Evolution of alpha q- and beta-tubulin genes as inferred by the nucleotide sequences of sea urchin cDNA clones

Intestinal immunoregulation: lessons from human mendelian diseases

The mechanisms that maintain intestinal homeostasis despite constant exposure of the gut surface to multiple environmental antigens and to billions of microbes have been scrutinized over the past 20 years with the goals to gain basic knowledge, but also to elucidate the pathogenesis of inflammatory bowel diseases (IBD) and to identify therapeutic targets for these severe diseases. Considerable insight has been obtained from studies based on gene inactivation in mice as well as from genome wide screens for genetic variants predisposing to human IBD. These studies are, however, not sufficient to delineate which pathways play key nonredundant role in the human intestinal barrier and to hierarchize their respective contribution. Here, we intend to illustrate how such insight can be derived from the study of human Mendelian diseases, in which severe intestinal pathology results from single gene defects that impair epithelial and or hematopoietic immune cell functions. We suggest that these diseases offer the unique opportunity to study in depth the pathogenic mechanisms leading to perturbation of intestinal homeostasis in humans. Furthermore, molecular dissection of monogenic intestinal diseases highlights key pathways that might be druggable and therapeutically targeted in common forms of IBD.

Soluble tubulin complexes, γ-tubulin, and their changing distribution in the zebrafish (Danio rerio) ovary, oocyte and embryo

Tubulin dynamics, i.e., the interchange of polymeric and soluble forms, is important for microtubule (MTs) cellular functions, and thus plays essential roles in zebrafish oogenesis and embryogenesis. A novel finding in this study revealed that there were soluble pools of tubulins in zebrafish oocytes that were sequestered and maintained in a temporary "oligomeric" state, which retained assembling and disassembling potential (suggested by undetected acetylated tubulin, marker of stable tubulin), but lacked abilities to assemble into MTs spontaneously in vivo. Using differential centrifugation, gel chromatography and DM1A-probed western blot, soluble alpha-tubulin was found to be associated with large molecular weight complexes (MW range to over 2 MDa) which were reduced in amount by the blastula stage, especially in some batches of embryos, with a concomitant decrease in soluble tubulin. Complexes (MW range less than 2 MDa) then increased in the gastrula with an increase in soluble alpha-tubulin. Two different anti-gamma-tubulin monoclonal antibodies, GTU 88 and TU 30, revealed the existence of soluble gamma-tubulin in both zebrafish oocytes and embryos, which also decreased by the blastula stage and increased in the gastrula stage. Soluble alpha-tubulin and gamma-tubulin extracted from zebrafish ovaries, oocytes and embryos co-localized in fractions on three different columns: S-200 Sephacryl, DEAE and Superose-6b. The soluble tubulin complexes were competent to assemble into MTs in vitro induced by taxol, and gamma-tubulin was co-localized with assembled MTs. These soluble tubulin complexes were stable during freeze-thaw cycles and resisted high ionic interaction (up to 1.5 M NaCl). Furthermore, some ovarian soluble alpha-tubulin could be co-immunoprecipitated with gamma-tubulin, and vice versa. Two antibodies specific for Xenopus gamma-tubulin ring complex proteins (Xgrip 109 and Xgrip 195) detected single bands from ovarian extracts in western blots, suggesting the existence of Xgrip 109 and Xgrip 195 homologues in zebrafish. These findings, together with recent work on gamma-tubulin ring complexes in oocytes, eggs and embryos of other species, suggest that soluble gamma-tubulin-associated protein complexes may be involved in regulating tubulin dynamics during zebrafish oogenesis and embryogenesis.

The carboxy-terminal sequence Asp427-Glu432 of beta-tubulin plays an important function in axonemal motility

Flagellar motility is the result of specific interactions between axonemal microtubular proteins and the dynein motors. Tubulin, the main component of microtubule, is a very polymorphic protein resulting from the expression of several isogenes and from the existence of various post-translational modifications. In order to characterize tubulin isoforms and tubulin domains that are important for flagellar movement, we prepared monoclonal antibodies against axonemal proteins from whole sea-urchin sperm tails. The monoclonal antibodies obtained were screened for their potency to inhibit demembranated-reactivated sperm models and for their monospecific immunoreactivity on immunoblot. Among the different antibodies we obtained, D66 reacted specifically with a subset of beta-tubulin isoforms. Limited proteolysis, HPLC, peptide sequencing, mass spectroscopy and immunoblotting experiments indicated that D66 recognized an epitope localized in the primary sequence Gln423-Glu435 of the C-terminal domain of Lytechinus pictus beta2-tubulin, and that this sequence belongs to class IVb. The use of synthetic peptides and immunoblotting analysis further narrowed the amino acids important for antibody recognition to Asp427-Glu432. Because the primary effect of this antibody on sperm motility is to decrease the flagellar beat frequency, we suggest that this sequence is involved in the tubulin-dynein head interaction.

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.

The A and B tubules of the outer doublets of sea urchin sperm axonemes are composed of different tubulin variants

The alpha beta-tubulin heterodimer, the structural unit of microtubules, comes in many variants. There are different alpha and beta isotypes encoded by multigene families. Additional heterogeneity is generated by a set of posttranslational modifications. Detyrosination of alpha-tubulin, removal of the carboxy-terminal Glu-Tyr dipeptide of alpha-tubulin, phosphorylation of some tubulins, polyglutamylation, and polyglycylation of alpha- and beta-tubulins all involve the acidic carboxy-terminal region. We have investigated the distribution of tubulin variants in the axonemal microtubules of sea urchin sperm flagella by immunological procedures and by direct sequence and mass spectrometric analysis of the carboxy-terminal peptides. The A and B tubules that comprise the nine outer doublets differ strongly in tubulin variants. A tubules contain over 95% unmodified, tyrosinated alpha beta-tubulin. In B tubules, alpha-tubulin is approximately 65% detyrosinated and both alpha- and beta-tubulin are 40-45% polyglycylated. These results show a segregation of tubulin variants between two different axonemal structures and raise the possibility that posttranslational modifications of tubulins reflect or specify structurally and functionally distinct microtubules.

