COLLAGEN STIMULATES TYROSINE PHOSPHORYLATION OF PHOSPHOLIPASE C-GAMMA-2 BUT NOT PHOSPHOLIPASE C-GAMMA-1 IN HUMAN PLATELETS

Collagen is an important primary stimulus of platelets during the process of hemostasis. As with many other platelet stimuli, collagen signal transduction involves the hydrolysis of inositol phospholipids; however, the mechanism which underlies this event is not well understood. Neither the collagen receptor nor the isoform of phospholipase C that is activated have been identified. We report that collagen-activation of platelets induces tyrosine phosphorylation of phospholipase C-gamma 2 but not phospholipase C-gamma l. We also show that the platelet low affinity Fc receptor (Fc gamma RII), which mediates activation of platelets by immune complexes, and wheat germ agglutinin, which binds non-specifically to glycoprotein, stimulate phospholipase C-gamma 2 tyrosine phosphorylation. In contrast, we could not detect phospholipase C-gamma 2 tyrosine phosphorylation in platelets stimulated by either thrombin or a stable thromboxane A(2) analogue, U46619.