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

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

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

Copolymerization of two distinct tubulin isotypes during microtubule assembly in vitro

Cells contain multiple tubulin isotypes that are the products of different genes and posttranslational modifications. It has been proposed that tubulin isotypes become segregated into different classes of microtubules each adapted to specific activities and functions. To determine if mixtures of tubulin isotypes segregate into different classes of polymers in vitro, we used immunoelectron microscopy to examine the composition of microtubule copolymers that assembled from mixtures of purified tubulin subunits from chicken brain and erythrocytes, each of which has been shown to exhibit distinct assembly properties in vitro. We observed that (a) the two isotypes coassemble rapidly and efficiently despite the fact that each isotype exhibits its own unique biochemical and assembly properties; (b) at low monomer concentrations the ratio of tubulin isotypes changes along the lengths of elongating copolymers resulting in gradients in immuno-gold labeling; (c) two distinct classes of copolymers each containing a distinct ratio of isotypes assemble simultaneously in the same subunit mixture; and (d) subunits and polymers of different isotypes associate nearly equally well with each other, there being only a slight bias favoring interactions among subunits and polymers of the same isotype. The observations agree with previous studies on the homogeneous distribution of multiple isotypes within cells and suggest that if segregation of isotypes does occur in vivo, it is most likely directed by cell-specific microtubule-associated proteins (MAPs) or specialized intracellular conditions.

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

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

Assembly properties of altered beta-tubulin polypeptides containing disrupted autoregulatory domains

beta-Tubulin synthesis in eucaryotic cells is subject to control by an autoregulatory posttranscriptional mechanism in which the first four amino acids of the beta-tubulin polypeptide act either directly or indirectly to control the stability of beta-tubulin mRNA. To investigate the contribution of this amino-terminal domain to microtubule assembly and dynamics, we introduced a series of deletions encompassing amino acids 2 to 5 of a single mammalian beta-tubulin isotype, M beta 1. Constructs carrying such deletions were inserted into an expression vector, and the ability of the altered polypeptide to coassemble into microtubules was tested by using an anti-M beta 1-specific antibody. We show that the M beta 1 beta-tubulin polypeptide was competent for coassembly into microtubules in transient transfection experiments and in stably transfected cell lines when it lacked either amino acid 2 or amino acids 2 and 3. The capacity of these mutant beta-tubulins to coassemble into polymerized microtubules was only slightly diminished relative to that of unaltered beta-tubulin, and their expression did not influence the viability or growth properties of cell lines carrying these deletions. However, more extensive amino-terminal deletions either severely compromised or abolished the capacity for coassembly. In analogous experiments in which alterations were introduced into the amino-terminal domain of a mammalian alpha-tubulin isotype, M alpha 4, deletion of amino acid 2 did not affect the ability of the altered polypeptide to coassemble, although removal of additional amino-terminal residues essentially abolished the capacity for competent coassembly. The stability of the altered assembly-competent alpha- and beta-tubulin polypeptides was measured in pulse-chase experiments and found to be indistinguishable from the stability of the corresponding unaltered polypeptides. An assembly-competent M alpha 4 polypeptide carrying a deletion encompassing the 12 carboxy-terminal amino acids also had a half-life indistinguishable from that of the wild-type alpha-tubulin molecule. These data suggest that the universally conserved amino terminus of beta-tubulin acts largely in a regulatory role and that the carboxy-terminal domain of alpha-tubulin is not essential for coassembly in mammalian cells in vivo.

Ethoxyformylation of tubulin with [3H]diethyl pyrocarbonate: a reexamination of the mechanism of assembly inhibition

In this study we reexamined the basis for the profound inhibitory effects of low concentrations of diethyl pyrocarbonate (DEP) on tubulin's ability to assemble into microtubules [cf. Lee, Y. C., Houston, L. I., & Himes, R. H. (1976) Biochem. Biophys. Res. Commun. 70, 50-56]. Assembly inhibition at low DEP concentrations can be resolved into two components: a component reversible with hydroxylamine (attributed to monoethoxyformylation of histidyl residues) that contributes approximately 40% of the inhibition and a hydroxylamine-resistant component (attributed to ethoxyformylation of non-histidyl residues) that contributes approximately 60% of the inhibition. Comparisons between the extent of assembly inhibition associated with each component and the degree of residue modification argue for the involvement of a small number of highly reactive residues in the inhibition process. To identify these residues, tubulin was reacted with limiting concentrations of [3H]DEP and subjected to tryptic digestion and HPLC analysis. Only one moderately reactive histidyl residue was detected. This residue (approximately 2-3-fold more reactive than the bulk histidyl residues) eluted in an apparently large, hydrophobic fragment. We failed to detect any non-histidyl residues that were exceptionally reactive to [3H]DEP. However, we did observe that the N-terminal methionyl residues in native protein were ethoxyformylated at rates comparable to that of the bulk histidyl residues. In denatured protein these methionyl residues were ethoxyformylated to a much larger extent (approximately 3-4-fold) than the bulk histidyl residues. We suggest that the N-terminal methionyl residues in tubulin are partly buried or are in a salt-bridge interaction in native protein and that ethoxyformylation of these residues disrupts tubulin structure and interferes with microtubule assembly.

