TYROSINE PHOSPHORYLATION IF CD45 PHOSPHOTYROSINE PHOSPHATASE BY P50(CSK) KINASE CREATES A BINDING-SITE FOR P56(LCK) TYROSINE KINASE AND ACTIVATES THE PHOSPHATASE

Src family protein tyrosine kinases (PTKs) play an essential role in antigen receptor-initiated lymphocyte activation. Their activity is largely regulated by a negative regulatory tyrosine which is a substrate for the activating action of the CD45 phosphotyrosine phosphatase (PTPase) or, conversely, the suppressing action of the cytosolic p50(csk) PTK. Here we report that CD45 was phosphorylated by p50(csk) on two tyrosine residues, one of them identified as Tyr-1193. This residue was not phosphorylated by T-cell PTKs p56(lck) and p59(fyn). Tyr-1193 was phosphorylated in intact T cells, and phosphorylation increased upon treatment with PTPase inhibitors, indicating that this tyrosine is a target for a constitutively active PTK. Cotransfection of CD45 and csk into COS-1 cells caused tyrosine phosphorylation of CD45 in the intact cells. Tyrosine-phosphorylated CD45 bound p56(lck) through the SH2 domain of the kinase. Finally, p50(csk)-mediated phosphorylation of CD45 caused a severalfold increase in its PTPase activity. Our results show that direct tyrosine phosphorylation of CD45 can affect its activity and association with Src family PTKs and that this phosphorylation could be mediated by p50(csk). If this is also true in the intact cells, it adds a new dimension to the physiological function of p50(csk) in T lymphocytes.