Characterization of the functional insulin binding epitopes of the full-length insulin receptor

Mutational analyses of the secreted recombinant insulin receptor extracellular domain have identified a ligand binding site composed of residues located in the L1 domain (amino acids 1 - 470) and at the C terminus of the alpha subunit (amino acids 705 - 715). To evaluate the physiological significance of this ligand binding site, we have transiently expressed cDNAs encoding full-length receptors with alanine mutations of the residues forming the functional epitopes of this binding site and determined their insulin binding properties. Insulin bound to wild-type receptors with complex kinetics, which were fitted to a two-component sequential model; the K-d of the high affinity component was 0.03 nM and that of the low affinity component was 0.4 nM. Mutations of Arg(14), Phe(64), Phe(705), Glu(706), Tyr(708), Asn(711), and Val(715) inactivated the receptor. Alanine mutation of Asn(15) resulted in a 20-fold decrease in affinity, whereas mutations of Asp(12), Gln(34), Leu(36), Leu(37), Leu(87), Phe(89), Tyr(91), Lys(121), Leu(709), and Phe(714) all resulted in 4-10-fold decreases. When the effects of the mutations were compared with those of the same mutations of the secreted recombinant receptor, significant differences were observed for Asn(15), Leu(37), Asp(707), Leu(709), Tyr(708), Asn(711), Phe714, and Val715, suggesting that the molecular basis for the interaction of each form of the receptor with insulin differs. We also examined the effects of alanine mutations of Asn(15), Gln(34), and Phe(89) on insulin-induced receptor autophosphorylation. They had no effect on the maximal response to insulin but produced an increase in the EC50 commensurate with their effect on the affinity of the receptor for insulin.