CD45 regulation of tyrosine phosphorylation and enzyme activity of src family kinases.

Previous analyses have suggested that the CD45 tyrosine phosphatase activates src family tyrosine kinases p56lck and p59fyn by dephosphorylating regulatory COOH-terminal residues. We have examined the status of p56lck and p59fyn in murine and human CD45- T cell lines. Surprisingly, despite the fact that p56lck and p59fyn were spontaneously hyperphosphorylated, the tyrosine kinase activity of both enzymes was increased in CD45- versus CD45+ cells. In vitro exposure of hyperphosphorylated p56lck to CD45 decreased enzyme activity to near-basal levels. Lck from CD45- cells was hyperphosphorylated on the cyanogen bromide digestion fragment that contains the negative regulatory residue Tyr-505, and the identity of this site of phosphorylation was confirmed by trypsin digestion followed by high performance liquid chromatography. Loss of CD45 results, therefore, in a paradoxical hyperphosphorylation of the COOH-terminal tyrosine and increased src family kinase enzymatic activity.