Role of phosphorylation in mediating the association of myosin with the cytoskeletal structures of human platelets.

The effect of myosin light chain phosphorylation on the association of myosin with the cytoskeletal structures of platelets was quantitated. In unstimulated platelets, little myosin light chain was phosphorylated and myosin remained in solution when cytoskeletons from Triton X-100 lysates of platelets were sedimented by centrifugation. In platelets activated by thrombin, the calcium ionophore A23187, or collagen, the rate and extent of myosin light chain phosphorylation paralleled the association of myosin with platelet cytoskeletal structures. Dephosphorylation of myosin light chain and myosin dissociation from the cytoskeleton occurred at comparable rates at longer times after addition of the stimulating agents to platelets. Quantitation of radioactive phosphate in the cytoskeleton-associated myosin and in the soluble myosin showed that the phosphorylated myosin light chain was selectively isolated with the Triton-insoluble cytoskeletons, whereas nonphosphorylated myosin was not associated. Inhibition of the light chain kinase with the calmodulin antagonist trifluoperazine inhibited myosin light chain phosphorylation and incorporation of myosin into the platelet cytoskeletons. Inhibition of light chain phosphorylation by prostaglandin E1 and prostacyclin produced similar effects. Thus, phosphorylation of the myosin light chain stabilizes the association of myosin with the contractile structures within platelets.