They serve as second messengers or transducers of the receptor-initiated response to intracellular elements such as enzymes to initiate an effect. They are also mediators of activated cell-surface receptors and their enzymes or of ion channels.
They are responsible for activating a chain of events that alters the concentration of intracellular signaling molecules such as cyclic amp and calcium. In turn, these intracellular messengers alter the behaviour of other target proteins within the cell.
Synonym: g-protein, GTP-binding proteins.