Structural determinants for binary and ternary complex formation between insulin-like growth factor-I (IGF-I) and IGF binding protein-3.
Structural analogs of recombinant human insulin-like growth factor-I (IGF-I), with alterations to each of the B, C, A, and D domains, have been tested for their ability to form binary complexes with IGF-binding protein-3 (IGFBP-3) and ternary complexes with IGFBP-3 and the acid-labile subunit (alpha-subunit). Two functionally distinct regions of IGF-I have been identified. The first, involving residues 3 and 4 and the alpha-helix between residues 8 and 18 of the B-domain, as well as residues 49-51 in the A-domain, appears important for IGFBP-3 binding, such that substitution of these residues results in decreased binary complex available for alpha-subunit binding. The second region, distal to the IGFBP-3-binding epitope and primarily involving the D-domain and B-domain near residue 24, with some involvement of the C-domain, appears slightly inhibitory to binary complex formation, such that analogs with a truncated D-domain or with a Gly4 bridge substituted for the C-domain show enhanced binding to IGFBP-3. However, binary complexes formed from these analogs bind the alpha-subunit with reduced affinity, the effect being most marked when substitution of the C-domain, or replacement of Tyr24, is superimposed on D-domain truncation. It is concluded that although the alpha-subunit does not itself bind IGF-I, its interaction with IGFBP-3 in the ternary complex is dependent on structural determinants on IGF-I distal to the IGFBP-3 binding domain.
Department of Endocrinology, Royal Prince Alfred Hospital, Camperdown, New South Wales, Australia.
R C
RC
Baxter
M L
ML
Bayne
M A
MA
Cascieri
D001667:Binding, Competitive; D002352:Carrier Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D000939:Epitopes / Q000276:immunology; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D011994:Recombinant Proteins / Q000378:metabolism; D013002:Somatomedins / Q000378:metabolism*; D013329:Structure-Activity Relationship
D002352:Carrier Proteins; D000939:Epitopes; D018969:Insulin-Like Growth Factor Binding Proteins; D011994:Recombinant Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I