Regulation of the epidermal growth factor receptor by phosphorylation

Abstract

The receptor for epidermal growth factor (EGF) is a glycosylated transmembrane phosphoprotein that exhibits EGF‐stimulable protein tyrosine kinase activity. On EGF stimulation, the receptor undergoes a self‐phosphorylation reaction at tyrosine residues located primarily in the extreme carboxyl‐terminal region of the protein. Using enzymatically active EGF receptor purified by immunoaffinity chromatography from A431 human epidermoid carcinoma cells, the self‐phosphorylation reaction has been characterized as a rapid, intramolecular process which is maximal at 30–37°C and exhibits a very low Km for ATP (0.2 μM). When phosphorylation of exogenous peptide substrates was measured as a function of receptor self‐phosphorylation, tyrosine kinase activity was found to be enhanced two to threefold at 1–2 mol of phosphate per mol of receptor. Analysis of the dependence of the tyrosine kinase activity on ATP concentration yielded hyperbolic kinetics when plotted in double‐reciprocal fashion, indicating that ATP can serve as an activator of the enzyme. Higher concentrations of peptide substrates were found to inhibit both the self‐ and peptide phosphorylation, but this inhibition could be overcome by first self‐phosphorylating the enzyme. These results suggest that self‐phosphorylation can remove a competitive/inhibitory constraint so that certain exogenous substrates can have greater access to the enzyme active site. In addition to self‐phosphorylation, the EGF receptor can be phosphorylated on threonine residues by the calcium‐ and phospholipid‐dependent protein kinase C. The sites on the EGF receptor phosphorylated in vitro by protein kinase C are identical to the sites phosphorylated on the receptor isolated from A431 cells exposed to the tumor promoters 12‐O‐tetradecanoylphorbol 13‐acetate or teleocidin. This phosphorylation of the EGF receptor results in a suppression of its tyrosine kinase and EGF binding activities both in vivo and in vitro. The EGF receptor can thus be variably regulated by phosphorylation: self‐phosphorylation can enhance tyrosine kinase activity whereas protein kinase C‐catalyzed phosphorylation can depress enzyme activity. Because these two phosphorylations account for only a fraction of the phosphate present in the EGF receptor in vivo, other protein kinases can apparently phosphorylate the receptor and these may exert additional controls on EGF receptor/kinase function.