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 proc ess of hemostasis. As with many other platelet stimuli, collagen signa l transduction involves the hydrolysis of inositol phospholipids; howe ver, 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 pla telets by immune complexes, and wheat germ agglutinin, which binds non -specifically to glycoprotein, stimulate phospholipase C-gamma 2 tyros ine phosphorylation. In contrast, we could not detect phospholipase C- gamma 2 tyrosine phosphorylation in platelets stimulated by either thr ombin or a stable thromboxane A(2) analogue, U46619.