Acanthamoeba profilin interacts with G-actin to increase the rate of exchange of actin-bound adenosine 5'-triphosphate

A sevenfold molar excess of Acanthamoeba profilin, a 12 000-dalton protein that inhibits actin polymerization, increases the rate of exchange of ATP bound to G-actin with ATP in solution about 17-fold, i.e., from 7.7 x 10(-4) to 1.3 x 10(-2) S-1, at 25 degrees C, 0.033 mM Ca2+, and 0.1 mM ATP, pH 7.5. Detailed analysis of the equilibrium isotope-exchange data shows that profilin and actin form a 1:1 complex with KD = 4.7 x 10(-5) M and that the binding of profilin to actin is rapid and reversible. The actin-profilin complex binds 1 mol of ATP/mol, as does G-actin. Profilin does not interact with ATP or Ca2+.