FIGURE 1.

Ligand binding to the EGF receptor. A, model for the binding of EGF to its receptor in a dimerizing system. The circles represent EGF receptor subunits. E represents a molecule of EGF. L₂₀, K₁₁, K₂₁, and K₂₂ refer to association constants for the indicated equilibria. Units for the three binding constants (K₁₁, K₂₁, and K₂₂) are m⁻¹. The unit for L₂₀ is D⁻¹ (mol/dm²), which represents the surface density of the receptor (22). B, ¹²⁵I-EGF binding to the wild type EGF receptor. CHO cells expressing increasing levels of wild type EGF receptor were subjected to ¹²⁵I-EGF radioligand binding as described under “Experimental Procedures.” The data were globally fit to the equation for binding in a dimerizing system as described. The fitted parameters are given in the inset.