Structural variation among human beta-tubulin genes

Tubulins and brain development - The origins of functional specification

The development of the vertebrate central nervous system is reliant on a complex cascade of biological processes that include mitotic division, relocation of migrating neurons, and the extension of dendritic and axonal processes. Each of these cellular events requires the diverse functional repertoire of the microtubule cytoskeleton for the generation of forces, assembly of macromolecular complexes and transport of molecules and organelles. The tubulins are a multi-gene family that encode for the constituents of microtubules, and have been implicated in a spectrum of neurological disorders. Evidence is building that different tubulins tune the functional properties of the microtubule cytoskeleton dependent on the cell type, developmental profile and subcellular localisation. Here we review of the origins of the functional specification of the tubulin gene family in the developing brain at a transcriptional, translational, and post-transcriptional level. We remind the reader that tubulins are not just loading controls for your average Western blot.

The Expression of Tubb2b Undergoes a Developmental Transition in Murine Cortical Neurons

The development of the mammalian brain requires the generation, migration, and differentiation of neurons, cellular processes that are dependent on a dynamic microtubule cytoskeleton. Mutations in tubulin genes, which encode for the structural subunits of microtubules, cause detrimental neurological disorders known as the tubulinopathies. The disease spectra associated with different tubulin genes are overlapping but distinct, an observation believed to reflect functional specification of this multigene family. Perturbation of the β-tubulin TUBB2B is known to cause polymicrogyria, pachygyria, microcephaly, and axon guidance defects. Here we provide a detailed analysis of the expression pattern of its murine homolog Tubb2b. The generation and characterization of BAC-transgenic eGFP reporter mouse lines has revealed that it is highly expressed in progenitors and postmitotic neurons during cortical development. This contrasts with the 8-week-old cortex, in which Tubb2b expression is restricted to macroglia, and expression is almost completely absent in mature neurons. This developmental transition in neurons is mirrored in the adult hippocampus and the cerebellum but is not a universal feature of Tubb2b; its expression persists in a population of postmitotic neurons in the 8-week-old retina. We propose that the dynamic spatial and temporal expression of Tubb2b reflects specific functional requirements of the microtubule cytoskeleton.

Microtubules and neurodevelopmental disease: the movers and the makers

The development of the mammalian cortex requires the generation, migration and differentiation of neurons. Each of these cellular events requires a dynamic microtubule cytoskeleton. Microtubules are required for interkinetic nuclear migration, the separation of chromatids in mitosis, nuclear translocation during migration and the outgrowth of neurites. Their importance is underlined by the finding that mutations in a host of microtubule associated proteins cause detrimental neurological disorders. More recently, the structural subunits of microtubules, the tubulin proteins, have been implicated in a spectrum of human diseases collectively known as the tubulinopathies. This chapter reviews the discovery of microtubules, the role they play in neurodevelopment, and catalogues the tubulin isoforms associated with neurodevelopmental disease. Our focus is on the molecular and cellular mechanisms that underlie the pathology of tubulin-associated diseases. Finally, we reflect on whether different tubulin genes have distinct intrinsic functions.

A transcript map encompassing a susceptibility locus for bipolar affective disorder on chromosome 4q35

Bipolar affective disorder is one of the most common mental illnesses with a population prevalence of approximately 1%. The disorder is genetically complex, with an increasing number of loci being implicated through genetic linkage studies. However, the specific genetic variations and molecules involved in bipolar susceptibility and pathogenesis are yet to be identified. Genetic linkage analysis has identified a bipolar disorder susceptibility locus on chromosome 4q35, and the interval harbouring this susceptibility gene has been narrowed to a size that is amenable to positional cloning. We have used the resources of the Human Genome Project (HGP) and Celera Genomics to identify overlapping sequenced BAC clones and sequence contigs that represent the region implicated by linkage analysis. A combination of bioinformatic tools and laboratory techniques have been applied to annotate this DNA sequence data and establish a comprehensive transcript map that spans approximately 5.5 Mb. This map encompasses the chromosome 4q35 bipolar susceptibility locus, which localises to a "most probable" candidate interval of approximately 2.3 Mb, within a more conservative candidate interval of approximately 5 Mb. Localised within this map are 11 characterised genes and eight novel genes of unknown function, which together provide a collection of candidate transcripts that may be investigated for association with bipolar disorder. Overall, this region was shown to be very gene-poor, with a high incidence of pseudogenes, and redundant and novel repetitive elements. Our analysis of the interval has demonstrated a significant difference in the extent to which the current HGP and Celera sequence data sets represent this region.

mRNA export correlates with activation of transcription in human subgroup C adenovirus-infected cells

