The functional importance of Leu15 of human epidermal growth factor in receptor binding and activation

The biological importance of Leu15 of epidermal growth factor (EGF) is suggested by its conservation through evolution, its critical location in the domain-domain interface of EGF and its close proximity to Arg41, a residue that is crucial for receptor binding and activation. Mutagenesis of Leu15 of human EGF (hEGF) was employed to examine the role of this residue in the ligand-receptor interaction. The relative receptor affinities of the hEGF variants, as determined by radioreceptor competition assays, varied depending on the amino acid substitution. The L15F, L15W and L15V hEGF analogues had receptor affinities 45, 26 and 18% respectively of wild type hEGF. The L15A and L15R analogues displayed receptor affinities of only 2.4 and 1.6% relative to wild type hEGF. No binding of the L15E analogue was detected. The relative agonist activities, as measured by receptor tyrosine kinase stimulation assays, generally followed a similar trend. The L15F, L15W and L15V analogues stimulated the receptor kinase to a level (Vmax) similar to that for wild type hEGF. A striking difference was observed between the L15A and L15R variants; although having similar binding affinities, the L15A mutant activated the receptor to only approximately 5% of the wild type Vmax in contrast to 53% for the L15R mutant. 1H-NMR analysis of the L15R and L15A mutants showed only minor structural alterations that were not sufficient to account for the dramatic losses in binding and agonist activities. The results indicate that both the size and hydrophobicity of the gamma-branched aliphatic side chain of Leu15 of hEGF are important in the formation of a catalytically active ligand-receptor complex.