Sequence of a highly divergent beta tubulin gene reveals regional heterogeneity in the beta tubulin polypeptide

Mutations in the autoregulatory domain of β-tubulin 4a cause hereditary dystonia

Dystonia type 4 (DYT4) was first described in a large family from Heacham in Norfolk with an autosomal dominantly inherited whispering dysphonia, generalized dystonia, and a characteristic hobby horse ataxic gait. We carried out a genetic linkage analysis in the extended DYT4 family that spanned 7 generations from England and Australia, revealing a single LOD score peak of 6.33 on chromosome 19p13.12-13. Exome sequencing in 2 cousins identified a single cosegregating mutation (p.R2G) in the β-tubulin 4a (TUBB4a) gene that was absent in a large number of controls. The mutation is highly conserved in the β-tubulin autoregulatory MREI (methionine-arginine-glutamic acid-isoleucine) domain, highly expressed in the central nervous system, and extensive in vitro work has previously demonstrated that substitutions at residue 2, specifically R2G, disrupt the autoregulatory capability of the wild-type β-tubulin peptide, affirming the role of the cytoskeleton in dystonia pathogenesis.

The roles of cys124 and ser239 in the functional properties of human betaIII tubulin

Tubulin is the target for some very powerful anti-mitotic and anti-tumor drugs. The betaIII tubulin isotype is found in very few normal tissues, but is often found in tumors, where it has been implicated in resistance to anti-tumor drugs. The betaIII isotype occurs in fish, amphibians, birds and mammals and its unique features are highly conserved in evolution. One of these features is the replacement of cys239 by ser239. Cys239 is unusual in being highly sensitive to oxidation; in fact, oxidation of this residue inhibits microtubule assembly. The betaIII isotype also has a very unusual cys124, where other beta isotypes have ser/ala124. The striking conservation in betaIII of vertebrates strongly suggests that cys124 and ser239 play functional roles. We have prepared the C124S and S239C mutants of betaIII and tested their effects on the functional properties of tubulin. We have found that both the betaIII C124S and betaIII S239C mutants bind colchicine less well than does wild-type alphabetaIII, and also make transfected HeLa cells more resistant to colchicine. However, the double mutant, betaIII C124S/S239C, binds colchicine still less well than do either of the single mutants, but in contrast to the former, the double mutant increases the cells' sensitivity to colchicine. Our results indicate that the roles that these residues play in colchicine binding and microtubule integrity are far more complex than previously imagined and that the specific residues at which betaIII differs from the other isotypes act collectively to keep betaIII in a functional conformation.

Non-lethal active caspase-3 expression in Bergmann glia of postnatal rat cerebellum

Caspase-3, an apoptotic executor, has been shown in recent years to mediate non-lethal events like cellular proliferation and differentiation, primarily in studies related to non-neural tissue. In central nervous system development, the role of active caspase-3 is still unclear. We provide the first evidence for a potential new role of active (cleaved) caspase-3 in promoting differentiation of Bergmann glia. This study was predicated on the hypothesis that active caspase-3 is important for the differentiation of glia. We addressed the hypothesis through the following specific aims: (1) to establish the expression of active caspase-3 in glia; (2) to determine the developmental phenotype of the active caspase-3-expressing glia; and (3) to confirm that active caspase-3 expression is not mediating an apoptotic event. Through a temporal investigation from postnatal day 8 to 21, we observed that Bergmann glia express active caspase-3 without compromising their survival. Potential apoptotic fate of active caspase-3-positive Bergmann glia were ruled out based on immunohistochemical exclusion of phosphatidylserine exposure (Annexin V), DNA fragmentation (TUNEL), and DNA compaction (TOPRO-3). More than 90% of the active caspase-3-positive cells lacked colabeling for one of the apoptotic markers. Correlative studies using a proliferation marker Ki67 and a differentiation marker brain lipid-binding protein suggest that the expression of active caspase-3 was mostly associated with differentiating rather than proliferating Bergmann glia at all ages. Thus, this study supports the hypothesis that active caspase-3 may be regulating both differentiation of Bergmann glia by allowing the cells to exit the cell cycle and their morphogenesis.

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.

Regulation of five tubulin isotypes by thyroid hormone during brain development

Nucleic acid probes derived from the 3' noncoding region of five tubulin cDNAs were used to study the effects of thyroid hormone deficiency on the expression of the mRNAs encoding two alpha (alpha 1 and alpha 2)- and three beta (beta 2, beta 4, and beta 5)-tubulin isotypes in the developing cerebral hemispheres and cerebellum. The content of alpha 1, which markedly declines during development in both brain regions, is maintained at high levels in the hypothyroid cerebellum, whereas it is decreased in the cerebral hemispheres. The alpha 2 level also declines during development and is decreased in both regions by thyroid hormone deficiency, but only during the two first postnatal weeks. Thyroid hormone deficiency slightly increases at all stages the beta 2 level in the cerebellum, whereas a decrease is observed at early stages in the cerebral hemispheres. The beta 5 level seems to be independent of thyroid hormone in the cerebral hemispheres, whereas it decreases at early stages in the hypothyroid cerebellum. Finally, the expression of the brain-specific beta 4 isotype is markedly depressed by thyroid hormone deficiency, particularly in the cerebellum. These data suggest that the genes encoding the tubulin isotypes are, directly or not, differently regulated by thyroid hormone during brain development. This might contribute to abnormal neurite outgrowth seen in the hypothyroid brain and therefore to impairment in brain functions produced by thyroid hormone deficiency.

