CHARACTERIZATION OF THE INTERACTION OF CRYPTOPHYCIN-1 WITH TUBULIN - BINDING IN THE VINCA DOMAIN, COMPETITIVE-INHIBITION OF DOLASTATIN-10 BINDING, AND AN UNUSUAL AGGREGATION REACTION

The antimitotic depsipeptide cryptophgcin 1 (CP1) was compared to the antimitotic peptide dolastatin 10 (D10) as an antiproliferative agent and in its interactions with purified tubulin. The potent activity of CP1 as an inhibitor of cell growth was confirmed. The agent had an IC5 0 of 20 pM against L1210 murine leukemia cells versus 0.5 nM for D10. Both drugs were comparable as inhibitors of the glutamate-induced asse mbly of purified tubulin, with D10 being slightly more potent. CP1, li ke D10, was a noncompetitive inhibitor of the binding of [H-3]vinblast ine to tubulin (apparent K-i, 3.9 mu M); and the depsipeptide was a co mpetitive inhibitor of the binding of [H-3]D10 to tubulin (apparent K- i, 2.1 mu M). CP1 was less potent than D10 as an inhibitor of nucleoti de exchange on tubulin, but the two drugs were equivalent in stabilizi ng the colchicine binding activity of tubulin, CP1, like D10, caused t he formation of extensive structured aggregates of tubulin when presen t in stoichiometric amounts relative to the protein. Whereas at lower concentrations the drugs were equivalent in causing formation of small oligomers detected by gel permeation highperformance liquid chromatog raphy, there were notable differences in the aggregation reactions ind uced by the two drugs. The electron micrographic appearance of the D10 -induced aggregate differed substantially from that of the CP1 induced aggregate. With D10, but not CP1, aggregate morphology was greatly al tered in the presence of microtubule-associated proteins. Finally, alt hough CP1 caused the formation of massive aggregates, as did D10, ther e was little turbidity change with the depsipeptide as opposed to the peptide.