Gelsolin inhibits nucleotide exchange from actin.

The effects of platelet gelsolin on the state and exchangeability of the nucleotide bound to skeletal muscle actin monomer have been investigated. In the presence of Ca2+, a stable ternary complex consisting of two actins and one gelsolin is formed. Removal of Ca2+ from this species results in the formation of a highly stable binary gelsolin-actin complex. The interaction of gelsolin with actin monomer has no effect on the virtually negligible [less than 0.01 mol of Pi X h-1 X (mol of actin)-1] intrinsic ATPase activity of actin monomer (in the absence of Mg2+). A single molecule of ATP is bound to the binary complex while two molecules of ATP are bound to the actins within the ternary complex. The ATP within the binary complex is nonexchangeable, and only one of the two ATP molecules in the ternary complex is exchangeable. In the latter case the rate constant for this nucleotide exchange is decreased compared to that for free actin monomer. These results demonstrate the nonequivalence of actin monomers within the ternary complex. The involvement of these oligomeric complexes of gelsolin and actin in the expression of the activity(ies) of gelsolin is discussed.