Competition for binding to insulin-like growth factor (IGF) binding protein-2, 3, 4, and 5 by the IGFs and IGF analogs.
The insulin-like growth factors (IGF) I and II bind to IGF binding proteins (BP) with high affinity. The affinity of each of the IGFs for individual BPs and the regions of the IGF-I molecule that are required for this high affinity binding have been defined only for IGFBP-1 and IGFBP-3. The present studies have determined the affinity of several IGF analogs (prepared using in vitro mutagenesis) for pure IGFBP-2, 3, 4, and 5. The results show IGFBP-2 binds these analogs in a manner similar to IGFBP-1. For example, a mutation in the A chain region (positions 49, 50, 51) or B chain (positions 3, 4) results in greater than 20-fold reduction in affinity for either IGFBP-1 or 2. In contrast, mutations in the A chain region have minimal effect on binding to IGFBP-3, whereas substitutions at the 3, 4, 15, 16 positions of the B chain reduce IGF-I affinity by at least 50-fold. At pH 7.4, binding of the analogs to IGFBP-4 is less affected by substitutions at the B chain 3, 4 positions compared to IGFBP-1, 2, and 3, but IGFBP-4 affinity for analogs containing the A chain substitutions is greatly reduced similarly to IGFBP-1 and 2. Binding to IGFBP-5 is greatly reduced by either A or B chain substitutions and most of the mutations result in greater than 100-fold reduction in affinity. Acidic pH 6.0 was associated with increased affinity of IGFBP-4 for the A chain containing mutants. The results indicate that only IGFBP-1 and 2 have nearly identical affinity for each of these analogs, whereas IGFBP-3, 4, and 5 have similarities and significant differences. The findings suggest that different binding proteins have differential structural requirements for optimal IGF-I binding.
Clemmons
D R
DR
Department of Medicine, University of North Carolina School of Medicine, Chapel Hill 27599.
Dehoff
M L
ML
Busby
W H
WH
Bayne
M L
ML
Cascieri
M A
MA
D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D006801:Humans; D006863:Hydrogen-Ion Concentration; D018971:Insulin-Like Growth Factor Binding Protein 2; D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000737:chemistry / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000737:chemistry / Q000378:metabolism; D015394:Molecular Structure; D016296:Mutagenesis; D011487:Protein Conformation; D013002:Somatomedins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013329:Structure-Activity Relationship
D002352:Carrier Proteins; D018971:Insulin-Like Growth Factor Binding Protein 2; D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D018969:Insulin-Like Growth Factor Binding Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II