Point mutation of a tyrosine in the linker region of Syk results in a gainof function

The protein tyrosine kinase Syk plays an essential role in Fc epsilon RI-me diated histamine release in mast cells by regulating the phosphorylation of other proteins. We investigated the functional role of a putative Syk phos phorylation site, Tyr(317). This tyrosine in the linker region of Syk is a possible site for binding by the negative regulator Chi, Syk with Tyr(317) mutated to Phe (Y317F) was expressed in a Syk-negative variant of the RBL-2 H3 mast cells. Compared with cells expressing wild-type Syk, expression of the Y317F mutant resulted in an increase in the Fc epsilon RI-mediated tyro sine phosphorylation of phospholipase C-gamma and a dramatic enhancement of histamine release. The in vivo Fc epsilon RI-induced tyrosine phosphorylat ion of wild-type Syk and that of the Y317F mutant were similar. Although th e FceRI-induced tyrosine phosphorylation of total cellular proteins was enh anced in the cells expressing the Y317F Syk, the phosphorylation of some ot her molecules, including the receptor subunits, Vav and mitogen-activated p rotein kinase, was not increased, The Fc epsilon RI-induced phosphorylation of Cbl was downstream of Syk kinase activity and was unchanged by expressi on of the Y317F mutation. These data indicate that Tyr(317) in the linker r egion of Syk functions to negatively regulate the signals leading to degran ulation.