The polyglutamylated lateral chain of alpha-tubulin plays a key role in flagellar motility

To investigate whether a specific isotype of tubulin is involved in flagellar motility, we have developed and screened a panel of monoclonal antibodies (mAb) generated against sea urchin sperm axonemal proteins. Antibodies were selected for their ability to block the motility of permeabilized sperm models. The antitubulin mAb B3 completely inhibited, at low concentrations, the flagellar motility of permeabilized sperm models from four sea urchin species. On immunoblots, B3 recognized predominantly alpha-tubulin in sea urchin sperm axonemes and equally well brain alpha- and beta-tubulins. Subtilisin cleavage of tubulin removed the B3 epitope, indicating that it was restricted to the last 13 amino acid residues of the C-terminal domain of alpha-tubulin. In enzyme-linked immunosorbant assays, B3 reacted with glutamylated alpha-tubulin peptides from sea urchin or mouse brain but did not bind to the unmodified corresponding peptide, indicating that it recognized polyglutamylated motifs in the C-terminal domain of alpha-tubulin. On the other hand, other tubulin antibodies directed against various epitopes of the C-terminal domain, with the exception of the antipolyglutamylated mAb GT335, had no effect on motility while having binding properties similar to that of B3. B3 and GT335 acted by decreasing the beating amplitude without affecting the flagellar beat frequency. B3 and GT335 were also capable of inhibiting the motility of flagella of Oxyrrhis marina, a 400,000,000 year old species of dinoflagellate, and those of human sperm models. Localization of the antigens recognized by B3 and GT335 by immunofluorescence techniques revealed their presence along the whole axoneme of sea urchin spermatozoa and flagella of O. marina, except for the distal tip and the cortical microtubule network of the dinoflagellate. Taken together, the data reported here indicate that the polyglutamylated lateral chain of alpha-tubulin plays a dynamic role in a dynein-based motility process.

Monoclonal anti-dipeptide antibodies cross-react with detyrosinated and glutamylated forms of tubulins

Two monoclonal antibodies, GLU-1 and A1.6, raised against gamma-L-glutamyl-L-glutamic acid dipeptide (Glu-Glu) and Ca(2+)-dependent ATPase from Paramecium, respectively, recognized the dipeptide Glu-Glu sequence. Whereas the antibodies immunofluorescently stained very few, if any, cytoskeletal fibers in cultured mammalian cells, almost all interphase as well as mitotic spindle microtubules became visible after treatment of cells with carboxypeptidase A. Immunoblot analysis demonstrated intense cross-reaction of the antibodies to the alpha-tubulin subunit. alpha-Tubulin isotypes produced as fusion proteins in bacteria were labeled by both the antibodies only when the proteins did not contain a tyrosine residue at the C terminus, indicating that GLU-1 and A1.6 specifically recognize the detyrosinated form of alpha-tubulin. When microtubule protein purified from brain was probed, not only alpha-but also, to a lesser extent, beta-tubulin were revealed by the dipeptide antibodies. A synthetic tripeptide YED containing one glutamyl group linked to the second residue of the peptide via the gamma position was also recognized by the antibodies. Since this peptide sequence corresponds to the amino acid sequence of polyglutamyated class III beta isotype at amino acid position 437 to 439, it is suggested that GLU-1 and A1.6 are able to recognize the glutamylated form of beta-tubulin. These results indicate that the C-terminal Glu-Glu sequence displays strong antigenicity, and the antibodies recognize the sequence present in the C terminus of the detyrosinated form of alpha-tubulin and the glutamyl side chain of beta-tubulin. Particularly strong immunoreaction was detected with ciliary and flagellar microtubules; thus, stable axonemal microtubules appear to be rich in post-translationally modified tubulin subunits.

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

Vertebrate beta-tubulins have been classified into six classes on the basis of their C-terminal sequences [(1987) J. Cell Biol. 105, 1707-1720]. In particular, the sequences starting at residue 430 differ between isotypes of the same animal but are conserved between species. We extend this analysis and show that there are three 'hot spots', at residues 35, 55-57 and 124 which exhibit intra-species heterogeneity but inter-species conservation. There is a remarkable correlation between the identity of these residues and the C-terminal sequences, and suggests that the vertebrate beta-tubulins fall into three broad types. This correlation extends to those non-vertebrate organisms which have the Type 1 C-terminal sequence. We propose that these three 'hot spots' and the C-terminal peptide interact in the tertiary structure. We have also noted that the C-terminal peptide almost always contains a single phenylalanine or tyrosine residue, and that there is a strong correlation between this residue and the amino acids at positions 217/218, in both the vertebrate and non-vertebrate sequences. We propose that the C-terminal aromatic amino acid interacts with residues 217/218 in the tertiary structure. Analysis of conditions which stabilise microtubules and/or lower the steady state critical concentration strongly suggests that these two sets of coordinated amino acid substitutions are directly involved in effecting the conformational change associated with GTP hydrolysis which results in dynamic instability. We propose that there is an interaction between the highly acidic sequence between residue 430 and the aromatic amino acid (termed peptide A) and conserved basic amino acids located close to the 'hot spots'. We suggest that this interaction is altered in response to the assembly-dependent GTP hydrolysis, with the consequential increase in the subunit dissociation rate constant.

