Figure 3.

Op18 at 1.7 μM increases microtubule plus end catastrophes over a range of tubulin concentrations in Buffer D. (A) Mean plus end catastrophe frequency ± SD is plotted as a function of tubulin concentration in the absence (open symbols) or presence of 1.7 μM Op18 (closed symbols). Op18 increased catastrophe frequency twofold (9 and 10 μM tubulin samples) to sixfold (12 μM tubulin). (B) Mean plus end elongation velocity ± SD. Symbols are the same as in A. Error bars are shown in only one direction for clarity. Regression lines through the data are shown, and these were used to calculate tubulin association and dissociation rate constants (Table 3).