CRYSTAL-STRUCTURE OF CHOLERA-TOXIN B-PENTAMER BOUND TO RECEPTOR G(M1)PENTASACCHARIDE

Cholera toxin (CT) is an AB(5) hexameric protein responsible for the s ymptoms produced by Vibrio cholerae infection. In the first step of ce ll intoxication, the B-pentamer of the toxin binds specifically to the branched pentasaccharide moiety of ganglioside G(M1) on the surface o f target human intestinal epithelial cells. We present here the crysta l structure of the cholera toxin B-pentamer complexed with the G(M1) p entasaccharide. Each receptor binding site on the toxin is found to li e primarily within a single B-subunit, with a single solvent-mediated hydrogen bond from residue Gly 33 of an adjacent subunit. The large ma jority 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(beta 1 -3)GalNAc moiety representing the ''forefinger'' and the sialic acid r epresenting the ''thumb.'' The residues forming the binding site are c onserved between cholera toxin and the homologous heat-labile enteroto xin from Escherichia coli, with the sole exception of His 13. Some rep orted differences in the binding affinity of the 2 toxins for ganglios ides other than G(M1) may be rationalized by sequence differences at t his residue. The CTB5:G(M1) pentasaccharide complex described here pro vides a detailed view of a protein:ganglioside specific binding intera ction, and as such is of interest not only for understanding cholera p athogenesis and for the design of drugs and development of vaccines bu t also for modeling other protein:ganglioside interactions such as tho se involved in G(M1)-mediated signal transduction.