Molecular basis of the differences in binding properties of the highly related C-type lectins DC-SIGN and L-SIGN to Lewis x trisaccharide and Schistosoma mansoni egg antigens

The dendritic cell-specific C-type lectin DC-SIGN functions as a pathogen receptor that recognizes Schistosoma mansoni egg antigens through its major glycan epitope Galbeta1,4(Fucalpha1,3) GlcNAc (Le(x)). Here we report that L-SIGN, a highly related homologue of DC-SIGN found on liver sinusoidal endothelial cells, binds to S. mansoni egg antigens but not to the Le(x) epitope. L-SIGN does bind the Lewis antigens Le(a), Le(b), and Le(y), similar as DC-SIGN. A specific mutation in the carbohydrate recognition domain of DC-SIGN ( V351G) abrogates binding to all Lewis antigens. In L-SIGN Ser(363) is present at the corresponding position of Val(351) in DC-SIGN. Replacement of this Ser into Val resulted in a gain of function L-SIGN mutant that binds to Le(x), and shows increased binding to the other Lewis antigens. These data indicate that Val(351) is important for the fucose specificity of DC-SIGN. Molecular modeling and docking of the different Lewis antigens in the carbohydrate recognition domains of L-SIGN, DC-SIGN, and their mutant forms, demonstrate that Val(351) in DC-SIGN creates a hydrophobic pocket that strongly interacts with the Fucalpha1,3/4-GlcNAc moiety of the Lewis antigens. The equivalent amino acid residue Ser(363) in L-SIGN creates a hydrophilic pocket that prevents interaction with Fucalpha1,3-GlcNAc in Le(x) but supports interactions with the Fucalpha1,4-GlcNAc moiety in Le(a) and Le(b) antigens. These data demonstrate for the first time that DC-SIGN and L-SIGN differ in their carbohydrate binding profiles and will contribute to our understanding of the functional roles of these C-type lectin receptors, both in recognition of pathogen and self-glycan antigens.