G-protein subunit dissociation is not an integral part of G-protein action.

Abstract

The concept that a guanosine triphosphate (GTP) binding protein (or G protein) is a transducer of receptor-to-effector signal transduction was formulated in the 1970s for hormonedependent adenylyl cyclase. It has been shown that binding of the hormone to the receptor triggers exchange of guanosine diphosphate (GDP) for GTP on the G protein, thereby converting the G protein from the inactive conformation to the activated form. In its GTP-bound state, the G protein activates adenylyl cyclase to produce cyclic adenosine monophosphate (cAMP). Hydrolysis of the GTP terminates the signal. Since the rate of GTP hydrolysis is approximately 100 times slower than the rate of production of cAMP by the catalytic unit, one hormone± receptor complex is able to generate many cAMP molecules per minute. Another amplification step is between the receptor and the G protein, since the hormone ± receptor complex interacts transiently with the G protein and dissociates from it, once the G protein has been loaded with GTP. The receptor then interacts with other G protein molecules. The amplification factor of this step has been estimated to be about 10 in the -adrenergic-dependent adenylyl cyclase system. For the light-dependent activation of cyclic guanosine monophosphate (cGMP) dependent phosphodiesterase by rhodopsin, it was found that each activated molecule of rhodopsin activates approximately 300 phosphodiesterase molecules. This mechanism of activation is known as TMcollision coupling∫. Many other G-protein-coupled receptor systems have since been discovered, but the main features of the signaling pathway remain essentially similar to those initially described for hormone-dependent adenylyl cyclases. The prevailing dogma for the action of G-protein-coupled receptors is still based on the detailed biochemical studies performed on hormone-dependent adenylyl cyclase. The Dogma