Studies of crystallization conditions for native and subtilisin-cleaved pig brain tubulin

A survey of crystallization conditions for pig brain tubulin, using standard vapor diffusion techniques in sitting drops or capillaries, has resulted in irregular, fragile needles or plates with a largest dimension of 0.5 mm. These occurred in 2.5% PEG (MW 3350), 0.1 M Pipes, pH 6.2 and 6.4, 2-16 mM MgSO4, 10-15 mM DTE, and 0.1 mM GDP at 8 degrees C. When GTP replaced GDP these aggregates did not form under any of the conditions surveyed (temperature: 8-10 degrees C; MgSO4: 2-16 mM; pH 6-7; PEG, MW 3350: 1.25-12.5%). EM observations demonstrated that sheets of rings appear in crystal solutions in the presence of GDP or GTP. These results are consistent with the results of Howard and Timasheff (1986, Biochemistry 25, 8292-8300) that tubulin rings form in the presence of GDP or GTP but more readily in GDP. Tubulin crystallization experiments are hampered by tubulin's high degree of heterogeneity. Much of the variability lies in the carboxyl terminal region. Conditions for limited digestion of the heterodimer by subtilisin, removing only the carboxyl terminus, were determined. Reduction of heterogeneity was demonstrated by isoelectric focusing. The solubilities of native and subtilisin-cleaved tubulin in MgSO4, (NH4)2SO4, PEG (MW 1450, 3350, 10,000), DMSO, and MPD were compared. Subtilisin-cleaved tubulin precipitated more readily than native tubulin under all conditions surveyed, consistent with the removal of the highly acidic carboxyl terminus. Vapor diffusion experiments using subtilisin-cleaved tubulin under conditions where native tubulin forms needles or plates resulted in similar aggregates.

Identification of an amino acid substitution in the benA, beta-tubulin gene of Aspergillus nidulans that confers thiabendazole resistance and benomyl supersensitivity

We are using molecular genetic techniques to identify sites of interaction of beta-tubulin with benzimidizole anti-microtubule agents. We have developed a marker-rescue technique for cloning mutant alleles of the benA, beta-tubulin gene of Aspergillus nidulans and have used the technique to clone two mutant benA alleles, benA16 and benA19. These are the only A. nidulans alleles known to confer resistance to the benzimidazole antimicrotubule agent thiabendazole and supersensitivity to other benzimidazole antimicrotubule agents including benomyl and its active breakdown product, carbendazim. benA16 has been shown, moreover, to reduce thiabendazole binding to beta-tubulin. We have sequenced the two mutant alleles and have found that they carry different nucleotide changes that cause the same single amino acid substitution, valine for alanine at amino acid 165. Since thiabendazole and carbendazim differ at only one side chain, the R2 group, we conclude that the region around amino acid 165 is involved in the binding of the R2 group of benzimidazole antimicrotubule agents to beta-tubulin.

Genetic and molecular analysis of a Caenorhabditis elegans beta-tubulin that conveys benzimidazole sensitivity

Benzimidazole anti-microtubule drugs, such as benomyl, induce paralysis and slow the growth of the nematode Caenorhabditis elegans. We have identified 28 mutations in C. elegans that confer resistance to benzimidazoles. All resistant mutations map to a single locus, ben-1. Virtually all these mutations are genetically dominant. Molecular cloning and DNA sequence analysis established that ben-1 encodes a beta-tubulin. Some resistant mutants are completely deleted for the ben-1 gene. Since the deletion strains appear to be fully resistant to the drugs, the ben-1 product appears to be the only benzimidazole-sensitive beta-tubulin in C. elegans. Furthermore, since animals lacking ben-1 are viable and coordinated, the ben-1 beta-tubulin appears to be nonessential for growth and movement. The ben-1 function is likely to be redundant in the nematode genome.

mec-7 is a beta-tubulin gene required for the production of 15-protofilament microtubules in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, microtubules with 15 protofilaments are a specialized feature of six touch-receptor neurons; microtubules found in other C. elegans neurons have 11 protofilaments. Mutations in the gene mec-7 result in touch-insensitive animals whose touch cells lack the 15-protofilament microtubules. We have characterized 54 mutations in the mec-7 gene. The absence of mec-7 activity results selectively in the recessive loss of touch sensitivity. Partial loss-of-function alleles result in a partial loss of touch sensitivity. Dominant mutations, which are isolated at an unusually high proportion, may encode abnormal products. We have cloned the mec-7 gene; it encodes a beta-tubulin which is 90-93% identical to vertebrate beta-tubulin. Our results are consistent with the hypothesis that tubulin heterogeneity contributes to the formation of structurally and functionally distinct sets of microtubules.

Structure and expression of an alpha-tubulin gene of Physarum polycephalum

Fragments of Physarum polycephalum DNA generated by partial digestion with Sau3A were cloned into phage-lambda EMBL4. A recombinant (phage-lambda E alpha Tu) containing an alpha-tubulin (E alpha-tubulin) gene was isolated. The E alpha-tubulin gene is part of the alt B locus. The gene was sequenced and was found to contain seven intervening sequences. The alpha-tubulin isotype (E alpha-tubulin) encoded by the gene has a methionine residue at the C-terminus. The E alpha-tubulin gene has much in common with the N alpha-tubulin gene cloned into phage-lambda NM1149 [M. J. Monteiro & R. A. Cox (1987) J. Mol. Biol. 193, 427-438]. However, the gene products E alpha-tubulin and N alpha-tubulin differ in amino acid sequence near to the C-terminus. E-peptide (corresponding to amino acids 440-448 of E alpha-tubulin) and N-peptide (corresponding to amino acids 437-445 of N alpha-tubulin) were synthesised and used to raise antibodies (E-antibodies and N-antibodies). The antibodies were used to show that on two-dimensional gel electrophoresis N alpha-tubulin travels to the alpha 1 position and E alpha-tubulin moves to the alpha 2 position. Gene-specific DNA probes were used to show that transcripts of the E alpha-tubulin gene were present in the plasmodial but not in the amoebal phase of the life cycle. The E- and N-antibodies detected E alpha- and N alpha-tubulins in plasmodia but not in amoebae, confirming that the expression of the E alpha- and N alpha-tubulin genes is regulated during development. E alpha- and N alpha-tubulin were shown to be components of spindle microtubules by indirect immunofluorescence microscopy.

