Figure 5.

Op18 phosphorylations attenuate both the MT growth rate inhibitory activity at pH 6.8 and the specific catastrophe-promoting activity at pH 7.5. (A) Elongation velocity at plus ends for MTs assembled at pH 6.8 and pH 7.5 with 11 μM tubulin alone (open bars), or 11 μM tubulin plus 2 μM nonphosphorylated Op18-wt (black bars), or 2 μM and 5 μM Ser-16 and Ser-63 phosphorylated Op18 (Op18–16, 63-PO₄, shaded bars). Op18-wt mean growth rate at pH 6.8 is significantly different from all other rates (p < 0.05), while tubulin alone, 2 and 5 μM Op18–16, 63-PO₄ mean growth rates are not significantly different. (B) Catastrophe frequency at MT plus ends measured at pH 7.5 in the same experiment as shown in panel A. Data represent means ±SD. Data shown in Figures 4 and 5 are obtained using different tubulin preparations, which explain the slight difference in dynamic properties.