RHAZINILAM MIMICS THE CELLULAR EFFECTS OF TAXOL BY DIFFERENT MECHANISMS OF ACTION

We have investigated the effects of the microtubule poison rhazinilam on microtubule assembly in vivo and in vitro. In mammalian cells, rhaz inilam mimics the effects of taxol and leads to microtubule bundles, m ultiple asters, and microtubule cold stability. In vitro, rhazinilam p rotected preassembled microtubules from cold-induced disassembly, but not from calcium ion-induced disassembly. Moreover, both at 0 degrees C and at 37 degrees C, rhazinilam induced the formation of anomalous t ubulin assemblies (spirals). This process was prevented by maytansine and vinblastine, but not by colchicine. Preferential saturable and sto ichiometric binding of radioactive rhazinilam to tubulin in spirals wa s observed with a dissociation constant of 5 mu M. This binding was ab olished in the presence of vinblastine and maytansine. In contrast, sp ecific binding of radioactive rhazinilam to tubulin assembled in micro tubules was undetectable. These results demonstrate that rhazinilam al ters microtubule stability differently than taxol, and that the overal l similar effects of rhazinilam and taxol on the cellular cytoskeleton are the consequence of two distinct mechanisms of action at the molec ular level. (C) 1994 Wiley-Liss, Inc.