IDENTIFICATION OF THE SITE IN THE SYK PROTEIN-TYROSINE KINASE THAT BINDS THE SH2 DOMAIN OF LCK

The Syk protein tyrosine kinase (PTK) is expressed in many hematopoiet ic cells and is involved in signaling from various receptors for antig en and Fc portions of IgG and IgE. Upon cross-linking of these recepto rs, Syk is rapidly phosphorylated on tyrosine residues and enzymatical ly activated. We and others have found that the Lck kinase, a member o f the Src family of PTKs, binds through its Src homology (SH) 2 domain to tyrosine phosphorylated Syk and to the related Zap kinase. Here we report that this interaction is direct and identify the two tandem ty rosines at the autophosphorylation site of Syk, Tyr(518), and Tyr(519) , as the binding site for the SH2 domain of Lck. Mutation of either or both tyrosines to phenylalanines abrogated binding, while mutation of a second repetition of the motif at Tyr(539) and Tyr(540), or of the three tyrosines in the C terminus of Syk, did not. The SH2 domain of L ck bound the autophosphorylation site only when both Tyr(518) and Tyr( 519) were phosphorylated. In intact cells the binding of the SH2 domai n of Lck correlated with the ability of Syk to induce tyrosine phospho rylation of cellular proteins.