A pore-forming β-trefoil lectin with specificity for the tumor-related glycosphingolipid Gb3
Lectins are efficient multivalent glycan receptors, deciphering the glyco-code on cell surfaces. The {beta}-trefoil fold, characterized by three lobe-shaped repeats, is adopted by several classes of lectins, often associated with other domains having enzymatic or toxic activity. Based on the UniLectin3D database classification, the sequence signature of trefoil lobes was defined and used to predict 44714 lectins from 4497 species. Among them, SaroL-1 from the lower eukaryote Salpingoeca rosetta was predicted to contain both {beta}-trefoil and aerolysin-like pore-forming domain. Recombinant SaroL-1 binds to galactose and derivatives, with a stronger affinity for cancer-related -galactosylated epitopes such as glycosphingolipid Gb3 embedded in giant unilamellar vesicles or cell membranes. Crystal structures in complex with Gb3 trisaccharide and GalNAc show similarity with pore-forming toxins. Recognition of the Gal epitope on glycolipids was necessary for hemolysis of rabbit erythrocytes and toxicity on cancer cells through carbohydrate-dependent pore-formation.