Toxins as Tools in the Characterization of Heterotrimeric G Proteins

Two bacterial toxins, namely pertussis toxin and cholera toxin, were of great importance in determining the function of G proteins. Both toxins catalyze ADP ribosylation of proteins. During ADP ribosylation, an ADP-ribose residue is transferred from NAD+ to an amino acid residue of a substrate protein (Fig. 5.17). 

Cholera toxin catalyzes the ADP-ribosylation of an arginine residue (Argl74 in Gat, Arg201 in Gas) in various a-subunits. The Argl74 residue of G t contacts the phosphate group of the bound GTP and is thus directly involved in GTP binding and possibly also in GTP hydrolysis. Modification of Argl74 by ADP-ribosylation interferes with this 

Constitutive activation of Gs-proteins by cholera toxin is the cause of the devastating effect of the cholera bacterium, Vibrio cholerae, on the water content of the intestine. Because of the lack of deactivation of the Gs-protein, adenylyl cyclase next in the reaction sequence is constantly activated, so that the level of cAMP in the cells of the intestinal epithelium is greatly increased. This, in turn, leads to increased active transport of ions, and an excessive efflux of water and Na+ takes place in the intestine. 

Pertussis toxin, formed by Bortedella pertussis, the causative organism of whooping cough, carries out an ADP-ribosylation at a cysteine residue close to the C-terminus of a-subunits. The modification prevents activation of the G protein G protein by the receptor, whereby the signal transmission is blocked. 

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