Figure 3.

Glu-substitutions only partially mimic phosphorylation-inactivation of tubulin-directed in vitro activities of Op18. (A) Op18-tubulin equilibrium binding curves were determined at pH 7.5 for Op18-wt, Op18-tetraE and Ser-16 and Ser-63 phosphorylated Op18 (Op18–16,63-PO₄) as described in MATERIALS AND METHODS. The contribution of nonspecific binding (3% of free tubulin) is subtracted. Curves assuming two-site positive cooperative binding were fitted to the data points. (B) Scatchard conversions of the binding curves. (C) Hydrolysis of [α³²P]GTP preloaded onto tubulin (3 μM) was determined at various concentrations of the indicated Op18 derivatives. Data are representative for at least two independent experiments.