COLLAGEN-LIKE PEPTIDE STIMULATES TYROSINE PHOSPHORYLATION OF SYK AND PHOSPHOLIPASE C-GAMMA-2 IN PLATELETS INDEPENDENT OF THE INTEGRIN ALPHA(2)BETA(1)

Activation of platelets by collagen is mediated through a tyrosine kin ase-dependent pathway that is associated with phosphorylation of the P c receptor gamma chain, the tyrosine kinase syk, and phospholipase C g amma 2 (PLC gamma 2). We recently described a collagen-related triple- helical peptide (CRP) with the sequence GCP(GPP*)GCP*G (single letter amino acid code: P = hydroxyproline; Morton et al, Biochem J 306:337 , 1995). The cross-linked peptide is a potent stimulus of platelet act ivation but, unlike collagen, does not support alpha(2) beta(1)-mediat ed, Mg2+-dependent adhesion, suggesting that its action is independent of the integrin alpha(2) beta(1). This finding suggests the existence of a platelet receptor other than alpha(2) beta(1) that underlies act ivation. In the present study, we show that CRP stimulates tyrosine ph osphorylation of the same pattern of proteins in platelets as collagen , including syk and PLC gamma 2. Protein tyrosine phosphorylation indu ced by CRP is not altered in the absence of Mg2+ or the presence of mo noclonal antibodies (MoAbs) to the integrin alpha(2) beta(1) (MoAb 6F1 and MoAb 13), conditions that prevent the interaction of collagen wit h the integrin. In contrast, phosphorylation of syk and PLC gamma 2 by collagen is partially reduced by MoAb 6F1 and MoAb 13 or by removal o f Mg2+. This may reflect a direct role of alpha(2) beta(1) in collagen -induced signaling events or an indirect role in which the integrin fa cilitates the binding of collagen to its signaling receptor. The resul ts show an alpha(2) beta(1)-independent pathway of platelet activation by CRP that involves phosphorylation of syk and PLC gamma 2. This pat hway appears to contribute to platelet activation by collagen. (C) 199 7 by The American Society of Hematology.