesized in regenerating neurites. Reduction of caltubin levels by gene silencing inhibits the outgrowth and regenerative ability of adult Lymnaea neurons and decreases local α- and β-tubulin levels in neurites. Caltubin binds to α- and/or β-tubulin in both Lymnaea and rodent neurons. Expression of caltubin in PC12 cells and mouse cortical neurons promotes NGF-induced axonal outgrowth and attenuates axonal retraction after injury. This is the first study illustrating that a xenoprotein can enhance outgrowth and prevent degeneration of injured mammalian neurons. These results may open up new avenues in molecular repair strategies through the insertion of molecular components of invertebrate regenerative pathways into mammalian neurons.

The design and synthesis of guanosine compounds with in vitro activity against the colon cancer cell line SW480: non-taxane derived mimics of taxol?

In the course of our investigation into the use of taxol as a lead compound to design new molecules with anti-cancer activity, we have synthesized four compounds based on protected guanosine coupled to taxol isoserine side-chain analogues. These analogues show in vitro anti-cancer activity against the colon cancer cell line SW480 that their constituent parts do not.

Structure of tubulin C-terminal domain obtained by subtilisin treatment The major α and β tubulin isotypes from pig brain are glutamylated

Limited subtilisin digestion of the tubulin alpha, beta heterodimer has been used in this work to reduce the total number of tubulin isotypes from 20 for native to 9 for subtilisin-cleaved tubulin. This indicates that the major part of tubulin heterogeneity is located at the C-terminus of the molecule. The C-terminal peptides of both alpha and beta subunits of tubulin were purified by anion-exchange HPLC. Combined use of Edman degradation chemistry and mass spectrometry on the isolated peptides shows that subtilisin cleavage occurs at position Asp-438 and His-406 of alpha and Gln-433 and His-396 of beta tubulin chains. Quantitative analysis of our data show that cleavage at positions His-406 (alpha) and His-396 (beta) occurs with a low efficiency and indicates that the major isotypes of pig brain tubulin are modified by sequential attachment of 1 to 5 glutamic acid residues at positions Glu-445 or -435 of alpha and beta tubulin, respectively.

Differential subcellular distribution of tubulin epitopes in boar spermatozoa: recognition of class III beta-tubulin epitope in sperm tail

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 beta-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 beta-tubulin were present in the flagellum. A tubulin epitope from the C-terminal domain of beta-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 beta-tubulin that is widely used as a neuronal marker, stained the flagella. The reactivity of TU-20 was further confirmed by absorbing the antibody with the immunizing peptide and by immunoelectron microscopy. Immunoblotting after two-dimensional electrophoresis revealed that the corresponding epitope was not present on all beta-tubulin isoforms. These results suggest that various tubulins are involved in the functional organization of the mammalian sperm flagellum and head.

Tubulins in the primate retina: evidence that xanthophylls may be endogenous ligands for the paclitaxel-binding site

The xanthophylls-lutein, zeaxanthin, and meso-zeaxanthin (L&Z)-are found in the central region of the primate retina, which is called the macula lutea (yellow spot). How they are anchored there and what their function is has been debated for over 50 years. Here, we present evidence that they may be bound to the paclitaxel (Taxol) binding site of the beta-tubulin subunit of microtubules and that a major function may be to modulate the dynamic instability of microtubules in the macula. Also, we compare nucleic acid and amino acid sequences of tubulins that are in human brain with those we have isolated from human-retina and monkey-macula cDNA libraries. In so doing, we suggest that in primates, class I beta-tubulin consists of at least two subtypes (beta(Ia) and beta(Ib)). Alignment analysis of the sequences of the genes for beta(Ia) and beta(Ib) indicates that the corresponding mRNAs may have other functions in addition to that of coding for proteins. Furthermore, we show that there are at least five different types of beta-tubulin in the macula lutea of rhesus monkey.

Identification of a novel beta-tubulin subfamily with one member (TUBB4Q) located near the telomere of chromosome region 4q35

The human beta-tubulin supergene family consists of several isotypes with many associated pseudogenes. Here we report the identification of yet another beta-tubulin sequence designated TUBB4Q. This tubulin maps 80 kb proximal to the facioscapulohumeral muscular dystrophy (FSHD1) associated D4Z4 repeats on chromosome 4q35. The genomic structure contains four exons encoding a putative protein of 434 amino acids. The TUBB4Q nucleotide and protein sequence show 87% and 86% homology to beta2-tubulin, respectively. Although the genomic structure shows all functional aspects of a genuine gene, no transcript could be detected. TUBB4Q-related sequences were identified on multiple chromosomes. Since these sequences mutually exhibit a high nucleotide sequence homology, they presumably belong to a novel subfamily of beta-tubulin genes. Although the chromosome 4q35 tubulin-member probably represents a pseudogene, ectopic expression due to a postulated position effect variegation (PEV), makes TUBB4Q an ideal dominant-negative candidate gene for FSHD1.

