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 alpha- and a beta-subunit, both of which consist of isotypes encoded by different genes. We have purified three isotypes of bovine brain tubulin, namely, alphabeta(II), alphabeta(III) and alphabeta(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 alphabeta(II) tubulin was resistant to 1 muM vinblastine. Vinblastine at low conentrations (<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 approximately 40 μM vinblastine. Our results suggest that among the tubulin isotypes, αβII is the most sensitive to vinblastine in the presence of MAPs while αβ(III) is the most resistant and this intrinsic resistance of αβ(III) dimers persists in the polymeric form of αβ(III) tubulin as well. These results may be relevant to the the therapeutic and toxic actions of vinblastine.