G proteins and the mechanism of action of hormones, neurotransmitters, and autocrine and paracrine regulatory factors.

Signal transduction by G proteins is a fundamental and widespread mechanism used by a wide variety of hormones, neurotransmitters, and autocrine and paracrine factors to regulate cellular functions. G proteins modulate not only cAMP formation, but also intracellular Ca2+ mobilization, arachidonic acid release, and, very importantly, membrane potential. The mechanism by which G proteins are activated provides for amplification, reversal of action, and continued monitoring of incoming signals. Not all G proteins are known and some are known but their functions are still unknown. More G proteins and more effector functions affected by them will surely be found. We discuss these features of signal transduction by G proteins.