Proteins with an important role in relaying signals in mammalian cells. The proteins occur on the inner surface of the plasma membrane and transmit signals from outside the membrane, via transmembrane receptors, to adenylate cyclase, which catalyses the formation of the second messenger, cyclic AMP, inside the cell. G-proteins derive their name from their ability to bind to guanine nucleotides, namely GTP and GDP; the GTP-protein complex is able to activate adenylate cyclase, whereas the GDP-protein complex is unable. G-proteins are activated when the signalling molecule (typically a hormone) binds to the transmembrane receptor.