title: 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 degrees C and exhibits a very low Km for ATP (0.2 microM). 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.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000804:Angiotensin II / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D004789:Enzyme Activation; D066246:ErbB Receptors; D006801:Humans; D007700:Kinetics; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D011956:Receptors, Cell Surface / Q000502:physiology*; D013379:Substrate Specificity; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism
chemical_list: D011956:Receptors, Cell Surface; D000804:Angiotensin II; D013912:Threonine; D000255:Adenosine Triphosphate; D066246:ErbB Receptors; D011505:Protein-Tyrosine Kinases; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate