Structural characterisation of human galectin-4 C-terminal domain -elucidating the molecular basis for recognition of glycosphingolipids, sulfated saccharides and blood group antigens.

Human galectin-4 is a lectin that is expressed mainly in the gastrointestinal tract and exhibits metastasis-promoting roles in some cancers. Its tandem-repeat nature exhibits two distinct carbohydrate recognition domains allowing cross-linking by simultaneous binding to sulfated and non-sulfated (but not sialylated) glycosphingolipids and glycoproteins; facilitating stabilisation of lipid rafts. Critically, galectin-4 exerts favourable or unfavourable effects depending upon the cancer. Here we report the first X-ray crystallographic structural information on human galectin-4, specifically the C-terminal carbohydrate recognition domain of human (galectin-4C) in complex with lactose, lactose-3'-sulfate, 2'-fucosyllactose, lacto-N-tetraose and lacto-N-neotetraose. These structures enable elucidation of galectin-4C binding fine-specificity towards sulfated and non-sulfated lacto- and neolacto-series sphingolipids as well as to human blood group antigens. Analysis of the lactose-3'-sulfate complex structure shows that galectin-4C does not recognise the sulfate group using any specific amino acid, but binds the ligand nonetheless. Complex structures with lacto-N-tetraose and lacto-N-neotetraose displayed differences in binding interactions exhibited by the non-reducing-end galactose. That of lacto-N-tetraose points outward from the protein surface whereas that of lacto-N-neotetraose interacts directly with the protein. Recognition patterns of human galectin-4C towards lacto- and neolacto-series glycosphingolipids are similar to those of human galectin-3, however detailed scrutiny revealed differences stemming from the extended binding-site that offer distinction in ligand profiles of these two galectins. Structural characterisation of the complex with 2'-fucosyllactose, a carbohydrate with similarity to the H-antigen, and molecular dynamics studies highlight structural features that allow specific recognition of A- and B-antigens, whilst a lack of interaction with the 2'-fucose of blood group antigens was revealed. This article is protected by copyright. All rights reserved