To investigate the mechanisms by which viral mRNA species are distinguished from their cellular counterparts for export to the cytoplasm during the late phase of subgroup C adenovirus infection, we have examined the metabolism of several cellular and viral mRNAs in human cells productively infected by adenovirus type 5 (Ad5). Several cellular mRNAs that were refractory to, or could escape from, adenovirus-induced inhibition of export of mRNA from the nucleus have been identified. This group includes Hsp70 mRNAs synthesized upon heat shock of Ad5-infected 293 or HeLa cells during the late phase of infection. However, successful export in Ad5-infected cells is not a specific response to heat shock, for beta-tubulin and interferon-inducible mRNAs were also refractory to virus-induced export inhibition. The export of these cellular mRNAs, like that of viral late mRNAs, required the E1B 55-kDa protein. Export to the cytoplasm during the late phase of Ad5 infection of several cellular mRNAs, including members of the Hsp70 family whose export was inhibited under some, but not other, conditions, indicates that viral mRNA species cannot be selectively exported by virtue of specific sequence or structural features. Cellular and viral late mRNAs that can be exported from the nucleus to the cytoplasm were expressed from genes whose transcription was induced or activated during the late phase of Ad5 infection. Consistent with the possibility that successful export is governed by transcriptional activation in the late phase of adenovirus infection, newly synthesized viral early E1A mRNA was subject to export inhibition during the late phase of infection.

The Pisum sativum TubA1 gene, a member of a small family of alpha-tubulin sequences

alpha- and beta-tubulin proteins are subunits of microtubules, which as primary elements of the plant cytoskeleton play major roles in plant cell division and cell morphogenesis. Several higher-plant alpha- and beta-tubulin gene families have been reported to have at least six to nine members each. Using genomic Southern hybridizations and polymerase chain reaction (PCR) experiments, we have found that the Pisum sativum (garden pea) genome has only four copies of alpha-tubulin sequences and a similar number of beta-tubulin sequences. We have characterized the pea alpha-tubulin gene TubA1. Its nucleotide sequence predicts a 452 amino acid product which is 89-98% identical to those predicted for other plant alpha-tubulins. By S1 nuclease analysis we have located the transcript start site at 102 bases upstream of the ATG. We have also shown that the TubA1 gene is expressed by northern hybridization with a gene-specific probe.

Cloning and sequence of a beta-tubulin cDNA from Pneumocystis carinii: possible implications for drug therapy

This work describes the isolation and characterization of a full-length cDNA clone encoding beta-tubulin from the pathogen Pneumocystis carinii. P. carinii contains a single gene encoding beta-tubulin. The complete sequence of this cDNA has been determined and its inferred amino acid sequence compared with the beta-tubulins from other organisms. This analysis augments the data indicating that P. carinii should be classified as a fungal organism. Further comparisons between the P. carinii beta-tubulin and those of fungal beta-tubulins resistant to benomyl, a beta-tubulin-binding drug, indicate a difference which may be exploited in the development of a new drug therapy for P. carinii pneumonitis. These results suggest that, theoretically, a drug presently administered for treatment of nematode worm infections may be an effective agent against P. carinii, without being toxic to the mammalian host. This possibility is currently being investigated.

Cell-surface galactosyltransferase acts as a modulator of rat and human acinar cell proliferation

Several physiological parameters were examined for inducing acinar cell proliferation and corresponding increased expression of beta 1-4 galactosyltransferase. In this study, dietary changes causing acinar cell proliferation included the following: the introduction of animals to a liquid diet (causing gland atrophy) followed by re-introduction of solid chow, gustatory stimulation provided by the introduction of 0.5% citric acid to animal drinking water, and removal of the submandibular gland with subsequent reliance on the parotid gland for saliva protein and fluid. Alterations in growth factor levels were produced by injecting animals with a chronic (three-day) regimen of either nerve growth factor (NGF) or epidermal growth factor (EGF). In all cases of acinar cell proliferation in vivo, generated by the above treatments, cell-surface galactosyltransferase was detected along with the unique expression of a 4.5-kb proliferation-associated mRNA. Parotid gland proliferation could be blocked in all cases by the injection of the galactosyltransferase specific modifier protein, alpha-lactalbumin. Propranolol, a beta-adrenergic receptor antagonist, blocked proliferation in all cases except EGF treatment. EGF-induced proliferation could, however, be prevented if the animals were treated with monoclonal antibody to EGF receptor or with the galactosyltransferase modifier alpha-lactalbumin. As a comparison, human parotid tissue samples obtained from neoplastic pleomorphic adenomas, muco-epidermoid carcinoma, adenoid cystic carcinoma, and a bulimia patient were analyzed for galactosyltransferase expression by Northern blot of mRNA and plasma membrane isolation. Elevated levels of galactosyltransferase were found in all neoplastic tissue preparations as well as in the bulimia sample. Amylase synthesis was reduced in samples compared with surrounding normal tissue from the same patient. In vitro cell culturing of pleomorphic adenoma cells in the presence of galactosyltransferase modifier alpha-lactalbumin and substrate UDP-galactose inhibited proliferation in a dose-dependent fashion. Southern blot analysis of DNA from neoplastic parotid cells showed an alteration in chromosomal gene structure for the galactosyltransferase activator cDNA from the adenoid cystic carcinoma. These results for induced acinar cell proliferation as well as human neoplastic pathologies suggest a direct role for cell surface beta 1-4 galactosyltransferase in signaling growth. Furthermore, the proliferation-associated activity of galactosyltransferase suggests that it may be considered as a new type of cell growth regulator.