Loss of tumorigenic potential by human lung tumor cells in the presence of antisense RNA specific to the ectopically synthesized alpha subunit of human chorionic gonadotropin

A clonal strain of human lung tumor cells in culture (ChaGo), derived from a bronchogenic carcinoma, synthesizes and secretes large amounts of alpha (alpha) and a comparatively lower level of beta (beta) subunit of the glycoprotein hormone, human chorionic gonadotropin (HCG). ChaGo cells lost their characteristic anchorage-independent growth phenotype in the presence of anti-alpha-HCG antibody. The effect of the antibody was partially reversed by addition of alpha-HCG to the culture medium. ChaGo cells were transfected with an expression vector (pRSV-anti-alpha-HCG), that directs synthesis of RNA complementary to alpha-HCG mRNA. The transfectants produced alpha-HCG antisense RNA which was associated with the reduced level of alpha-HCG. Transfectants also displayed several altered phenotypic properties, including altered morphology, less mitosis, reduced growth rate, loss of anchorage-independent growth, and loss of tumorigenicity in nude mice. Treatment of transfectants with 8,bromo-cAMP resulted in increased accumulation of alpha-HCG mRNA, no change in the level of alpha-HCG antisense RNA, release of the inhibition of [3H]thymidine incorporation, and restoration of anchorage-independent growth phenotype. The overexpression of c-myc, observed in ChaGo cells, was unaffected by the reduced level of alpha-HCG. These results suggest that ectopic synthesis of the alpha subunit of HCG plays a functional role in the transformation of these human lung cells.

Stabilization of tubulin mRNA by inhibition of protein synthesis in sea urchin embryos

An increased level of unpolymerized tubulin caused by depolymerization of microtubules in sea urchin larvae resulted in a rapid loss of tubulin mRNA, which was prevented by nearly complete inhibition of protein synthesis. Results of an RNA run-on assay indicated that inhibition of protein synthesis does not alter tubulin gene transcription. Analysis of the decay of tubulin mRNA in embryos in which RNA synthesis was inhibited by actinomycin D indicated that inhibition of protein synthesis prevents the destabilization of tubulin mRNA. The effect was similar whether mRNA was maintained on polysomes in the presence of emetine or anisomycin or displaced from the polysomes in the presence of puromycin or pactamycin; thus, the stabilization of tubulin mRNA is not dependent on the state of the polysomes after inhibition of protein synthesis. Even after tubulin mRNA declined to a low level after depolymerization of microtubules, it could be rescued by treatment of embryos with inhibitors of protein synthesis. Tubulin mRNA could be induced to accumulate prematurely in gastrulae but not in plutei if protein synthesis was inhibited, an observation that is indicative of the importance of the autogenous regulation of tubulin mRNA stability during embryogenesis. Possible explanations for the role of protein synthesis in the control of mRNA stability are discussed.

Sequential expression and cooperative interaction of c-Ha-ras and c-erbB genes in in vivo chemical carcinogenesis

The level of expression of several cellular protooncogenes is examined at different stages of 7,12-dimethylbenzanthracene (DMBA)-induced tumor development in hamster buccal pouch epithelium (HBPE). Results presented demonstrate overexpression of c-Ha-ras gene at a very early stage of tumor development, and this elevated level of expression of the gene persists throughout the tumorigenesis process. The expression of the cellular protooncogene c-erbB, on the other hand, can be detected only after 8-10 weeks of DMBA treatment of the tissue and increases with the progression of the disease. The overexpression of c-erbB gene can be correlated with the stage of extensive proliferation and subsequent invasion of the HBPE cells into the underlying connective tissue. This sequential pattern of stage-specific expression of the two cellular protooncogenes can be observed in (i) treated tissues, (ii) stage-representative cultured cells, and (iii) NIH 3T3 transformants derived with DNA from HBPE cells. The low-level expression of c-myc and c-sis genes detected in control tissues remains unaffected, while c-fos gene activity cannot be detected at any stage of tumor development. The overexpression of c-Ha-ras gene alone in HBPE cells derived from tissues treated for 5 weeks (DM5) is not sufficient to induce tumors in athymic mice, whereas expression of c-Ha-ras and c-erbB genes at later stages of tumor development (DM10 and HCPC cells) induce histopathologically defined epithelial cell carcinoma in athymic mice within 2-3 weeks. The sequential overexpression of c-Ha-ras and c-erbB genes in a stage-specific manner and their cooperative interaction in the DMBA-induced in vivo oral carcinogenesis have been demonstrated.