In vitro assembled plant microtubules exhibit a high state of dynamic instability

Higher plants possess four distinct microtubule arrays. One of these, the cortical array, is involved in orienting the deposition of cellulose microfibrils. This plant interphase array is also notable because it contains exceptionally dynamic microtubules. Although the primary sequence of plant and animal tubulin is similar (79-87% amino acid identity overall) there are some regions of divergence. Thus, one possible explanation for the high state of polymer assembly and turnover that is observed in plant interphase arrays is that the tubulins have evolved differently and possess a higher intrinsic dynamic character than their animal counterparts. This hypothesis was tested using highly purified plant tubulin assembled in vitro. Using high-resolution DIC video-enhanced microscopy, we quantified the four characteristic parameters of dynamic instability of plant microtubules and compared them with animal microtubules. The elongation velocities between plant and animal microtubules are similar, but plant microtubules undergo catastrophes more frequently, do not exhibit any rescues, and have an average shortening velocity of 195 microm/min (compared with 21 microm/min for animal microtubules). These data support the hypothesis that plant tubulin forms microtubules that are intrinsically more dynamic than those of animals.

Identification of betaIII- and betaIV-tubulin isotypes in cold-adapted microtubules from Atlantic cod (Gadus morhua): antibody mapping and cDNA sequencing

Isolated microtubule proteins from the Atlantic cod (Gadus morhua) assemble at temperatures between 8 and 30 degrees C. The cold-adaptation is an intrinsic property of the tubulin molecules, but the reason for it is unknown. To increase our knowledge of tubulin diversity and its role in cold-adaptation we have further characterized cod tubulins using alpha- and beta-tubulin site-directed antibodies and antibodies towards posttranslationally modified tubulin. In addition, one cod brain beta-tubulin isotype has been sequenced. In mammals there are five beta-tubulins (betaI, betaII, betaIII, betaIVa and betaIVb) expressed in brain. A cod betaIII-tubulin was identified by its electrophoretic mobility after reduction and carboxymethylation. The betaIII-like tubulin accounted for more than 30% of total brain beta-tubulins, the highest yield yet observed in any animal. This tubulin corresponds most probably with an additional band, designated beta(x), which was found between alpha- and beta-tubulins on SDS-polyacrylamide gels. It was found to be phosphorylated and neurospecific, and constituted about 30% of total cod beta-tubulin isoforms. The sequenced cod tubulin was identified as a betaIV-tubulin, and a betaIV-isotype was stained by a C-terminal specific antibody. The amount of staining indicates that this isotype, as in mammals, only accounts for a minor part of the total brain beta-tubulin. Based on the estimated amounts of betaIII- and betaIV-tubulins in cod brain, our results indicate that cod has at least one additional beta-tubulin isotype and that beta-tubulin diversity evolved early during fish evolution. The sequenced cod betaIV-tubulin had four unique amino acid substitutions when compared to beta-tubulin sequences from other animals, while one substitution was in common with Antarctic rockcod beta-tubulin. Residues 221, Thr to Ser, and 283, Ala to Ser, correspond in the bovine tubulin dimer structure to loops that most probably interact with other tubulin molecules within the microtubule, and might contribute to cold-adaptation of microtubules.

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.

Preparation of a monoclonal antibody specific for the class I isotype of beta-tubulin: the beta isotypes of tubulin differ in their cellular distributions within human tissues

