Tubulin bound to colchicine forms polymers different from microtubules.

The purified tubulin-colchicine complex undergoes in vitro polymerization under the same conditions that promote the assembly of microtubules from purified tubulin. The need for a critical concentration, the apparent free energy change of the reaction, and the effects of divalent cations and nucleotide binding indicate interactions similar to those involved in microtubule formation. The large polymers formed are not microtubules, suggesting that the mode of action of the antimitotic drug may be the production of an incorrect bonding geometry between tubulin molecules.