Characterization of TCR-induced receptor-proximal signaling events negatively regulated by the protein tyrosine phosphatase PEP

The proline-, glutamic acid-, serine- and threonine-enriched protein tyrosi ne phosphatase PEP, which is expressed primarily in hematopoietic cells, wa s recently discovered to be physically associated with the 50-kDa cytosolic protein tyrosine kinase (PTK) Csk, an important suppressor of Src family P TK, including Lck and Fyn in T cells. We report that this phosphatase has a n inhibitory effect on TCR-induced transcriptional activation of the c-fos protooncogene and elements from the IL-2 gene promoter. Catalytically inact ive mutants of PEP had no effects in these assays. Expression of PEP also r educed activation of the N-terminal c-Jun kinase Jnk2 in response to recept or ligation, but not in response to UV light. In agreement with a more rece ptor-proximal site of action, we found that PEP reduced the TCR-induced inc rease in tyrosine phosphorylation of an Lck mutant, Lck-Y505F, which is onl y phosphorylated on tyrosine 394, the positive regulatory site. Finally, we observed that PEP reduced c-fos activation in a synergistic manner with Cs k, supporting the notion that these two enzymes form a functional team acti ng on Src family kinases involved in TCR signaling.