Thy-1 , a glycosyl-phosphatidylinositol-anchored surface glycoprotein, has been shown to possess transmembrane signaling capacity. In rat mast cells and rat basophilic leukemia cells (RBL) aggregation of surface Thy-1 with antibodies triggers a series of intracellular events, resembling those induced by aggregation of the high-affinity receptor for IgE (FceRI), including tyrosine phosphorylation of multiple proteins and release of secretory components. Unlike the FceRI-mediated activation, where both the membrane-associated protein tyrosine kinase (PTK) Lyn and the cytoplasmic PTK Syk are responsible for initiating the signaling cascade, only Lyn has been implicated in Thy-1 -mediated activation in RBL cells.
Here we report that Syk is also rapidly tyrosine phosphorylated upon Thy-1 cross-linking. Increased Syk tyrosine phosphorylation is observed only in cells in which extensive aggregation of Thy-I is induced by two layers of cross-linking reagents. RBL-derived mutant cells deficient in the expression of surface Thy-1 and transfectants re-expressing surface Thy-I were used to exclude the possibility that Syk activation reflects an interaction of the crosslinking reagents with surface molecules other than Thy-1. As FceRl y subunits are well known to promote activation of Syk and its recruitment to membrane complexes, we also investigated the role of these subunits in Thy-1 -mediated Syk activation, using RBL-derived mutant cells deficient in the expression of FceRl y subunits and their revertants. Consistent with the lack of FceRl expression, no IgE-induced response could be elicited, while Thy-1inducible Syk phosphorylation was preserved. Our results suggest that Syk might be one of the kinases responsible for signal propagation upon Thy-1 cross-linking in a FceRIindependent pathway.