Structural basis for the inhibition of insulin-like growth factors by insulin-like growth factor-binding proteins

Abstract

structure ͉ cell growth T he insulin-like growth factor-binding protein (IGFBP) family comprises six soluble proteins (IGFBP1-6) of Ϸ250 residues that bind to IGFs with nanomolar affinities (1-4). Because of their sequence homology, IGFBPs are assumed to share a common overall fold and are expected to have closely related IGF-binding determinants. Each IGFBP can be divided into three distinct domains of approximately equal lengths: highly conserved cysteinerich N and C domains and a central linker domain unique to each IGFBP species. Both the N and C domains participate in the binding to IGFs, although the specific roles of each of these domains in IGF binding have not been decisively determined (1-13). The C-terminal domain may be responsible for preferences of IGFBPs for one species of IGF over the other (2, 3-7, 9-13); the C-terminal domain is also involved in regulation of the IGF-binding affinity through interaction with extracellular matrix components (1,2,14) and is most probably engaged in mediating IGF1-independent actions (1, 4, 14). The central linker domain is the least conserved region and has never been cited as part of the IGF-binding site for any IGFBP. This domain is the site of posttranslational modifications, specific proteolysis (4), and the acid-labile subunit (1) and extracellular matrix associations (1, 2, 14) known for IGFBPs. Proteolytic cleavage in this domain is believed to produce loweraffinity N-and C-terminal fragments that cannot compete with IGF receptors for IGFs, and, thus, the proteolysis is assumed to be the predominant mechanism for IGF release from IGFBPs (4, 9).However, recent studies indicate that the resulting N-and Cterminal fragments still can inhibit IGF activity and have functional properties that differ from those of the intact proteins (1, 3, 5, 9).The structure of the N-terminal domain of IGFBP-5, free (15) and complexed to IGF1 (16), was solved some time ago. More recently, low-resolution structures of the C-terminal domain of IGFBP6 (12) and its binding surface on IGF2 (3, 12) have been determined with NMR spectroscopy. There is, however, no x-ray structure of a ternary complex of the C-terminal domain of any IGFBPs bound to both the N-terminal domain and IGF, although the C-terminal fragment of IGFBP4 was crystallized recently (9), and also the x-ray structure of the isolated C-terminal fragment of IGFBP1 has been solved (17). We recently reported the x-ray structure of the ternary complex of the N-and C-terminal domains of IGFBP4 bound to IGF1 (10) and described ordered structures for the N-terminal domain of IGFBP4 and IGF1. The C domain was represented by disconnected patches of electron density and could not be interpreted. We describe here the long-sought, highresolution x-ray structure of a complex of the N-and C-terminal domains of IGFBP4 bound to IGF1. We also present the structure of the C-terminal domain of IGFBP1 bound to the N-terminal domain of IGFBP4 and IGF1 and the structure of the binary complex of the N-terminal domain of IGFBP4 (residues 1-92)...

Antagonists of IGF:Vitronectin Interactions Inhibit IGF-I–Induced Breast Cancer Cell Functions

“…24), and the residues identified from IGFBP-3 peptide array screening were mapped out on its surface in blue. The somatomedin B domain of vitronectin obtained from PDB ID 1OC0 (25) and a de novo model of the VN P8 peptide (RVNLRTRRVDTVDPPYPRS covering VN residues 425 to 443) were aligned with these regions based on shape and charge complementarity, and then docked in interactive molecular dynamics simulations.…”

Defining the Disulfide Bonds of Insulin-like Growth Factor-binding Protein-5 by Tandem Mass Spectrometry with Electron Transfer Dissociation and Collision-induced Dissociation

“…Limited insights into the three-dimensional organization of IGFBPs have come from results of high-resolution x-ray crystallographic analyses of the isolated N-terminal domain of IGFBP-4 and the C-terminal segments of IGFBP-1 and IGFBP-4 (13,14). One consistent observation from these data is that IGFBPs lack inter-domain disulfide bonds (12)(13)(14)(15). However, as the structure of a fulllength IGFBP has not been solved, possibly because of the dis-ordered nature of the linker segment, this conclusion remains provisional.…”

“…Exceptions include IGFBP-4, with two cysteines in its linker segment (10), and IGFBP-6, with only 10 cysteines in its N-terminal domain (11). In addition, IGFBPs 1-5 share a cysteine-rich motif, GCGCCXXC (where X is any amino acid), within the N-terminal domain (6,8,12). Limited insights into the three-dimensional organization of IGFBPs have come from results of high-resolution x-ray crystallographic analyses of the isolated N-terminal domain of IGFBP-4 and the C-terminal segments of IGFBP-1 and IGFBP-4 (13,14).…”

First Structural Glimpse of CCN3 and CCN5 Multifunctional Signaling Regulators Elucidated by Small Angle X-ray Scattering

“…The IGFBP domain is a flat domain that has a two-lobe arrangement with the active site cleft between the lobes and the N-terminal lobe of primarily random coil stabilized by a ladder-like arrangement of disulfide bonds and an anti-parallel ␤-sheet forming the C-terminal half. For CCN3 and CCN5, the IGFBP domain was modeled from the NMR structure of IGFBP4 (PDB 1DSP) (54). The VWC domain is related to fibronectin (55) and has two small ␤-sheets stabilized by disulfides at the N-terminal half and a C-terminal half stabilized by three disulfides without any major secondary structure elements.…”