A dominant negative mutation in two proteins created by ectopic expression of an AU-rich 3' untranslated region

We have found that the ectopic expression of a specific gene's 3' untranslated region leads to the dose dependent loss, relative to gene copy number, of that specific mRNA and protein, as well as an associated protein, in a eukaryotic cell line. The loss of these proteins from the eukaryotic cell line also results in specific phenotypic changes in these cells, suggesting that we have created a dominant negative mutant. This gene's 3' untranslated region is known to contain numerous AU sequences, reminiscent of other eukaryotic genes whose expression may be regulated by these sequences. The apparent control of gene expression by a truncated 3' untranslated region sequence provides further evidence supporting the regulatory function of these regions. The resulting decrease in steady-state mRNA levels by the overexpression of a portion of that gene's 3' untranslated region further suggests the possible existence of a factor(s) that may bind to this region, and thus regulate gene function via its mRNA. The use of a gene's 3' untranslated sequence to create a specific dominant negative mutation may also be applicable to other eukaryotic genes whose expression is controlled by similar regulatory sequences.

Cloning of a beta-tubulin gene from Plasmodium falciparum

We describe the isolation and characterization of a gene for beta-tubulin from the malaria parasite, Plasmodium falciparum. This organism appears to contain a single gene encoding beta-tubulin. A single transcript from this gene can be detected in the total RNA of the parasite's asexual blood stages. The complete sequence for the gene has been elucidated. It has two introns, one of which has a position identical to that of a related parasite, Toxoplasma gondii. The gene shows the usual preference for codons with A or T in the third position. The predicted amino acid sequence is compared with that of T. gondii and the human host. Further comparisons between these and fungal sequences of beta-tubulins resistant to benomyl, a drug binding this protein, highlight differences that could be exploited in the development of parasite-specific antitubulin drugs.

Isoprenaline-induced transcription of 4 beta-galactosyltransferase is inhibited by both cycloheximide and actinomycin D in rat parotid acinar cells

A cDNA clone for the Golgi enzyme 4 beta-galactosyltransferase (EC 2.4.1.38) was used to determine the steady-state mRNA content in cultured rat parotid acinar cells. Isoprenaline, a beta-adrenergic-receptor agonist, caused an increase in steady-state amounts of mRNA for 4 beta-galactosyltransferase in cultured acinar cells as well as in specific activity of the enzyme. The amount of 4 beta-galactosyltransferase-specific mRNA was dependent on transcription of the gene, as determined by incubation of cells with the RNA polymerase inhibitor actinomycin D, concomitant with the time of isoprenaline treatment. Transcription of the 4 beta-galactosyltransferase gene also required the active biosynthesis of additional cellular factors, since isoprenaline-induced increases in mRNA amounts were not observed on co-incubation with the protein-synthesis inhibitor cycloheximide.

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.

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.

Identification and functional analysis of beta-tubulin genes by site specific integrative transformation in Aspergillus nidulans

We have cloned two different beta-tubulin sequences from the filamentous fungus Aspergillus nidulans. Each was used in the construction of transforming plasmids that carry the pyr4 gene of Neurospora crassa. We used these plasmids to transform a pyrG-strain of Aspergillus to uridine prototrophy. Both plasmids were shown to integrate site specifically into the homologous chromosomal sequences. We then used transformant strains in genetic crosses to demonstrate that one of the cloned beta-tubulin sequences was the benA beta-tubulin gene, which codes for the beta 1-and beta 2-tubulins. The other cloned beta-tubulin sequence was shown to be the structural gene for beta 3-tubulin by gene disruption and to participate in conidial development. This is the first report of a gene disruption by site specific, integrative recombination in Aspergillus nidulans.

Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

Changes in the expression of the genes encoding alpha-tubulin and a 94,000-dalton protein (p94) specified by a cDNA clone, p4-30, were examined in a differentiated teratocarcinoma-derived parietal endoderm cell line, PYS-2, and an undifferentiated teratocarcinoma stem cell line, F9. Relative to other proteins or mRNA species, the synthesis rate of the alpha-tubulins and of p94, as well as the levels of their corresponding cytoplasmic mRNAs, were lower in PYS-2 than in F9 cells. The decrease was greater for the relative abundance of cytoplasmic alpha-tubulin mRNA than for p94 mRNA. Similarly, induction of differentiation of F9 cells by simultaneous exposure to retinoic acid (RA) and dibutyryl cyclic AMP resulted in reduced relative levels of the cytoplasmic mRNAs for these proteins. The reduction in abundance of the two RNA species was not due to a decrease in growth rate since the differentiated cells, PYS-2, RA-treated F9, and RA plus dibutyryl cyclic AMP-treated F9 cells, grew at a rate similar to that of undifferentiated F9 cells. However, induction of differentiation of F9 cells by treatment with RA alone did not cause down-regulation of the two RNA species. The relative levels of total cellular RNA encoding alpha-tubulin and p94 in PYS-2 cells were also lower than those in F9 cells to an extent comparable to the decrease in the cytoplasmic RNAs. Since the apparent relative rates of RNA transcription were similar in both cell types, we conclude that the reduction in relative levels of the alpha-tubulin and p94 RNAs in the cell depends largely on the relative stability of the two RNAs and not on the relative rates of transcription. The faster disappearance of the two RNA species relative to other cellular RNAs from actinomycin D-treated PYS-2 compared with F9 cells is consistent with this interpretation.

Structure and arrangement of the beta-tubulin genes of Leishmania tropica

We studied the organization and arrangement of the genes encoding beta-tubulin in the protozoan parasite Leishmania tropica and examined the structure and orientation of the beta-tubulin mRNA relative to the gene. There were found to be eight to nine beta-tubulin genes arranged in an array of direct tandem repeat units with a length of 3.8 kilobase pairs, and they were extremely homologous, if not identical, in sequence. These repeat units did not contain the alpha-tubulin genes. The transcribed sequences within the beta-tubulin genes were localized, and the orientation of the major alpha-tubulin mRNA was mapped on the gene by S1 nuclease analysis.

Differential expression of two neural cell-specific beta-tubulin mRNAs during rat brain development

We recently demonstrated that beta-tubulin mRNA expression is regulated during rat brain development. This is manifested by a dramatic decrease in both 1.8- and 2.9-kilobase (kb) mRNAs when extensive neurite elongation is occurring. Coincident with these decreases is the increased production of a 2.5-kb mRNA. (J.F. Bond and S.R. Farmer, Mol. Cell. Biol. 3:1333-1342, 1983). In the present study, we have isolated and characterized three different cDNAs corresponding to beta-tubulin mRNAs (R beta T.1, R beta T.2, and R beta T.3). Hybridization of 3' untranslated region subclones of R beta T.1 and R beta T.2 cDNAs to RNA from a variety of rat tissues and cells revealed that these two cDNAs are neural cell specific. R beta T.1 corresponds to an abundant 1.8-kb mRNA expressed only at early stages of rat brain development. R beta T.2 corresponds to the 2.5-kb mRNA expressed at later stages. These data strongly suggest that there is differential expression of the beta-tubulin multigene family during rat brain development.

Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

Changes in the expression of the genes encoding alpha-tubulin and a 94,000-dalton protein (p94) specified by a cDNA clone, p4-30, were examined in a differentiated teratocarcinoma-derived parietal endoderm cell line, PYS-2, and an undifferentiated teratocarcinoma stem cell line, F9. Relative to other proteins or mRNA species, the synthesis rate of the alpha-tubulins and of p94, as well as the levels of their corresponding cytoplasmic mRNAs, were lower in PYS-2 than in F9 cells. The decrease was greater for the relative abundance of cytoplasmic alpha-tubulin mRNA than for p94 mRNA. Similarly, induction of differentiation of F9 cells by simultaneous exposure to retinoic acid (RA) and dibutyryl cyclic AMP resulted in reduced relative levels of the cytoplasmic mRNAs for these proteins. The reduction in abundance of the two RNA species was not due to a decrease in growth rate since the differentiated cells, PYS-2, RA-treated F9, and RA plus dibutyryl cyclic AMP-treated F9 cells, grew at a rate similar to that of undifferentiated F9 cells. However, induction of differentiation of F9 cells by treatment with RA alone did not cause down-regulation of the two RNA species. The relative levels of total cellular RNA encoding alpha-tubulin and p94 in PYS-2 cells were also lower than those in F9 cells to an extent comparable to the decrease in the cytoplasmic RNAs. Since the apparent relative rates of RNA transcription were similar in both cell types, we conclude that the reduction in relative levels of the alpha-tubulin and p94 RNAs in the cell depends largely on the relative stability of the two RNAs and not on the relative rates of transcription. The faster disappearance of the two RNA species relative to other cellular RNAs from actinomycin D-treated PYS-2 compared with F9 cells is consistent with this interpretation.

