Tubulin-colchicine complex inhibits microtubule elongation at both plus and minus ends

We studied the mechanism by which tubulin-colchicine complex (TC) inhibits microtubule polymerization in vitro by using the axoneme-directed polymerization system (Bergen, L. G., and Borisy, G. G. (1980) J. Cell Biol. 84, 141-150). With this system, the growth properties of each microtubule end can be determined from the direct visual analysis of changes in lengths of seeded microtubules. The rate of growth at both ends was inhibited equally by TC and the magnitude of the inhibition increased progressively with the molar ratio of TC to tubulin dimer (TC:T). At a TC:T ratio of approximately 0.12, all microtubule polymerization was inhibited at both ends. Therefore, substoichiometric poisoning of microtubule elongation is both a nonpolar and graded phenomenon. We determined the four association and dissociation rate constants in the presence and absence of TC and found that TC inhibits the overall growth of microtubules by reducing the association rate constants at both ends under conditions that do not alter the dissociation rate constants. Therefore, by an independent analytical method, we have confirmed Sternlicht and Ringel's hypothesis of TC action (Sternlicht, H., and Ringel, I. (1979) J. Biol. Chem. 254, 10540-10550), and have extended this hypothesis 1) by demonstrating that net growth of both ends are equally inhibited by TC, and 2) by determining which changes in the separate rate constants were responsible for the net inhibition.