B-ring of colchicine and its role in taxol-induced tubulin polymerization.

Taxol-induced assembly of purified tubulin is not inhibited by the colchicine analogue 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone. Colchicine analogues having intact A, C and B-rings (without NH-CO-CH3) such as desacetamidocolchicine have also been found to be inactive. It has been observed that these two colchicine analogues are incorporated into polymers when incubated in the presence of taxol. Furthermore, preformed taxol-induced polymers of tubulin have been found to bind these two colchicine analogues. These results suggest that colchicine-binding domains on the tubulin molecule are mostly (if not completely) exposed in the taxol-induced polymers.