Structural Characterization of the DC-SIGN–Lewis<sup>X</sup> Complex

Dendritic cell-specific intracellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) is a C-type lectin highly expressed on the surface of antigen-presenting dendritic cells. DC-SIGN mediates interactions among dendritic cells, pathogens, and a variety of epithelia, myeloid cells, and endothelia by binding to high mannose residues on pathogenic invaders or fucosylated residues on the membranes of other immune cells. Although these interactions are normally beneficial, they can also contribute to disease. The structural characterization of binding geometries is therefore of interest as a basis for the construction of mimetics that can mediate the effects of abnormal immune response. Here, we report the structural characteristics of the interaction of the DC-SIGN carbohydrate recognition domain (CRD) with a common fucosylated entity, the Lewis<sup>X</sup> trisaccharide (Le<sup>X</sup>), using NMR methods. Titration of the monomeric DC-SIGN CRD with Le<sup>X</sup> monitored by 2D NMR revealed significant perturbations of DC-SIGN cross-peak positions in <sup>1</sup>H–<sup>15</sup>N heteronuclear single quantum coherence (HSQC) spectra and identified residues near the binding site. Additionally, saturation transfer difference (STD) and transferred nuclear Overhauser effect (trNOE) NMR experiments, using a tetrameric form of DC-SIGN, identified binding epitopes and bound conformations of the Le<sup>X</sup> ligand. The restraints derived from these multiple experiments were used to generate models for the binding of Le<sup>X</sup> to the DC-SIGN CRD. Ranking of the models based on the fit of model-based simulations of the trNOE data and STD buildup curves suggested conformations distinct from those seen in previous crystal structures. The new conformations offer insight into how differences between binding of Lewis<sup>X</sup> and mannose-terminated saccharides may be propagated.