Mannose receptor (MR) is widely expressed on macrophages, immature dendriticcells, and a variety of epithelial and endothelial cells. It is a 180 kD type I transmembranereceptor whose extracellular region consists of three parts: the amino-terminal cysteine-richdomain (Cys-MR); a fibronectin type II-like domain; and a series of eight tandem C-typelectin carbohydrate recognition domains (CRDs). Two portions of MR have distinctcarbohydrate recognition properties: Cys-MR recognizes sulfated carbohydrates and thetandem CRD region binds terminal mannose, fucose, and N-acetyl-glucosamine (GlcNAc).The dual carbohydrate binding specificity allows MR to interact with sulfated and nonsulfatedpolysaccharide chains, and thereby facilitating the involvement of MR inimmunological and physiological processes. The immunological functions of MR includeantigen capturing (through binding non-sulfated carbohydrates) and antigen targeting(through binding sulfated carbohydrates), and the physiological roles include rapid clearanceof circulatory luteinizing hormone (LH), which bears polysaccharide chains terminating withsulfated and non-sulfated carbohydrates.
We have crystallized and determined the X-ray structures of unliganded Cys-MR (2.0Å) and Cys-MR complexed with different ligands, including Hepes (1.7 Å), 4SO_4-N-Acetylgalactosamine(4SO_4-GalNAc; 2.2 Å), 3SO_4-Lewis^x (2.2 Å), 3S04-Lewis^a (1.9 Å),and 6SO_4-GalNAc (2.5 Å). The overall structure of Cys-MR consists of 12 anti-parallel β-strandsarranged in three lobes with approximate three fold internal symmetry. The structurecontains three disulfide bonds, formed by the six cysteines in the Cys-MR sequence. Theligand-binding site is located in a neutral pocket within the third lobe, in which the sulfategroup of ligand is buried. Our results show that optimal binding is achieved by acarbohydrate ligand with a sulfate group that anchors the ligand by forming numeroushydrogen bonds and a sugar ring that makes ring-stacking interactions with Trpll7 of CysMR.Using a fluorescence-based assay, we characterized the binding affinities between CysMRand its ligands, and rationalized the derived affinities based upon the crystal structures.These studies reveal the mechanism of sulfated carbohydrate recognition by Cys-MR andfacilitate our understanding of the role of Cys-MR in MR recognition of its ligands.
