G protein-coupled receptors are transmembrane receptors. Signal molecules bind to a domain located outside the cell. An intracellular domain activates a G protein. The G protein activates a cascade of further compounds, and finally causes a change downstream in the cell.

G proteins function as molecular switches. When they bind guanosine triphosphate (GTP), they are 'on', and, when they bind guanosine diphosphate (GDP), they are 'off. GTP- ON