Cholera toxin (CT) is an AB, 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 GM, on the surface of target human intestinal epithelial cells. We present here the crystal structure of the cholera toxin B-pentamer complexed with the GMI 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 GM1, galactose and sialic acid, with a smaller contribution from the N-acetyl galactosamine residue. The binding of GMI to cholera toxin thus resembles a 2-fingered grip: the Gal(P1-3)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 GM1 may be rationalized by sequence differences at this residue. The CTBs:GMl pentasaccharide complex described here provides a detailed view of a pr0tein: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 OM,-mediated signal transduction.Keywords: cholera toxin; crystal structure; ganglioside GM1; sugar binding specificity Cholera is a severe disease that can lead to death within a few hours. The major clinical symptoms are caused by a toxin released after adhesion of the noninvasive Vibrio cholerue bacteria to the proximal small intestine of the host. Cholera toxin (CT) acts intracellularly to catalyze ADP-ribosylation of residue Arg 187 in the a subunit of the trimeric protein G,. The modified G,, loses its GTPase activity and remains constitutively in its GTP-bound state (Cassel & Pfeuffer, 1978), which in turn causes a continuous stimulation of adenylate cyclase. The resulting elevated levels of cyclic AMP lead to massive loss of fluids, the characteristic pathology of enterotoxigenic disease.Cholera toxin is an AB, hexamer consisting of 5 identical B subunits and a single A subunit. It is structurally and functionally related to a larger group of bacterial enterotoxins that includes the closely related Escherichia coli heat-labile enterotoxin (LT) as well as pertussis toxin, diphtheria toxin, shigella toxin, and Pseudomonas aeruginosa exotoxin A. In this class of tox- ins, the biological functions of target cell recognition and enzymatic activity are separated into distinct domains. In the case of CT and LT, cell recognition and binding are carried out by the ...

Combining this new structure for the E3-E2BD complex with previously determined structures of the E2 catalytic domain and the E2 lipoyl domain creates a model of the E2 core showing how the lipoyl domain can move between the active sites of E2 and E3 in the multienzyme complex.

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The structure of LT-IIb provides insight into the sequence diversity and structural similarity of the AB5 toxin family. New knowledge has been gained regarding the assembly of AB5 toxins and their active-site architecture.