Crystal Structure of Human RANKL Complexed with Its Decoy Receptor Osteoprotegerin

Abstract

Receptor activator of NF-κB ligand (RANKL), its signaling receptor RANK, and its decoy receptor osteoprotegerin (OPG) constitute a molecular triad that is critical in regulating bone remodeling, and also plays multiple roles in the immune system. OPG binds RANKL directly to block its interaction with RANK. In this article, we report the 2.7-Å crystal structure of human RANKL trimer in complex with the N-terminal fragment of human OPG containing four cysteine-rich TNFR homologous domains (OPG-CRD). The structure shows that RANKL trimer uses three equivalent grooves between two neighboring monomers to interact with three OPG-CRD monomers symmetrically. A loop from the CRD3 domain of OPG-CRD inserts into the shallow groove of RANKL, providing the major binding determinant that is further confirmed by affinity measurement and osteoclast differentiation assay. These results, together with a previously reported mouse RANKL/RANK complex structure, reveal that OPG exerts its decoy receptor function by directly blocking the accessibilities of important interacting residues of RANKL for RANK recognition. Structural comparison with TRAIL/death receptor 5 complex also reveals structural basis for the cross-reactivity of OPG to TRAIL.

Osteoprotegerin: Multiple partners for multiple functions

“…However, it has been shown that the homodimeric form of OPG exhibits a much greater higher affinity for the homotrimeric form of RANKL (Schneeweis et al 2005;Theoleyre et al 2006). More recently, the crystal structure of the interaction between RANKL and OPG was described (Luan et al 2012). In this study, the authors showed that OPG (Domains 1 to 4) is able to interact with RANKL in a 1:1 ratio, and blocks the accessibilities of important biding sites of RANKL to RANK.…”

Characterisation of Osteoprotegerin Autoantibodies in Coeliac Disease

“…The recombinant plasmids pET-OPG1 and pET-OPG2 express the amino-terminal cysteine rich domains (D1-D4) which are necessary for binding to RANKL (15,16). The third recombinant plasmid, pET-OPG3, was designed to express the carboxy-terminal portion of the protein that contains two putative death domain homologous regions (D5 and D6) which have been related to cytotoxic signals in mammalian cells (15,17), and a heparin-binding region (D7). To verify protein expression we analyzed the whole cell extract by anti-His monoclonal antibody (moAb) immunoblotting.…”

Structural principles of tumor necrosis factor superfamily signaling

“…The corresponding receptor monomers bind on the outside at the interface between two ligand monomers ( Fig. 1) (17)(18)(19)(20)(21)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40). This is critically important for receptor activation, as it ensures that only an intact trimeric ligand can trigger signaling because even the loss of a single ligand monomer results in the loss of two receptor-binding sites.…”