Copolymerization of tubulin-colchicine complex and unliganded tubulin in a nonmicrotubular polymer

Abstract

The tubulin-colchicine [TC] complex is able to polymerize in vitro into peculiar curly polymers, under the solution conditions permitting polymerization of unliganded tubulin [T] into microtubules. Unliganded T can be incorporated into these curly polymers. The partial critical concentration of TC is decreased upon incorporation of unliganded T into the copolymer. GTP hydrolysis occurs on unliganded T upon incorporation in the copolymer. T-podophyllotoxin does not copolymerize with TC to form a large polymer but interacts with it, preventing TC polymerization. The data were analyzed within a model of random copolymerization of unliganded T and T-C into curly polymers. A corollary is that unliganded T is virtually able to self-assemble into curly polymers with a critical concentration 10-fold higher than the critical concentration found for microtubule assembly. These peculiar T homopolymers cannot be observed except as transients at high concentrations, or when microtubule assembly is inhibited. Kinetic measurements of the T-TC copolymerization process and associated GTP hydrolysis at different T/TC ratios provide supplementary information about some privileged interactions between T and TC molecules. A comprehensive phase diagram of the various possible polymers formed in the presence of T and TC is presented.