Phosphorylation of Human Platelet Myosin

A preparation extracted from human blood platelets, which incorporates ³²P from γ-labeled AT³²P into one of the two light chains of platelet myosin and platelet myosin head is described. This phosphorylation, which appears to be due to an endogenous kinase, is specific for the myosin light chain in that no other protein extracted in 0.6 M KCl-15 mM Tris·HCl (pH 7.5) is phosphorylated. The phosphorylated light chain, which has been purified by gel filtration, releases the covalently bound phosphate after incubation in alkali and not after incubation in acid.