Cellular mechanism of action of cholera toxin. (a) The toxin, through its B subunits, which encircle the enzymically active A subunit, binds to the ubiquitous glycolipid membrane receptor, the monosialoganglioside GM1. (b) The entire complex is endocytosed by both clathrin-dependent and clathrin-independent means. (c) Inwardly directed proton pumps acidify the CT-containing endocytic vesicle, causing the toxin subunits to dissociate. Then the enzymically active A1 peptide is inserted into the vesicle membrane, with its catalytic site exposed to cytoplasm. (d) The A1 peptide is an ADP-ribosyltransferase that cleaves NAD into adenosine diphosphoribose (ADPR) and nicotinamide and covalently bonds the former to the α subunit of the G_s adenylyl cyclase–stimulatory G protein. (e) The intrinsic GTPase activity of the ADPR-modified α subunit of G_s is markedly inhibited, allowing GTP to remain bound to it; the G_sα-GTP complex separates from its membrane mooring to G_sβ and G_sγ, attaching to and activating adenylyl cyclase (AdCy).