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Two low-affinity Ca(2+)-binding sites of gelsolin that regulate association with actin.

The time course of binding of actin to gelsolin or 1:1 gelsolin-actin complex was measured at defined Ca2+ concentrations in the range 0.5-500 microM. The rate of association was followed by the fluorescence increase of a fluorescent label covalently linked to actin. Free Ca2+ was determined by titration with EGTA in the presence of Fura-2 as indicator. The experimental data were quantitatively evaluated by calculations of the kinetics of association of actin with gelsolin thereby taking into account the equilibrium of binding of Ca2+ ions to gelsolin. It was found that association of gelsolin with one actin monomer is regulated by a Ca(2+)-binding site with a dissociation constant Kd1 = 25 microM. Binding of the second actin monomer was found to be controlled by a Ca(2+)-binding site of which the dissociation constant Kd2 was 200 microM. Mg2+ ions in the concentration range 0-1 mM did not compete with Ca2+ for binding to gelsolin. More complex interactions of gelsolin with actin such as nucleated actin polymerization were found to occur even at Ca2+ concentrations below Kd1 (e.g. 10 microM) at almost maximal rates.