Activation of Epidermal Growth Factor Receptor by Epidermal Growth Factor

The binding of epidermal growth factor (EGF) to epidermal growth factor receptor (EGF receptor) induces dimerization of the receptor and activation of its protein tyrosine kinase. Each of these three steps was followed as a function of the concentrations of EGF and of EGF receptor. Binding of EGF was followed by sedimentation of the complex between [3H]EGF and EGF receptor, dimerization was measured by quantitative cross-linking with glutaraldehyde, and the activation of the protein tyrosine kinase was monitored under the same conditions by following the initial velocity of the phosphorylation of peptides containing tyrosine. The binding of epidermal growth factor to its receptor was measured as a function of the concentration of epidermal growth factor, and the relationship was sigmoid with an average value of 1.7 for the Hill coefficient. Both dimerization and the activation of the tyrosine kinase displayed saturation as a function of the concentration of EGF. The ranges of the concentrations of EGF where dimerization and activation of the tyrosine kinase activity were half-maximal were 15−30 and 50−200 nM, respectively, but the value for the concentration of EGF at the half-maximum for the activation of the tyrosine kinase was a complex function of the concentration of EGF receptor. The observed behavior of the binding of EGF, the dimerization of EGF receptor, and the activation of the tyrosine kinase were used as criteria against which to test mechanisms for the process of activation. Equations were derived for various reversible and irreversible mechanisms and used to calculate the theoretical behaviors of the three properties. In direct comparisons of the experimental and the theoretical data, several of the previously proposed reversible and irreversible mechanisms for the activation of EGF receptor were found to be inadequate, but a reasonable mechanism was formulated that was compatible with the experimental data. In this mechanism, dimeric EGF receptor must be occupied by two molecules of EGF for enzymatic activity to be expressed.