An immunohistochemical characterization of the primitive and maturing neuroepithelial components in the OTT-6050 transplantable mouse teratoma

The neuroepithelial component of the OTT-6050 mouse teratoma has previously been characterized as an experimental system for the study of differentiation and cytologic maturation in embryonal tumours of the human central nervous system. A number of transplantable tumours composed of primitive stem cells and of a neuroepithelial component displaying a spectrum of differentiation were previously produced by centrifugal elutriation of the dissociated OTT-6050 teratoma. These tumours have provided a reproducible cell population that has permitted the study of both the early and later stages of neoplastic neurocytogenesis. The purpose of the present study was to detect, by immunohistochemistry, the earliest stages of neurocytogenesis in these tumours as shown by the expression of neuron-associated microtubule proteins. This was correlated to the appearance and localization of other markers associated with neuronal and glial differentiation. The primitive neuroepithelial structures resembling neural tubes (medulloepithelial rosettes) contained single or small groups of cells which reacted with the monoclonal antibody TUJ1, specific for the neuron-associated class III beta-tubulin isotype. Immature neuroblasts and maturing polar neurons also showed immunoreactivity with TUJ1, whereas reactivity for microtubule-associated protein 2 (MAP2), tau, the 200 kilodalton isoform of neurofilament protein, neuron-specific enolase and synaptophysin was primarily seen in maturing neurons. By comparison, both medulloepithelial and ependymoblastic rosettes, neuroblasts and glial cells were immunopositive with monoclonal antibody TU27, which defines an antigenic site shared by most mammalian beta-tubulin isotypes. Astroglia were reactive with antisera to glial fibrillary acidic and S-100 proteins, but not with monoclonal antibody (MAb) TUJ1, or with MAbs to the other neuron-associated cytoskeletal proteins, MAP2, tau and the 200 kilodalton subunit of neurofilament protein. Our findings suggest that (1) expression of the class III beta-tubulin isotype is an early event during neoplastic neurocytogenesis, (2) this isotype is subsequently preserved in maturing neuronal populations, and (3) it is not present at detectable levels in stem cells or glial cells. The observation that morphologically undifferentiated neuroepithelial cells express a neuron-associated beta-tubulin isotype signifies the value of examining tubulin isotype expression in the characterization of normal and neoplastic neuroepithelial differentiation.

Localization and immunogenicity of tubulin in the filarial nematodes Brugia malayi and B. pahangi

Tubulin was identified in the filarial nematodes Brugia malayi and B. pahangi by several approaches. Initially, a monoclonal antibody (6D8) was selected for its unusual binding to B. malayi microfilariae in indirect immunofluorescence assays: 6D8 showed granular, heterogeneously dispersed fluorescence on fixed parasites but did not bind to unfixed microfilariae. The microfilarial sheath did not bind 6D8, although it did bind fluoresceinated wheatgerm agglutinin. By Western blotting against microfilarial sonicate, 6D8 reacted with a 50,000-55,000 mol. wt protein, and also bound to purified chicken brain beta-tubulin. Additionally, this monoclonal antibody reacted with a recombinant fusion protein expressed by a clone (Bpa-7) originally isolated from an adult B. pahangi cDNA expression library by its reaction with chronic human filariasis serum. This clone encodes a small 40 amino acid C-terminal segment corresponding to residues 409-449 of beta-tubulin, and shows complete amino acid sequence homology with vertebrate beta-tubulin from 409 to 430 but 55% divergence (six amino acid substitutions, four insertions and one deletion) from human and chicken beta-tubulin over positions 431-449 at the C terminus. Antibody to both parasite and vertebrate (chicken) tubulin was found in filarial infection sera, with higher levels of autoreactive antibody apparent in amicrofilaraemic individuals. Immunogold electron microscopy was then used to localize beta-tubulin in B. malayi microfilariae and adult worms. Tubulin was shown not to be exposed on the microfilarial sheath or in the cuticle of either stage, but was found to be abundant in the somatic tissues. In microfilariae, 6D8 bound myofibril structures under the hypodermal layer, and also bound within cell nuclei. In the adult stage, tubulin was associated with muscle blocks, as well as the intestinal brush border and the embryonic uterine microfilariae.