DNA sequence and pattern of expression of the sea urchin (Paracentrotus lividus) alpha-tubulin genes

To study the molecular aspects of the regulation of transcription of a multigene family, we have isolated and sequenced cDNA and genomic clones coding for the alpha-tubulin of the sea urchin Paracentrotus lividus. Two cDNA clones, P alpha 10 and P alpha 4, contain respectively the coding information for 391 C-terminal and for 338 N-terminal amino acids of the 452 residues that constitute the complete protein. They show silent nucleotide substitutions only, suggesting that P alpha 10 and P alpha 4 represent the cloned copies of two allelic gene transcripts, which encode for two alpha-tubulin isoforms with identical amino acid sequence in the region of the overlap. The comparison of the predicted amino acid sequence of the composite P alpha 4-10 and of the mouse M alpha-6 (Villasante et al., Mol Cell Biol 1986; 6:2409-2419) reveals a conservation of 97% between the two polypeptides. By RNA blotting hybridization six major alpha-tubulin transcripts were identified. Two, of 3.5 kb and 2.0 kb, are expressed in the unfertilized eggs and during early cleavage. The other two maternal mRNAs, of 2.4 kb and 1.8 kb, are expressed in both early and late cleavage embryos, but in the intestine the 1.8 kb RNA, which specifically reacted with the 3' specific probe of the P alpha 10 cDNA, is the only transcript detected. Finally, the 1.5 kb and 1.9 kb mRNAs represent the transcription of stage- and tissue-specific genes, respectively. In fact, the former becomes detectable at blastula stage and accumulates during late development, whereas the latter is found in the testis only. The sequence data of the 3' terminus of the alpha-3 genomic clone suggests that it encodes for a divergent alpha-tubulin, and it most probably corresponds to the testis-specific gene.

Multiple levels of regulation of tubulin gene expression during sea urchin embryogenesis

The expression of the tubulin genes during embryogenesis of the sea urchin Lytechinus pictus has been analyzed. Single strand tracer excess titrations of alpha- and beta-tubulin mRNA and RNA gel blot hybridizations indicate that tubulin mRNA remains at a constant 1.3 X 10(5) transcripts per embryo during cleavage stages, increases during ciliogenesis shortly before hatching (12 hr PF), declines until midgastrula (30-35 hr PF), and then gradually increases 3-fold to about 6 X 10(5) per pluteus larva (72 hr PF). Tubulin synthesis changes in concert with its mRNA, except that during cleavage the relative rate of tubulin synthesis increases without a corresponding increase in tubulin mRNA abundance. The relative rates of tubulin gene transcription were assayed by a run-on assay in isolated nuclei. The synthesis of alpha- or beta-tubulin RNA results in little supplementation of maternal tubulin RNA during cleavage stages, but the rate increases at least 18-fold during ciliogenesis and then gradually decreases thereafter. The accumulation of tubulin mRNA after gastrulation can be accounted for by an ontogenetic increase in tubulin RNA stability, assayed by actinomycin D chase and RNA gel blot hybridization. The rates of synthesis, stabilities, and abundances of alpha- and beta-tubulin mRNAs were similar, suggesting coordinate regulation. These observations indicate the importance of translational regulation during cleavage, transcriptional regulation during ciliogenesis, and regulation of mRNA stability by the level of unpolymerized tubulin during later development.

Sequence of one alpha- and two beta-tubulin genes of Tetrahymena pyriformis. Structural and functional relationships with other eukaryotic tubulin genes

Macronuclear DNA of the ciliate Tetrahymena pyriformis contains only one size class of fragments coding for alpha-tubulin, alpha TT. We have isolated alpha TT from a partial plasmid library, using Chlamydomonas reinhardtii alpha-tubulin gene as a probe. This gene as well as the two beta-tubulin genes, beta TT1 and beta TT2, have been sequenced. None of these genes contains introns and all use TGA as the stop codon. In the coding region of the two beta-tubulin genes, there are several TAA and TAG stop codons that probably code for glutamine. The codon usage is very biased. Regions flanking the tubulin coding sequences are A + T-rich (75%) and quite different among themselves. In these regions there are several putative transcription-regulatory sequences. Nuclear transcripts begin and terminate at multiple sites. The beta-tubulin proteins differ only in two amino acid residues. Primary structure of Tetrahymena tubulins as well as their hydropathy indexes show a high degree of homology with tubulins from other organisms. Two-dimensional electrophoretic analysis of the ciliary tubulins shows the presence of eight alpha-tubulins and four beta-tubulins. The alpha-tubulins migrate faster than the beta-tubulins, in contrast with what happens with brain tubulins. We suggest that there are several alpha- and beta-tubulin isoforms and the migratory inversion observed may be due to post-translational modifications.

Stabilization of tubulin mRNA by inhibition of protein synthesis in sea urchin embryos

An increased level of unpolymerized tubulin caused by depolymerization of microtubules in sea urchin larvae resulted in a rapid loss of tubulin mRNA, which was prevented by nearly complete inhibition of protein synthesis. Results of an RNA run-on assay indicated that inhibition of protein synthesis does not alter tubulin gene transcription. Analysis of the decay of tubulin mRNA in embryos in which RNA synthesis was inhibited by actinomycin D indicated that inhibition of protein synthesis prevents the destabilization of tubulin mRNA. The effect was similar whether mRNA was maintained on polysomes in the presence of emetine or anisomycin or displaced from the polysomes in the presence of puromycin or pactamycin; thus, the stabilization of tubulin mRNA is not dependent on the state of the polysomes after inhibition of protein synthesis. Even after tubulin mRNA declined to a low level after depolymerization of microtubules, it could be rescued by treatment of embryos with inhibitors of protein synthesis. Tubulin mRNA could be induced to accumulate prematurely in gastrulae but not in plutei if protein synthesis was inhibited, an observation that is indicative of the importance of the autogenous regulation of tubulin mRNA stability during embryogenesis. Possible explanations for the role of protein synthesis in the control of mRNA stability are discussed.

