Regulatory features of the epidermal growth factor receptor.

The role of EGF receptor concentration in tumor growth was investigated in athymic mice by measuring the rate of growth of clonal human epidermoid carcinoma A431 cells containing different extents of EGF receptor gene amplification and protein expression. A direct correlation-between the rate of tumor growth and EFG receptor concentration was found, supporting previous cell culture studies that quantitated the relationship between activated EGF receptors and cell proliferation. Holo EGF receptor is activated by ligand binding to the extracellular domain to activate cytoplasmic tyrosine protein kinase activity. A model of single molecule transmembrane signaling is proposed. The function of two phosphorylation sites on the EGF receptor has been analyzed by use of site-directed mutagenesis. Comparison of normal and mutant hEGF receptors expressed in rodent cells lacking endogenous EGF receptors indicates that: 1) Thr654, located 10 amino acids carboxyl terminal to the inner membrane boundary, is a major site of heterologous regulation via protein kinase C catalyzed phosphorylation, and 2) Tyr1173, the major site of self-phosphorylation, located at the carboxyl terminus, provides a secondary level of regulation of receptor function by acting as a competitive inhibitor with exogenous substrates.