Crystal structure of the Marasmius oreades mushroom lectin in complex with a xenotransplantation epitope.

MOA, a lectin from the mushroom Marasmius oreades, is one of the few reagents that specifically agglutinate blood group B erythrocytes. Further, it is the only lectin known to have exclusive specificity for Galalpha(1,3)Gal-containing sugar epitopes, which are antigens that pose a severe barrier to animal-to-human organ transplantation. We describe here the structure of MOA at 2.4 A resolution, in complex with the linear trisaccharide Galalpha(1,3)Galbeta(1,4)GlcNAc. The structure is dimeric, with two distinct domains per protomer: the N-terminal lectin module adopts a ricinB/beta-trefoil fold and contains three putative carbohydrate-binding sites, while the C-terminal domain serves as a dimerization interface. This latter domain, which has an unknown function, reveals a novel fold with intriguing conservation of an active site cleft. A number of indications suggest that MOA may have an enzymatic function in addition to the sugar-binding properties.