(A) Domain structure of insulin-like growth factor-binding proteins (IGFBPs). IGFBPs contain conserved N- and C-terminal domains and a variable linker domain between them. The N-domain contains an insulin-like growth factor (IGF)-binding motif and the C-domain contains a thyroglobulin type-I repeat. The N-domain usually contains 12 conserved cysteine residues and the C-domain contains 6. (B) In extracellular environments, most IGFs are bound with IGFBPs, either in a binary complex containing one IGF and one IGFBP or a ternary complex consisting of an IGF, IGFBP-3 (or less often IGFBP-5), and a glycoprotein called acid labile subunit (ALS).
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