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta consist of various isotypes. Although the isotypes differ in their tissue distributions, the question of whether the isotypes perform different functions in vivo is unanswered. In mammals, the betaI and betaIV isotypes are quite widespread, and betaII is less so, while betaIII and betaVI have narrow distributions and betaV distribution is unknown. As a tool for localizing the isotypes, we report the preparation of a monoclonal antibody specific for betaI, to add to our previously described monoclonal antibodies specific for betaII, betaIII, and betaIV [Banerjee et al., J. Biol. Chem. 263:3029-3034, 1988; 265:1794-1799, 1990; 267:5625-5630, 1992]. In order to prepare this antibody, we have purified betaI-rich rat thymus tubulin. We have used our battery of antibodies to localize the beta isotypes in four human tissues: oviduct, skin, colon, and pancreas. We have found striking differences in their tissue distributions. There is little or no betaIII in these tissues, except for the columnar epithelial cells of the colon. BetaII is restricted to very few cells, except in the skin, where it is concentrated in the stratum granulosum. BetaI is widespread in all the epithelia. In the skin it is found in the entire stratum malpighii. In the oviduct, betaI is found largely in the nonciliated epithelial cells. In the exocrine pancreas, betaI occurs only in the centroacinar cells and not in the acinar cells; the latter do not stain with any of these antibodies. BetaIV is present at very low levels in skin and pancreas. By contrast, it is prominent in the colon and also in the oviduct, where it occurs in all the epithelial cells, especially in the ciliated cells, with the highest concentrations in the cilia themselves. These results suggest that the regulation of the expression and localization of isotypes in tissues is very complex.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Microtubule assembly in cold-adapted organisms: functional properties and structural adaptations of tubulins from antarctic fishes

Fishes native to the coastal waters of the Antarctic have adapted to habitat and body temperatures in the range -1.8 to +2 degrees C. Their cytoplasmic microtubules, unlike those of mammals and temperate poikilotherms, have evolved to assemble efficiently at these low temperatures. To learn about the underlying molecular adaptations, my laboratory is studying microtubule proteins [tubulin alpha beta dimers and microtubule-associated proteins (MAPs)] and tubulin genes from several Antarctic fishes, including the rockcods Notothenia coriiceps and Gobionotothen gibberifrons. We find that the assembly-enhancing adaptations of the fish microtubule proteins are intrinsic to the tubulin subunits themselves. Furthermore, microtubule formation by Antarctic fish tubulins is strongly entropy driven, due in part to an increased reliance, relative to tubulins from other species, on hydrophobic interactions. Based on analyses of tubulin polypeptides and cDNAs, we suggest that the structural adaptations of Antarctic fish tubulins most likely involve alterations in the primary sequences of tubulin isotypes. With respect to neural beta tubulins from other vertebrates, for example, the class II beta-tubulin isotype of N. coriiceps brain contains seven unique amino acid substitutions and one novel insertion in its 446-residue primary sequence. Most of these changes are located in a structural domain that forms contacts between tubulin dimers during microtubule assembly and would be expected to enhance polypeptide flexibility, thereby facilitating addition of tubulin to microtubule ends. The acidic carboxy-terminal tails of the alpha and beta tubulins, by contrast, appear not to be sites of cold adaptation of polymerization. We have also found that brain and egg tubulins from Antarctic fishes differ strikingly in their polymerization efficiencies, which demonstrates, in agreement with the multitubulin hypothesis, that tissue-specific tubulin isoforms can possess distinct functional properties. Thus, study of microtubule proteins from organisms, such as the Antarctic fishes, that have adapted to extreme thermal regimes should contribute significantly to an understanding of the quaternary interactions that control microtubule assembly in all eukaryotes.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

Interaction of phomopsin A with normal and subtilisin-treated bovine brain tubulin

Tubulin, the major component of microtubules, has a tendency to lose its ability to assemble or to bind to ligands in a time-dependent process known as "decay." The decay process also causes tubulin to expose sulfhydryl groups and hydrophobic areas. The antimitotic drug phomopsin A strongly protects the tubulin molecule from decay. Here we have studied the interaction of phomopsin A with alpha beta tubulin and tubulin which has been treated with subtilisin to remove selectively the C-termini of the alpha and beta chains (alpha(s) beta(s)). The binding of phomopsin A to alpha beta tubulin decreases the sulfhydryl titer by approximately 1.0 mol/mol. Selective removal of the peptides from the C-terminal ends does not affect phomopsin A's interaction with tubulin. Moreover, the alpha(s) beta(s) tubulin-phomopsin A complex appears to be more stable than the alpha bet tubulin-phomopsin A complex as determined by the time-dependent increase in exposure of sulfhydryl groups and hydrophobic areas on tubulin. In fact, phomopsin A inhibits the decay process of alpha(s) beta(s) tubulin completely. This observation raises the possibility of determining the conformation of this configuration of tubulin.