Assembly properties of altered beta-tubulin polypeptides containing disrupted autoregulatory domains

beta-Tubulin synthesis in eucaryotic cells is subject to control by an autoregulatory posttranscriptional mechanism in which the first four amino acids of the beta-tubulin polypeptide act either directly or indirectly to control the stability of beta-tubulin mRNA. To investigate the contribution of this amino-terminal domain to microtubule assembly and dynamics, we introduced a series of deletions encompassing amino acids 2 to 5 of a single mammalian beta-tubulin isotype, M beta 1. Constructs carrying such deletions were inserted into an expression vector, and the ability of the altered polypeptide to coassemble into microtubules was tested by using an anti-M beta 1-specific antibody. We show that the M beta 1 beta-tubulin polypeptide was competent for coassembly into microtubules in transient transfection experiments and in stably transfected cell lines when it lacked either amino acid 2 or amino acids 2 and 3. The capacity of these mutant beta-tubulins to coassemble into polymerized microtubules was only slightly diminished relative to that of unaltered beta-tubulin, and their expression did not influence the viability or growth properties of cell lines carrying these deletions. However, more extensive amino-terminal deletions either severely compromised or abolished the capacity for coassembly. In analogous experiments in which alterations were introduced into the amino-terminal domain of a mammalian alpha-tubulin isotype, M alpha 4, deletion of amino acid 2 did not affect the ability of the altered polypeptide to coassemble, although removal of additional amino-terminal residues essentially abolished the capacity for competent coassembly. The stability of the altered assembly-competent alpha- and beta-tubulin polypeptides was measured in pulse-chase experiments and found to be indistinguishable from the stability of the corresponding unaltered polypeptides. An assembly-competent M alpha 4 polypeptide carrying a deletion encompassing the 12 carboxy-terminal amino acids also had a half-life indistinguishable from that of the wild-type alpha-tubulin molecule. These data suggest that the universally conserved amino terminus of beta-tubulin acts largely in a regulatory role and that the carboxy-terminal domain of alpha-tubulin is not essential for coassembly in mammalian cells in vivo.

Differential utilization of beta-tubulin isotypes in differentiating neurites

beta-Tubulin is encoded in vertebrate genomes by a family of six to seven functional genes that produce six different polypeptide isotypes. We now document that although rat PC-12 cells express five of these isotypes, only two (classes II and III) accumulate significantly as a consequence of nerve growth factor-stimulated neurite outgrowth. In contrast to previous efforts that have failed to detect in vivo distinctions among different beta-tubulin isotypes, we demonstrate using immunoblotting with isotype-specific antibodies that three beta-tubulin polypeptides (classes I, II, and IV) are used preferentially for assembly of neurite microtubules (with approximately 70% of types I and II assembled but only approximately 50% of type III in polymer). Immunofluorescence localization shows that an additional isotype (V) is partially excluded from neurites. Distinctions in in vivo localization of the neuron-specific, class III isotype have also been directly observed using immunofluorescence and immunogold electron microscopy. The sum of these efforts documents that some in vivo functional differences between tubulin isotypes do exist.

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

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

Inhibition of immunoglobulin gene rearrangement by the expression of a lambda 2 transgene

The rearrangement of Ig genes is known to be regulated by the production of H and kappa L chains. To determine whether lambda L chains have a similar effect, transgenic mice were produced with a lambda 2 gene. It was necessary to include the H chain enhancer, since a lambda gene without the added enhancer did not result in transgene expression. The lambda 2 transgene with the H enhancer was expressed in lymphoid cells only. The majority of the B cells of newborn transgenic mice produced lambda, whereas kappa + cells were reduced. Concomitantly, serum levels of kappa and kappa mRNA were diminished. By 2 wk after birth the proportion of kappa-expressing cells was dramatically increased. Adults had reduced proportions of B cells that produced lambda only, but the levels of lambda were still higher than in normal littermates. Also, kappa + cells were still lower than in normal mice. Analysis of hybridomas revealed that reduction of kappa gene rearrangement was the basis for the decreased frequency of kappa + cells. Furthermore, many cells also contained an unrearranged H chain allele. It was concluded that feedback inhibition by the lambda 2 together with endogenous H protein may have inhibited recombinase activity in early pre-B cells, leading to inhibition of both H chain and kappa gene rearrangement. Thus, lambda 2 can replace kappa in a feedback complex. The levels of serum lambda 1 and, to a lesser degree, of spleen lambda 1 mRNA were reduced in the lambda 2 transgenic mice. However, the proportion of hybridomas with endogenous lambda gene rearrangement was at least as high as in normal mice. It was therefore concluded that the suppression of functional lambda 1 may be a consequence of decreased selection of endogenous lambda-producing cells because of the excess of transgenic lambda. The escape of kappa-producing cells from feedback inhibition may be the result of several mechanisms that operate to varying degrees, among them: (a) kappa rearrangement during a period in which the recombinase is still active after appearance of a lambda 2/mu stop signal; (b) a B cell lineage that is not feedback inhibited at the pre-B cell stage; (c) subthreshold levels of transgenic lambda 2 in some pre-B cells; and (d) loss of the lambda 2 transgenes in rare pre-B cells.

