Phosphorylation of bovine platelet myosin by protein kinase C

Bovine platelet myosin is phosphorylated by protein kinase C at multiple sites. Most of the phosphate is incorporated in the 20,000-dalton light chain although some phosphate is incorporated in the heavy chain. Phosphorylation of the 20,000-dalton light chain of platelet myosin is 10 times faster than the phosphorylation of smooth muscle myosin. Platelet myosin light chain is first phosphorylated at a threonine residue followed by a serine residue. Dominant phosphorylation sites of the 20,000-dalton light chain are estimated as serine-1, serine-2, and threonine-9. Prolonged phosphorylation by protein kinase C resulted in an additional phosphorylation site which, on the basis of limited proteolysis, appears to be either serine-19 or threonine-18. Phosphorylation by protein kinase C causes an inhibition of actin-activated ATPase activity of platelet myosin prephosphorylated by myosin light chain kinase. Inhibition of ATPase activity is due to a decreased affinity of myosin for actin, and no change in Vmax is observed. It is shown that platelet myosin also exhibits the 6S to 10S conformation transition as judged by viscosity and gel filtration methods. Mg2(+)-ATPase activity of platelet myosin is paralleled with the 10S-6S transition. Phosphorylation by protein kinase C affects neither the 10S-6S transition nor the myosin filament formation. Therefore, the inhibition of actin-activated ATPase activity of platelet myosin is not due to the change in the myosin conformation.