Modulation of actin polymerization by 47,000-dalton protein of human platelets.

We purified 47,000-dalton proteins from both thrombin-stimulated and unstimulated human platelets. The purity of the protein was almost 80% on SDS-polyacrylamide gel electrophoresis. The protein obtained from unstimulated platelets strongly inhibited actin gelation when its molar ratio to actin was 1:200 or higher. The protein obtained from thrombin-stimulated platelets had no inhibitory activity. The results suggest that the 47,000-dalton protein modulates actin polymerization through phosphorylation.