Gelsolin activates DNase I in vitro and in cystic fibrosis sputum

Because actin can form a complex in vitro containing both gelsolin and DNase I, gelsolin and DNase I have been assumed to bind independently to actin. Although this assumption is consistent with the known crystalline structures of gelsolin with one actin and of actin with DNase I, which suggest that the binding sites on actin for both gelsolin and DNase I are distinct and separate, we propose that a second actin binding site on gelsolin competes with DNase I for actin. Since actin is an inhibitor of DNase I, competition at the second binding site results in activation of DNase I by gelsolin. Covalent cross-linking experiments confirm that DNase I prevents dimerization of actin by gelsolin, consistent with displacement of one actin from, gelsolin by DNase I. Activation of DNase I by gelsolin is a novel function for a cytoskeletal protein and could have broad implications for biology, such as a role in initiating apoptosis. These results also may explain why both gelsolin and DNase I decrease sputum viscosity in cystic fibrosis (CF). While the activity of DNase I had originally been attributed to fragmentation of DNA, subsequent data suggested that both gelsolin and DNase I may affect viscosity by depolymerizing filamentous actin. The current results alternatively suggest that dissociation of the actin-DNase I complex by gelsolin in CF sputum results in activation of the nuclease activity of constitutive DNase I. The nuclease activity of DNase I alone is therefore sufficient to explain the effects of both gelsolin and DNase I on CF sputum.