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In a preliminary study we demonstrated that the formation of the epidermal growth factor (EGF) receptor-ligand complex requires the participation of the highly conserved arginine 41 side chain of the growth factor peptide (Engler, D.A., Montelione, G.T., and Niyogi, S.K. (1990) FEBS Lett. 271, 47-50). In an attempt to gain further insight into the nature of this interaction(s), we used both site-directed mutagenesis and chemical modification reagents to produce human EGF (hEGF) analogues with altered chemical properties of the residue 41 side chain. Eight mutant analogues of hEGF were generated, substituting arginine 41 with lysine, glutamine, isoleucine, tyrosine, glycine, alanine, aspartate, or glutamate. Although each of the mutant analogues was able to displace wild-type hEGF fully in receptor competition binding assays, affinity of the receptor for the mutants was substantially reduced, varying from 0.4 to less than 0.01% of that observed for wild-type growth factor. At sufficiently high concentrations these mutants were able to stimulate DNA synthesis in mouse keratinocytes. Substitution of lysine for arginine 41 reduced the receptor affinity 250-fold from that observed for wild type, despite retention of the positive electrostatic charge. The lysine substitution leaves a reactive amine at position 41 and made it possible, using amine-specific chemical modification reagents, to produce selected arginine homologues that were tested for their effects on receptor binding, receptor tyrosine kinase activation, and stimulation of DNA synthesis in mouse keratinocytes. The reaction of lysine 41 with methyl acetimidate resulted in a lysineacetamidine product which only partially restored activity of the lysine hEGF mutant. However, reaction with O-methylisourea resulted in generation of an arginine 41 homologue (homoarginine) which restored full activity. The results indicate that the chemical properties inherent in the guanidinium group of the arginine 41 side chain of hEGF are responsible for optimal receptor-ligand association. |
