Tyrosine phosphorylation of the Fc receptor gamma-chain in collagen-stimulated platelets

Stimulation of platelets by the extracellular matrix protein collagen leads to activation of a tyrosine kinase-dependent mechanism resulting in secretion and aggregation. Tyrosine phosphorylation of the tyrosine kinase Syk. and phospholipase C(gamma)2 are early events in collagen-induced activation. We recently proposed that collagen-signaling in platelets involves a receptor or a receptor-associated protein containing an immunoreceptor tyrosine based activation motif (ITAM) enabling interaction with Syk. In this report we show that collagen stimulation of platelets causes rapid tyrosine phosphorylation of the ITAM containing Fe receptor gamma-chain and that this is precipitated by the tandem Src homology 2 (SH2) domains of Syk expressed as a fusion protein. In addition we demonstrate an association between the Fc receptor gamma-chain with endogenous Syk in collagen stimulated platelets. The Fc receptor gamma-chain undergoes tyrosine phosphorylation in platelets stimulated by a collagen-related peptide which does not bind the integrin alpha(2) beta(1) and by the lectin wheat germ agglutinin. In contrast, cross-linking of the platelet low affinity receptor for immune complexes, Fc gamma RIIA, or stimulation by thrombin does not induce phosphorylation of the Fc receptor gamma-chain. The present results provide a molecular basis for collagen activation of platelets which is independent of the integrin alpha(2) beta(1) and involves phosphorylation of the Pc receptor gamma-chain, its association with Syk and subsequent phosphorylation of phospholipase C(gamma)2. Collagen is the first example of a nonimmune receptor stimulus to signal through a pathway closely related to signaling by immune receptors.