Structural diversity in a conserved cholera toxin epitope involved in ganglioside binding

Abstract

Cholera is a widespread disease for which there is no efficient vaccine. A better understanding of the conformational rearrangements at the epitope might be very helpful for the development of a good vaccine. Cholera toxin (CT) as well as the closely related heat-labile toxin from Escherichia coli (LT) are composed of two subunits, A and B, which form an oligomeric assembly AB5. Residues 50-64 on the surface of the B subunits comprise a conserved loop (CTP3), which is involved in saccharide binding to the receptor on epithelial cells. This loop exhibits remarkable conformational plasticity induced by environmental constraints. The crystal structure of this loop is compared in the free and receptor-bound toxins as well as in the crystal and solution structures of a complex with TE33, a monoclonal antibody elicited against CTP3. In the toxins this loop forms an irregular structure connecting a &strand to the central a-helix. Ser 55 and Gln 56 exhibit considerable conformational variability in the five subunits of the unliganded toxins. Saccharide binding induces a change primarily in Ser 55 and Gln 56 to a conformation identical in all five copies. Thus, saccharide binding confers rigidity upon the loop. The conformation of CTP3 in complex with TE33 is quite different. The amino-terminal part of CTP3 forms a 0-turn that fits snugly into a deep binding pocket on TE33, in both the crystal and NMR-derived solution structure. Only 8 and 12 residues out of 15 are seen in the NMR and crystal structures, respectively. Despite these conformational differences, TE33 is cross-reactive with intact CT, albeit with a thousandfold decrease in affinity. This suggests a different interaction of TE33 with intact CT.Keywords: antipeptide antibody; conformational rearrangements at the epitope; cholera toxin; flexible loop involved in receptor binding Cholera toxin (CT) as well as the closely related heat-labile toxin from Escherichia coli (LT) are composed of two types of subunits, A and B, which form an oligomeric assembly AB, (Sixma et al., 1991). The initial step in the toxin-induced process leading to the symptoms of diarrhea is the attachment of the B subunits to intestinal epithelial cells. Immunogenic moieties on the surface of the B, molecule therefore constitute suitable targets for the development of peptide vaccines against cholera. Residues 50-64 of the B subunit form such a surface loop that is entirely conserved between CT and LT ( Fig. 1; Kinemage 1). This loop is directly involved in carbohydrate bind-

Bootstrap-Based Consensus Scoring Method for Protein–Ligand Docking

“…The binding mode of the crystal structure in Figure 8c shows two important histidine residues. It is likely that these imidazole rings are positively charged and make strong ion-ion interactions with two carboxylate ions of citrate, as discussed by Shoham et al 46 In our study, however, the histidine residues were treated as neutral. That is a main reason why the crystal structure was top-ranked.…”