6. Vibrio cholerae Cholera Toxin B subunit A subunit Binds GM1 ganglioside (surface of epithelial cells) Carried to ER (retrograde transport) Endocytosis Reduced by protein disulfide isomerase in ER Cytosol Unfolding Refolding Interact with cytosolic ADP ribosylation factors Activate G protein Stimulate adenylate cyclase Inc. cAMP Open CFTR Cl released in lumen; secretion of HCO 3 , Na + & water