SELECTIVE BINDING OF GELSOLIN TO ACTIN MONOMERS CONTAINING ADP

The rate of reaction and the stoichiometry of binding between gelsolin and actin monomers depends on adenine nucleotides. In the presence of Ca2+ but not Mg2+, gelsolin retains the ability to sever actin filame nts when incubated for more than 20 min with an excess of G-actin in t he presence of ATP but loses severing activity within seconds when mix ed with G-actin in ADP. Immunoprecipitation of gelsolin removes more a ctin from ADP than from ATP solutions. Monomeric ATP-actin in 2 mM MgC l2 and 150 mM KCl slowly destroys the filament-severing activity of ge lsolin with kinetics that are first order in actin concentration and w ith an apparent bimolecular rate constant of 0.021 +/-0.007 muM-1 s-1. Coincident with the slow complex formation in MgCl2, the actin bound to the calcium-sensitive actin binding domain of gelsolin hydrolyzes i ts ATP to ADP. These results suggest a further level of gelsolin regul ation and a functional similarity between actin and GTP-binding protei ns.