ABSTRACT
The amino acid sequences of the G protein-linked receptors have been determined and shown to consist of single polypeptide chains. The cell surface receptors are coupled to various intracellular enzymes, ion channels or transporters by means of G proteins. These are heterotrimeric proteins consisting of three individual protein units, each having separate genetic origins. G proteins linking receptors to adenylyl cyclase are either stimulatory or inhibitors. The a-subunit of stimulatory is the target for ADP-ribosylation by the ribosyl-transferase activity of cholera toxin, the diarrhoea-causing toxin from the bacterium, Vibrio cholerae. Coupling of the receptor to the G protein occurs through identifiable regions of receptor which have been determined by site-directed mutagenesis and deletion studies, whereby amino acid residues have been substituted or deleted, respectively, from the receptor.
