A study of protein phosphorylation in shape change and Ca++-dependent serotonin release by blood platelets

Upon treatment with agents such as thrombin, collagen or concanavalin A, blood platelets change shape, secrete serotonin and phosphorylate two proteins having molecular weights of approximately 20,000 and 40,000. We have analyzed the relationship of this protein phosphorylation to shape change and release aided by the fact that while shape change occurs independently of extracellular calcium, release of serotonin displays a rather strict calcium requirement. Under limited calcium conditions, where virtually no serotonin release occurs, (Con A)-stimulated phosphorylation is uninhibited. Divalent cations (Mg++, Co++ and Zn++) also inhibit release but not phosphorylation. The microtubule effectors colchicine and D2O show concomitant effects on release and phosphorylation, indicating a microtubule involvement prior to phosphorylation. Papaverine inhibits release and phosphorylation while not strongly influencing shape change, suggesting that shape change does not require phosphorylation. We therefore conclude that phosphorylation of these proteins takes place after shape change but prior to release, and although it may be required for secretion to occur, the two processes are easily separated. Thus phosphorylation of these proteins is not likely to be an integral component of the release mechanism.