The alpha-tubulin gene family expressed during cell differentiation in Naegleria gruberi

Genes that direct the programmed synthesis of flagellar alpha-tubulin during the differentiation of Naegleria gruberi from amebae to flagellates have been cloned, and found to be novel with respect to gene organization, sequence, and conservation. The flagellar alpha-tubulin gene family is represented in the genome by about eight homologous DNA segments that are exceptionally similar and yet are neither identical nor arrayed in a short tandem repeat. The coding regions of three of these genes have been sequenced, two from cDNA clones and one from an intronless genomic gene. These three genes encode an identical alpha-tubulin that is conserved relative to the alpha-tubulins of other organisms except at the carboxyl terminus, where the protein is elongated by two residues and ends in a terminal glutamine instead of the canonical tyrosine. In spite of the protein conservation, the Naegleria DNA sequence has diverged markedly from the alpha-tubulin genes of other organisms, a counterexample to the idea that tubulin genes are conserved. alpha-Tubulin mRNA homologous to this gene family has not been detected in amebae. This mRNA increases markedly in abundance during the first hour of differentiation, and then decreases even more rapidly with a half-life of approximately 8 min. The abundance of physical alpha-tubulin mRNA rises and subsequently falls in parallel with the abundance of translatable flagellar tubulin mRNA and with the in vivo rate of flagellar tubulin synthesis, which indicates that flagellar tubulin synthesis is directly regulated by the relative rates of transcription and mRNA degradation.

Stimulation of tubulin gene transcription by deciliation of sea urchin embryos

Deciliation by hypertonic shock of embryos of the sea urchin Lytechinus pictus resulted in an increase in synthesis of alpha- and beta-tubulins, the consequence of an increased concentration of RNA encoding the tubulins. RNA run-on assays in isolated nuclei indicated that this response is due to a transient increase in the rate of synthesis of tubulin RNA beginning within 5 min of deciliation. This enhancement of tubulin gene transcription also occurred in deciliated embryos treated with the microtubule-depolymerizing agent colcemid; thus the reaction to deciliation is not a response to a reduction in concentration of unpolymerized tubulin utilized for ciliogenesis. In deciliated embryos treated with colcemid, the elevated level of tubulin RNA declined rapidly, due to its destabilization by the elevated concentration of unpolymerized tubulin. The increased transcription of tubulin genes is a response to the loss of cilia, not to the hypertonic shock, and occurs even when cilium regeneration is prevented. Inhibition of protein synthesis with puromycin or emetine did not prevent the transcriptional enhancement but stabilized tubulin mRNA, resulting in increased accumulation of tubulin mRNA after deciliation.

Synonymous nucleotide substitution rates of beta-tubulin and histone genes conform to high overall genomic rates in rodents but not in sea urchins

Sea urchin and rodent genomes have been posited to evolve rapidly as indicated by divergences in single copy nuclear DNA sequences. We have examined whether the synonymous substitution rates of three highly conserved genes, beta-tubulin, histone H4, and histone H3, adhere to these high genomic substitution rates by comparing sequences between two sea urchins, Strongylocentrotus purpuratus and Lytechinus pictus, and between rodents and humans. Whereas the rate of change between the 3' untranslated regions of the beta-tubulin cDNA of S. purpuratus (Sp-beta 1), sequenced in this study, and of L. pictus (Lp-beta 3) was consistent with the overall rate of change estimated from previous DNA hybridization results between these species, the synonymous substitution rates for the carboxyl domains of these beta-tubulins, as well as for the late histones H4 and H3, were significantly depressed. In contrast, synonymous nucleotide substitution rates between rodents and between rodent and human for the carboxyl domain proper of identical beta-tubulin isotypes and for histone H4 and H3.1 did not differ from the overall rate of change for the rodent genomes. Moreover, an analysis of paralogous human and mouse beta-tubulin sequences supported the conclusion that the synonymous substitution rates in the mouse were higher than those in the human. Differences in constraint on evolutionary change were not evident strictly from the conserved amino acid sequences and base compositions of these genes. Other constraining influences seemed more relevant to the departure of the synonymous substitution rates of the sea urchin beta-tubulin and histone coding regions from the average genomic rate.

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.

Defective distal regulatory element at the 5' upstream of rat prolactin gene of steroid-nonresponsive GH-subclone

The prolactin-nonproducing (PRL-) GH cell strains (rat pituitary tumor cells in culture). GH12C1 and F1BGH12C1, do not respond to steroid hormones estradiol or hydrocortisone (HC). However, the stimulatory effect of estradiol and the inhibitory effect of hydrocortisone on prolactin synthesis can be demonstrated in the prolactin-producing GH cell strain, GH4C1. In this investigation we have examined the 5' end flanking region of rat prolactin (rat PRL) gene of steroid-responsive, GH4C1 cells to identify the positive and negative regulatory elements and to verify the status of these elements in steroid-nonresponsive F1BGH12C1 cells. Results presented in this report demonstrate that the basel level expression of the co-transferred Neo gene (neomycin phosphoribosyl transferase) is modulated by the distal upstream regulatory elements of rat PRL gene in response to steroid hormones. The expression of adjacent Neo gene is inhibited by dexamethasone and is stimulated by estradiol in transfectants carrying distal regulatory elements (SRE) of steroid-responsive cells. These responses are not observed in transfectants with the rat PRL upstream sequences derived from steroid-nonresponsive cells. The basal level expression of the host cell alpha-2 tubulin gene is not affected by dexamethasone. We report here the identification of the distal steroid regulatory element (SRE) located between 3.8 and 7.8 kb upstream of the transcription initiation site of rat PRL gene. Both the positive and the negative effects of steroid hormones can be identified within this upstream sequence. This distal SRE appears to be nonfunctional in steroid-nonresponsive cells. Though the proximal SRE is functional, the defect in the distal SRE makes the GH substrain nonresponsive to steroid hormones. These results suggest that both the proximal and the distal SREs are essential for the mediation of action of steroid hormones in GH cells.

