Protein-tyrosine kinase p72^syk in FcγRI receptor signaling

In this report we show that γ-interferon (IFN) induces the expression of the nonreceptor protein tyrosine kinase, p72^syk, and that cross-linking the FcγRI receptor in IFN-differentiated U937 cells (U937IF cells) results in the activation of syk kinase. We show that syk is tyrosine phosphorylated (12-fold increase) after FcγRI cross-linking. In vitro kinase assays demonstrate that the specific kinase activity of syk increased eightfold after FcγRI cross-linking. The activation of signal transduction through the FcγRI receptor, as measured by the respiratory burst, is associated with the tyrosine phosphorylation and catalytic activation of the syk kinase. We show that syk coprecipitates with the γ subunit of the FcγRI, FcγRIγ. The data suggest that p72^syk is involved in signal transduction through the FcγRI receptor, involving the FcγRIγ subunit.