REGULATION OF THE LCK SH2 DOMAIN BY TYROSINE PHOSPHORYLATION

Src homology 2 (SH2) domains bind to phosphotyrosine (Tyr(P)) residues in specific sequence contexts in other proteins and thereby mediate t yrosine phosphorylationdependent protein-protein interactions. The SH2 domain of the Src family kinase Lck is phosphorylated at tyrosine 192 in T cells upon T cell antigen receptor triggering. We have studied t he consequences of this phosphorylation on the properties of the SH2 d omain and on the function of Lck in T cell activation. We report that phosphorylation at Tyr(192) reduced the capacity of the isolated SH2 d omain to bind a high affinity peptide ligand and Tyr(P)-containing cel lular proteins. This effect was mimicked by mutation of Tyr(192) to an acidic residue. In intact T cells, where Lck participates in T cell a ntigen receptor signal transduction in an SH2 domain dependent manner, phosphorylation of Tyr(192) correlated with reduced downstream signal ing. Our results indicate that tyrosine phosphorylation of the SH2 dom ain of Lck terminates its high affinity binding to ligands, thereby ne gatively regulating its participation in T cell antigen receptor signa ling. This represents a novel mechanism for the regulation of the func tion of SH2 domains.