Identification of lymphocyte integral membrane proteins as substrates for protein kinase C. Phosphorylation of the interleukin-2 receptor, class I HLA antigens, and T200 glycoprotein.

The interleukin-2 (IL-2) receptor, the leukocyte-specific membrane glycoprotein, T200, and the class I major histocompatibility antigens (HLA) have been identified as substrates for protein kinase C in vitro. IL-2 receptors on normal human T lymphocytes and the leukemic cell line, HUT102B2, are rapidly phosphorylated in vivo in response to the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA). Tryptic peptide analysis showed that the in vitro and in vivo 32P-labeled IL-2 receptors were phosphorylated on the same sites. A synthetic peptide corresponding to the carboxyl-terminal cytoplasmic tail of the IL-2 receptor was shown to be phosphorylated in vitro by protein kinase C. Tryptic digestion of the peptide generated the same 32P-labeled species as those found for the IL-2 receptor. From these studies, it was concluded that Ser-247 is the major site of phosphorylation in the IL-2 receptor and that Thr-250 is a minor site. These results also provide direct evidence that the in vivo phosphorylation of the IL-2 receptor stimulated by TPA is catalyzed by protein kinase C. The sites phosphorylated in the HLA antigens in vitro by protein kinase C or in vivo after TPA stimulation were also localized to the carboxyl-terminal cytoplasmic domain of the heavy chain by limited proteolysis.