Co-localization of elements required for phorbol ester stimulation and glucocorticoid repression of proliferin gene expression

Proliferin (PLF) gene expression provides a model of growth-related transcriptional activation in mouse cells. Transcription from a cloned PLF promoter is inducible by phorbol esters, and this induction involves a region of 31 bp that includes an AP-1 site and a cluster of sites similar to the simian virus 40 (SV40) SphI element. The mutation of either the AP-1 or the SphI sites abolishes phorbol ester induction, and the transfer of this 31-bp sequence to a site upstream of a minimal promoter is sufficient to confer phorbol-ester responsiveness. In contrast to phorbol esters, glucocorticoids repress PLF transcription, which results in a reduced accumulation of PLF mRNA in serum-stimulated cells. Repression is dependent on the glucocorticoid receptor, which binds to the PLF promoter in a region that includes the AP-1 site, and the 31-bp phorbol ester 12-O-tetra decanoylphorbol-13-acetate (TPA)-inducible region is sufficient to mediate glucocorticoid repression. In addition, extracts from glucocorticoid-treated and untreated mouse cells are found to differ in the nature of the protein complexes that interact with the AP-1 site.

Neuroblastic differentiation potential of the human retinoblastoma cell lines Y-79 and WERI-Rb1 maintained in an organ culture system. An immunohistochemical, electron microscopic, and biochemical study

The differentiation potential of the human retinoblastoma cell lines Y-79 and WERI-Rb1 was evaluated in vitro for up to 120 days in a matrix system and in rotary suspension for 30 days. Matrix cultures were grown with 10% fetal calf serum (FCS), with and without differentiation-promoting agents. The latter were applied for a total of 5-45 days (usually 30 days) and included 7S nerve growth factor, dibutyryl cyclic AMP, sodium butyrate, retinoic acid, hydrocortisone, and ascorbic acid. Fully defined, serum-free medium and medium containing 5 or 15% FCS were also used for matrix cultures, and medium with 5 or 10% FCS for suspension cultures. By immunoperoxidase (performed on matrix cultures, both untreated and treated for 30 days with differentiation-promoting agents), the cells of both lines were positive for neuron-specific enolase (NSE), microtubule-associated protein 2 (MAP2), class III beta-tubulin (human h beta 4) isotype, and synaptophysin. In addition, the WERI-Rb1 cells expressed 200 kd neurofilament protein (NFP-H) and retinal S-antigen. Both lines were invariably negative for glial fibrillary acidic (GFA) protein, myelin-associated glycoprotein, myelin basic protein, the epitope recognized by the Leu-7 monoclonal antibody, opsin, and hydroxy-indole-O-methyltransferase. In the Y-79 line the presence of NSE and the absence of NF proteins-H, -M and -L, of GFA protein, and of retinal S-antigen were confirmed biochemically. No differentiated features were found by electron microscopy in either line. Thus, in the matrix system employed, both lines exhibited solely a potential for neuroblastic differentiation, which was more advanced in the WERI-Rb1 line, as reflected by the antigenic expression of NFP-H and of retinal S-antigen.

At least two mutant alleles of ornithine delta-aminotransferase cause gyrate atrophy of the choroid and retina in Finns

Gyrate atrophy of the choroid and retina (GA) is an inherited chorioretinal degeneration caused by deficiency of ornithine delta-aminotransferase (OAT; L-ornithine: 2-oxo-acid aminotransferase; EC 2.6.1.13). GA is one of the "Finnish genetic diseases," a group of several rare monogenic disorders that occur with increased frequency in the Finnish population. Using a combination of RNase A protection, genomic cloning, and polymerase chain reaction amplification of genomic DNA, we found one of two missense mutant OAT alleles to be present in each of 16 Finnish GA pedigrees. The first mutation R180T, in which arginine-180 is replaced by threonine, was present in homozygous form in patients from two pedigrees. The second mutation L402P, in which leucine-402 is replaced by proline, was present in homozygous form in patients from 14 pedigrees. Neither mutation was present in 19 Finnish controls. L402P was not present in 18 non-Finnish GA patients but R180T was found in an American GA patient. We constructed full-length mutant cDNAs by amplifying patient cDNA with the polymerase chain reaction and cloning a restriction fragment containing the mutation into an otherwise normal human OAT cDNA. These mutant cDNAs were then expressed in CHO-K1 cells, which lack endogenous OAT. Both R180T and L402P inactivate OAT. These results show molecular heterogeneity in GA alleles even in the Finnish population.

Gene expression in the embryonic nervous system of Xenopus laevis

Development of the nervous system in the amphibian embryo is initiated during gastrulation by an inductive interaction between chordamesoderm and dorsal ectoderm. The induced ectoderm forms the neural plate while uninduced ectoderm generates epidermis. We screened for genes activated during gastrulation and expressed specifically in the nervous system of Xenopus laevis in the expectation that clones representing such genes will constitute useful markers for the study of early neurogenesis. Probes were prepared from adult brain RNA by subtraction with RNA from ovary and from different combinations of adult kidney, muscle, and skin; cDNA libraries prepared from early to late neurula embryo RNA were screened with these probes. Six clones were chosen for further study. Three of these clones are not represented in the maternal RNA population but are activated at the late gastrula stage; the other three increase from a maternal base. Expression of five of the genes is restricted to the neural plate during embryogenesis, and all six are restricted to the central nervous system in premetamorphic tadpoles and adults. One of the clones encodes an apparently neurospecific isoform of beta-tubulin; the identity of the other clones is unknown. Expression of all six genes is suppressed in axis-deficient embryos that lack dorsal structures including the brain.

