Competitive inhibition of colchicine binding to tubulin by microtubule-associated proteins.

Abstract
Microtubule-associated proteins (MAP) promote tubulin polymerization, but colchicine inhibits this process. MAP inhibit colchicine binding to tubulin in a competitive manner. Attempts were made to identify which of the MAP fraction(s) was responsible: both tau protein (a thermostable molecule with a MW of .apprx. 70,000) and a high MW fraction completed with colchicine. In contrast, Mg2+, which also induces microtubule assembly in vitro, had no effect on colchicine binding to tubulin.