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A detergent extract isolated from the enriched fraction of integral membrane proteins of Jurkat cells showed an enhanced tyrosine phosphate level when phosphorylated in the presence of phorbol 12-myristate 13-acetate (TPA) and phorbol 12,13-dibutyrate (PDBu). The enhanced tyrosine phosphorylation was observed when the reaction time exceeded 6 min; at shorter incubation times, however, TPA inhibited tyrosine phosphorylation. When the reaction proceeded for a constant time period longer than 6 min and phorbol esters were added at different times after the start of the reaction, two phases of an enhanced tyrosine phosphorylation of a 50 kDa protein were observed. An increased phosphorylation of the 50 kDa protein was correlated with an enhanced phosphorylation of poly-(Glu 4 ,Tyr 1 ). The two phases of enhanced phosphorylation differed in their TPA and PDBu requirement and in the proteins that were tyrosine phosphorylated. Studies with protein kinase C (PKC) inhibitors showed a negatively correlated effect on the enhanced tyrosine phosphorylation in phase I; tyrosine phosphorylation was further augmented. In phase II the regulation of tyrosine phosphorylation correlated with the efficiency of the PKC inhibitors on the α-isoform of PKC which was found in the cell extract. Separation of the proteins present in the investigated cell extract by gel filtration revealed a co-migration of the α-PKC and the 50 kDa protein. The metabolic labeling of intact Jurkat cells with  32 P i  indicated that phorbol esters are also able to induce tyrosine phosphorylation of the 50 kDa protein under  in vivo  conditions.

Protein Kinase C-α Produces Reciprocal Effects on the Phorbol Ester Stimulated Tyrosine Phosphorylation of a 50 kDa Kinase in Jurkat Cells