Microtubule architecture specified by a beta-tubulin isoform

In Drosophila melanogaster, a testis-specific beta-tubulin (beta2) is required for spermatogenesis. A sequence motif was identified in carboxyl termini of axonemal beta-tubulins in diverse taxa. As a test of whether orthologous beta-tubulins from different species are functionally equivalent, the moth Heliothis virescens beta2 homolog was expressed in Drosophila testes. When coexpressed with beta2, the moth isoform imposed the 16-protofilament structure characteristic of that found in the moth on the corresponding subset of Drosophila microtubules, which normally contain only 13-protofilament microtubules. Thus, the architecture of the microtubule cytoskeleton can be directed by a component beta-tubulin.

Phosphorylation of beta III-tubulin

There is considerable evidence that mammalian beta-tubulin is phosphorylated. Specifically, of the seven beta isotypes, the phosphorylated one is beta III, the isotype found almost entirely in neurons. The phosphate is added at a serine and perhaps a tyrosine near the C-terminus. All the evidence to date has been gathered by growth of cells and tissues in the presence of radioactive inorganic phosphate followed by tubulin isolation and determination of the labeled tubulin; thus, the actual extent of phosphorylation of beta III is unknown. Nor is it known if alpha-tubulin and the other beta isotypes are phosphorylated by a mechanism which would not be revealed by previous experiments. In addition, the role of tubulin phosphorylation is unknown. We have purified the alpha beta II-, alpha beta III-, and alpha beta IV-tubulin dimers from bovine brain and have determined their phosphate content chemically. We have found that alpha-tubulin is not phosphorylated and neither are the beta II or beta IV isotypes. However, beta III is phosphorylated with a stoichiometry of about 1.52 mol/mol. We have found that the phosphate on beta III is resistant to a wide variety of phosphatases except for human erythrocyte phosphatase 2A and that removal of the phosphate inhibits microtubule assembly in vitro stimulated by microtubule-associated protein 2 (MAP 2). However such an inhibition was not evident when microtubule assembly was induced in the absence of microtubule-associated proteins. Our results suggest the possibility that beta III phosphorylation may play a role in regulating microtubule assembly in vivo.

Anti-beta-tubulin antibodies have no diagnostic value in patients with chronic inflammatory demyelinating polyneuropathy

High-titre anti-beta-tubulin antibodies were recently reported to occur in over 50% of sera from patients with chronic inflammatory demyelinating polyneuropathy (CIDP). It was concluded that these antibodies may help to distinguish CIDP from other neuropathies and that they are diagnostically useful. To verify these findings, we studied sera of 43 CIDP patients, only 3 of whom had anti-beta-tubulin antibodies. The differences between the results obtained cannot fully be explained by differences between patients or antigens but may be explained by the use of different techniques (Western blot versus ELISA). We used Western blot, which is less sensitive but far more specific, to detect these antibodies. The findings of our study did not confirm the high frequency of selective high-titre anti-beta-tubulin antibodies in CIDP patients. Therefore we conclude that binding to beta-tubulin by Western blot cannot serve as a marker of CIDP.

Effect of modification of the chromophore in retinochrome

14-Methyl-, 14-chloro-, 14-fluoro-retinals have been incorporated into apo-retinochrome to investigate the effect of the electronegativities of C-14 substituents on the protonation of the Schiff base. The extent of protonation decreased in going from 14-methyl- to 14-chloro- to 14-fluoro-retinal. There was usually no protonation in the case of the 14-fluoro analogue. The chromophore-protein interactions are also discussed.

Cold-stable and cold-adapted microtubules

Most mammalian microtubules disassemble at low temperature, but some are cold stable. This probably has little to do with a need for cold-stable microtubules, but reflects that certain populations of microtubules must be stabilized for specific functions. There are several routes by which to achieve cold stability. Factors that interact with microtubules, such as microtubule-associated proteins, STOPs (stable tubule only polypeptides), histones, and possibly capping factors, are involved. Specific tubulin isotypes and posttranslational modifications might also be of importance. More permanent stable microtubules can be achieved by bundling factors, associations to membranes, as well as by assembly of microtubule doublets and triplets. This is, however, not the explanation for cold adaptation of microtubules from poikilothermic animals, that is, animals that must have all their microtubules adapted to low temperatures. All evidence so far suggests that cold adaptation is intrinsic to the tubulins, but it is unknown whether it depends on different amino acid sequences or posttranslational modifications.

