Crystal Structure of the GalNAc/Gal-Specific Agglutinin from the Phytopathogenic Ascomycete Sclerotinia sclerotiorum Reveals Novel Adaptation of a β-Trefoil Domain

A lectin from the phytopathogenic ascomycete Sclerotinia sclerotiorum that shares only weak sequence similarity with characterized fungal lectins has recently been identified. S. sclerotiorum agglutinin (SSA) is a homodimeric protein consisting of two identical subunits of similar to 17 kDa and displays specificity primarily towards Gal/GalNAc. Glycan array screening indicates that SSA readily interacts with Gal/GalNAc-bearing glycan chains. The crystal structures of SSA in the ligand-free form and in complex with the Gal-beta 1,3-GalNAc (T-antigen) disaccharide have been determined at 1.6 and 1.97 angstrom resolution, respectively. SSA adopts a beta-trefoil domain as previously identified for other carbohydrate-binding proteins of the ricin B-like lectin superfamily and accommodates terminal non-reducing galactosyl and N-acetylgalactosaminyl glycans. Unlike other structurally related lectins, SSA contains a single carbohydrate-binding site at site a. SSA reveals a novel dimeric assembly markedly dissimilar to those described earlier for ricin-type lectins. The present structure exemplifies the adaptability of the beta-trefoil domain in the evolution of fungal lectins. (C) 2010 Elsevier Ltd. All rights reserved.