Conformational states of tubulin liganded to colchicine, tropolone methyl ether, and podophyllotoxin

The conformational effects of colchicine, podophyllotoxin, and tropolone methyl ether binding to tubulin have been studied. Conditions for the stability of the purified calf brain protein were established, and the effects of binding were examined by means of difference absorption spectroscopy, circular dichroism, fluorescence, activation of tubulin GTPase, and tubulin self-association reactions. The tubulin-colchicine complex was isolated and characterized. It displays quenched intrinsic protein fluorescence, ligand fluorescence, and GTPase activity, probably accompanied by minor perturbations in the secondary structure. The conformation of the tubulin-colchicine complex appears to be nonidentical with that of the unliganded protein. Podophyllotoxin was not found to induce any of the mentioned changes. This ligand seems to bind through a hydrophobic interaction of its trimethoxybenzene ring with tubulin, as does colchicine. Binding of tropolone methyl ether, which is the analogue of the other part of the colchicine molecule that binds to tubulin, produced effects consistent with a ligand-linked conformational change. The small perturbation by tropolone methyl ether of the circular dichroism spectrum of tubulin resembles changes induced by colchicine.