2-Chloroacetyl-2-demethylthiocolchicine (2CTC) and 3-chloroacetyl-3-demethylthiocolchicine (3CTC) resemble colchicine in binding to tubulin and react covalently with -tubulin, forming adducts with cysteine residues 239 and 354. The adducts at Cys-239 are less stable than those at Cys-354 during formic acid digestion. Extrapolating to zero time, the Cys-239 to Cys-354 adduct ratio is 77:23 for 2CTC and 27:73 for 3CTC. Using energy minimization modeling to dock colchicinoids into the electron crystallographic model of -tubulin in protofilaments (Nogales, E., Wolf, S. G., and Downing, K. H. (1998) Nature 391, 199 -203), we found two potential binding sites. At one, entirely encompassed within -tubulin, the C2-and C3-oxygen atoms of 2CTC and 3CTC overlapped poorly with those of colchicine and thiocolchicine, but distances from the reactive carbon atoms of the analogs to the sulfur atoms of the cysteine residues were qualitatively consistent with reactivity. The other potential binding site was located at the ␣/ interface. Here, the oxygen atoms of the analogs overlapped well with those of colchicine, but relative distances of the reactive carbons to the cysteine sulfur atoms did not correlate with the observed reactivity. A significant conformational change must occur in the colchicine binding site of tubulin in the transition from the unpolymerized to the polymerized state.Despite the interaction of tubulin with a large number of drugs that inhibit or promote its assembly into microtubules, precise definition of drug binding sites on the protein has not been possible. This is a consequence of the lack of success in crystallizing the protein, probably because of its sequence and post-translational heterogeneity, its instability, and its tendency to form oligomers and polymers of highly aberrant morphology in the presence of many of these drugs. The recent electron crystallographic determination of a relatively detailed structure for zinc-induced antiparallel tubulin protofilaments has provided insights into the paclitaxel/docetaxel site on these protofilaments, since docetaxel was used to enhance their stability during data accumulation (1), and provided a scaffold on which to model other drug sites (2). A limitation in such analysis, however, is that drugs that inhibit assembly have limited ability to bind to tubulin polymers containing linear protofilaments.Alternative approaches to obtain preliminary information about drug binding sites have included "direct" photoaffinity labeling (e.g. Ref.3), analog photoaffinity labeling (e.g. Ref. 4), and cross-link formation with chemically reactive analogs that retain biological activity (e.g. Ref. 5). In the first method, a drug-tubulin complex is exposed to light of an appropriate wave-length, and ligand-protein cross-link formation is evaluated. In the second, an active drug analog containing a photoreactive moiety is prepared and bound to tubulin, and crosslink formation is induced by exposure of the complex to light of an appropriate wavelength. In the third metho...