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

Overexpression of class I, II or IVb beta-tubulin isotypes in CHO cells is insufficient to confer resistance to paclitaxel

Recent studies have suggested a correlation between increased expression of specific beta-tubulin isotypes and paclitaxel resistance in drug-selected cell lines. In an attempt to establish a causal link, we have transfected Chinese hamster ovary cells with cDNAs encoding epitope-tagged class I, II, and IVb beta-tubulins, as well as a class I beta-tubulin with a mutation previously characterized in a paclitaxel resistant mutant. To eliminate possible toxicity that might be associated with overexpression of non-native tubulin, each of the cDNAs was placed under the control of a tetracycline regulated promoter. All transfected cDNAs produced assembly competent tubulin whose synthesis could be turned off or on by the presence or absence of tetracycline. Production of betaI, betaII, or betaIVb tubulin had no effect on the sensitivity of the cells to paclitaxel, but production of the mutant betaI-tubulin conferred clear resistance to the drug. We conclude from these experiments that simple overexpression of class I, II, or IVb isoforms of beta-tubulin is insufficient to confer resistance to paclitaxel.

Resistance to antimitotic agents as genetic probes of microtubule structure and function

Much of our knowledge about microtubules has come from detailed morphological, biochemical, and cell biological studies. As more is learned about these organelles, questions regarding the in vivo regulation of their expression and function become increasingly important. Genetics provides an approach to address these more subtle questions in the living cell. Mammalian mutants with microtubule alterations have been isolated using selections for resistance to the cytotoxic effects of a number of antimitotic drugs. A subset of these mutants have clearly defined alterations in alpha- or in beta-tubulin, and these have been used to explore the mechanisms by which mammalian cells acquire resistance to this class of drugs. In addition, the mutants are providing valuable insights into how tubulin expression is regulated, into what factors determine the extent of microtubule assembly in living cells, into the domains of tubulin that are involved in assembly, and into the role of microtubules in essential cellular processes.

Nucleotide sequences of three different isoforms of beta-tubulin cDNA from Chinese hamster ovary cells

The complete cDNA sequences of two clones encoding beta-tubulin isotypes and the partial sequence of a third isoform from Chinese hamster ovary cells have been determined. The deduced amino acid sequences of the three isoforms show extensive homology to each other as well as with other alpha and beta-tubulin sequences from various species. These results provide evidence for the expression of three different isoforms of beta-tubulin in Chinese hamster ovary cells.

Recognition of specific Physarum alpha-tubulin isotypes by a monoclonal antibody. Sequence heterogeneity around the acetylation site at lysine 40

The monoclonal antibody 6-11B-1 recognises specifically the acetylated form of alpha-tubulin. The acetylation event occurs on a unique lysine residue, lysine 40. Using 6-11B-1, acetylated alpha-tubulin was detected in myxamoebae but not plasmodia of Physarum polycephalum. Following chemical acetylation plasmodial alpha-tubulin was detected by 6-11B-1. The monoclonal antibody KMP-1 recognises certain Physarum alpha-tubulin isotypes but only in non-acetylated form. Whilst recognising all the non-acetylated fraction of myxamoebal alpha-tubulin only a proportion of plasmodial alpha-tubulin was recognised by KMP-1. Peptides were synthesised corresponding to the acetylation domains (containing lysine 40) of myxamoebal alpha-tubulin and the inferred acetylation domains of two plasmodial-specific alpha-tubulin isotypes. The only difference between the two peptides was at a single residue corresponding to amino acid 44 in the polypeptide. Tyrosine was present in myxamoebal alpha-tubulin and glycine was present in the plasmodial specific peptides; the peptides are referred to as the Tyr44 and Gly44 peptides respectively. Both peptides in acetylated form blocked 6-11B-1 reactivity towards acetylated myxamoebal alpha-tubulin. The Tyr44 but not the Gly44 peptide blocked KMP-1 reactivity towards non-acetylated myxamoebal alpha-tubulin. Tyrosine at position 44 is not found in any other known alpha-tubulin. Thus a unique antigenic determinant exists in certain Physarum alpha-tubulin isotypes, close to the acetylation site at lysine 40. This antigenic determinant forms part of the KMP-1 recognition epitope and explains the unique isotype selectivity of this monoclonal antibody.

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

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

Further characterization of a somatic cell mutant defective in regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase

Two enzymes of mammalian cellular mevalonate biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase and HMG-CoA reductase, have been shown to be regulated by exogenous sterols. It has been demonstrated that these enzymes are regulated, at least in part, by transcriptional control of their synthesis. We have previously described a somatic cell mutant (CR1) of the CHO-K1 cell line that is defective in regulation of the activity of these enzymes in response to exogenous sterols. In this report, we demonstrate that this mutant is defective in regulation of the mRNA levels for HMG-CoA reductase and HMG-CoA synthase by 25-hydroxycholesterol and mevinolin. In the case of HMG-CoA reductase, this loss of apparent transcriptional control is not accompanied by a comparable loss in regulation of synthesis of this enzyme. This observation is consistent with prior studies suggesting that HMG-CoA reductase can be regulated translationally. We also show that CR1 cells exhibit a constitutively rapid rate of degradation of HMG-CoA reductase.