A gene encoding the major beta tubulin of the mitotic spindle in Physarum polycephalum plasmodia

The multinucleate plasmodium of Physarum polycephalum is unusual among eucaryotic cells in that it uses tubulins only in mitotic-spindle microtubules; cytoskeletal, flagellar, and centriolar microtubules are absent in this cell type. We have identified a beta-tubulin cDNA clone, beta 105, which is shown to correspond to the transcript of the betC beta-tubulin locus and to encode beta 2 tubulin, the beta tubulin expressed specifically in the plasmodium and used exclusively in the mitotic spindle. Physarum amoebae utilize tubulins in the cytoskeleton, centrioles, and flagella, in addition to the mitotic spindle. Sequence analysis shows that beta 2 tubulin is only 83% identical to the two beta tubulins expressed in amoebae. This compares with 70 to 83% identity between Physarum beta 2 tubulin and the beta tubulins of yeasts, fungi, alga, trypanosome, fruit fly, chicken, and mouse. On the other hand, Physarum beta 2 tubulin is no more similar to, for example, Aspergillus beta tubulins than it is to those of Drosophila melanogaster or mammals. Several eucaryotes express at least one widely diverged beta tubulin as well as one or more beta tubulins that conform more closely to a consensus beta-tubulin sequence. We suggest that beta-tubulins diverge more when their expression pattern is restricted, especially when this restriction results in their use in fewer functions. This divergence among beta tubulins could have resulted through neutral drift. For example, exclusive use of Physarum beta 2 tubulin in the spindle may have allowed more amino acid substitutions than would be functionally tolerable in the beta tubulins that are utilized in multiple microtubular organelles. Alternatively, restricted use of beta tubulins may allow positive selection to operate more freely to refine beta-tubulin function.

A survey of the alpha-tubulin gene family in chicken: unexpected sequence heterogeneity in the polypeptides encoded by five expressed genes

To characterize the alpha-tubulin gene family in chicken, we have isolated five chicken alpha-tubulin genes and determined the majority of the sequences of the encoded polypeptides. Three of these (c alpha 3, c alpha 5/6 and c alpha 8) encode novel, expressed alpha-tubulins that have not previously been analyzed, whereas one gene segment is a pseudogene and another appears capable of encoding a functional subunit (although we were unable to document its expression in a survey of chicken tissues). Together with two additional expressed, functional alpha-tubulins reported earlier, we conclude that the chicken alpha-tubulin family is comprised of at least five functional genes whose polypeptide products are substantially more heterogeneous than found in preceding analyses of vertebrate alpha-tubulins. Comparison of the amino acid sequences reveals that the five polypeptides are between 96 and 83% identical, with the extreme carboxy-terminal residues representing a highly heterogeneous variable domain. Since some alpha-tubulins undergo cyclic post-translational removal and readdition of a carboxy-terminal tyrosine, the notable sequence divergence in this domain suggests that individual tubulins probably participate to different extents in this modification cycle.

Identification of the phosphorylated beta-tubulin isotype in differentiated neuroblastoma cells

The tubulin molecule consists of an alpha- and a beta-subunit, each of which exists in several isotypic forms. It has been previously shown that one of the isotypes of neuroblastoma beta-tubulin is phosphorylated at a serine residue in vivo [(1985) J. Cell Biol. 100, 764-774]. Here we identify the phosphorylated isotype as beta 2 (type III). Moreover, the large size of the phosphorylated tryptic peptide and sequence comparisons of vertebrate beta-tubulins suggest that one of the two serines in positions 444 and 446 is the phosphorylated residue. Our results raise the possibility that beta 2-tubulin differs functionally from the other beta-tubulin isotypes.

A gene encoding the major beta tubulin of the mitotic spindle in Physarum polycephalum plasmodia

The multinucleate plasmodium of Physarum polycephalum is unusual among eucaryotic cells in that it uses tubulins only in mitotic-spindle microtubules; cytoskeletal, flagellar, and centriolar microtubules are absent in this cell type. We have identified a beta-tubulin cDNA clone, beta 105, which is shown to correspond to the transcript of the betC beta-tubulin locus and to encode beta 2 tubulin, the beta tubulin expressed specifically in the plasmodium and used exclusively in the mitotic spindle. Physarum amoebae utilize tubulins in the cytoskeleton, centrioles, and flagella, in addition to the mitotic spindle. Sequence analysis shows that beta 2 tubulin is only 83% identical to the two beta tubulins expressed in amoebae. This compares with 70 to 83% identity between Physarum beta 2 tubulin and the beta tubulins of yeasts, fungi, alga, trypanosome, fruit fly, chicken, and mouse. On the other hand, Physarum beta 2 tubulin is no more similar to, for example, Aspergillus beta tubulins than it is to those of Drosophila melanogaster or mammals. Several eucaryotes express at least one widely diverged beta tubulin as well as one or more beta tubulins that conform more closely to a consensus beta-tubulin sequence. We suggest that beta-tubulins diverge more when their expression pattern is restricted, especially when this restriction results in their use in fewer functions. This divergence among beta tubulins could have resulted through neutral drift. For example, exclusive use of Physarum beta 2 tubulin in the spindle may have allowed more amino acid substitutions than would be functionally tolerable in the beta tubulins that are utilized in multiple microtubular organelles. Alternatively, restricted use of beta tubulins may allow positive selection to operate more freely to refine beta-tubulin function.

Sequence of chicken c beta 7 tubulin. Analysis of a complete set of vertebrate beta-tubulin isotypes

In chicken, beta-tubulin is encoded by a family of seven genes. We have now isolated and sequenced overlapping cDNA clones corresponding to gene c beta 7 (previously designated c beta 4'), the only chicken beta-tubulin not previously characterized. The inferred amino acid sequence of c beta 7 tubulin is identical with the class I beta-tubulin isotype found in human, mouse and rat. Moreover, c beta 7 is highly expressed in almost all tissue and cell types in chicken, a pattern similar to those of the genes for class I beta-tubulin isotypes in other vertebrates. Comparison of the complete family of chicken beta-tubulin gene sequences reveals that the heterogeneity of beta-tubulin polypeptides encoded in a higher eukaryote is confined to six distinct beta-tubulin isotypes. Five of these are members of evolutionarily conserved isotypic classes (I to V), whereas the sixth represents a divergent erythroid-specific tubulin whose sequence has not been conserved.