Diversity among beta-tubulins: a carboxy-terminal domain of yeast beta-tubulin is not essential in vivo

Sequences of genes for beta-tubulins from many different organisms demonstrate that they encode highly conserved proteins but that these proteins diverge considerably at their carboxyl termini. The patterns of interspecies conservation of this diversity suggest that it may have functional significance. We have taken advantage of the properties of Saccharomyces cerevisiae to test this hypothesis in vivo. The sole beta-tubulin gene of this species is one of the most divergent of all beta-tubulins and encodes 12 amino acids which extend past the end of most other beta-tubulin molecules. We have constructed strains in which the only beta-tubulin gene is an allele lacking these 12 codons. We show here that this carboxy-terminal extension is not essential. The absence of these 12 amino acids had no effect on a number of microtubule-dependent functions, such as mitotic and meiotic division and mating. It did confer dominant supersensitivity to a microtubule-depolymerizing drug.

Analysis of tubulin proteins and peptides in neuronal and non-neuronal tissues using immobilized pH gradients

The accurate identification of the individual protein products of multi-gene families is essential to the interpretation of data from a wide range of experimental approaches including molecular biology, protein chemistry, and cell biology. We have adapted immobilized pH gradient isoelectric focusing to provide high resolution of tubulin proteins. Here we use these techniques to investigate the heterogeneity of tubulin in several neuronal and non-neuronal tissues to provide an accurate evaluation of isotubulin composition. Of the ten sources examined, the greatest number of isotubulins was found in whole adult brain. Tubulin isolated from either neonate human or rat brain consists predominantly of the more basic alpha and beta isotubulins found in adult brain. Cerebrum, cerebellum, medulla and caudate nucleus all contain the same large number of isotubulins as in whole brain, but in varying proportions. Liver, kidney and spleen isotubulin populations are all similar to each other and consist of a simpler distribution than any neuronal tissue examined. The majority of the tubulin protein in these non-neuronal tissues is composed of only the most basic alpha tubulins and intermediately-charged beta tubulins. No isotubulins were identified that were unique to these three non-neuronal tissues. Tubulin from neuroblastoma cells has an isotubulin distribution grossly similar to non-neuronal sources but additionally contains two basic beta isotubulins found in adult brain that are absent from non-neuronal tissues.

Diversity among beta-tubulins: a carboxy-terminal domain of yeast beta-tubulin is not essential in vivo

Sequences of genes for beta-tubulins from many different organisms demonstrate that they encode highly conserved proteins but that these proteins diverge considerably at their carboxyl termini. The patterns of interspecies conservation of this diversity suggest that it may have functional significance. We have taken advantage of the properties of Saccharomyces cerevisiae to test this hypothesis in vivo. The sole beta-tubulin gene of this species is one of the most divergent of all beta-tubulins and encodes 12 amino acids which extend past the end of most other beta-tubulin molecules. We have constructed strains in which the only beta-tubulin gene is an allele lacking these 12 codons. We show here that this carboxy-terminal extension is not essential. The absence of these 12 amino acids had no effect on a number of microtubule-dependent functions, such as mitotic and meiotic division and mating. It did confer dominant supersensitivity to a microtubule-depolymerizing drug.

The α1-tubulin gene of Arabidopsis thaliana: primary structure and preferential expression in flowers

The primary structure of the α1-tubulin gene of Arabidopsis thaliana was determined and the 5' and 3' ends of its transcript were identified by S1 nuclease mapping experiments. The information obtained was used to (i) predict the amino acid sequence of the α1-tubulin, (ii) deduce the positions of introns within the α1-tubulin gene, and (iii) construct 3' noncoding gene-specific hybridization probes with which to study the pattern of α1-tubulin transcript accumulation in different tissues and at different stages of development. The predicted amino acid sequence of the α1-tubulin has 92% identity with the predicted product of the previously characterized A. thaliana α3-tubulin gene. The coding sequence of the α1-tubulin gene is interrupted by four introns located at positions identical to those of the four introns in the α3 gene. RNA blot hybridization studies carried out with an α1-tubulin gene-specific probe showed that the α1 gene transcript accumulates primarily in flowers, with little transcript present in RNA isolated from roots or leaves. In order to investigate the pattern of α-tubulin gene expression in developing flowers, RNA was isolated from flowers at five different stages of development: flower buds, unopened flowers with pollen, open flowers, flowers with elongating carpels, and green seed pods. RNA blot hybridizations performed with 3' noncoding gene-specific probes showed that the α3 tubulin gene transcript is present in flowers at all stages of development, whereas the α1-tubulin gene transcript could only be detected in RNA from unopened flowers with pollen, open flowers, and flowers with elongating carpels.

