Different effects of vinblastine on the polymerization of isotypically purified tubulins from bovine brain

Vinblastine, a highly successful antitumor drug, targets the tubulin molecule. Tubulin, the subunit protein of microtubules, consists of an α- and a β-subunit, both of which consist of isotypes encoded by different genes. We have purified three isotypes of bovine brain tubulin, namely, αβ_II, αβ_III and αβ_IV. Microtubule associated protein-2 (MAP2) and Tau-induced assembly of these isotypes were compared in the presence and absence of vinblastine. MAP2-induced assembly of unfractionated tubulin and all the isotypes except αβ_II tubulin was resistant to 1 μM vinblastine. Vinblastine at low concentrations (< 10 μM) progressively inhibited the assembly of all of the isotypes but the vinblastine concentration required for inhibition of MAP2-induced microtubule assembly was minimal for αβ_II. The tau-induced assembly of unfractionated tubulin and αβ_III were equally sensitive to 1 μM vinblastine whereas αβ_II and αβ_IV were much more sensitive to vinblastine. The microtubules obtained in the presence of tau from unfractionated tubulin, αβ_II and αβ_IV could be easily aggregated by 20 μM vinblastine whereas such as aggregation of microtubules obtained from αβ_III and tau required approximatedly 40 μM vinblastine. Our results suggest that among the tubulin isotypes, αβ_III is the most sensitive to vinblastine in the presence of MAPs while αβ_III is the most resistant and this intrinsic resistance of αβ_II dimers persists in the polymeric form of αβ_III tubulin as well. These results may be relevant to the therapeutic and toxic actions of vinblastine.