TAXOL (Fig. 1) was isolated from the plant Taxus brevifolia (western yew) by Wani et al., who reported that the molecule has antitumour activity in several experimental systems1. In our laboratory we have found that taxol, a low molecular weight neutral compound, completely inhibits division of exponentially growing HeLa cells at low concentrations of drug (0.25 M) that have no significant effects on DNA, RNA or protein synthesis during a 4-h incubation with the cells. HeLa cells incubated with taxol for 20 h are blocked in late G2 and/or M (ref. 2). We report here that taxol acts as a promoter of calf brain microtubule assembly in vitro, in contrast to plant products such as colchicine and podophyllotoxin, which inhibit assembly. Taxol decreases the lag time for microtubule assembly and shifts the equilibrium for assembly in favour of the microtubule, thereby decreasing the critical concentration of tubulin required for assembly. Microtubules polymerised in the presence of taxol are resistant to depolymerisation by cold (4 °C) and CaCl2 (4 mM).
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