Comparative analysis of tubulin sequences

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

Free intermingling of mammalian beta-tubulin isotypes among functionally distinct microtubules

Mammalian cells express a spectrum of tubulin isotypes whose relationship to the diversity of microtubule function is unknown. To examine whether different isotypes are segregated into functionally distinct microtubules, we generated immune sera capable of discriminating among the various naturally occurring beta-tubulin isotypes. Cloned fusion proteins encoding each isotype were used first to tolerogenize animals against shared epitopes, and then as immunogens to elicit a specific response. In experiments using these sera, we show that there is neither complete nor partial segregation of beta-tubulin isotypes: both interphase cytoskeletal and mitotic spindle microtubules are mixed copolymers of all expressed beta-tubulin isotypes. Indeed, a highly divergent isotype normally expressed only in certain hematopoietic cells is also indiscriminately assembled into all microtubules both in their normal context and when transfected into HeLa cells.

Homology between P-glycoprotein and a bacterial haemolysin transport protein suggests a model for multidrug resistance

Increased expression of P-glycoprotein, a plasma membrane glycoprotein of relative molecular mass (Mr) 170,000 (170K), occurs in a wide variety of cell lines that exhibit pleiotropic resistance to unrelated drugs. The presence of P-glycoprotein in human cancers refractory to chemotherapy suggests that tumour cells with multidrug resistance can arise during malignant progression. We have discovered striking homology between P-glycoprotein and the HlyB protein, a 66K Escherichia coli membrane protein required for the export of haemolysin (protein of Mr 107K). P-glycoprotein can be viewed as a tandem duplication of the HlyB protein. The hydropathy profiles of the two proteins are similar and reveal an extensive transmembrane region resembling those found in pore-forming plasma membrane proteins. The C-terminal region of P-glycoprotein and the HlyB protein contain sequences homologous to the nucleotide-binding domains of a group of closely related bacterial ATP-binding proteins. We propose a model for multidrug resistance in which P-glycoprotein functions as an energy-dependent export pump to reduce intracellular levels of anticancer drugs.

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

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

Differential expression of alpha-tubulin mRNA in rat cerebellum as revealed by in situ hybridization

Nucleotide sequence analysis of two rat alpha-tubulin cDNA clones showed a marked divergence in their 3'-untranslated regions. However, each of the alpha-tubulin isotypes shows a high interspecies homology in this region, when compared with an isotubulin sequence from human and Chinese hamster. In situ hybridization of rat cerebellum with alpha-tubulin cDNA revealed differential expression in various cell layers. The mitotically active cells in the external granular layer show the highest level of alpha-tubulin mRNA, while lower levels are observed in the migrating cells in the molecular layer and in the differentiating cells in the internal granular layer. Very low levels of the mRNA are observed in the prenatally differentiated Purkinje cells.

Tubulin composition and microtubule nucleation of a griseofulvin-resistant Chinese hamster ovary cell mutant with abnormal spindles

A griseofulvin-resistant Chinese hamster ovary (CHO) mutant (Grs-2) which has an altered beta-tubulin subunit as well as wild-type beta-tubulin is temperature-sensitive (ts) for growth at 40.5 degrees C. This growth defect appears to result from the formation of abnormal mitotic spindles at the non-permissive temperature (Abraham, I et al., J cell biol 97 (1983) 1055) [19]. Light microscopy of spindles isolated from mutant cells cultured at the permissive temperature showed a typical bipolar morphology, whereas spindles isolated at the non-permissive temperature were multipolar. In order to study the role of tubulin in spindle formation, we analyzed the tubulin composition of the multipolar spindles. Two-dimensional gels and immunoblotting analysis of one-dimensional electrophoretic gels stained with monoclonal anti-Chinese hamster brain beta-tubulin antibody revealed that both mutant and wild-type beta-tubulins were present in similar proportions in both bipolar spindles at 37 degrees C and multipolar spindles at 40.5 degrees C. The ratio between wild-type and mutant tubulin in spindles was also found to be the same as in the cytoplasmic microtubule network in interphase cells, providing evidence that the mutant beta-tubulin appeared to be incorporated in a similar manner into both interphase and mitotic microtubule structures. In vitro microtubule polymerization onto centrosomes prepared from mutant Grs-2 demonstrated that 80% of the sites for microtubule nucleation were without centrioles, suggesting fragmentation of pericentriolar material away from centrioles. This may be one of the causes of multipolar spindle formation in the mutant cells. These results, therefore, suggest that abnormal formation of spindles in mutant cells is due not to the presence of the mutant tubulin per se, but to the abnormal behavior of this mutant tubulin in the cellular environment during mitosis or abnormal interaction with other components in the spindle at 40.5 degrees C.