P56(LCK) INTERACTS VIA ITS SRC HOMOLOGY-2 DOMAIN WITH THE ZAP-70 KINASE

p56kk, a member of the src family of protein tyrosine kinases, is an e ssential component in T cell receptor (TCR) signal transduction. p56lc k contains a src homology 2 (SH2) domain found in a number of proteins involved in intracellular signaling. SH2 domains have been implicated in protein-protein interactions by binding to sequences in target pro teins containing phosphorylated tyrosine. Using an in vitro assay, we have studied specific binding of tyrosine-phosphorylated proteins to a recombinant p56lck SH2 domain. In nonactivated Jurkat cells, two tyro sine-phosphorylated proteins were detected. Stimulation with anti-CD3 monoclonal antibodies induced the binding of seven additional tyrosine -phosphorylated proteins to the SH2 domain of p56lck. We have identifi ed the zeta-associated tyrosine kinase, ZAP-70, as one of these protei ns. Evidence suggests that binding of ZAP-70 to p56lck SH2 is direct a nd not mediated by zeta. The significance of this interaction was furt her investigated in vivo. p56lck could be coprecipitated with the zeta /ZAP-70 complex and conversely, ZAP-70 was detected in p56lck immunopr ecipitates of activated Jurkat cells. The physical association of p56l ck and ZAP-70 during activation supports the recently proposed functio nal cooperation of these two tyrosine kinases in TCR signaling.