REGULATION OF VAV-SLP-76 BINDING BY ZAP-70 AND ITS RELEVANCE TO TCR-ZETA CD3 INDUCTION OF INTERLEUKIN-2/

T cell activation stimulates p56(lck), p59(fyn), ZAP-70, Vav-SLP-76 bi nding, and IL-2 transcription. Major questions concern the tyrosine-ki nase and relevant site(s) needed for Vav-SLP-76 complex formation and its role in IL-2 production. Here, we show that of the three kinases, only ZAP-70 phosphorylates SLP-76 at specific sites that allow Vav SH2 domain binding. Therefore, while p56(lck) regulates proximal events, ZAP-70 acts downstream on targets such as SLP-76. We also show by in v itro and in vivo analysis that two SLP-76 pYESP motifs (Y113 and Y128) mediate binding, the first being more efficient. A third pYEPP motif (Y145) failed to bind. Finally, TCR zeta/CD3 ligation of T cell hybrid oma DC27.10 induces IL-2 production without detectable Vav-SLP-76 bind ing. Therefore, despite effects of Vav-SLP-76 on IL-2 expression, Vav- SLP-76 binding per se is not essential for IL-2 production in all T ce lls.