INDUCTION OF HYPERPHOSPHORYLATION AND ACTIVATION OF THE P56(LCK) PROTEIN-TYROSINE KINASE BY PHENYLARSINE OXIDE, A PHOSPHOTYROSINE PHOSPHATASE INHIBITOR

The T cell protein tyrosine kinase p56(lck) is implicated in thymic de velopment and mitogenic activation of T lymphocytes, and is itself reg ulated by reversible tyrosine phosphorylation. When phenylarsine oxide (PAO), a membrane-permeable inhibitor of phosphotyrosine phosphatases , was added to Jurkat T leukemia or LSTRA thymoma cells, the phosphate content of p56(lck) increased rapidly. The sites of increased phospho rylation were mapped to Tyr-192, Tyr-394 and Tyr-505. Hyperphosphoryla ted p56(lck) displayed retarded mobility on SDS gels, unaltered or mar ginally increased cytoskeletal association, and its catalytic activity changed in a biphasic manner; during the first 10-20 min of PAC-treat ment the activity increased and then it declined to very low values wi thin 1-2 hr. Our data suggest that p56(lck) contains both positive and negative regulatory sites which are constantly dephosphorylated at an unexpectedly high rate by cellular phosphotyrosine phosphatases.