Effect of specific proteolytic cleavages on tubulin polymer formation

The capacity for self-polymerization and shape of the tubulin polymers assembled after digestion with trypsin, Pronase, chymotrypsin, subtilisin, Staphylococcus aureus proteinase V8 and proteinase K were investigated. Digestion with trypsin, Pronase or chymotrypsin resulted in a decrease in the ability of tubulin for self-assembly, whereas limited proteolysis with subtilisin, S. aureus proteinase V8 or proteinase K resulted in an increase in such ability. The shape of the assembled polymers varied from typical microtubules (after the treatment with trypsin or Pronase) to sheets (after the treatment with chymotrypsin) and from hooked microtubules with a constant polarity (after the treatment with subtilisin) to the disappearance of a defined polarity of such polymers (after the treatment with S. aureus V8 proteinase or proteinase K). These results indicate that the tubulin C-terminal regions are involved in the regulation of microtubule polymerization, shape, directional growth and lateral interactions between tubulin protofilaments.

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

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

Autoregulated changes in stability of polyribosome-bound beta-tubulin mRNAs are specified by the first 13 translated nucleotides

The expression of tubulin polypeptides in animal cells is controlled by an autoregulatory mechanism whereby increases in the tubulin subunit concentration result in rapid and specific degradation of tubulin mRNAs. We have now determined that the sequences that are necessary and sufficient to specify mouse beta-tubulin mRNAs as substrates for this autoregulated instability reside within the first 13 translated nucleotides (which encode the first four beta-tubulin amino acids Met-Arg-Glu-Ile). This domain has been functionally conserved throughout evolution, inasmuch as sequences isolated from the analogous region of human, chicken, and yeast beta-tubulin mRNAs also confer autoregulation. Further, for an RNA to be a substrate for regulation, not only must it carry the 13-nucleotide coding sequence, but it must also be ribosome bound and its translation must proceed 3' to codon 41.

Generation of antisera that discriminate among mammalian alpha-tubulins: introduction of specialized isotypes into cultured cells results in their coassembly without disruption of normal microtubule function

To assay the functional significance of the multiple but closely related alpha-tubulin polypeptides that are expressed in mammalian cells, we generated three specific immune sera, each of which uniquely recognizes a distinct alpha-tubulin isotype. All three isotypes are expressed in a tissue-restricted manner: one (M alpha 3/7) only in mature testis, one (M alpha 4) mainly in muscle and brain, and the third (M alpha 6) in several tissues at a very low level. A fourth specific antiserum was also generated that distinguishes between the tyrosinated and nontyrosinated form of a single alpha-tubulin isotype. Because individual tubulin isotypes cannot be purified biochemically, these sera were raised using cloned fusion proteins purified from host Escherichia coli cells. To suppress the immune response to shared epitopes, animals were first rendered tolerant to fusion proteins encoding all but one of the known mammalian alpha-tubulin isotypes. Subsequent challenge with the remaining fusion protein then resulted in the elicitation of an immune response to unique epitopes. Three criteria were used to establish the specificity of the resulting sera: (a) their ability to discriminate among cloned fusion proteins representing all the known mammalian alpha-tubulin isotypes; (b) their ability to uniquely detect alpha-tubulin in whole extracts of tissues; and (c) their capacity to stain microtubules in fixed preparations of cells transfected with sequences encoding the corresponding isotype. The transfection experiments served to demonstrate (a) the coassembly of M alpha 3/7, M alpha 4, and M alpha 6 into both interphase and spindle microtubules in HeLa cells and NIH 3T3 cells, and (b) that the M alpha 4 isotype, which is unique among mammalian alpha-tubulins in that it lacks an encoded carboxy-terminal tyrosine residue, behaves like other alpha-tubulin isotypes with respect to the cycle of tyrosination/detyrosination that occurs in most cultured cells.

Neurofilament and tubulin expression recapitulates the developmental program during axonal regeneration: induction of a specific beta-tubulin isotype

We examined the differential expression of genes encoding three beta-tubulin isotypes (classes I, II, and IV) and the 68-kDa neurofilament protein (NF68) in rat sensory neurons during development, maturation, and axonal regeneration. Expression of the specific beta-tubulin gene encoding the class II isotype was induced to high levels during development and axonal regeneration, whereas the expression of genes encoding the two other isotypes (classes I and IV) remained comparable to mature levels. Conversely, expression of the NF68 gene was relatively low during development and regeneration. Thus, the developmental program for cytoskeletal gene expression is recapitulated during axonal regeneration. The high level of class II beta-tubulin expression found in developing and regenerating neurons occurs during the longitudinal growth of axons. In contrast, induction of NF68 gene expression is associated with the radial growth of axons in maturing neurons.