Comparative analysis of tubulin sequences

1. Information on the structure and evolution of tubulin has been obtained by comparing the available sequence data on 31 alpha-tubulins and 31 beta-tubulins. 2. Similar numbers of conserved amino acids are found amongst both alpha- and beta-tubulins (alpha: 48%, plus conservative substitutions: 72%; beta: 48%, plus conservative substitutions: 70%). About half of them are common to both subunits (23%, plus conservative substitutions: 45%). Four cysteines in the alpha-tubulins and 2 cysteines in the beta-tubulins are conserved. Only one cysteine (position 129) is conserved in all alpha- and beta-tubulins. 3. The longest unbroken stretch of identical amino acids between all the alpha- and beta-tubulins is found in positions 180-186 (Val-Val-Glu-Pro-Tyr-Asn), a region that appears to be important for binding the ribose moiety of GTP. Two other groups of amino acids implicated in GTP binding, one near position 70 and a glycine cluster at position 144 are also quite conserved. 4. Extra length differences between tubulin subunits, presumably present as extensions on the dimer surface, have been observed at position 50 and near position 360 in alpha-tubulins and in one case at position 57 in a beta-tubulin. 5. The introns of tubulin genes, many of them clustered in the first quarter of the tubulin coding region, do not appear to correspond to any particular structural or functional regions. 6. Mutation rates of tubulins vary considerably. The lowest alpha-tubulin homology (62.3%) is between a very divergent Drosophila alpha-tubulin and an alpha-tubulin from the yeast S. cerevisiae. The lowest beta-tubulin homology (63.3%) is between a yeast (S. cerevisiae) beta-tubulin and a mouse beta-tubulin expressed in hematopoietic tissue. In contrast, some mammalian and bird tubulins are almost identical. 7. Tubulin's heterogeneous C-termini are useful for identifying corresponding tubulins of different vertebrate species, many of which are remarkably conserved. Exceptions are the divergent beta-tubulins of erythrocyte and thrombocyte marginal bands. 8. We have proposed a model for tubulin evolution in metazoan organisms in which the release of structural constraints after gene duplication is a major cause of relatively rapid change.

Structural differences between blood-platelet tubulin and other mammalian tubulins

Carboxymethylated pig blood-platelet tubulin subunits have been characterized by comparing their electrophoretic mobilities and two-dimensional tryptic peptide maps with those of tubulin subunits isolated from four other mammalian tissues. These were pig brain, pig lung, pig sperm and bovine kidney. In addition to the major blood-platelet alpha- and beta-tubulin subunits, minor alpha- and beta-components could be resolved on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The major blood-platelet beta-tubulin, as well as the minor brain beta 2-tubulin subunit, were clearly tissue specific. In both cases, their two-dimensional peptide maps contained unique heavily labelled peptides. Distinct peptide maps were also observed for the major blood-platelet alpha-tubulin subunit as well as for sperm tail alpha-tubulin, both of which were missing peptides (different in each case) that were present in all of the other alpha-tubulin maps. Under our conditions, a protein with a molecular weight of 70,000 and an isoelectric point of pH 6.4 co-purified with blood-platelet microtubules.

In vivo coassembly of a divergent beta-tubulin subunit (c beta 6) into microtubules of different function

alpha- and beta-Tubulin are encoded in vertebrate genomes by a family of approximately 6-7 functional genes whose polypeptide products differ in amino acid sequence. In the chicken, one beta-tubulin isotype (c beta 6) has previously been found to be expressed only in thrombocytes and erythroid cells, where it is assembled into a circumferential ring of marginal band microtubules. In light of its unique in vivo utilization and its divergent assembly properties in vitro, we used DNA transfection to test whether this isotype could be assembled in vivo into microtubules of divergent functions. Using an antibody specific to c beta 6, we have found that upon transfection this polypeptide is freely coassembled into an extensive array of interphase cytoplasmic microtubules and into astral and pole-to-chromosome or pole-to-pole microtubules during mitosis. Further, examination of developing chicken erythrocytes reveals that both beta-tubulins that are expressed in these cells (c beta 6 and c beta 3) are found as co-polymers of the two isoforms. These results, in conjunction with efforts that have localized various other beta-tubulin isotypes, demonstrate that to the resolution limit afforded by light microscopy in vivo microtubules in vertebrates are random copolymers of available isotypes. Although these findings are consistent with functional interchangeability of beta-tubulin isotypes, we have also found that in vivo microtubules enriched in c beta 3 polypeptides are more sensitive to cold depolymerization than those enriched in c beta 6. This differential quantitative utilization of the two endogenous isotypes documents that some in vivo functional differences between isotypes do exist.

In vivo microtubules are copolymers of available beta-tubulin isotypes: localization of each of six vertebrate beta-tubulin isotypes using polyclonal antibodies elicited by synthetic peptide antigens

beta-Tubulin is encoded in the genomes of higher animals by a small multigene family comprising approximately seven functional genes. These genes produce a family of closely related, but distinct polypeptide isotypes that are distinguished principally by sequences within the approximately 15 carboxy-terminal amino acid residues. By immunizing rabbits with chemically synthesized peptides corresponding to these variable domain sequences, we have now prepared polyclonal antibodies specific for each of six distinct isotypes. Specificity of each antiserum has been demonstrated unambiguously by antibody binding to bacterially produced, cloned proteins representing each isotype and by the inhibition of such binding by preincubation of each antiserum only with the immunizing peptide and not with heterologous peptides. Protein blotting of known amounts of cloned, isotypically pure polypeptides has permitted accurate quantitative measurement of the amount of each beta-tubulin isotype present in the soluble and polymer forms in various cells, but has not revealed a bias for preferential assembly of any isotype. Localization of each isotype in three different cell types using indirect immunofluorescence has demonstrated that in vivo each class of microtubules distinguishable by light microscopy is assembled as copolymers of all isotypes expressed in a single cell.

