Citation

ADP-ribosyltransferases constitute a class of enzymes which are responsible for the toxicity of a number of bacterial pathogens including cholera toxin, pertussis toxin, diphtheria toxin, and exotoxin A from Pseudomonas aeruginosa. The present study is focused on cholera toxin (CT) and the development of a sensitive, reliable assay method to measure ADP-ribosyltransferase activity. Cholera toxin is a 85,620 kD heterohexameric protein consisting of a single 27, 234 kD A subunit (CTA) and five identical 11,677 kD B subunits (CTB). The B-pentamer binds specifically to the ganglioside, GM1, on the surface of human intestinal epithelial cells. The A subunit is the ADP-ribosylating enzyme. The transferase activity is dependent on proteolytic cleavage (nicking) of the A-subunit which gives rise to two domains, A1 and A2, connected by a single disulfide bond. Reduction of the disulfide occurs in the endoplasmic reticulum, activating the A1 domain. A1 crosses the ER membrane, enters the cytosol, and catalyzes the transfer of an ADP ribosyl group from NAD to an arginine of a specific protein in the cell stimulatory signaling pathway, Gs (see representations below). This signaling pathway is permanently activated resulting in elevated levels of cyclicAMP and massive loss of fluids