Effects of mitotic and tubulin mutations on microtubule architecture in actively growing protoplasts of Aspergillus nidulans

We used immunofluorescent microscopy to characterize microtubule (MT) architecture in wild-type and mutant protoplasts of Aspergillus nidulans at interphase and at mitosis. Because the visualization of MTs by immunofluorescence is technically difficult in intact hyphae of A. nidulans, we developed a method for removing the cell wall under conditions that do not perturb cell physiology, as evidenced by the fact that the resulting protoplasts undergo nuclear division at a normal rate and that cell cycle mutant phenotypes are expressed at restrictive temperature. Interphase cells exhibited an extensive network of cytoplasmic MTs. During mitosis the cytoplasmic MTs mostly disappeared and an intranuclear mitotic spindle appeared. We have previously shown that the benA 33 beta-tubulin mutation causes hyperstabilization of the mitotic spindle, and we have presented additional indirect evidence that suggested that the tubA1 and tubA4 alpha-tubulin mutations destabilize spindle MTs. In this paper, we show that the benA33 mutation increases the stability of cytoplasmic MTs as well as spindle MTs and that the tubA1 and tubA4 mutations destabilize both spindle and cytoplasmic MTs.

Sequence of an expressed human beta-tubulin gene containing ten Alu family members

The complete sequence of a functionally expressed human beta-tubulin gene (5 beta) is presented. The amino acid sequence encoded by this gene constitutes a distinct isotype, differing from a previously described human beta-tubulin sequence at 21 positions throughout the polypeptide chain. The beta-tubulin coding sequence in 5 beta is interrupted by three intervening sequences of 1014, 117 and 4826 nucleotides. The largest of these contains ten members of the Alu family of middle repetitive sequences. Together, these regions account for sixty percent of this intervening sequence. Two of the Alu elements are juxtaposed head to tail, and share the same flanking direct repeat. The ten Alu sequences are substantially divergent, both from each other and from an Alu consensus sequence, and several contain deletions of up to half the entire sequence.

Structure and arrangement of the beta-tubulin genes of Leishmania tropica

We studied the organization and arrangement of the genes encoding beta-tubulin in the protozoan parasite Leishmania tropica and examined the structure and orientation of the beta-tubulin mRNA relative to the gene. There were found to be eight to nine beta-tubulin genes arranged in an array of direct tandem repeat units with a length of 3.8 kilobase pairs, and they were extremely homologous, if not identical, in sequence. These repeat units did not contain the alpha-tubulin genes. The transcribed sequences within the beta-tubulin genes were localized, and the orientation of the major alpha-tubulin mRNA was mapped on the gene by S1 nuclease analysis.

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.

Differential expression of two neural cell-specific beta-tubulin mRNAs during rat brain development

We recently demonstrated that beta-tubulin mRNA expression is regulated during rat brain development. This is manifested by a dramatic decrease in both 1.8- and 2.9-kilobase (kb) mRNAs when extensive neurite elongation is occurring. Coincident with these decreases is the increased production of a 2.5-kb mRNA. (J.F. Bond and S.R. Farmer, Mol. Cell. Biol. 3:1333-1342, 1983). In the present study, we have isolated and characterized three different cDNAs corresponding to beta-tubulin mRNAs (R beta T.1, R beta T.2, and R beta T.3). Hybridization of 3' untranslated region subclones of R beta T.1 and R beta T.2 cDNAs to RNA from a variety of rat tissues and cells revealed that these two cDNAs are neural cell specific. R beta T.1 corresponds to an abundant 1.8-kb mRNA expressed only at early stages of rat brain development. R beta T.2 corresponds to the 2.5-kb mRNA expressed at later stages. These data strongly suggest that there is differential expression of the beta-tubulin multigene family during rat brain development.

Tubulin heterogeneity in the trypanosome Crithidia fasciculata

The interphase cell of Crithidia fasciculata has three discrete tubulin populations: the subpellicular microtubules, the axonemal microtubules, and the nonpolymerized cytoplasmic pool protein. These three tubulin populations were independently and selectively purified, yielding, in each case, microtubule protein capable of self-assembly. All three preparations polymerized to form ribbons and sheets rather than the more usual microtubular structures. Analyses of the tubulin by two-dimensional polyacrylamide gel electrophoresis, isoelectric focusing, and peptide mapping indicated that the beta-tubulin complex remained constant regardless of source but that some heterogeneity was present in the alpha subunit. Cytoplasmic pool alpha tubulins (alpha 1/alpha 2) were the only alpha isotypes in the cytoplasm and also formed most of the alpha tubulin species in the pellicular fraction. Flagellar alpha tubulin (alpha 3) was the sole alpha isotype in the flagella; it appeared in small amounts in the pellicular fraction but was completely absent from the cytoplasm. In vitro translation products from polyadenylated RNA from C. fasciculata were also examined by two-dimensional polyacrylamide gel electrophoresis and possessed a protein corresponding to alpha 1/alpha 2 tubulin but lacked any alpha 3 tubulin. The alpha 3 polypeptide arose from a post-translational modification of a precursor polypeptide not identifiable by two-dimensional polyacrylamide gel electrophoresis as alpha 3. Peptide mapping data indicated that cytoplasmic alpha tubulin is the most likely precursor. These results demonstrate alpha-tubulin heterogeneity in this organism and also how close the relationship between flagellar and cytoskeletal tubulins can be among lower eucaryotes.

