Crystal structure of cholera toxin B‐pentamer bound to receptor GM1 pentasaccharide

Abstract

Cholera toxin (CT) is an AB 5 hexameric protein responsible for the symptoms produced by Vibrio cholerae infection. In the first step of cell intoxication, the B‐pentamer of the toxin binds specifically to the branched pentasaccharide moiety of ganglioside G M1 on the surface of target human intestinal epithelial cells. We present here the crystal structure of the cholera toxin B‐pentamer complexed with the G M1 pentasaccharide. Each receptor binding site on the toxin is found to lie primarily within a single B‐subunit, with a single solvent‐mediated hydrogen bond from residue Gly 33 of an adjacent subunit. The large majority of interactions between the receptor and the toxin involve the 2 terminal sugars of G M1, galactose and sialic acid, with a smaller contribution from the N‐acetyl galactosamine residue. The binding of G M1 to cholera toxin thus resembles a 2‐fingered grip: the Gal(β1‐3)CalNAc moiety representing the “forefinger” and the sialic acid representing the “thumb.” The residues forming the binding site are conserved between cholera toxin and the homologous heat‐labile enterotoxin from Escherichia coli, with the sole exception of His 13. Some reported differences in the binding affinity of the 2 toxins for gangliosides other than G m1 may be rationalized by sequence differences at this residue. The CTB 5:G M1 pentasaccharide complex described here provides a detailed view of a protein:ganglioside specific binding interaction, and as such is of interest not only for understanding cholera pathogenesis and for the design of drugs and development of vaccines but also for modeling other protein:ganglioside interactions such as those involved in G M1‐mediated signal transduction.