The effect of myelin basic protein on the endogenous phosphorylation of platelets.

Bovine myelin basic protein (BP) induced a shape change and endogenous phosphorylation of a 45,000 (45K) molecular weight protein of intact human platelets. This effect occurred rapidly over an effective concentration range of 5-100 microM BP. BP peptides encompassing residues 1-42, 43-88, and 89-169 from the BP molecule of 169 residues, neither induced phosphorylation of platelets nor blocked the effect of intact BP. Subfractionation of disrupted platelets demonstrated the phosphorylated 45K protein in the 100,000 xg supernate. When isolated platelet membranes were used, no BP induced phosphorylation of a 45K protein could be detected. The amino acid composition of the purified, phosphorylated 45K protein differed from those of other known platelet proteins. BP itself was also phosphorylated by an endogenous platelet protein kinase(s) present both in the 100,000 xg supernatant and in the isolated membrane fraction of platelets. These results indicate that the normal or pathological release of BP from myelin may lead to phosphorylation of an internal protein of platelets and possibly other tissue elements with resultant metabolic and functional changes.