Stimulation of tubulin gene transcription by deciliation of sea urchin embryos

Deciliation by hypertonic shock of embryos of the sea urchin Lytechinus pictus resulted in an increase in synthesis of alpha- and beta-tubulins, the consequence of an increased concentration of RNA encoding the tubulins. RNA run-on assays in isolated nuclei indicated that this response is due to a transient increase in the rate of synthesis of tubulin RNA beginning within 5 min of deciliation. This enhancement of tubulin gene transcription also occurred in deciliated embryos treated with the microtubule-depolymerizing agent colcemid; thus the reaction to deciliation is not a response to a reduction in concentration of unpolymerized tubulin utilized for ciliogenesis. In deciliated embryos treated with colcemid, the elevated level of tubulin RNA declined rapidly, due to its destabilization by the elevated concentration of unpolymerized tubulin. The increased transcription of tubulin genes is a response to the loss of cilia, not to the hypertonic shock, and occurs even when cilium regeneration is prevented. Inhibition of protein synthesis with puromycin or emetine did not prevent the transcriptional enhancement but stabilized tubulin mRNA, resulting in increased accumulation of tubulin mRNA after deciliation.

Spec3: embryonic expression of a sea urchin gene whose product is involved in ectodermal ciliogenesis

We have characterized the temporal and spatial expression of Spec3 mRNA in embryos of the sea urchin, Strongylocentrotus purpuratus. This mRNA, 2.0 kb in length, is present at low levels in unfertilized eggs but accumulates rapidly during cleavage, increasing 50-fold by hatching blastula stage. Message levels then decline abruptly, remain constant during mesenchyme blastula and gastrula stages, and increase again during prism and pluteus stages. This accumulation pattern is quite similar to that of the ectodermally expressed beta-tubulin mRNAs described recently by Harlow and Nemer (1987a). In situ hybridization shows that although Spec3 message accumulates in all blastomeres at early blastula stages, it later becomes restricted to ectoderm. By late blastula stage, hybridization is strongest in the animal hemisphere. At gastrula, signals are variable over ectoderm, and by pluteus, grains are concentrated in the ciliary band, though present in other ectodermal cells as well. Deciliation and regeneration of cilia in gastrula-stage embryos results in a four- to fivefold increase in Spec3 mRNA levels, implying that the Spec3 gene product is associated with ciliogenesis. Spec3 mRNA is encoded by a single gene in the haploid genome, and characterization of the gene shows that it contains three exons that encode an open reading frame for a hydrophobic protein of 21.6 kD. The reading frame reveals that the carboxy-terminal part of the protein contains two long hydrophobic stretches, 31 and 37 residues long, separated by short hydrophilic regions of six to eight residues. The presence of these two distinct hydrophobic stretches suggests that the Spec3 protein contains two alpha-helical domains that either span the lipid bilayer or are associated with some other hydrophobic environment.

Developmental and tissue-specific regulation of beta-tubulin gene expression in the embryo of the sea urchin Strongylocentrotus purpuratus

Four beta-tubulin mRNAs in the embryo of the sea urchin Strongylocentrotus purpuratus are transcribed from at least 3 of the 9-12 beta-tubulin genes. A beta 1 tubulin mRNA of 1.8 kb, transcribed from a unique beta 1 gene, is expressed with high specificity in the pluteus ectoderm. Another 1.8-kb mRNA, beta 2, and a 2.5-kb beta 3 mRNA are moderately ectoderm specific. In contrast, a 3.0-kb beta 4 mRNA is highly specific for the endomesoderm tissue fraction. Certain similarities in developmental and tissue-specific expression suggest that these beta-tubulin genes may be related in their mode of regulation to counterparts among the genes for actin, another cytoskeletal protein. Measurements of absolute amounts revealed a distinct developmental profile for each beta-tubulin mRNA. An increase in the total amount of beta-tubulin mRNA in the early blastula was correlated with an increase in transcription rate per nucleus; whereas, later in the mesenchyme blastula stage, the beta-tubulin mRNA level decreased sharply as the rate of beta-tubulin gene transcription on a per embryo basis remained constant. Thus, during development through the blastula stages, there was a switch to a predominantly posttranscriptional regulation of beta-tubulin mRNA expression, probably through a decrease in mRNA stability.

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

About 96% of the amino acid sequence of an alpha-tubulin from the slime mould Physarum polycephalum has been determined. Of 430 sequenced amino acids, 30 differ from the deduced amino acid sequence of a recently published alpha-tubulin complementary DNA from the plasmodial form of P. polycephalum. The myxamoebal alpha-tubulin differs from all other known alpha-tubulins in one of the last three C-terminal amino acids that are Gly-Glu-Tyr instead of the usual Glu-Glu-Tyr. These last three amino acids are preceded by 11 residues that appear to be particularly susceptible to mutation. No heterogeneity was found whilst sequencing the myxamoebal alpha-tubulin, indicating that only one type of alpha-tubulin is present in myxamoebae. This alpha-tubulin appears to be less conserved than the previously described plasmodial alpha-tubulin, supporting the hypothesis that the structural constraints on tubulin in axonemes have a significant effect on its rate of mutation.

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.

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.

