Curacin A, the major lipid constituent of a strain of the marine cyanobacterium Lyngbya majuscula obtained off the coast of Curaçao, is a potent antimitotic agent that we have previously shown to inhibit microtubule assembly and colchicine binding to tubulin. In the present study, we report that curacin A probably binds in the colchicine site because it competitively inhibits the binding of [3H]colchicine to tubulin with an apparent Ki value of 0.6 microM and stimulates tubulin-dependent GTP hydrolysis, as do most other colchicine-site agents. The binding of curacin A to tubulin resembled the binding reactions of combretastatin A-4 and podophyllotoxin in contrast to that of colchicine in that it occurred as extensively on ice as at higher temperatures. However, once bound, the dissociation rate of curacin A from tubulin is very slow, more closely resembling that observed with colchicinoids (thiocolchicine was the drug examined) than the faster dissociation that occurs with combretastatin A-4 and podophyllotoxin. Because the molecular structure of curacin A is so different from that of previously described colchicine-site drugs (e.g., there is no aromatic moiety, and there are only two conjugated double bonds in its linear hydrocarbon chain), we have been examining the activities of natural isomers and synthetic derivatives. So far, only modest enhancement or reduction of activity has been observed with a variety of structural changes.