Sumn'lary p56 kk, a member of the src family of protein tyrosine kinases, is an essential component in T cell receptor (TCR) signal transduction, p56 kk 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 proteins containing phosphorylated tyrosine. Using an in vitro assay, we have studied specific binding of tyrosine-phosphorylated proteins to a recombinant p56 ~k SH2 domain. In nonactivated Jurkat cells, two tyrosine-phosphorylated proteins were detected. Stimulation with anti-CD3 monoclonal antibodies induced the binding of seven additional tyrosine-phosphorylated proteins to the SH2 domain of p56 ~k. We have identified the ~'-associated tyrosine kinase, ZAP-70, as one of these proteins. Evidence suggests that binding of ZAP-70 to p56 ~k SH2 is direct and not mediated by ~'. The significance of this interaction was further investigated in vivo. p56 kk could be coprecipitated with the ~'/ZAP-70 complex and conversely, ZAP-70 was detected in p56 kk immunoprecipitates of activated Jurkat cells. The physical association of p56 k~ and ZAP-70 during activation supports the recently proposed functional cooperation of these two tyrosine kinases in TCR signaling.

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