Three Drosophila beta-tubulin sequences: a developmentally regulated isoform (beta 3), the testis-specific isoform (beta 2), and an assembly-defective mutation of the testis-specific isoform (B2t8) reveal both an ancient divergence in metazoan isotypes and structural constraints for beta-tubulin function

The genomic DNA sequence and deduced amino acid sequence are presented for three Drosophila melanogaster beta-tubulins: a developmentally regulated isoform beta 3-tubulin, the wild-type testis-specific isoform beta 2-tubulin, and an ethyl methanesulfonate-induced assembly-defective mutation of the testis isoform, B2t8. The testis-specific beta 2-tubulin is highly homologous to the major vertebrate beta-tubulins, but beta 3-tubulin is considerably diverged. Comparison of the amino acid sequences of the two Drosophila isoforms to those of other beta-tubulins indicates that these two proteins are representative of an ancient sequence divergence event which at least preceded the split between lines leading to vertebrates and invertebrates. The intron/exon structures of the genes for beta 2- and beta 3-tubulin are not the same. The structure of the gene for the variant beta 3-tubulin isoform, but not that of the testis-specific beta 2-tubulin gene, is similar to that of vertebrate beta-tubulins. The mutation B2t8 in the gene for the testis-specific beta 2-tubulin defines a single amino acid residue required for normal assembly function of beta-tubulin. The sequence of the B2t8 gene is identical to that of the wild-type gene except for a single nucleotide change resulting in the substitution of lysine for glutamic acid at residue 288. This position falls at the junction between two major structural domains of the beta-tubulin molecule. Although this hinge region is relatively variable in sequence among different beta-tubulins, the residue corresponding to glu 288 of Drosophila beta 2-tubulin is highly conserved as an acidic amino acid not only in all other beta-tubulins but in alpha-tubulins as well.

Three Drosophila beta-tubulin sequences: a developmentally regulated isoform (beta 3), the testis-specific isoform (beta 2), and an assembly-defective mutation of the testis-specific isoform (B2t8) reveal both an ancient divergence in metazoan isotypes and structural constraints for beta-tubulin function

The genomic DNA sequence and deduced amino acid sequence are presented for three Drosophila melanogaster beta-tubulins: a developmentally regulated isoform beta 3-tubulin, the wild-type testis-specific isoform beta 2-tubulin, and an ethyl methanesulfonate-induced assembly-defective mutation of the testis isoform, B2t8. The testis-specific beta 2-tubulin is highly homologous to the major vertebrate beta-tubulins, but beta 3-tubulin is considerably diverged. Comparison of the amino acid sequences of the two Drosophila isoforms to those of other beta-tubulins indicates that these two proteins are representative of an ancient sequence divergence event which at least preceded the split between lines leading to vertebrates and invertebrates. The intron/exon structures of the genes for beta 2- and beta 3-tubulin are not the same. The structure of the gene for the variant beta 3-tubulin isoform, but not that of the testis-specific beta 2-tubulin gene, is similar to that of vertebrate beta-tubulins. The mutation B2t8 in the gene for the testis-specific beta 2-tubulin defines a single amino acid residue required for normal assembly function of beta-tubulin. The sequence of the B2t8 gene is identical to that of the wild-type gene except for a single nucleotide change resulting in the substitution of lysine for glutamic acid at residue 288. This position falls at the junction between two major structural domains of the beta-tubulin molecule. Although this hinge region is relatively variable in sequence among different beta-tubulins, the residue corresponding to glu 288 of Drosophila beta 2-tubulin is highly conserved as an acidic amino acid not only in all other beta-tubulins but in alpha-tubulins as well.

Location of two cysteines in brain beta 1-tubulin that can be cross-linked after removal of exchangeable GTP

Two cysteines of the major neuronal beta-tubulin subunit (beta 1) can be specifically cross-linked with the bifunctional sulfhydryl reagent N',N'-ethylenebis(iodoacetamide) after removal of GTP. A cysteine in position 12 cross-links with one of the cysteines in position 201 or 211. Although the two cross-linked cysteines are separated by at least 189 residues in the primary structure of tubulin, they are maximally 9 A apart in the tertiary structure.

Sequence and expression of the chicken beta 5- and beta 4-tubulin genes define a pair of divergent beta-tubulins with complementary patterns of expression

We have determined the nucleotide sequence of the chicken beta 5 (c beta 5)-tubulin gene. The gene displayed the coding structure common to all previously studied vertebrate beta-tubulin genes and was divided into four exon sequences interrupted by three intervening sequences (located between codons 19 and 20, within codon 56, and within codon 93). Comparison of the predicted polypeptide sequence encoded by c beta 5 with those of four other available chicken beta-tubulin sequences revealed that c beta 5 encoded a highly divergent beta-tubulin polypeptide isotype which was distinguished from previously known sequences primarily by two discrete variable sequence domains. However, c beta 5 uniquely shared identity in 16 residue positions with another divergent chicken beta-tubulin gene, c beta 4. These common sequences distinguished c beta 4 and c beta 5 from the remaining three chicken beta-tubulin genes. Analysis of the expression of c beta 5 and c beta 4 revealed a strikingly complementary pattern of gene expression: c beta 5 was expressed in a wide variety of cell and tissue types but not in neurons, whereas c beta 4 expression was detected uniquely in neuronal cells. Overall, these findings suggest the existence of two divergent families of beta-tubulin sequences in the chicken and further raise the possibility that the complementary expression of the c beta 4 and c beta 5 genes may fulfill a requirement for the presence of a divergent beta-tubulin polypeptide isotype in all cell types.