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

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

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

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

Expression of a human alpha-tubulin: properties of the isolated subunit

We examined the in vitro expression and biochemical properties of the isolated alpha subunit of tubulin both in rabbit reticulocyte lysates and in Escherichia coli extracts. Both systems produce soluble, full-length human alpha-tubulin polypeptide. When alpha-tubulin mRNA is translated in rabbit reticulocyte lysates, the isolated alpha subunit is fully functional as assayed by coassembly with bovine brain tubulin using temperature-dependent or taxol/salt assembly procedures. The conformation of the isolated alpha subunit was probed by limited proteolytic digestion with chymotrypsin and by reductive methylation. Limited proteolysis studies indicated that the "monomeric" alpha subunit is highly susceptible to chymotrypsin digestion and becomes resistant to chymotrypsin cleavage following incorporation into the heterodimer. Reductive methylation indicated that the unassociated alpha subunit has a highly reactive lysyl residue essential for microtubule assembly similar to that observed in the heterodimer. In contrast, alpha-tubulin expressed in E. coli lysates was incapable of coassemblying with bovine brain tubulin. Differences in assembly competence of the two alpha-tubulin products appear to be related to formylation of the N-terminal methionine in the procaryotic synthesized subunit. These findings suggest that the amino-terminal methionine of alpha-tubulin plays an essential role in the isolated subunit and/or in the heterodimer, a hypothesis supported by chemical reactivity studies [Sherman, G., Rosenberry, T.L., & Sternlicht, H. (1983) J. Biol. Chem. 258, 2148-2156] which imply that this residue is in a salt-bridge interaction in the dimer.

Autoregulated changes in stability of polyribosome-bound beta-tubulin mRNAs are specified by the first 13 translated nucleotides

The expression of tubulin polypeptides in animal cells is controlled by an autoregulatory mechanism whereby increases in the tubulin subunit concentration result in rapid and specific degradation of tubulin mRNAs. We have now determined that the sequences that are necessary and sufficient to specify mouse beta-tubulin mRNAs as substrates for this autoregulated instability reside within the first 13 translated nucleotides (which encode the first four beta-tubulin amino acids Met-Arg-Glu-Ile). This domain has been functionally conserved throughout evolution, inasmuch as sequences isolated from the analogous region of human, chicken, and yeast beta-tubulin mRNAs also confer autoregulation. Further, for an RNA to be a substrate for regulation, not only must it carry the 13-nucleotide coding sequence, but it must also be ribosome bound and its translation must proceed 3' to codon 41.

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.

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

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

Subcellular sequestration of an antigenically unique beta-tubulin

Tubulin from Trypanosoma brucei was characterized by Western blotting using well defined monoclonal antibodies reacting with alpha- or beta-tubulin and a new monoclonal antibody, 1B41, raised against a microtubule-enriched fraction of T. brucei, which specifically reacts with the beta-subunit of tubulin from either T. brucei or rat brain. This antibody has been used to examine the subcellular distribution of the corresponding antigen in T. brucei by indirect immunofluorescence. The epitope recognized by 1B41 is restricted to a thin line extending from the basal body region to the anterior end of the cell body. To determine the relationship between the immunoreactive zone and the flagellum, double-label immunofluorescence was performed in both interphase and mitotic cells with 1B41 and a flagellar marker, the monoclonal antibody 5E9, specific for the paraflagellar rod polypeptides of trypanosomes. These experiments revealed that the immunoreactive tubulin was contained in a part of the subpellicular cytoskeleton that remained in a constant spatial correspondence with the flagellum throughout the cell division cycle. The beta-tubulin recognized by 1B41 may be segregated into the microtubular structures associated with a cisterna of the endoplasmic reticulum forming the subflagellar microtubule quartet (SFMQ). These results suggest that the presence of an antigenically unique beta-tubulin defines a subpopulation of microtubules possessing specific dynamic properties that may be involved in the morphogenesis of daughter cells during the division of T. brucei.

The expression of beta 1 and beta 3 tubulin genes of Drosophila melanogaster is spatially regulated during embryogenesis

In Drosophila beta tubulins are encoded by a small gene family and the four members of this family are differentially expressed. mRNAs transcribed from two of these genes, namely the beta 1 and beta 3 tubulin genes, are abundant during embryogenesis. While the beta 1 tubulin gene is constitutively expressed during development, beta 3 mRNA is restricted to two distinct phases: mid embryogenesis and metamorphosis. The transcription initiation sites are identical in both these stages and comparison of presumptive promoter regions reveals no extensive homologies between the genes. In situ localization shows beta 1 tubulin mRNA to be maternally expressed in the nurse cells of the egg chambers and evenly distributed during early embryogenesis. In contrast, during later stages of embryogenesis beta 1 tubulin transcripts are predominantly expressed in neural derivatives. The beta 3 tubulin gene expression is also spatially regulated, beta 3 mRNA being restricted to the mesoderm.

Coordinate and selective beta-tubulin gene expression associated with cilium formation in sea urchin embryos

beta-Tubulin mRNAs associated with cilium formation in Strongylocentrotus purpurpatus sea urchin embryos are expressed selectively from a multiple gene family. The accumulations of three beta-tubulin mRNAs (beta 1, beta 2, and beta 3) are temporally coordinated with ciliogenesis during blastula development and with the regeneration of cilia after their amputation. In contrast, another beta-tubulin mRNA, beta 4, is not induced in either case. The zinc-animalized embryo with its exaggerated blastula phenotype forms longer cilia through a protracted period of ciliogenesis, in which the beta-tubulin mRNAs, principally beta 1, accumulate to higher than normal levels. The rate of beta-tubulin transcription per nucleus in the animalized embryo is greater than that of the normal embryo and is not changed through deciliation, although the tubulin mRNAs accumulate to higher levels. However, deciliation raises the beta-tubulin transcription rate in the normal embryo to that in the animalized embryo. Thus, the induction of beta-tubulin mRNA by cilium amputation is regulated transcriptionally in the normal embryo, but post-transcriptionally in the zinc-animalized embryos. Moreover, the beta-tubulin genes that are expressed in association with cilium formation appear to be induced selectively within the framework of ectodermal cell-type specificity.