Tubulin heterogeneity in the trypanosome Crithidia fasciculata

The interphase cell of Crithidia fasciculata has three discrete tubulin populations: the subpellicular microtubules, the axonemal microtubules, and the nonpolymerized cytoplasmic pool protein. These three tubulin populations were independently and selectively purified, yielding, in each case, microtubule protein capable of self-assembly. All three preparations polymerized to form ribbons and sheets rather than the more usual microtubular structures. Analyses of the tubulin by two-dimensional polyacrylamide gel electrophoresis, isoelectric focusing, and peptide mapping indicated that the beta-tubulin complex remained constant regardless of source but that some heterogeneity was present in the alpha subunit. Cytoplasmic pool alpha tubulins (alpha 1/alpha 2) were the only alpha isotypes in the cytoplasm and also formed most of the alpha tubulin species in the pellicular fraction. Flagellar alpha tubulin (alpha 3) was the sole alpha isotype in the flagella; it appeared in small amounts in the pellicular fraction but was completely absent from the cytoplasm. In vitro translation products from polyadenylated RNA from C. fasciculata were also examined by two-dimensional polyacrylamide gel electrophoresis and possessed a protein corresponding to alpha 1/alpha 2 tubulin but lacked any alpha 3 tubulin. The alpha 3 polypeptide arose from a post-translational modification of a precursor polypeptide not identifiable by two-dimensional polyacrylamide gel electrophoresis as alpha 3. Peptide mapping data indicated that cytoplasmic alpha tubulin is the most likely precursor. These results demonstrate alpha-tubulin heterogeneity in this organism and also how close the relationship between flagellar and cytoskeletal tubulins can be among lower eucaryotes.

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

Evolutionary studies on the tubulin multigene families were initiated by nucleotide sequence analysis of cDNA clones complementary to sea urchin (Lytechinus pictus) testis alpha- and beta-tubulin cDNA clones (p beta 1, p beta 2, p beta e) demonstrated the existence of tubulin mRNA heterogeneity. p beta 2 and p beta 3 contain identical tubulin-coding regions and extremely similar 3' untranslated sequences, including a polyadenylation signal (AAUAAA). However, p beta 2 contains an additional region of 3' untranslated sequence which includes a second polyadenylation signal. These two sequences may be allelic, representing products of alternative transcription termination or processing pathways. p beta 1 and p beta 2 (or p beta 3) cDNAs almost certainly correspond to transcripts of distinct but evolutionarily related genes. Examination of the available coding portions showed that they differ only by a few silent nucleotide substitutions and the deletion/insertion of one codon; most of the differences are clustered within the last 15 3'-end codons. In contrast, their 3' untranslated sequences are considerably divergent. Nucleotide alignment in this region was feasible by considering specific point and segmental mutations, mainly T in equilibrium or formed from C transitions and small deletions/insertions associated with small direct repeats. The sea urchin alpha- and beta-tubulin cDNA and corresponding protein sequences were compared with previously described tubulin cDNA and protein sequences from other organisms. Both alpha and beta tubulins are very conserved proteins, evolving with a rate comparable to that of histones. Analysis of the nucleotide divergence of the coding cDNA regions showed that replacement sites have changed with a rate 20-175 times lower than that of the silent sites. Among the 177 codons compared between the sea urchin testis and chick brain beta-tubulin cDNAs, there are 7 conservative amino acid replacements and the deletion/insertion of two codons. Most of these changes are clustered near the C-terminus. The 161-amino acid portion of chick brain, rat and porcine alpha-tubulin sequences differs by 3 conservative amino acid replacements from the corresponding sea urchin testis alpha-tubulin sequence. The compared interspecies 3' untranslated sequences are very divergent.