Multiple cis-acting elements modulate the translational efficiency of GCN4 mRNA in yeast

The expression of the GCN4 gene in yeast is regulated at the translational level: growth of yeast cells under amino acid starvation conditions results in an increase in the translational efficiency of GCN4 mRNA. A sequence within the 5' untranslated region of this mRNA, which contains four small open reading frames, acts in cis to suppress translation when growth occurs in rich media. In this report, we have analyzed the effects on translation of a series of deletion, insertion, and substitution mutations in the 5' untranslated region of GCN4 mRNA. This analysis showed that at least two distinct cis elements located within the region of the small upstream open reading frames are required, in conjunction with trans positive elements, for the translational activation of GCN4 mRNA. We propose that the translational efficiency of GCN4 mRNA is modulated by the rate of translation initiation at the upstream AUG codons.

Sequence homologies of glucose-dehydrogenases of Bacillus megaterium and Bacillus subtilis

The sequence homologies of the glucose dehydrogenase subunits of B. megaterium and B. subtilis are compared. From the known B. megaterium aminoacid sequence and the base sequence of the cloned B. subtilis structural gene we predict the B. megaterium structural glucose dehydrogenase gene. Assuming the minimal mutational changes to convert one gene into the other 23 transitions, 30 transversions, 1 inversion, 3 insertion-deletions, but no frameshifts are postulated necessary to interconvert the structural genes. The homology of both enzyme subunits of 85% reflects the close evolutionary distance between B. subtilis and B. megaterium.

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.

Tubulin evolution: ciliate-specific epitopes are conserved in the ciliary tubulin of Metazoa

In spite of their overall evolutionary conservation, the tubulins of ciliates display electrophoretic and structural particularities. We show here that antibodies raised against Paramecium and Tetrahymena ciliary tubulins fail to recognize the cytoplasmic tubulins of all the metazoans tested. Immunoblotting of peptide maps of ciliate tubulins reveals that these antibodies react with one or very few ciliate-specific epitopes, in contrast to polyclonal antibodies against vertebrate tubulins, which are equivalent to autoantibodies and recognize several epitopes in both ciliate and vertebrate tubulins. Furthermore, we show that the anti-ciliate antibodies recognize ciliary and flagellar tubulins of metazoans ranging from sea urchin to mammals (with the exception of humans). The results support the conclusion that although duplication and specialization of tubulin genes in metazoans may have led to distinct types of tubulins, the axonemal one has remained highly conserved.

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.

A plasmodial alpha-tubulin cDNA from Physarum polycephalum. Nucleotide sequence and comparative analysis

As the first step towards correlating structure and function of tubulin in the slime mold Physarum polycephalum we have elucidated the nucleotide sequence of a cDNA that appears to code for all but the last 25 to 30 C-terminal amino acids of a plasmodial alpha-tubulin. Differences in amino acid sequence from those of other alpha-tubulins are distributed fairly evenly throughout the sequence, although a relatively extensive conserved region is found in position 396 to 426 near the C terminus. A small region in position 298 to 307 contains a cluster of amino acid residues unique to Physarum alpha-tubulin. The sequence is 70% homologous to two yeast alpha-tubulins and about 83% homologous to five animal alpha-tubulins. A comparison of the homologies of all the known alpha-tubulins indicates that a large decrease in the accepted point mutation rate has occurred during the evolution of the metazoa, suggesting a major functional specialization of microtubules.

Expression of alpha- and beta-tubulin genes during development of sea urchin embryos

Mature unfertilized eggs of the sea urchin Lytechinus pictus contain multiple alpha-tubulin mRNAs, which range in size from 1.75 to 4.8 kb, and two beta-tubulin mRNAs, 1.8 and 2.25 kb. These mRNAs were found at similar levels throughout the early cleavage stages. RNA gel blot hybridizations showed that prominent quantitative and qualitative changes in tubulin mRNAs occurred between the early blastula and hatched blastula stages. The overall amounts of alpha- and beta-tubulin mRNAs increased two- to fivefold between blastula and pluteus. These increases were due mainly to a rise in a 1.75-kb alpha RNA and a new 2.0-kb beta RNA. Other, minor changes also occurred during subsequent development. All size classes of alpha- and beta-tubulin RNAs in early and late embryos contained poly(A)+ translatable sequences. As reported earlier, some of each of the alpha RNAs, but neither of the beta RNAs, are translated in the egg and a small portion of each of the stored alpha and beta RNAs is recruited onto polysomes within 30 min of fertilization. In the work described here, subsequent development up to the morula stage was accompanied by a gradual recruitment of tubulin mRNAs into polysomes. By the early blastula stage, most of the maternal tubulin sequences were associated with polysomes. In contrast to the gradual recruitment of maternal sequences throughout cleavage, the tubulin mRNAs which appeared at the blastula stage showed no delay in entering polysomes. The exact fraction of each mRNA that was translationally active at later stages varied somewhat among the individual mRNAs. From the differential hybridization patterns of egg, embryo, and testis RNAs to various tubulin cDNA and genomic DNA probes, it is concluded that at least one gene producing maternal alpha mRNA is different from a second one which is expressed only in testis. Each of the three embryonic beta RNAs is encoded by a different beta gene; at least two of these different beta genes are also expressed in testis.