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

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

Testis-specific beta 2 tubulins are identical in Drosophila melanogaster and D. hydei but differ from the ubiquitous beta 1 tubulin

In Drosophila as in many organisms beta tubulins are encoded by a gene family. We have determined the complete nucleotide sequences coding for the beta 1 and beta 2 tubulins of Drosophila melanogaster and the beta 2 tubulin of D. hydei, and found these insect beta tubulins to be highly conserved and like beta tubulins of other organisms. This is discussed with reference to the possible functional domains of these proteins. The beta 1 tubulin gene of Drosophila is constitutively expressed, whereas the beta 2 tubulin is expressed specifically in the testes. In D. melanogaster the amino acid sequences of these proteins are 95% homologous, differing at only 25 positions. In the testes the beta 2 tubulin participates in different microtubules as shown by genetic analysis (Kemphues et al. 1982). Interestingly, all of the amino acids characteristic of the testis-specific beta 2 tubulin are also present in the corresponding gene of D. hydei. Of special interest is the high degree of conservation of the carboxy-terminal domain in these functionally equivalent beta tubulins.

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

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

Tubulin phosphorylation by casein kinase II is similar to that found in vivo

Purified brain tubulin subjected to an exhaustive phosphatase treatment can be rephosphorylated by casein kinase II. This phosphorylation takes place mainly on a serine residue, which has been located at the carboxy-terminal domain of the beta-subunit. Interestingly, tubulin phosphorylated by casein kinase II retains its ability to polymerize in accordance with descriptions by other authors of in vivo phosphorylated tubulin. Moreover, the V8 phosphopeptide patterns of both tubulin phosphorylated in vitro by casein kinase II and tubulin phosphorylated in vivo in N2A cells are quite similar, and different from that of tubulin phosphorylated in vitro by Ca/calmodulin-dependent kinase II. On the other hand, we have found an endogenous casein kinase II-like activity in purified brain microtubule protein that uses GTP and ATP as phosphate donors, is inhibited by heparin, and phosphorylates phosphatase-treated tubulin. Thus it appears that a casein kinase II-like activity should be considered a candidate for the observed phosphorylation of beta-tubulin in vivo in brain or neuroblastoma cells.

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

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

Characterization of the alpha-tubulin gene family of Arabidopsis thaliana

The genome of Arabidopsis thaliana (Linnaeus) Heynhold was shown to contain an alpha-tubulin gene family consisting of at least four genes and/or pseudogenes. The primary structure of a transcribed alpha-tubulin gene was determined. A comparison of the predicted amino acid sequence of the A. thaliana alpha-tubulin with the predicted amino acid sequences of alpha-tubulins of Chlamydomonas reinhardtii, Stylonychia lemnae, and Homo spaiens reveals a high degree of homology; 90%, 87%, and 83% identity, respectively. Thus, a plant alpha-tubulin exhibits a high degree of homology to the alpha-tubulins of protists and animals. The coding sequence of the A. thaliana alpha-tubulin gene is interrupted by four introns, which occur at positions different from those of the less numerous introns of C. reinhardtii and rat alpha-tubulin genes. S1 nuclease mapping data showed that transcription is initiated 99 +/- 1 base pairs upstream from the translation initiation codon. Both 5' and 3' noncoding gene-specific probes were used to examine the expression of the alpha-tubulin gene in leaves, roots, and flowers by hybridization to total RNA isolated from these tissues. The results showed that the alpha-tubulin gene was transcribed in all three tissues.

Kinetics of association and dissociation of colchicine-tubulin complex from brain and renal tubulin. Evidence for the existence of multiple isotypes of tubulin in brain with differential affinity to colchicine

The kinetics of colchicine binding to bovine brain tubulin have been reported to be biphasic under pseudo first order conditions [(1978) Biochemistry 17, 4466-4472]. Unlike brain tubulin, the kinetics of colchicine binding to bovine renal tubulin are monophasic. The apparent on-rate constant for the binding of colchicine to renal tubulin is found to be very close to that of the faster binding component in brain tubulin. Similarly, the dissociation of colchicine-tubulin complex in the presence of iodide is biphasic for brain tubulin but monophasic for renal tubulin. Since brain and renal tubulin apparently differ in beta-tubulin, our results suggest that the biphasic nature of the kinetics for bovine brain tubulin could possibly originate from the existence of multiple isotypes of tubulin differing in drug binding affinity.

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

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