Regulation of beta-tubulin function and expression in Drosophila spermatogenesis

In this study we examined two aspects of beta-tubulin function in Drosophila spermatogenesis: 1) beta-tubulin structural requirements for assembly of different categories of microtubules and 2) regulatory requirements for production of the correct tubulin protein level. In normal Drosophila spermatogenesis, the testis-specific beta 2-tubulin isoform supports multiple microtubule functions. Our previous work showed that another Drosophila isoform, beta 3, cannot support spermatogenesis, whereas a carboxyl-truncated form of beta 2, beta 2 delta C, can at least to some extent provide all of beta 2's normal functions, save one: beta 2 delta C cannot support organization of axonemal microtubules into the supramolecular architecture of the axoneme. Here, to test whether beta 2 carboxyl sequences can rescue the functional failure of the beta 3 isoform in spermatogenesis, we constructed a gene encoding a chimeric protein, beta 3 beta 2C, in which beta 3 sequences in the carboxyl region are replaced with those of beta 2. Unlike either beta 3 or beta 2 delta C, beta 3 beta 2C can provide partial function for both assembly of axonemal microtubules and their organization into the supramolecular architecture of the axoneme. In particular, the beta 2 carboxyl sequences mediate morphogenesis of the axoneme doublet tubule complex, including accessory microtubule assembly and attachment of spokes and linkers. However, our data also reveal aspects of beta 2-specific function that require structural features other than the primary sequence of the isotype-defining variable regions, the C terminus and the internal variable region. Tests of fecundity in males that coexpress beta 2 and the chimeric beta 3 beta 2C protein showed that in Drosophila there are differential requirements for sperm motility in the male and in the female reproductive tract. Since some aspects of microtubule function in spermatogenesis are sensitive to the tubulin pool size, we examined the mechanisms for control of tubulin protein levels in the male germ cells. We found that both beta 2-tubulin mRNA accumulation and protein synthesis are dependent on gene dose, and that the level of expression is regulated by 3' noncoding sequences in the beta 2 gene. Our data show that the regulatory mechanisms that control tubulin pool levels in the Drosophila male germ line differ from those observed in cultured animal somatic cells. Finally, expression of transgenic constructs is consistent with early cessation of X chromosome expression in Drosophila spermatogenesis.

A beta-tubulin gene of Naegleria encodes a carboxy-terminal tyrosine. Aromatic amino acids are conserved at carboxy termini

A gene that directs the programmed synthesis of flagellar beta-tubulin during the rapid differentiation of Naegleria gruberi from amoebae to flagellates has been cloned and sequenced. The intronless gene is one of 8 to 10 similar but non-identical genes that are dispersed in the genome. beta-Tubulin mRNA homologous to this gene family is expressed transiently during differentiation, and has not been detected in amoebae. The encoded beta-tubulin is strongly conserved, with features that closely resemble the beta-tubulins of diverse organisms, especially organisms that, like Naegleria, use tubulin to assemble flagellar axonemes. In most sequenced alpha-tubulins, the encoded carboxy-terminal amino acid is tyrosine, which undergoes post-translational removal and readdition, conserved processes of unknown function. In N. gruberi, unusually, the terminus of alpha-tubulin is encoded as glutamine while that of beta-tubulin is tyrosine. The presence of these divergent termini on subunits of a conserved tubulin provoked us to re-examine aromatic amino acids at the termini of alpha- and beta-tubulins. Although evolution has tinkered extensively with the carboxy-terminal domains of tubulin subunits, we find an unexpected conservation. In every organism or cell type for which both tubulin subunits have been sequenced, except the ciliate Stylonychia lemnae, at least one tubulin subunit of some or all tubulin heterodimers terminates in an aromatic amino acid, either tyrosine or phenylalanine. This remarkable conservation of carboxy-terminal aromatic amino acids suggests that these residues serve some crucial function.

The highly divergent alpha- and beta-tubulins from Dictyostelium discoideum are encoded by single genes

