Direct phosphorylation of the IL-2 receptor Tac antigen epitope by protein kinase C

Phorbol esters induce a rapid phosphorylation of the antigenic epitope of the human IL-2 receptor identified by anti-Tac monoclonal antibody. The physiological activator of protein kinase C, diacylglycerol also stimulated the phosphorylation of the Tac epitope in intact activated human T lymphocytes. Stable derivatives of cyclic nucleotides had no effect on the stimulation of Tac phosphorylation with cultured lymphocytes. Immunoprecipitated Tac derived from particulate membranes could serve as a direct substrate for purified protein kinase C in vitro. The Ca2+/phospholipid dependency of the in vitro phosphorylation reaction substantiated that the phosphorylation of Tac observed in intact cells stimulated by phorbol ester or diacylglycerol was the result of the physiological activation of protein kinase C.