The signal transduction between beta-receptors and adenylyl cyclase

The beta-adrenergic receptor-dependent adenylyl cyclase system is the most extensively studied G-protein-coupled system. Studies of the coupling between the receptor and effector can provide an insight into the nature of all these systems in general. In the activation of adenylyl cyclase by the receptor, the binding of an agonist to the stimulatory receptor (Rs) and the binding of GTP to the G-protein (Gs) are both required to activate the catalytic moiety (C). The active state decays as GTP is hydrolysed to GDP and inorganic phosphate (Pi), but reactivation occurs as GTP is replenished. The receptor acts as a catalyst, i.e. one agonist-bound receptor can activate numerous adenylyl cyclase molecules. Kinetic studies led to the formulation of the 'collision coupling' model of receptor activation and show that Gs protein does not shuttle between the receptor and cyclase. The Gs protein appears to undergo conformational changes between an 'open' state in which it can bind with GTP, and a 'closed' state unable to achieve this binding. This mechanism of activation does not involve the dissociation of Gs or of Gi. A model which fits the experimental data suggests that Gi*GTP affects cyclase only in its Gs-activated state via the G alpha 1 subunit, but that the oligomeric state of Gi is required for inhibition. The site on C which interacts with Gi is formed only when C is activated by Gs.