Effect of Acanthamoeba profilin on the pre-steady state kinetics of actin polymerization and on the concentration of F-actin at steady state.

The interaction of Acanthamoeba actin and Acanthamoeba profilin was evaluated as a function of ionic conditions. In the presence of 2 mM MgCl2 or 1 mM MgCl2 and 50 mM KCl, profilin decreased the concentration of F-actin at steady state, and inhibited the rates of filament elongation and spontaneous nucleation and polymerization. All of the experimental data were quantitatively accounted for on the basis of a 1:1 complex between profilin and monomeric actin with a Kr between 4 and 9 microM, the same value obtained previously in the absence of MgCl2. Therefore, the Mg2+ concentration did not affect the KD of the profilin-actin complex in these experiments. On the other hand, profilin did greatly amplify the decrease in concentration of F-actin at steady state caused by lowering the Mg2+ concentration. This results from the effect of Mg2+ on the critical concentration of the actin monomer with which the profilin-actin complex is in equilibrium. When the Mg2+ concentration is lowered, the critical concentration of actin monomer increases so that more profilin-actin complex is formed. Thus, appreciably more F-actin depolymerizes than in the absence of profilin. In this way, profilin could function intracellularly to convert small changes in critical concentration into large changes in the concentration of F-actin.