TI - 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 . AB - The interleukin-2 ( IL-2 ) receptor , the leukocyte-specific membrane glycoprotein , Thr200 , 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 .