Protein phosphorylation and platelet secretion.

Platelet secretion in response to physiologic stimuli appears to result from the complementary stimulation of two processes--granule centralization and granule membrane fusion. Granule centralization is produced by actin-myosin contraction which is initiated by a movement of calcium ions into the cytoplasm. The calcium binds to calmodulin to form a complex which activates myosin light chain kinase to phosphorylate myosin light chain (MLC). Once phosphorylated in this fashion, actin-myosin contraction occurs. Granule membrane fusion can be produced selectively by phorbol myristate acetate and oleoyl-acetyl diglyceride, both of which activate protein kinase C. Phosphorylation of a 47,000 dalton intracellular protein (47K) by protein kinase C may be critical to granule membrane fusion. The mechanism of action of 47K is presently unknown. The combined phosphorylation of MLC and 47K in response to most physiologic agonists which cause granule secretion, and the synergistic effects on granule secretion of agents which independently stimulate MLC and 47K phosphorylation, suggests secretion usually results from the interaction of granule centralization and granule membrane fusion.