Tyrosine phosphorylation of protein kinase C delta-isoform and its association with membrane

Stimulation of CHO cells stably overexpressing the delta-isoform of protein kinase C (delta PKC) by phorbol ester resulted in the tyrosine phosphorylation and association of the enzyme with particulate fraction. This tyrosine phosphorylation of delta PKC occurred preferentially in the enzyme prephosphorylated at serine/threonine residue(s). The enzymatic activity of tyrosine-phosphorylated delta PKC was dependent on both phospholipid and diacylglycerol which is the same as the non-tyrosine-phosphorylated form, and no significant difference was observed between the two forms for their kinetic properties and specific activities. When delta PKC was phosphorylated at tyrosine in vitro, phosphatidylserine and either diacylglycerol or phorbol ester were needed for the maximal rate of the reaction, suggesting that the tyrosine phosphorylation is a consequence of, but not a prerequisite for the enzyme activation. The tyrosine-phosphorylated delta PKC was associated with the particulate fraction, and presumably exists as a large complex in the membrane of the stimulated cells. It may be possible that the tyrosine phosphorylation is related to sustained activation and/or targeting of the delta PKC isoform.