Six mouse alpha-tubulin mRNAs encode five distinct isotypes: testis-specific expression of two sister genes

Five mouse alpha-tubulin isotypes are described, each distinguished by the presence of unique amino acid substitutions within the coding region. Most, though not all of these isotype-specific amino acids, are clustered at the carboxy terminus. One of the alpha-tubulin isotypes described is expressed exclusively in testis and is encoded by two closely related genes (M alpha 3 and M alpha 7) which have homologous 3' untranslated regions but which differ at multiple third codon positions and in their 5' untranslated regions. We show that a subfamily of alpha-tubulin genes encoding the same testis-specific isotype also exists in humans. Thus, we conclude that the duplication event leading to a pair of genes encoding a testis-specific alpha-tubulin isotype predated the mammalian radiation, and both members of the duplicated sequence have been maintained since species divergence. A second alpha-tubulin gene, M alpha 6, is expressed ubiquitously at a low level, whereas a third gene, M alpha 4, is unique in that it does not encode a carboxy-terminal tyrosine residue. This gene yields two transcripts: a 1.8-kilobase (kb) mRNA that is abundant in muscle and a 2.4-kb mRNA that is abundant in testis. Whereas the 1.8-kb mRNA encodes a distinct alpha-tubulin isotype, the 2.4-kb mRNA is defective in that the methionine residue required for translational initiation is missing. Patterns of developmental expression of the various alpha-tubulin isotypes are presented. Our data support the view that individual tubulin isotypes are capable of conferring functional specificity on different kinds of microtubules.

Alternative 5' exons either provide or deny an initiator methionine codon to the same alpha-tubulin coding region

The primary structures of two overlapping novel alpha-tubulin cDNA clones isolated from a Macaca fascicularis testis cDNA library and the corresponding human gene are presented. Although the general structure of the human gene conforms to that of previously described mammalian alpha-tubulin genes, there is a surprising difference: the ATG initiator codon is conspicuously absent. The macaque testis cDNA similarly lacks the initiator methionine, but otherwise encodes a variant alpha-tubulin isotype precisely conserved in the human gene. RNA blot analysis in the macaque, using a 3' untranslated region probe, revealed the existence of two additional related transcripts expressed in every tissue examined except the adult testis. Sequence comparisons indicate that the 2.0 kb testis transcript and one of the additional transcripts result from differential transcription of the same gene. The two transcripts differ only at the 5' end as a result of the recruitment of different 5' exons. Curiously, the 5' exon utilized outside the testis encodes an initiator methionine in the expected location.

Complete sequence of three alpha-tubulin cDNAs in Chinese hamster ovary cells: each encodes a distinct alpha-tubulin isoprotein

The genome of Chinese hamster ovary (CHO) cells contains a complex family of approximately 16 alpha-tubulin genes, many of which may be pseudogenes. We present here the complete cDNA sequences of three expressed alpha-tubulin genes; one of these genes has been identified only in CHO cells. The noncoding regions of these three CHO alpha-tubulin genes differed significantly, but their coding regions were highly conserved. Nevertheless, we observed differences in the predicted amino acid sequences for the three genes. A comparison of the CHO alpha-tubulin sequences with all of the sequences available for mammals allowed assignment of the alpha-tubulin genes to three classes. The proteins encoded by the members of two of these classes showed no class-specific amino acids among the mammalian species examined. The gene belonging to the third class encoded an isoprotein which was clearly distinct, and members of this class may play a unique role in vivo. Sequencing of the three alpha-tubulin genes was also undertaken in CMR795, a colcemid-resistant clonal CHO cell line which has previously been shown to have structural and functional alterations in its tubulin proteins. We found differences in the tubulin nucleotide sequence compared with the parental line; however, no differences in the alpha-tubulin proteins encoded in the two cell lines were observed.

Amino-acid sequence data of beta-tubulin from Physarum polycephalum myxamoebae

Starting with 7.7 mg of a beta-tubulin isolated from myxamoebae of the slime mould Physarum polycephalum, 90% of the sequence has been determined by the Edman degradation of peptides generated by cyanogen bromide, trypsin and Staphylococcus aureus protease. Differences to other beta-tubulins are mainly conservative and spread evenly throughout the chain except for a high concentration at the C-terminus. The Physarum beta-tubulin shows most homology to Chlamydomonas beta-tubulin (90.5%) and least homology to yeast beta-tubulin (S. cerevisiae, 73.4%). Two tryptic peptides were isolated in approximately equal quantities which were identical except in one position (S/ALTVPELTQRMFDA) showing that at least two beta-tubulins are present in myxamoebae. However, since this was the only heterogeneity found, these beta-tubulins are probably very similar.

Sequence and expression of the chicken beta 5- and beta 4-tubulin genes define a pair of divergent beta-tubulins with complementary patterns of expression

We have determined the nucleotide sequence of the chicken beta 5 (c beta 5)-tubulin gene. The gene displayed the coding structure common to all previously studied vertebrate beta-tubulin genes and was divided into four exon sequences interrupted by three intervening sequences (located between codons 19 and 20, within codon 56, and within codon 93). Comparison of the predicted polypeptide sequence encoded by c beta 5 with those of four other available chicken beta-tubulin sequences revealed that c beta 5 encoded a highly divergent beta-tubulin polypeptide isotype which was distinguished from previously known sequences primarily by two discrete variable sequence domains. However, c beta 5 uniquely shared identity in 16 residue positions with another divergent chicken beta-tubulin gene, c beta 4. These common sequences distinguished c beta 4 and c beta 5 from the remaining three chicken beta-tubulin genes. Analysis of the expression of c beta 5 and c beta 4 revealed a strikingly complementary pattern of gene expression: c beta 5 was expressed in a wide variety of cell and tissue types but not in neurons, whereas c beta 4 expression was detected uniquely in neuronal cells. Overall, these findings suggest the existence of two divergent families of beta-tubulin sequences in the chicken and further raise the possibility that the complementary expression of the c beta 4 and c beta 5 genes may fulfill a requirement for the presence of a divergent beta-tubulin polypeptide isotype in all cell types.

