Substitutions for hydrophobic amino acids in the N-terminal domains of IGFBP-3 and-5 markedly reduce IGF-I binding and alter their biologic actions

Insulin-like growth factor-binding protein-3 and -5 (IGFBP-3 and -5) have been shown to bind insulin-like growth factor-I and -II (IGF-I and -II) with high affinity. Precious studies have proposed that the N-terminal region of IGFBP-5 contains a hydrophobic patch between residues 49 and 74 that is required for high affinity binding. These studies were undertaken to determine if mutagenesis of several of these residues resulted in a reduction of the affinity of IGFBP-3 and -5 for IGF-I. Substitutions for residues 68, 69, 70, 73, and 74 in IGFBP-5 (changing one charged residue, Lys(68), to a neutral one and the four hydrophobic residues to nonhydrophobic residues) resulted in an similar to 1000-fold reduction in the affinity of IGFBP-5 for IGF-I. Substitutions for homologous residues in IGFBP-3 also resulted in a >1000-fold reduction in affinity, The physiologic consequence of this reduction was that IGFBP-5 and -5 became very weak inhibitors of IGF-I-stimulated cell migration and DNA synthesis. Likewise, the ability of IGFBP-5 to inhibit IGF-I-stimulated receptor phosphorylation was attenuated. These changes did not appear to be because of alterations in protein folding induced by mutagenesis, because the IGFBP-5 mutant was fully susceptible to proteolytic cleavage by a specific IGFBP-5 protease, In summary, residues 68, 69, 70, 73, and 74 in IGFBP-5 appear to be critical for high affinity binding to IGF-I. Homologous residues in IGFBP-3 are also required, suggesting that they form a similar binding pocket and that for both proteins these residues form an important component of the core binding site. The availability of these mutants will make it possible to determine if there are direct, non-IGF-I-dependent effects of IGFBP-3 and -5 on cellular physiologic processes in cell types that secrete IGF-I.