MURINE T-CELL DIFFERENTIATION ANTIGEN CD8 IS A DIRECT SUBSTRATE OF PROTEIN KINASE-C

Murine T cell differentiation antigen CD8α (Lyt-2) is phosphorylated in vivo after phorbol 12-myristate 13-acetate (PMA) treatment of cells. Concanavalin A, dibutyryl cAMP and calcium ionophore are unable to stimulate phosphate incorporation into CD8α. Depletion of cellular protein kinase C (PKC) by prolonged PMA treatment abolished this phosphorylation, suggesting that PKC is required for this effect. Using the amino acid sequence derived from cloning CD8α, peptides encompassing both possible intracellular phosphorylation sites were made and used to test the ability of various kinases to phosphorylate CD8α sequences. Only the proximal serine peptide was a kinase substrate, and of PKC, cAMP-dependent kinase and the multifunctional calcium/calmodulin-dependent kinase, only PKC was able to phosphorylate this peptide. These studies provide the first definitive evidence that CD8α is a direct substrate of PKC. © 1990.