G protein Any one of a group of proteins that have an important role in relaying signals in mammalian cells (see signal transduction). They occur on the inner surface of the plasma membrane and transmit signals from hormone receptors on the outer surface of the cell to adenylate cyclase, which catalyses the formation of the second messenger, cyclic AMP, inside the cell. G proteins bind to GTP and GDP (see guanosine); the GTP–protein complex is able to activate adenylate cyclase, whereas the GDP–protein complex does not. Normally, deactivation of a G protein is controlled by the hormone receptor on the cell surface and it also occurs as the GTP of a GTP–protein complex gradually hydrolyses to GDP. The cholera toxin exerts its effects by changing the G protein in the epithelial cells of the intestine so that it is continually activated, which causes an abnormal increase in cellular adenylate cyclase levels. One consequence of this is that sodium ions are actively pumped into the intestine, causing water to follow by osmosis: the result is diarrhoea and dehydration.
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