Cooperative inhibition of T-cell antigen receptor signaling by a complex between a kinase and a phosphatase

Antigen receptor-triggered T-cell activation is mediated by the sequential action of the Src and Syk/Zap-70 families of protein tyrosine kinases (PTKs ). Previously, we reported that another PTK termed p50(csk) was a potent ne gative regulator of T-cell receptor (TCR) signaling because of its ability to inactivate Src-related kinases. This inhibitory effect required the cata lytic activity of Csk, as well as its Src homology (SH)3 and SH2 domains. S ubsequent studies uncovered that, via its SH3 domain, p50(csk) was associat ed with PEP, a proline-enriched protein tyrosine phosphatase (PTP) of unkno wn function expressed in hemopoietic cells. Herein, we have attempted to id entify the role of the Csk-PEP complex in T lymphocytes. The results of our experiments showed that, like Csk, PEP was a strong repressor oi TCR signa ling. This property was dependent on the phosphatase activity of PEP, as we ll as oil the sequence mediating its binding to p50(csk). Through reconstit ution experiments in Cos-l cells, evidence was obtained that Csk and PEP ac t synergistically to inhibit protein tyrosine phosphorylation by Src-relate d kinases, and that this effect requires their association. Finally, experi ments with a substrate-trapping mutant of PEP suggested that PEP functions by dephosphorylating and inactivating the PTKs responsible for T-cell activ ation, in addition to giving novel insights into the mechanisms involved in the negative regulation of T-cell activation, these findings indicate that the association of all inhibitory PTK with a PTP constitutes a more effici ent means of inhibiting signal transduction by Src family kinases in vivo.