Some features of the vinblastine-induced assembly of porcine tubulin.

Porcine tubulin precipitated by 10^?3, m vinblastine (VLB) contains approximately 0.50 molecule of VLB bound per 110,000-molecular-weight tubulin dimer. The amount of precipitate, followed by turbidity, is a linear function of the initial tubulin concentration. The rate of precipitation is roughly first order in protein concentration. Vindoline and velbanamine halves of VLB are ineffective separately or together in producing the tubular aggregates observed for VLB precipitates by electron microscopy. At 10^?3, m concentrations no turbidity is observed nor is there any competition with VLB-induced turbidity. Removal of GTP from tubulin by dialysis or incubation of tubulin in the absence of added GTP blocks VLB-induced assembly. Readdition of GTP at room temperature or above restores sensitivity to VLB precipitation. The β,γ methylene analog of GTP cannot substitute for GTP in this process. About 0.7 mol of added GTP is found bound per mole of tubulin dimer. During the course of VLB-induced assembly, roughly half of this GTP is displaced. These results show interesting similarities and differences in the VLB-induced assembly of tubulin and the normal in vitro assembly of microtubules. Further comparisons between both assembly processes should be useful.