G protein
any of a class of proteins that are coupled to the intracellular portion of a type of membrane receptor (G-protein-coupled receptor) and are activated when the receptor binds an appropriate ligand (e.g., a neurotransmitter) on the extracellular surface. G proteins thus have a role in signal transduction, being involved, for example, in indirect chemical neurotransmission. They work in conjunction with the nucleotides guanosine diphosphate (GDP) and guanosine triphosphate (GTP) and serve to transmit the signal from the receptor to other cell components (e.g., ion channels) in various ways, such as by controlling the synthesis of second messengers within the cell.