2-(m-Azidobenzoyl)taxol binds differentially to distinct β-tubulin isotypes

There are seven beta-tubulin isotypes present in distinct quantities in mammalian cells of different origin. Altered expression of beta-tubulin isotypes has been reported in cancer cell lines resistant to microtubule stabilizing agents (MSAs) and in human tumors resistant to Taxol. To study the relative binding affinities of MSAs, tubulin from different sources, with distinct beta-tubulin isotype content, were specifically photolabeled with a tritium-labeled Taxol analog, 2-(m-azidobenzoyl)taxol, alone or in the presence of MSAs. The inhibitory effects elicited by theseMSAs on photolabeling were distinct for beta-tubulin from different sources. To determine the exact amount of drug that binds to different beta-tubulin isotypes, bovine brain tubulin was photolabeled and the isotypes resolved by high-resolution isoelectrofocusing. All bands were analyzed by mass spectrometry following cyanogen bromide digestion, and the identity and relative quantity of each beta-tubulin isotype determined. It was found that compared with other beta-tubulin isotypes, beta III-tubulin bound the least amount of 2-(m-azidobenzoyl) taxol. Analysis of the sequences of beta-tubulin near the Taxol binding site indicated that, in addition to the M-loop that is known to be involved in drug binding, the leucine cluster region of beta III-tubulin contains a unique residue, alanine, at 218, compared with other isotypes that contain threonine. Molecular dynamic simulations indicated that the frequency of Taxol-accommodating conformations decreased dramatically in the T218A variant, compared with other beta-tubulins. Our results indicate that the difference in residue 218 in beta III-tubulin may be responsible for inhibition of drug binding to this isotype, which could influence downstream cellular events.