Actin Filament Barbed End Elongation with Nonmuscle MgATP−Actin and MgADP−Actin in the Presence of Profilin

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Abstract
We have quantitated the in vitro interactions of profilin and the profilin−actin complex (PA) with the actin filament barbed end using profilin and nonmuscle β,γ-actin prepared from bovine spleen. Actin filament barbed end elongation was initiated from spectrin seeds in the presence of varying profilin concentrations and followed by light scattering. We find that profilin inhibits actin polymerization and that this effect is much more pronounced for β,γ-actin than for α-skeletal muscle actin. Profilin binds to β,γ-actin filament barbed ends with an equilibrium constant of 20 μM, decreases the filament elongation rate by blocking addition of actin monomers, and increases the dissociation rate of actin monomers from the filament end. PA containing bound MgADP supports elongation of the actin filament barbed end, indicating that ATP hydrolysis is not necessary for PA elongation of filaments. Initial analysis of the energetics for these reactions suggested an apparent greater negative free energy change for actin filament elongation from PA than elongation from monomeric actin. However, we calculate that the free energy changes for the two elongation pathways are equal if the profilin-induced weakening of nucleotide binding to actin is taken into consideration.