Effect of subtilisin on vinblastine-induced tubulin polymerization. Rat brain tubulin (2 mg/ml) was polymerized at 25°C, with 10% dimethyl sulfoxide and 50 μM vinblastine. Curves: 1, control, 1 mM GTP added at arrow; 2, with 0.5 mM GTP, subtilisin (1:100) added at thick arrow; 3, subtilisin added at beginning, left arrow indicates 1 mM GTP was added, and right arrow indicates 1 mM sodium triphosphate was added; 4, with 0.5 mM GTP and subtilisin added at beginning.