Tubulin aggregation and disaggregation: mediation by two distinct vinblastine-binding sites

Abstract

Rat brain tubulin possesses two distinct binding sites for vinblastine per molecule: a high-affinity site with an affinity constant of 6.2 X 10⁶ M^-1 and a low-affinity site with an affinity constant of 8 X 10⁴ M^-1. The high-affinity site is labile, with a t_1/2^37° of 3.5 hr, is protected by colchicine, and is unaffected by salt, whereas the ow-affinity site is stable but is inhibited by salt. Binding to both sites is rapid. The high-affinity binding constant of vinblastine to tubulin (6.2 X 10⁶M^-1) corresponds to the half-maximal concentration of vinblastine need to prevent polymerization of tubulin in vitro, whereas the low-affinity binding constant (8 X 10⁴ M^-1) corresponds to the half-maximal concentration of vinblastine required to aggregate tubulin. We conclude that vinblastine binding to the high- and low-affinity sites, respectively, accounts for the depolymerization and aggregation behavior of tubulin.