Tubulin polymerization and colchicine-binding assays were performed as described previously (D'Amato et al., 1994). In the polymerization assay, 12 μM tubulin was preincubated with varying drug concentrations for 15 min at 26°C before the addition of GTP. The assembly reactions were followed turbidimetrically for 20 min at 26°C, and drug concentrations required for 50% reduction in the plateau turbidity reading were determined. In the colchicine-binding assays, reaction mixtures contained 1.0 μM tubulin, 5.0 μM [³H]colchicine, and inhibitors, if present, at 50 μM. Incubation was for 10 min at 37°C. Previous studies have shown that at this time point, the colchicine-binding reaction was about 50% complete in the control reaction mixtures under the reaction conditions used. All experiments were performed twice, and standard deviations are shown. In the colchicine-binding experiments, the individual assays were also performed with duplicate samples.

• ↵1-a Same value obtained in both experiments.