[5] Activation of cholera toxin by ADP-ribosylation factors: 20-kDa guanine nucleotide-binding proteins

This chapter discusses activation of cholera toxin by adenosine diphosphate (ADP)-ribosylation factors. Cholera toxin, a secretory product of Vibrio cholerae, is responsible in large part for the devastating fluid and electrolyte loss characteristic of cholera. The toxin is an oligomeric protein of 84 kDa, consisting of one A subunit (∼29 kDa) and five B subunits (11.6 kDa). The B oligomer is responsible for toxin binding to cell surface ganglioside GM1 [galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl) galactosylglucosylceramide]. The A subunit is a latent ADP-ribosyltransferase; activation requires proteolysis near the carboxyl terminus in a domain between two cysteines. Reduction of the disulfide then releases the larger (22 kDa) catalytically active A1 protein (CTA1) and a smaller, carboxyl-terminal A2 protein (CTA2). ADP-ribosylation is responsible for the effects of the toxin on cells. The major ADP-ribose acceptor substrates for the A subunit are the regulatory guanine nucleotide-binding (G) proteins that couple membrane associated cell surface receptors with their intracellular effectors.