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

Abstract

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(γ)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 Fc receptor γ-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 γ-chain with endogenous Syk in collagen-stimulated platelets. The Fc receptor γ- chain undergoes tyrosine phosphorylation in platelets stimulated by a collagen-related peptide which does not bind the integrin α2β1 and by the lectin wheat germ agglutinin. In contrast, cross-linking of the platelet low affinity receptor for immune complexes, FcγRIIA, or stimulation by thrombin does not induce phosphorylation of the Fc receptor γ-chain. The present results provide a molecular basis for collagen activation of platelets which is independent of the integrin α2β1 and involves phosphorylation of the Fc receptor γ-chain, its association with Syk and subsequent phosphorylation of phospholipase C(γ)2. Collagen is the first example of a nonimmune receptor stimulus to signal through a pathway closely related to signaling by immune receptors.