Mutations in alpha- and beta-tubulin affect spindle formation in Chinese hamster ovary cells

Two Chinese hamster ovary cell lines with mutated beta-tubulins (Grs-2 and Cmd-4) and one that has a mutation in alpha-tubulin (Tax-1) are temperature sensitive for growth at 40.5 degrees C. To determine the functional defect in these mutant cells at the nonpermissive temperature, they were characterized with respect to cell cycle parameters and microtubule organization and function after relatively short periods at 40.5 degrees C. At the nonpermissive temperature all the mutants had normal appearing cytoplasmic microtubules. Premature chromosome condensation analysis failed to show any discrete step in the interphase cell cycle in which these mutants are arrested. These cells, however, show several defects at the nonpermissive temperature that appear related to the function of microtubules during mitosis. Time-lapse studies showed that mitosis was lengthened in the three mutant lines at 40.5 degrees C as compared with the wild-type cells at this temperature, resulting in a higher proportion of cells in mitosis after temperature shift. There was also a large increase in multinucleated cells in mutant populations after incubation at the nonpermissive temperature. Immunofluorescent studies using a monoclonal anti--alpha-tubulin antibody showed that the mutant cells had a high proportion of abnormal spindles at the nonpermissive temperature. The two altered beta-tubulins and the altered alpha-tubulin all were found to cause a similar phenotype at the high temperature that results in mitotic delay, defective cytokinesis, multinucleation, and ultimately, cell death. We conclude that spindle formation is the limiting microtubule function in these mutant cell lines at the nonpermissive temperature and that these cell lines will be of value for the study of the precise role of tubulin in mammalian spindle formation.

Tandem arrangement of tubulin genes in the protozoan parasite Leishmania enriettii

The arrangement of developmentally regulated alpha- and beta-tubulin genes has been studied in the parasitic protozoan Leishmania enriettii by using Southern blot hybridization analysis. The alpha-tubulin genes occur in a tandem repeat whose monomeric unit may be represented by a 2-kilobase PstI fragment. Similarly, the beta-tubulin genes probably occur in a separate tandem repeat consisting of approximately 4-kilobase units unlinked to the alpha-tubulin repeats.

Identification of two human beta-tubulin isotypes

The sequence of a human beta-tubulin cDNA clone (D beta-1) is described; our data revealed 95.6% homology compared with the sequence of a human beta-tubulin processed pseudogene derived by reverse transcription of a processed mRNA (Wilde et al., Nature [London] 297:83-84, 1982). However, the amino acid sequence encoded by this cDNA showed less homology with pig and chicken beta-tubulin sequences than the latter did to each other, with major divergence within the 15 carboxy-terminal amino acids. On the other hand, an independently isolated, functionally expressed genomic human beta-tubulin sequence (5 beta) possessed a very high degree of homology with chicken and pig beta-tubulins in this region. Thus, human cells appear to contain two distinct beta-tubulin isotypes. Both the intact beta-tubulin cDNA clone and a subclone containing only the 3' untranslated region detected two mRNA species in HeLa cells; these mRNAs were 1.8 and 2.6 kilobases long and were present in about equal amounts. Two independently subcloned probes constructed from the 3' untranslated region of the 5 beta genomic sequence also detected a 2.6-kilobase beta-tubulin mRNA. However, the 3'-untranslated-region probes from the cDNA clone and the genomic sequence did not cross-hybridize. Thus, at least two human beta-tubulin genes, each specifying a distinct isotype, are expressed in HeLa cells, and the 2.6-kilobase mRNA band is a composite of at least two comigrating beta-tubulin mRNAs.

Three organizations of human DNA

Human DNA was denatured, annealed to low C0t, and fractionated by centrifugation to equilibrium on gradients of NaI. Three well-defined zones resulted. The topmost, least dense zone contained satellite DNA. The bottom zone contained molecules bearing infrequent sequences and short repeated DNA. The middle zone contained molecules bearing infrequent sequences, short repeated DNA, and long repeated DNA. The NaI pattern was independent of single-strand DNA chain length from 5 to 30 kilobases. Structural genes were found in the organization bearing long repeats.

Tandem arrangement of tubulin genes in the protozoan parasite Leishmania enriettii

The arrangement of developmentally regulated alpha- and beta-tubulin genes has been studied in the parasitic protozoan Leishmania enriettii by using Southern blot hybridization analysis. The alpha-tubulin genes occur in a tandem repeat whose monomeric unit may be represented by a 2-kilobase PstI fragment. Similarly, the beta-tubulin genes probably occur in a separate tandem repeat consisting of approximately 4-kilobase units unlinked to the alpha-tubulin repeats.

Identification of two human beta-tubulin isotypes

The sequence of a human beta-tubulin cDNA clone (D beta-1) is described; our data revealed 95.6% homology compared with the sequence of a human beta-tubulin processed pseudogene derived by reverse transcription of a processed mRNA (Wilde et al., Nature [London] 297:83-84, 1982). However, the amino acid sequence encoded by this cDNA showed less homology with pig and chicken beta-tubulin sequences than the latter did to each other, with major divergence within the 15 carboxy-terminal amino acids. On the other hand, an independently isolated, functionally expressed genomic human beta-tubulin sequence (5 beta) possessed a very high degree of homology with chicken and pig beta-tubulins in this region. Thus, human cells appear to contain two distinct beta-tubulin isotypes. Both the intact beta-tubulin cDNA clone and a subclone containing only the 3' untranslated region detected two mRNA species in HeLa cells; these mRNAs were 1.8 and 2.6 kilobases long and were present in about equal amounts. Two independently subcloned probes constructed from the 3' untranslated region of the 5 beta genomic sequence also detected a 2.6-kilobase beta-tubulin mRNA. However, the 3'-untranslated-region probes from the cDNA clone and the genomic sequence did not cross-hybridize. Thus, at least two human beta-tubulin genes, each specifying a distinct isotype, are expressed in HeLa cells, and the 2.6-kilobase mRNA band is a composite of at least two comigrating beta-tubulin mRNAs.

