Structural basis for carbohydrate recognition by the Gal/GalNAc lectin of Entamoeba histolytica

Abstract

Intestinal amoebiasis is caused by Entamoeba histolytica, one of the deadliest human-infective parasites. Central to its pathogenicity is its binding to mucosal carbohydrates, which precedes tissue damage by trogocytosis. Carbohydrate binding is mediated by a single adhesin, the galactose/N-acetylgalactosamine (Gal/GalNAc) lectin, which is the leading vaccine candidate for amoebiasis. We present the structure of the native heterodimeric lectin, revealing an ordered core containing the light chain and the N-terminal region of the heavy chain. Structures in the presence of ligand show that the Gal/GalNAc binding site is in the light chain. An elongated arm emerges from the heavy chain, which adopts multiple positions. This arm contains the epitope for a monoclonal antibody which blocks binding to target cells, located unexpectedly away from the Gal/GalNAc binding site. This study reveals the molecular basis for sugar binding by the Entamoeba histolytica Gal/GalNAc lectin and will guide future vaccine development against amoebiasis.