INVOLVEMENT OF PHENYLALANINE-23 IN THE BINDING OF IGF-1 TO THE INSULIN AND TYPE-I IGF RECEPTOR

The hydrophobic residue Phe-23 lies on a surface of insulin like growt h factor 1 (IGF-1) which may be involved in binding to the type I IGF and insulin receptors. The possibility that Phe-23 participates direct ly in binding to these receptors has been investigated by comparing th e properties of [F23G]IGF-1, a mutant of insulin-like growth factor 1 in which Phe-23 has been replaced by Gly, with those of IGF-1 and insu lin. [F23G]IGF-I has a 48-fold lower affinity for the type I IGF recep tor, a markedly reduced affinity for the insulin receptor and a 100-fo ld reduced affinity for insulin-like growth factor binding proteins (I GFBPs) compared to IGF-1. [F23G]IGF-1 is a full IGF-1 agonist as measu red by its ability to stimulate cell proliferation. Its potency was on ly 4.5-fold less than IGF-1, probably because its bioavailability was increased as a result of its reduced affinity for IGFBPs. The large re duction in the affinity of [F23G]IGF-1 for the type I IGF receptor and insulin receptor contrasts with the lack of effect of the correspondi ng alteration to insulin [FB24G]. Phe-B24 is not thought to be involve d directly in the binding of insulin to the insulin receptor and the C -terminus of the B-chain of insulin is suggested to be displaced on bi nding. We suggest that for IGF-1, the C-terminus of the B chain cannot be displaced because of the presence of the C-domain and the large re duction in the binding affinities of [F23G]IGF-1 suggest that the Phe- 23 side chain may be directly involved in binding of IGF-1 to the type I IGF and insulin receptors.