A COMPARISON OF THE INTERACTION OF SHC AND THE TYROSINE KINASE ZAP-70WITH THE T-CELL ANTIGEN RECEPTOR ZETA-CHAIN TYROSINE-BASED ACTIVATIONMOTIF

Tyrosine-based activation motifs (TAMs) define a conserved signaling s equence, EX(2)YX(2)L/IX(7)L/I, that couples the T cell antigen recepto r to protein tyrosine kinases and adapter molecules. The present study shows that phosphorylation of both tyrosines within the motif is requ ired for high affinity binding of the tyrosine kinase ZAP-70 whereas p hosphorylation of the single COOH-terminal tyrosine within the motif i s optimal for the binding of the adapter Shc. There were also quantita tive differences in the ZAP-70 and Shc association with the zeta 1-TAM since nM concentrations of the doubly phosphorylated zeta 1-TAM are s ufficient for ZAP-70 recruitment whereas micromolar levels of singly p hosphorylated TAMs are necessary for Shc binding. Shc is tyrosine phos phorylated in antigen receptor-activated T cells and can potentially f orm a complex with the adapter molecule Grb2 and could thus recruit th e Ras guanine nucleotide exchange protein Sos into the antigen recepto r complex. The present data show that Grb2 can bind to the phosphoryla ted TAM, but this binding is independent of Shc and there is no format ion of zeta 1-TAM . Shc . Grb2 . Sos complexes in antigen receptor-act ivated cells. Accordingly, Shc function should not be considered in th e context of Grb2/Sos recruitment to the T cell antigen receptor compl ex.