Insulin-like growth factors (IGFs) are important mediators of growth and IGF-binding proteins (IGFBPs) 1-6 regulate IGF actions. As lGFBP C-terminal domains contribute to high-affinity IGF binding, we have defined the binding site for the C-domain of IGFBP-6 on IGF-II using NMR. This site lies adjacent to and between the binding sites for the IGFBP N-domain and IGF-I receptor (IGFIR), which have previously been found on opposite sides of the IGF molecule. The C-domain is therefore likely to interfere with IGF binding to the IGFIR, providing a structural basis for the potent inhibitory effects of intact IGFBPs on IGF actions. (C) 2004 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.