Differential expression of three alpha-tubulin genes in Chinese hamster ovary cells

Chinese hamster ovary cells contain a complex family of ca. 16 unique alpha-tubulin sequences and a similar multiplicity of beta sequences. To examine which members of this multigene family are expressed, we constructed cDNA libraries from two Chinese hamster ovary cell lines according to the method of H. Okayama and P. Berg (Mol. Cell. Biol. 3:280-289, 1983). Each library consisted of 5.5 X 10(5) transformants and contained a high percentage of full-length tubulin clones. Three different alpha-tubulin genes were identified by sequence analysis of the 3' noncoding regions of these tubulin clones. The relative abundance of the transcripts corresponding to the three genes was estimated by gene-specific dot blotting of 96 cDNA alpha-tubulin clones and was found to be 71, 24, and 5%. There is little homology in the 3' noncoding sequences of these genes; however, a strong interspecies homology exists in this region for two of the Chinese hamster ovary genes with the two alpha-tubulin genes previously described in other systems. The third Chinese hamster ovary gene, with an expression frequency of 24%, is unique in that its 3' noncoding region is unlike that of the other mammalian alpha-tubulin genes. In addition, limited sequence data from the coding region of this gene indicates it codes for a unique alpha-tubulin protein.

Differential expression of three alpha-tubulin genes in Chinese hamster ovary cells

Chinese hamster ovary cells contain a complex family of ca. 16 unique alpha-tubulin sequences and a similar multiplicity of beta sequences. To examine which members of this multigene family are expressed, we constructed cDNA libraries from two Chinese hamster ovary cell lines according to the method of H. Okayama and P. Berg (Mol. Cell. Biol. 3:280-289, 1983). Each library consisted of 5.5 X 10(5) transformants and contained a high percentage of full-length tubulin clones. Three different alpha-tubulin genes were identified by sequence analysis of the 3' noncoding regions of these tubulin clones. The relative abundance of the transcripts corresponding to the three genes was estimated by gene-specific dot blotting of 96 cDNA alpha-tubulin clones and was found to be 71, 24, and 5%. There is little homology in the 3' noncoding sequences of these genes; however, a strong interspecies homology exists in this region for two of the Chinese hamster ovary genes with the two alpha-tubulin genes previously described in other systems. The third Chinese hamster ovary gene, with an expression frequency of 24%, is unique in that its 3' noncoding region is unlike that of the other mammalian alpha-tubulin genes. In addition, limited sequence data from the coding region of this gene indicates it codes for a unique alpha-tubulin protein.

Multiple polymorphic alpha- and beta-tubulin mRNAs are present in sea urchin eggs

Multiple alpha- and beta-tubulin RNAs were found in the mature unfertilized eggs of the sea urchin Lytechinus pictus. The alpha-tubulin RNAs were polymorphic in number, size, and relative amounts in the eggs of different females. Five to seven different size classes [1.75-4.2 kilobases (kb)] were detected on RNA gel blots. All egg preparations contained variable amounts of 1.8- and 2.25-kb beta-tubulin RNAs, and a few of them contained an additional 2.9-kb beta-tubulin RNA. The total amount of alpha-tubulin RNA did not always parallel that of beta-tubulin RNA. A portion of all of the various alpha- and beta-tubulin RNAs were polyadenylylated. RNase H digestions ruled out the possibility that some of these RNAs represented a single transcript bearing different lengths of 3' poly(A). One class of alpha-tubulin RNAs (2.4-2.65 kb) was reduced to 2 kb by RNase H, suggesting the presence of internal oligo(A) regions. All of the egg beta-tubulin RNAs sedimented as free ribonucleoprotein particles. Only a small portion of the 1.75- to 3.6-kb alpha-tubulin RNAs, but most of the 4.2-kb alpha-tubulin RNA, were found on polysomes before fertilization. In the 30-min embryo, small amounts of each of the various alpha- and beta-tubulin RNAs were recruited onto polysomes. Thus, each of the multiple polymorphic alpha- and beta-tubulin RNAs in the egg represent translationally competent mRNA.

An evaluation of tubulin as a molecular clock

The available sequence data for tubulin indicates that it cannot be used as a molecular clock. Apparent alpha-tubulin mutation rates, for example, vary from 0.16 to 3.8 PAMs per 100 million years depending on which two alpha-tubulins are compared. All animal alpha-tubulin mutation rates seem to be quite low, whereas those of non-animals are relatively high. A similar division is not present amongst the beta-tubulins; their apparent mutation rates, however, vary just as much. For any given tubulin, the largest number of amino acid sequence differences are obtained when comparing it to the tubulins of yeasts. Sequence comparisons with the tubulins of unicellular algae and chelates show far fewer differences. Cytochrome c data, however, show that the ciliates diverged from animals well before the yeasts. This means, therefore, that the average tubulin mutation rates in yeasts and ciliates since the time they shared a common ancestor must be quite different. The high mutation rate of yeast tubulins may possibly reflect the absence of cilia. Structural constraints imposed on tubulin by the large number of interactions with other components of the complex ciliary axoneme probably have a significant effect on its rate of mutation.

Genetics of microtubule systems

In most eucaryotes the tubulin genes comprise small multigene families with approximately equal numbers of genes for alpha- and beta-tubulin, the structural proteins of microtubules. The recent isolation of tubulin mutations in several species is proving to be a powerful tool for examining the structure and function of specific sets of microtubules. In Drosophila melanogaster, genetic analysis of a testis-specific beta-tubulin gene has shown that a single tubulin gene product may fulfill a number of different microtubule functions. In addition to tubulin mutations, mutations in other genes whose products are involved in the regulation or structure of specific microtubule arrays have also been isolated. The combination of analysis of both classes of mutations is beginning to allow a molecular description of the construction and function of three-dimensional cellular structures. In addition, such studies may also shed light on the evolutionary pressures that gave rise to and serve to maintain small families of genes encoding very similar proteins.