Structure of the Shiga-like toxin I B-pentamer complexed with an analogue of its receptor Gb3

Shiga-like toxin I (SLT-I) is a virulence factor of Escherichia coli strains that cause disease in humans, Like other members of the Shiga toxin family, it consists of an enzymatic (A) subunit and five copies of a binding subunit (the B-pentamer). The R-pentamer binds to a specific glycolipid, globotriaosylceramide (Gb(3)), on the surface of target cells and thereby plays a crucial role in the entry of the toxin. Here we present the crystal structure at 2.8 Angstrom resolution of the SLT-I B-pentamer complexed with an analogue of the Gb(3) trisaccharide, The structure reveals a surprising density of binding sites, with three trisaccharide molecules bound to each B-subunit monomer of 69 residues. All 15 trisaccharides bind to one side of the B-pentamer, providing further evidence that this side faces the cell membrane. The structural model is consistent with data from site-directed mutagenesis and binding of carbohydrate analogues, and allows the rational design of therapeutic Gb(3) analogues that block the attachment of toxin to cells.