Human tubulin α/β heterodimers are encoded by six and seven genes for the α- and β-subunit, respectively. Each of these isotypes can undergo various posttranslational modifications. Most of the sequence specificity for each isotype and posttranslational modifications occur at the C-terminal part of tubulin. The biological significance of this so-called “tubulin code” and its regulation are unresolved questions in basic cytoskeleton research and in pathologies such as cancer. βIII-tubulin repeatedly appears as a potential marker of poor prognosis in different tumor types and of drug resistance. Nevertheless, because of limitations in present methods of analysis, it is still unclear if tubulin isotype expression profiles will provide useful biomarkers for the stratification of cancer patients prior to treatments including microtubulo-interacting drugs. Recent progresses in mass spectrometry-based analyses of tubulin isotype expression in cancer cells are presented in this chapter. Such approaches allow the validation of tools such as antibodies used in immunohistochemistry, identifies tubulin sequences expressed in human cells and posttranslational modifications, and offer new avenues for tubulin isotype quantitation in tumor and normal tissues.

ABSTRACT Microsporidia form environmentally resistant spores that are critical for their host-to-host transmission and persistence in the environment. The spore walls of these organisms are composed of two layers, the exospore and the endospore. Two spore wall proteins (SWP1 and SWP2) have been previously identified in members of the Encephalitozoonidae family. These proteins localize to the exospore. The endospore is known to contain chitin, and a putative glycosylphosphatidylinositol (GPI)-anchored chitin deacetylase has been localized to the plasmalemma-endospore interface. Using proteomic techniques, we have identified a new spore wall protein (SWP3) that is located in the endospore. The gene for this protein is located on chromosome 1 and corresponds to the open reading frame ECU01_1270. SWP3 is predicted to have a signal peptide and to be GPI anchored. Consistent with these modifications, two-dimensional electrophoresis demonstrated that SWP3 has an acidic pI and a molecular mass of <20 kDa. By immunoelectron microscopy, this protein was found on the cell surface during sporogony and in the endospore in mature spores. SWP3 has several potential O-glycosylation sites, and it is possible that it is a mannosylated protein like the major polar tube protein (PTP1).

A 95-fold epothilone B (EpoB)–resistant, but not dependent, A549 human lung carcinoma cell line, A549.EpoB40 (EpoB40), has a Gln to Glu mutation at residue 292 that is situated near the M-loop of βI-tubulin. Further selection of this cell line with higher concentrations of EpoB produced A549.EpoB480 (EpoB480), which is ∼900-fold resistant to EpoB. This cell line, like EpoB40, exhibits cross-resistance to Taxol and extreme sensitivity to vinblastine, but in contrast to EpoB40 it is unusually dependent on EpoB, requiring a minimum of 125 nmol/L EpoB to maintain normal growth. Sequence analysis of the β-tubulin and Kα1-tubulin genes in EpoB480 showed that, in addition to the β292 mutation, β60 was mutated from Val to Phe and α195 was mutated from Leu to Met. Mass spectrometry indicated that both the Val60Phe and Leu195Met mutations in βI- and Kα1-tubulin, respectively, were expressed at the protein level. Molecular modeling indicated that β60 is located at the end of the H1-S2 loop that has been implicated as a principal partner of the M-loop for contacts between protofilaments. A mutation at β60 could inhibit the lateral contacts between protofilaments, thereby destabilizing microtubules. α195 is located at the external surface of the microtubule that has been proposed as the domain that interacts with a variety of endogenous proteins, such as stathmin and microtubule-associated protein 4. A mutation at α195 could modulate the interactions between tubulin and regulatory proteins. We propose that the βVal60Phe mutation plays a critical role in the drug-dependent phenotype of EpoB480 cells.

Tubulin, the constitutive protein of microtubules, is a heterodimeric protein with an alpha and beta subunit, encoded in vertebrates by six and seven different genes, respectively. Each tubulin isotype can be identified by its divergent C-terminal sequence. Nevertheless, two groups of beta-tubulin isotypes can be distinguished by sequence alignment; one includes betaI-, betaII-, betaIVa-, and betaIVb-tubulin, and the other includes betaIII-, betaV-, and betaVI-tubulin. betaIII-tubulin overexpression has been associated with microtubule destabilization and resistance to Taxol. Recent data indicate that mouse betaV-tubulin overexpression in CHO cells results in profound microtubule disorganization and dependence of cells on Taxol for growth. Mouse and human betaV-tubulin sequences display several differences, such as their respective extreme C-terminus, suggesting that they may have different effects on microtubule stability and different affinities for drugs. When high-resolution isoelectric focusing, in-gel CNBr cleavage, and mass spectrometry were combined, we detected for the first time the betaV-tubulin protein in human cell lines and found that it was highly expressed in Hey, an epithelial ovarian cancer cell line. Our data confirm that human and rodent betaV-tubulins are distinct and indicate that, regardless of species, betaIII- and betaV-tubulin may be expressed in a complementary pattern at the protein level. Therefore, both betaIII- and betaV-tubulin expression levels should be systematically determined to assess the role of differential tubulin isotype expression in the response of tumors to drugs targeting microtubules.

Six human alpha-tubulin and seven human beta-tubulin isotypes, each of which can undergo posttranslational modifications, have been detected by the reverse transcriptase-polymerase chain reaction. This repertoire of tubulin isotypes plays a role in development and in the building of specialized microtubule-based structures. In cell lines, the relationship between resistance to microtubule-interacting drugs and altered tubulin isotype expression profiles is often established by quantitation of cDNA and/or Western blot analysis. Tubulin mutations in major isotypes are detected by sequencing cDNA, but more analysis of expression of tubulin mutations at the protein level, to assess their role in drug resistance, is needed. We utilized a Taxol-based purification and high-resolution isoelectrofocusing combined with a mass spectrometry-based analysis of tubulin. This approach has allowed the separation and relative quantitation of tubulin isotypes having a difference in isoelectric point values of 0.01, without the need for two-dimensional gel electrophoresis. The specificity of tubulin isotype antibodies also has been established. In cell lines resistant to microtubule-stabilizing drugs that express heterozygous tubulin mutations, the relative amount of mutant tubulin expression has been determined. In these cell lines, the absence of betaII- and betaIVa-tubulin has been demonstrated, and an increased level of expression of betaIII-tubulin in resistant cells has been confirmed, indicating that this tubulin isotype is a unique marker of resistance.