ATP Hydrolysis Energy Transfer in the Profilin-Mediated Actin Polymerization

Abstract

Profilin regulates actin polymerization in cells playing important role in cell motility and division. Actin polymerization involves ATP hydrolysis which occurs both in the absence and the presence of profilin. There is a hypothesis suggested in literature that profilin promotes actin polymerization through direct transfer of the energy of this spontaneous hydrolysis to polymerization utilizing tight coupling of the hydrolysis and corresponding polymerization events. Recently we suggested an alternative hypothesis based on an indirect energy transfer and pointed out that recent experimental and theoretical findings require re-evaluation of the direct transfer hypothesis.Our thermodynamically rigorous model of actin steady state dynamics in the presence of profilin describes all events in terms of chemical reactions and allows both energy transfer mechanisms, each corresponding to certain ranges of the rate constants (parameters) for these reactions. In fact, the difference between the two mechanisms is defined by a single ratio r of the two rates: the rate of hydrolysis by the profilin-and-ATP-bound subunit at the filament end, and the rate of dissociation of this subunit (in complex with profilin) from the end. The direct transfer cannot exist unless r>1 while indirect transfer can occur at any r.Our model predicts specific shapes for the dependence of actin critical concentration on profilin concentration depending on the ratio r. For values r>1, curves have peaks, which persist for wide ranges of other model parameters. Our experimental dependence (obtained with our new technique) shows no peak but a steady decline, contrary to the direct transfer hypothesis. This result provides strong support for our indirect transfer hypothesis.Moreover, using our theory and routine experimental techniques we determined relative activities of the two specific molecular mechanisms of profilin action (both based on indirect transfer) predicted earlier with our model.