INSULIN-LIKE GROWTH-FACTOR (IGF)-I A-DOMAIN AND B-DOMAIN ANALOGS WITHALTERED TYPE-1 IGF AND INSULIN-RECEPTOR BINDING SPECIFICITIES

Insulin-like growth factor-I (IGF-I) analogues were produced with the aim of identifying IGF-I residues that contribute to the specificity o f binding to the type I IGF receptor as opposed to the insulin recepto r. Receptor binding properties of a series of A- and B-domain analogue s were compared using rat L6 myoblasts, soluble human IGF type 1 recep tors and soluble human insulin receptor isoforms HIR-A (-Ex11) and HIR -B (+Ex11). IGF-I analogues, [Leu(8)] IGF-I and [Phe(59)] IGF-I, were shown to exhibit respectively, a 28- and 17-fold decrease in affinity for the HIR-A with only a 6- and 5-fold decrease in affinity for the h uman IGF type 1 receptor. In contrast, the analogue [His(4)] IGF-I was equivalent to IGF-I in binding to the soluble type 1 IGF receptor whi le showing 7-fold and 4-fold increases in HIR-A and HIR-B binding resp ectively. Furthermore, [Leu(62)] IGF-I was 8-fold less potent than IGF -I in soluble IGF type 1 receptor binding but only showed a 2-fold dec rease in HIR-A and HIR-B binding. Our study supports the conclusion th at the co-evolution of the IGF-I and insulin receptor/ligand systems h as resulted in subtle structural differences in the A- and B-regions o f each ligand important for defining receptor binding specificity.