Figure 4.

Analysis of MT plus end dynamics reveals that the Op18-tetraE mutant retains the MT growth rate inhibitory activity at pH 6.8 but has lost specific catastrophe-promoting activity at pH 7.5. (A) Elongation velocity at the plus end for MTs assembled at pH 6.8 or pH 7.5 with 11 μM tubulin alone (open bars) or 11 μM tubulin plus 1.7 μM of Op18-wt (black bars) or Op18-tetraE (shaded bars). (B) Catastrophe frequencies at MT plus ends measured at pH 7.5 in the same experiment, as shown in panel A. Data for tubulin alone and in the presence of Op18-wt have previously been published (Howell et al., 1999b), but are shown here for comparison. Values represent means +/-SD.