Arrangement of the CRDs from human SRCL in crystals. a, Diagram of multiple P3₂ unit cells. The four monomers A, B, C and D are shown in red, blue, orange, and green. The relative heights of the molecules along the z axis are indicated for A and B, and for C and D. Note that monomers C and D are translated along the z axis relative to A and B by ∼5.5 Å, such that the local twofold axis relating A to C or B to D is not at z=0. A given unit cell can only have a copy of C or D, which would otherwise overlap. Thus, copies C and D are randomly distributed throughout the crystal, each with a net occupancy of 50%. Dashed lines indicate crosslinks mediated by Zn²⁺. b, Crosslinking of h-SR-CRD monomer A and its symmetry related molecules. His⁶¹⁰ from molecule A and His⁶⁴¹ from a symmetry related molecule A bind the same Zn⁺² ion. The same crosslinking occurs for the other monomers B, C and D. c, Crosslinking of molecules A and B forms a 6₅-symmetric hexamer in the unit cell. A Zn²⁺ binds to His⁷⁰⁰ of one molecule and Asp⁶¹⁶ and Asp⁷³³ from the other. D, Monomer C and its symmetry mates, forming a chain of monomers with a 3₂ in the centre of the hexamer formed by molecules A and B, in yellow. A similar arrangement occurs for monomer D.

Sugar binding by SRCL. A, identification of glycan ligands for SRCL. A glycan array was screened with fluorescein-labeled extracellular domain from mouse SRCL. The level of fluorescense was normalized to glycan 130 (Lewis^x). Glycans binding to SRCL are shown as red bars, glycans with terminal Gal or GalNAc residues are shown as blue bars and all other glycans are shown as black bars. A full list of glycans on the array is available in the Supplemental Material. B, alignment of human and mouse SRCL CRD sequences. The sequences that form the principal Ca²⁺- and sugar-binding site are highlighted in yellow, and residues that serve as Ca²⁺ ligands are highlighted in green.

Comparison of galactose-binding sites in C-type CRDs. Carbon, nitrogen, oxygen, and calcium are represented as white, blue, red, and green spheres, respectively. Hydrogen bonds are shown as dashed gray lines, Ca²⁺ coordination bonds are dashed black lines and hydrophobic interactions are in dashed blue lines. a, mouse SRCL. For simplicity only the galactose residue of Lewis^x is shown. b, Gal/GalNAc-binding mutant of mannose-binding protein complexed with GalNAc (1FIH, copy A) (16). c, Rattlesnake venom lectin complexed with lactose (1JZN) (17). d, Tunicate lectin complexed with galactose (1TLG) (18).

Structure of the CRD from SRCL. The CRD is shown as a color ramp starting with blue at the N-terminus and ending in red at the C-terminus. Disulfide bonds are shown in yellow. A, human SRCL. Zn²⁺ are shown as orange spheres. B, mouse SRCL bound to Lewis^x. The oligosaccharide is shown in a stick representation. Ca²⁺ are shown as large green spheres.

Lewis^x binding. A, The structure of the CRD from mouse SRCL bound to Lewis^x. B, DC-SIGN bound to the pentasaccharide lacto-N-fucopentaose III (LNFP III), which contains the Lewis^x trisaccharide (PDB 1SL5) (7). The mouse CRD and DC-SIGN are shown in cyan. The oligosaccharide and selected side chains are shown in stick representation. The green sphere is the conserved Ca²⁺. Selected coordination bonds between the carbohydrate or protein side chains and the Ca⁺² ion are shown in black dashed lines, selected hydrogen bonds between the protein and the carbohydrate are shown in gray. Hydrophobic interactions between the sugar and the protein or within the protein are in blue. C, Superposition of Lewis^x in the two structures. D, The position of Lewis^x after superposition of the CRDs of mouse SRCL and DC-SIGN. E, superposition of Lewis^a from PDB ID 1W8H (24) onto Lewis^x observed bound to the mouse SRCL. The Gal residues of each oligosaccharide were superimposed.

Structural changes in SRCL in the absence of Ca²⁺. a, Superposition of the CRDs from mouse SRCL (red) and human SRCL (yellow). b, Gal/GalNAc-binding mutant of mannose-binding-protein-A (PDB 1FIF) (16), copy A in cyan and copy B with a rearranged Ca⁺² site in yellow. c, Mannose-binding protein-C with bound Ca²⁺ (PDB 1RDO copy A) (20) in cyan and without Ca²⁺ (PDB 1BV4, copy B) (22) in yellow. d, DC-SIGN-R (PDB 1K9J) (8) with bound Ca²⁺ (copy A) in cyan and apo (copy B) in yellow.