As a step in the characterization of the microtubule system of Dictyostelium discoideum, we have isolated and sequenced full-length cDNA clones that encode the Dictyostelium alpha- and beta-tubulins, as well as the Dictyostelium alpha-tubulin gene. Southern blot analysis suggests that Dictyostelium is unusual in that its genome contains single alpha- and beta-tubulin genes, rather than the multi-gene family common in most eukaryotic organisms. The complete alpha-tubulin cDNA contains 1558 nucleotides, with an open reading frame, that encode a protein of 457 amino acids. The complete beta-tubulin cDNA contains 1572 nucleotides and encodes a protein of 456 amino acids. Analysis of the deduced protein sequences indicates that while there is a significant degree of sequence similarity between the Dictyostelium tubulins and other known tubulins, the Dictyostelium alpha-tubulin displays the greatest sequence divergence yet described. Single alpha- and beta-tubulin transcripts are detected by northern blot analysis during all stages of Dictyostelium development. The highest levels of message accumulate late in germinating spores and vegetative amoebae. Despite changes in alpha- and beta-tubulin mRNA levels, protein levels remain constant throughout development. We have expressed the carboxy-terminal two-thirds of the alpha- and beta-tubulins as trpE fusions in Escherichia coli and used this protein to produce polyclonal antisera specific for the Dictyostelium alpha- and beta-tubulins. These antisera recognize one alpha- and two beta-tubulin spots on western blots of 2-D gels and, by indirect immunofluorescence, both recognize the interphase and mitotic microtubule arrays in vegetative amoebae.

Role of the carboxy terminal region of beta tubulin on microtubule dynamics through its interaction with the GTP phosphate binding region

The dynamic instability of microtubules depends on the GTP binding to tubulin, the rate of hydrolysis of GTP bound to tubulin molecules, at the microtubule caps, and on the affinity and exchange rate of tubulin for GTP versus GDP. It has been demonstrated that the binding of microtubule-associated proteins (MAPs) such as Tau or MAP2 notably enhances microtubule stability in vivo. These MAPs bind to the tubulin carboxy terminal domain. Consequently, an attractive hypothesis to explain the modulation of microtubule dynamics by MAPs is that the carboxy terminal domain of tubulin interacts with a region close to the GTP binding site, preventing the binding of GTP or exchange of GDP for GTP. By carrying out a combined analysis of crosslinking and limited proteolysis, an intramolecular interaction between the carboxy terminus and the tubulin region containing the GTP binding site in beta tubulin has been observed. It is proposed that this interaction modifies the binding of GTP to the tubulin beta-subunit and, therefore, affects tubulin assembly dynamics. This suggests a molecular explanation for the effect of MAPs in facilitating tubulin polymerization through the regulation of the interaction of GTP.

Divergent neural beta tubulin from the Antarctic fish Notothenia coriiceps neglecta: potential sequence contributions to cold adaptation of microtubule assembly

The cytoplasmic microtubules of the cold-adapted Antarctic fishes, unlike those of homeotherms and temperate poikilotherms, assemble and function at body temperatures in the range -1.8 to +2 degrees C. To determine whether alterations to the primary sequence of beta tubulin may contribute to enhancement of microtubule assembly at cold temperatures, we have cloned and sequenced a 1.8-kilobase neural beta-chain cDNA, Ncn beta 1, from an Antarctic rockcod, Notothenia coriiceps neglecta. Based on nucleotide sequence homology, Ncn beta 1 probably corresponds to a class-II beta-tubulin gene. The 446-residue beta chain encoded by Ncn beta 1 is closely related (sequence homology approximately 95%) both to the neural class-I/II isotypes and to the neural/testicular class-IV variants of higher vertebrates, but the sequence of its carboxy-terminal isotype-defining region (residues 431-446) has diverged markedly (> or = 25% change relative to the I/II/IV referents). Furthermore, the Ncn beta 1 polypeptide contains six unique amino-acid substitutions (five conservative, one nonconservative) not found in other vertebrate brain isotypes, and the carboxy-terminal region possesses a unique tyrosine inserted at position 442. We conclude that Ncn beta 1 encodes a class-II beta tubulin that contains sequence modifications, located largely in its interdimer contact domain, that may contribute to cold adaptation of microtubule assembly.

Analysis of the colchicine-binding site of beta-tubulin

Comparison of the beta-tubulin sequences with the equilibrium colchicine Ka and the Ki for inhibition by podophyllotoxin suggests that residue beta:316 is directly involved in binding the common trimethoxyphenyl-(or A-) ring. By contrast, the analysis indicates that the local hydrophobicity affects the rate of one of the two conformational changes associated with colchicine binding but does not determine the affinity of the colchicine-binding site.

Tubulin structure and biochemistry

In the past year, much has been learned about structure-function correlations in the tubulin molecule, and specifically about the nature and roles of post-translational modifications and tubulin isotypes. The interactions between tubulin and its ligands--both microtubule-associated proteins and anti-mitotic drugs--are becoming clearer at the molecular level.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

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.

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.