activity. The CTA1 fragment catalyses ADP-ribosylation of the Gs alpha subunit (Gαs) proteins using NAD. The ADP-ribosylation causes the Gαs subunit to lose its catalytic activity of GTP hydrolysis into GDP + Pi, thus maintaining Gαs in its activated state. Increased Gαs activation leads to increased adenylate cyclase activity, which increases the intracellular concentration of 3',5'-cyclic AMP (cAMP) to more than 100-fold over normal and over-activates cytosolic PKA. These active PKA then phosphorylate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel proteins, which leads to ATP-mediated efflux of chloride ions and leads to secretion of H2O, Na+, K+, and HCO3− into the intestinal lumen. In addition, the entry of Na+ and consequently the entry of water into enterocytes are diminished. The combined effects result in rapid fluid loss from the intestine, up to 2 liters per hour, leading to severe dehydration and other factors associated with cholera, including a rice-water stool. The pertussis toxin (also an AB5 protein) produced by Bordetella pertussis acts in a similar manner with the exception that it ADP-ribosylates the Gαi subunit, rendering it unable to inhibit cAMP production. Origin The gene encoding the cholera toxin is introduced into V. cholerae by horizontal gene transfer. Virulent strains of V. cholerae hold a virus known as a CTXφ Bacteriophage. Applications Because the B subunit appears to be relatively non-toxic, researchers have found a number of applications for it in cell and molecular biology. It is routinely used as a neuronal tracer. Treatment of cultured rodent neural stem cells with cholera toxin induces changes in the localization of the transcription factor Hes3 and increases their numbers. GM1 gangliosides are found in lipid rafts on the cell surface. B subunit complexes labelled with fluorescent tags or subsequently targeted with antibodies can be used to identify rafts. Vaccine There are currently two vaccines for cholera: Dukoral and Shanchol. Both vaccines use whole killed V. cholerae cells however, Dukoral also contains recombinant cholera toxin β (rCTB). Some studies suggest that the inclusion of rCTB may improve vaccine efficacy in young children (2-10) and increase the duration of protection. This is countered by the costs of protecting and storing rCTB against degradation. Vaccine adjuvant Another application of the CTB subunit may be as a vaccine adjuvant to other vaccines. It has been shown that coupling CTB and antigens improves the response of the vaccine. Currently the adjuvant potential of CTB has