Purification and characterization of a gelsolin-actin complex from human platelets. Evidence for Ca2+-insensitive functions.

Extracts of human platelets contain a 90,000-Da protein that is retained by DNase I-agarose in the presence of Ca2+. The 90-kDa protein, tightly complexed with platelet actin, can be eluted from DNase I-agarose by ethylene glycol bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid (EGTA). The platelet 90-kDa protein is immunologically related to rabbit macrophage gelsolin. The 90-kDa protein-actin complex was purified from platelet extracts using DEAE-Sephacel, Sephadex G-200, and hydroxyapatite and is stable in EGTA and 0.8 M KCl. The purified complex will modulate the assembly of fluorescently labeled 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole-actin in the presence of both Ca2+ and EGTA. In addition, the complex affects the low shear viscosity of F-actin solutions in the presence of both Ca2+ and EGTA. Finally, the complex increases the critical concentration for actin assembly about 4-fold. The results are consistent with a strong preferential binding to or capping of the barbed end of actin filaments by the complex in either Ca2+ or EGTA.