Figure 2.

Limited subtilisin digestion of tubulin, vinblastine-treated tubulin, or maytansine-treated tubulin as a function of time. Tubulin (2.0 mg/ml) in Mes assembly buffer containing 10% dimethyl sulfoxide was exposed to 50 μM vinblastine or 50 μM maytansine for 25 min at 25°C followed by addition of 1:100 subtilisin. Digestion was stopped with 1% phenylmethylsulfonyl fluoride, and samples were electrophoresed on SDS/10% gels. (Left) Coomassie stain. (Right) Fraction of tubulin as native or digested monomers. (Top) Control without drugs. (Middle) Pretreatment with vinblastine. (Bottom) Pretreatment with maytansine. •, ▴, ○, and ▵ refer to α-, β-, α_s-, and β_s-tubulins, respectively.