C-kinase phosphorylates the epidermal growth factor receptor and reduces its epidermal growth factor-stimulated tyrosine protein kinase activity.

The Ca2+- and phospholipid-dependent protein kinase (C-kinase) binds tightly in the presence of Ca2+ to purified membranes of A431 human epidermoid carcinoma cells. The major membrane substrate for C-kinase is the epidermal growth factor (EGF) receptor. Phosphorylation of the EGF receptor is Ca2+-dependent and occurs at threonine and serine residues. After tryptic digestion of the receptor, three major phosphothreonine-containing peptides were identified. These are identical with three new phosphopeptides present in the EGF receptor isolated from A431 cells treated with either of the tumor promoters 12-O-tetradecanoylphorbol 13-acetate or teleocidin. C-kinase catalyzes phosphorylation at these same sites in purified EGF receptor protein. These results indicate that, in A431 cells exposed to tumor promoters, C-kinase catalyzes phosphorylation of a significant population of EGF receptor molecules. This phosphorylation of EGF receptors results in decreased self-phosphorylation of the EGF receptor at tyrosine residues both in vivo and in vitro and in decreased EGF-stimulated tyrosine kinase activity in vivo.