ROLE OF TYR518 AND TYR519 IN THE REGULATION OF CATALYTIC ACTIVITY ANDSUBSTRATE PHOSPHORYLATION BY SYK PROTEIN-TYROSINE KINASE

The Syk protein-tyrosine kinase is expressed in many hematopoietic cel ls and is involved in signaling from various receptors for antigen and Fc portions of IgG and IgE. After cross-linking of these receptors, S yk is rapidly phosphorylated on tyrosine residues. We have previously reported that Syk expressed in COS cells is predominantly phosphorylat ed at both Tyr518 and Tyr519 at its putative autophosphorylation site. In this study, we have examined the role of each of these two residue s for the catalytic activity of Syk in vitro and for the Syk-induced p hosphorylation of cellular proteins in intact cells. Mutation of eithe r residue had minor effects on the catalytic activity of Syk, and even the double mutant [F518, F519]Syk was about 60% as active as the wild -type enzyme. In intact cells, however, all three mutants consistently failed to induce the extensive tyrosine phosphorylation of cellular p roteins typically observed with wild-type Syk. We have recently shown that the doubly phosphorylated Y518/Y519 site is also the site for ass ociation of Syk with the SH2 domain of the Lck kinase, which suggests that although phosphates at Y518/Y519 may enhance the catalytic activi ty of Syk, its interaction with Src family protein-tyrosine kinases is at least equally important for the induction of downstream substrate phosphorylation.