Occurrence of two beta-tubulin isoforms with different polymerizing abilities in L5178Y cells

Mouse lymphoma L5178Y cells express at least two isoforms of beta-tubulin, designated M beta I and M beta II, as revealed by isoelectrofocusing, whereas two independently isolated normal T-cell clones, 3D10 and K23, express only M beta I. M beta II-tubulin is more acidic (pI, 5.10) than M beta I-tubulin (pI, 5.15). L5178Y cells were disrupted under the microtubule-stabilizing conditions, followed by centrifugation to separate fractions containing polymerized and unpolymerized tubulin. We found that a proportion of M beta II to total beta-tubulins is larger in the fraction containing unpolymerized tubulin than in that containing polymerized tubulin. In addition, when tubulin was purified from extracts of L5178Y cells by repeated cycles of polymerization-depolymerization, the M beta II-tubulin isoform was gradually lost during the successive purification steps. The low recovery of M beta II-tubulin was observed, irrespective of the presence or absence of MAPs, and even in the presence of an excess amount of essentially polymerizable porcine brain tubulin. These results indicate that M beta II-tubulin is less able to polymerize than is M beta I-tubulin, both in vivo and in vitro.

Apparent gene conversion between beta-tubulin genes yields multiple regulatory pathways for a single beta-tubulin polypeptide isotype

We have determined the complete nucleotide sequences of two chicken beta-tubulin genes, beta 1 and beta 2. These genes display an unusual pattern of segmental homology which indicates that they originally arose by gene duplication and have subsequently coevolved by a process that included localized gene conversion or intergenic recombination. Since the beta-tubulin polypeptides encoded by the two genes are virtually identical (99.5%), particularly in the major beta-tubulin isotype defining regions, they almost certainly constitute a single isotypic class of beta tubulin. However, the regulatory properties of the two genes are highly divergent as indicated by analysis of their patterns of expression in different chicken cell types. beta 1 is the major transcript detected in skeletal muscle myoblasts, whereas beta 2 is the major beta-tubulin transcript in cultured sympathetic neurons. The existence of these two genes appears to derive from a regulatory requirement whereby the expression of a single tubulin isotype is mediated through different regulatory programs in development and differentiation. These results thus provide direct experimental support for the hypothesis that gene conversion and intergenic recombination play an important role in evolution by uncoupling the evolution of structural genes from the regulatory sequences which control them.

The two alpha-tubulin genes of Chlamydomonas reinhardi code for slightly different proteins

Full-length cDNA clones corresponding to the transcripts of the two alpha-tubulin genes in Chlamydomonas reinhardi were isolated. DNA sequence analysis of the cDNA clones and cloned gene fragments showed that each gene contains 1,356 base pairs of coding sequence, predicting alpha-tubulin products of 451 amino acids. Of the 27 nucleotide differences between the two genes, only two result in predicted amino acid differences between the two gene products. In the more divergent alpha 2 gene, a leucine replaces an arginine at amino acid 308, and a valine replaces a glycine at amino acid 366. The results predicted that two alpha-tubulin proteins with different net charges are produced as primary gene products. The predicted amino acid sequences are 86 and 70% homologous with alpha-tubulins from rat brain and Schizosaccharomyces pombe, respectively. Each gene had two intervening sequences, located at identical positions. Portions of an intervening sequence highly conserved between the two beta-tubulin genes are also found in the second intervening sequence of each of the alpha genes. These results, together with our earlier report of the beta-tubulin sequences in C. reinhardi, present a picture of the total complement of genetic information for tubulin in this organism.

Six mouse alpha-tubulin mRNAs encode five distinct isotypes: testis-specific expression of two sister genes

Five mouse alpha-tubulin isotypes are described, each distinguished by the presence of unique amino acid substitutions within the coding region. Most, though not all of these isotype-specific amino acids, are clustered at the carboxy terminus. One of the alpha-tubulin isotypes described is expressed exclusively in testis and is encoded by two closely related genes (M alpha 3 and M alpha 7) which have homologous 3' untranslated regions but which differ at multiple third codon positions and in their 5' untranslated regions. We show that a subfamily of alpha-tubulin genes encoding the same testis-specific isotype also exists in humans. Thus, we conclude that the duplication event leading to a pair of genes encoding a testis-specific alpha-tubulin isotype predated the mammalian radiation, and both members of the duplicated sequence have been maintained since species divergence. A second alpha-tubulin gene, M alpha 6, is expressed ubiquitously at a low level, whereas a third gene, M alpha 4, is unique in that it does not encode a carboxy-terminal tyrosine residue. This gene yields two transcripts: a 1.8-kilobase (kb) mRNA that is abundant in muscle and a 2.4-kb mRNA that is abundant in testis. Whereas the 1.8-kb mRNA encodes a distinct alpha-tubulin isotype, the 2.4-kb mRNA is defective in that the methionine residue required for translational initiation is missing. Patterns of developmental expression of the various alpha-tubulin isotypes are presented. Our data support the view that individual tubulin isotypes are capable of conferring functional specificity on different kinds of microtubules.

Chlamydomonas reinhardtii tubulin gene structure

DNA sequencing studies have provided a picture of the total information available at the gene level for tubulin production in C. reinhardtii. The data indicates that diversity at the gene level is very limited and that all the microtubules in the cell are composed of a very similar set of tubulins. These studies contrast with similar studies of S. pombe alpha-tubulin genes and chicken beta-tubulin genes that show much heterogeneity among members of the same gene family. Further studies will be needed to investigate whether the high degree of conservation of tubulin genes is unique or common among lower eukaryotes, and what mechanisms are used to maintain homogeneity in C. reinhardtii tubulin gene families. Our DNA sequence analysis, in addition to the work of Brunke et al., has provided information on the noncoding, and possibly regulatory, portions of the tubulin genes. For example, the promoter regions of the 4 tubulin genes share a consensus sequence of 16 nucleotides upstream of the TATA box. This sequence could be involved in regulating the coordinate expression of the genes. Although little homology exists generally in the noncoding region of the genes, striking homology between the third IVS in each beta-tubulin gene is observed. Small elements homologous to the beta-tubulin IVS 3 also exist in the second IVS of each alpha-tubulin gene. In addition, considerable homology in the 5' noncoding portion of the alpha-tubulin transcripts has been noted. These homologies may be the result of recent gene conversion events, and may not have functional significance. The possibility, however, must also be considered in future experiments that these elements may play a role in regulating the expression of the tubulin genes.

