Enterotoxin, bacterial <p> Cholera toxin produced by <taxon tax_id="666">Vibrio cholerae</taxon> and heat-labile enterotoxin, produced by enterotoxigenic <taxon tax_id="562">Escherichia coli</taxon>, are AB5 heterohexamers, consisting of one A polypeptide and five identical B polypeptides, with an ADP-ribosylating A subunit and a GM1 receptor binding B pentamer. These toxins are among the most potent mucosal adjuvants known. The B pentamer is required for binding to the cell surface receptor ganglioside GM1. The A subunit can be proteolytically cleaved within the single disulphide-linked loop between two cysteine residues to produce the enzymatically active A1 polypeptide and the smaller A2 polypeptide that links fragment A1 to the B pentamer. Upon entry into enterocytes by endocytosis and following reduction and translocation, CT-A1 ADP-ribosylates a regulatory G-protein (Gsalpha), which leads to constitutive activation of adenylate cyclase, increased intracellular concentration of cyclic AMP, and secretion of fluid and electrolytes into the lumen of the small intestine [<cite idref="PUB00011767"/>]. </p><p> All family members contain a common OB-fold, a five-stranded beta-sheet coiled to form a closed beta-barrel capped by an alpha-helix located between the third and fourth strands.</p>