THE INTERACTION OF AN EPIDERMAL GROWTH-FACTOR TRANSFORMING GROWTH-FACTOR-ALPHA TAIL CHIMERA WITH THE HUMAN EPIDERMAL GROWTH-FACTOR RECEPTORREVEALS UNEXPECTED COMPLEXITIES

It has been assumed that substitution of homologous regions of transfo rming growth factor alpha (TGF-alpha) into epidermal growth factor (EG F) can be used to probe ligand-receptor recognition without detrimenta l effects on ligand characteristics for the human EGF receptor (EGFR), We show that a chimera of murine (m) EGF in which the carboxyl-termin al tail is substituted for that of TGF-alpha (mEGF/TGF-alpha(44-50)) r esults in complex features that belie this initial simplistic assumpti on, Comparison of EGF and mEGF/TGF-alpha(44-50) in equilibrium binding assays showed that although the relative binding affinity of the chim era was reduced 80-200-fold, it was more potent than EGF in mitogenesi s assays using NR6/HER cells, This superagonist activity could not be attributed to differences in ligand processing or to binding to other members of the c-erbB family. It appeared to be due, in part, to choic e of an EGFR overexpressing target cell where high receptor number com pensated for the low affinity of the ligand; it also appeared to be re lated to the ability of the chimera to activate the EGFR tyrosine kina se, Thus, when EGFR autophosphorylation was measured, mEGF/TGF-alpha(4 4-50) was more potent than EGF, despite its low affinity, When tested using chicken embryo fibroblasts, substitution of the TGF-alpha carbox yl-terminal tail into mEGF failed to enhance its binding affinity for chicken EGFRs; however, the chimera was intermediate in potency betwee n TGF-alpha and mEGF in mitogenesis assays, Our results suggest a cont extual requirement for EGFR recognition which is ligand-specific, Furt her, the unpredictable responses to chimeric ligands underline the com plex nature of the processes of ligand recognition, receptor activatio n, and the ensuing cellular response.