The roles of tyrosines 24, 31, and 60 in the high affinity binding of insulin-like growth factor-I to the type 1 insulin-like growth factor receptor

A series of insulin-like growth factor I (IGF-I) structural analogs in which one or more of the three tyrosine residues were replaced with nonaromatic residues were produced and their binding properties characterized. The single point mutations, Leu24]IGF-I, Ala31]IGF-I, and Leu60]IGF-I result in an 18-, 6-, or 20-fold loss in affinity, respectively, for the type 1 IGF receptor. Multiple mutations, Ala31,Leu60]IGF-I, Leu24, Ala31]IGF-I, Leu24, Leu60]IGF-I, or Leu24, Ala31, Leu60]IGF-I result in a 520-, 240-, 1200-, or greater than 1200-fold loss in affinity, respectively, at the type 1 IGF receptor. In contrast, none of the analogs display greater than a 2-fold loss in affinity for the acid-stable human serum binding proteins. At the insulin receptor, Ala31]IGF-I and Leu24]IGF-I are equipotent to and 5-fold less potent than IGF-I, whereas Leu60]IGF-I and the multiple mutation analogs are inactive up to 10 microM. Analogs Leu24]IGF-I, Ala31]IGF-I, and Leu24, Ala31]IGF-I are equipotent to IGF-I at the type 2 IGF receptor, whereas all analogs containing Leu60 demonstrate little measurable affinity at this receptor. Thus, Tyr24, Tyr31, and Tyr60 are involved in the high affinity binding of IGF-I to the type 1 IGF receptor, while Tyr60 is important for maintaining binding to the type 2 IGF receptor.