Role of protein methylation in agonist-induced signal transduction in human platelets.

Possible role of methylation of proteins in platelet activation was examined in this study. Electropermeabilized platelets incorporated radioactivity in the presence of [methyl-3H]-S-adenosylmethionine. Thrombin, PDBu and GTP gamma S increased incorporation of radioactivity in a time-dependent manner. In other experiments, 23 kD membrane proteins incorporated radioactivity in the presence of [methyl-3H]-S-adenosylmethionine and platelet cytosol. Using rap specific antisera the 23 kD methylated proteins were characterized as low Mr G proteins, known as rap1 proteins. N-Acetyl-S-farnesyl-L-cysteine (AFC), an inhibitor of the methyltransferase, inhibited carboxyl methylation of platelet rap1 proteins and also inhibited platelet aggregation and mobilization of cytosolic calcium induced by a variety of agonists in a concentration-dependent manner. Inhibition of methylation of rap1 proteins as well as inhibition of platelet activation by AFC suggests that methylation and consequently translocation of rap1 proteins to plasma membrane may be important for agonist-induced signal transduction in human platelets.