T-CELL RECEPTOR-ZETA CD3-P59(FYN(T))-ASSOCIATED P120 130 BINDS TO THESH2 DOMAIN OF P59(FYN(T))/

Intracellular signaling from the T cell receptor (TCR)zeta/CD3 complex is likely to be mediated by associated protein tyrosine kinases such as p59fyn(T), ZAP-70, and the CD4:p56lck and CD8:p56lck coreceptors. T he nature of the signaling cascade initiated by these kinases, their s pecificities, and downstream targets remain to be elucidated. The TCR- zeta/CD3:p59fyn(T) complex has previously been noted to coprecipitate a 120/130-kD doublet (p120/130). This intracellular protein of unknown identity associates directly with p59fyn(T) within the receptor compl ex. In this study, we have shown that this interaction with p120/130 i s specifically mediated by the SH2 domain (not the fyn-SH3 domain) of p59fyn(T). Further, based on the results of in vitro kinase assays, p1 20/130 appears to be preferentially associated with p59fyn(T) in T cel ls, and not with p56lck. Antibody reprecipitation studies identified p 120/130 as a previously described 130-kD substrate of pp60v-src whose function and structure is unknown. TCR-zeta/CD3 induced activation of T cells augmented the tyrosine phosphorylation of p120/130 in vivo as detected by antibody and GST:fyn-SH2 fusion proteins. p120/130 represe nts the first identified p59fyn(T):SH2 binding substrate in T cells, a nd as such is likely to play a key role in the early events of T cell activation.