The sequences of an expressed rat alpha-tubulin gene and a pseudogene with an inserted repetitive element

The rat genome contains two segments closely related to a rat alpha-tubulin mRNA. Both have been cloned and complete nucleotide sequences are presented. Analysis of the structure and sequence of one of these establishes it as a functional alpha-tubulin gene. The second segment is a processed alpha-tubulin pseudogene. Comparison of this pseudogene to the mRNA and gene coding for alpha-tubulin strongly suggests that a mature mRNA was involved in its origin. Features of the pseudogene and a dispersed repetitive element inserted within it possibly reflect a common RNA-mediated process of insertion.

Relationship between the total size of exons and introns in protein-coding genes of higher eukaryotes

We have attempted to ascertain the correlation between the genetic information content in the exons and the surrounding intron sequences with regard to their spatial arrangement within a gene. A comparison is made of the sizes, taken from recent publications, of exons and introns of approximately equal to 80 different protein-coding chromosomal genes, mostly from higher eukaryotes. The exons of these genes do not show very marked variation in size and can be classified into three major discrete and two minor additional size groups, whereas individual introns vary considerably in size within and between genes. Notwithstanding, the overall length of all introns present within a given gene is a function of the total size, mostly corresponding to the total genetic information content, of the exons. Three cases that violate this exon-size dependency of introns are genes coding for (i) histone H1, feather keratin, and interferons, (ii) tubulin and actin, and (iii) silk fibroin. The exons of these genes are larger than 0.7 kilobase pair in total size and the genes show a strong sequence homogeneity among the repetitious family members or internal repeats of coding sequences within the gene. We propose that conservation of sequences, which is required by the family members, internal repeats, or the entire gene, would actually motivate the removal of introns.

Coordinate regulation of the four tubulin genes of Chlamydomonas reinhardi

During cell division and during the induction of tubulin synthesis that accompanies flagellar regeneration in Chlamydomonas reinhardi, four tubulin mRNAs of discrete molecular sizes are produced. During induction two beta tubulin mRNAs (2.47 kb and 2.34 kb) and two alpha tubulin mRNAs (2.26 kb and 2.13 kb) are synthesized in high abundance and in a closely coordinated fashion. Combined data from restriction enzyme mapping (i.e., Southern analysis) of genomic DNA and of Charon 30 recombinant clones bearing inserts of Chlamydomonas tubulin genes provide direct evidence for four distinct tubulin genes in this organism. Dot-blot analysis of the level of hybridization of a 32p nick-translated beta tubulin cDNA to genomic DNA from gametic cells and to a clone containing the beta 1 tubulin gene indicate that each beta 1 tubulin gene is present in one copy per cell. Additional hybridization experiments employing fragments of cDNA clones which selectively anneal to either the 3' or 5' portions of the two alpha tubulin genes or to one or both of the two beta tubulin genes suggest that each tubulin gene is actively transcribed to give rise to one of the four tubulin mRNAs. These observations further suggest that at most four basic types of tubulin subunits are produced by Chlamydomonas and that the heterogeneity of tubulin subunits reported to exist in the flagellar axoneme must arise as a result of post-translational modification.

Structure of two human alpha-tubulin genes

The ability of a chicken alpha-tubulin cDNA probe to cross-hybridize with human DNA under stringent conditions has been exploited to screen two independently constructed human genomic libraries. Nine clones were isolated, accounting for 60% of the bands observed in a whole genomic Southern blot of human DNA. Two clones were selected for further analysis by restriction mapping, orientation experiments using 3'- or 5'-specific probes, and electron microscopy of heteroduplexes. One clone, 2 alpha, contains an alpha-tubulin-specific region of 5.0 kilobases that includes three intervening sequences. The second clone, 19 alpha, contains an alpha-tubulin-specific region of 5.4 kilobases and has somewhat diverged 5' and 3' ends. Clone 19 alpha has only two intervening sequences that correspond to the first two in clone 2 alpha. However, these intervening sequences differ in size between clones 2 alpha and 19 alpha and show no detectable sequence homology. The sum of the lengths of sequences in either clone that hybridize to the cDNA probe accounts for essentially the entire length of the cDNA molecule.