THE FC RECEPTOR GAMMA-CHAIN AND THE TYROSINE KINASE SYK ARE ESSENTIALFOR ACTIVATION OF MOUSE PLATELETS BY COLLAGEN

Activation of mouse platelets by collagen is associated with tyrosine phosphorylation of multiple proteins including the Fc receptor gamma-c hain, the tyrosine kinase Syk and phospholipase C gamma 2, suggesting that collagen signals in a manner similar to that of immune receptors, This hypothesis has been tested using platelets from mice lacking the Fc receptor gamma-chain or Syk, Tyrosine phosphorylation of Syk and p hospholipase C gamma 2 by collagen stimulation is absent in mice lacki ng the Fe receptor gamma-chain, Tyrosine phosphorylation of phospholip ase C gamma 2 by collagen stimulation is also absent in mice platelets which lack Syk, although phosphorylation of the Fc receptor gamma-cha in is maintained, In contrast, tyrosine phosphorylation of platelet pr oteins by the G protein-coupled receptor agonist thrombin is maintaine d in mouse platelets deficient in Fc receptor gamma-chain or Syk, The absence of Fe receptor gamma-chain or Syk is accompanied by a loss of secretion rind aggregation responses in collagen- but not thrombin-sti mulated platelets, These observations provide the first direct evidenc e of an essential role for the immunoreceptor tyrosine-based activatio n motif (ITAM) in signalling by a non-immune receptor stimulus.