Identification of conserved isotype-defining variable region sequences for four vertebrate beta tubulin polypeptide classes

We report the determination of the complete sequences for two chicken beta tubulin genes, beta 3 and beta 5. Taken with the previously published efforts, we have determined the primary structures of five of the seven beta tubulin genes in this vertebrate species. A comparison of these sequences unambiguously reveals that amino acid sequence variations among different beta tubulin gene products are distinctly clustered within an otherwise highly conserved framework of the beta tubulin molecule. To determine the extent to which this pattern of structural heterogeneity is conserved among vertebrates, we have isolated novel beta tubulin sequences from human and mouse cDNA libraries and compared these and all other known vertebrate beta tubulin sequences with the family of chicken polypeptide sequences. What emerges from such comparison is the recognition of distinct, evolutionarily conserved isotypes of beta tubulin that are distinguished primarily by their characteristic carboxyl-terminal variable region sequence and, to a lesser extent, by sequence in an amino-terminal variable domain as well. These correlations represent a convincing demonstration that multiple beta tubulin genes in vertebrates encode a family of closely related but structurally distinct beta tubulin isotypes and further serve to define the sequences of four classes of polypeptide isotypes that constitute that family.

Complete sequence of three alpha-tubulin cDNAs in Chinese hamster ovary cells: each encodes a distinct alpha-tubulin isoprotein

The genome of Chinese hamster ovary (CHO) cells contains a complex family of approximately 16 alpha-tubulin genes, many of which may be pseudogenes. We present here the complete cDNA sequences of three expressed alpha-tubulin genes; one of these genes has been identified only in CHO cells. The noncoding regions of these three CHO alpha-tubulin genes differed significantly, but their coding regions were highly conserved. Nevertheless, we observed differences in the predicted amino acid sequences for the three genes. A comparison of the CHO alpha-tubulin sequences with all of the sequences available for mammals allowed assignment of the alpha-tubulin genes to three classes. The proteins encoded by the members of two of these classes showed no class-specific amino acids among the mammalian species examined. The gene belonging to the third class encoded an isoprotein which was clearly distinct, and members of this class may play a unique role in vivo. Sequencing of the three alpha-tubulin genes was also undertaken in CMR795, a colcemid-resistant clonal CHO cell line which has previously been shown to have structural and functional alterations in its tubulin proteins. We found differences in the tubulin nucleotide sequence compared with the parental line; however, no differences in the alpha-tubulin proteins encoded in the two cell lines were observed.

A chicken-yeast chimeric beta-tubulin protein is incorporated into mouse microtubules in vivo

The role of divergent primary sequences in restricting tubulin function was tested in vivo by a gene transfection experiment. A chicken-yeast chimeric beta-tubulin DNA was introduced into 3T3 cells using the transfection vector pSV2. The 5' end of this gene, from chicken, is similar but not identical with that of mouse beta-tubulins; the 3' end, from yeast, contains a carboxyl terminus that is very different from other known beta-tubulin sequences. The chimeric protein is incorporated efficiently into each of the microtubule structures and each of the microtubules in the host cells. The presence of the protein has no apparent effect on either growth rate or cell morphology. The results suggest that the divergent sequences in this chimeric tubulin molecule place no restrictions on its activities in mouse cells.

Multiple forms of tubulin in the cilia and cytoplasm of Tetrahymena thermophila

Most higher eukaryotic tubulins are separated into alpha- and beta-tubulin when electrophoresed in NaDodSO4- denaturing gels, while many lower eukaryotic tubulins are poorly resolved under these conditions, which include a stacking gel (pH 6.80) and a separating gel (pH 8.80). By lowering the pH of the separating gel to 8.25, we have found that tubulin isolated from the protozoan Tetrahymena thermophila is resolved by one-dimensional polyacrylamide gel electrophoresis into two alpha-tubulins and one beta-tubulin. Moreover, at least five alpha- and two beta-tubulin isotypes are identified in Tetrahymena by isoelectric focusing and two-dimensional polyacrylamide gel electrophoresis. Three of these alpha isotypes and one beta isotype are found specifically in ciliary microtubules, while the other two isotypes are found only in the cytoplasmic tubulin pool that was isolated and induced to self-assemble into microtubules in vitro. Peptide mapping by limited proteolytic digestion indicates that the tubulins are closely related. Possible mechanisms for the generation and selection of these tubulin isotypes are discussed.

Apparent gene conversion between beta-tubulin genes yields multiple regulatory pathways for a single beta-tubulin polypeptide isotype

We have determined the complete nucleotide sequences of two chicken beta-tubulin genes, beta 1 and beta 2. These genes display an unusual pattern of segmental homology which indicates that they originally arose by gene duplication and have subsequently coevolved by a process that included localized gene conversion or intergenic recombination. Since the beta-tubulin polypeptides encoded by the two genes are virtually identical (99.5%), particularly in the major beta-tubulin isotype defining regions, they almost certainly constitute a single isotypic class of beta tubulin. However, the regulatory properties of the two genes are highly divergent as indicated by analysis of their patterns of expression in different chicken cell types. beta 1 is the major transcript detected in skeletal muscle myoblasts, whereas beta 2 is the major beta-tubulin transcript in cultured sympathetic neurons. The existence of these two genes appears to derive from a regulatory requirement whereby the expression of a single tubulin isotype is mediated through different regulatory programs in development and differentiation. These results thus provide direct experimental support for the hypothesis that gene conversion and intergenic recombination play an important role in evolution by uncoupling the evolution of structural genes from the regulatory sequences which control them.