Allosteric equilibrium model explains steady-state coupling of beta-adrenergic receptors to adenylate cyclase in turkey erythrocyte membranes.
We used a simple experimental approach to clarify some contradictory predictions of the collision coupling and equilibrium models (e.g. ternary complex, two-state ternary complex or quinternary complex), which describe G-protein-mediated beta-adrenergic receptor signalling in essentially different manners. Analysis of the steady-state coupling of beta-adrenoceptors to adenylate cyclase in turkey erythrocyte membranes showed that: (1) in the absence of an agonist, Gpp(NH)p (a hydrolysis-resistant analogue of GTP) can activate adenylate cyclase very slowly; (2) this activity reaches a steady state in approx. 5 h, the extent of activity depending on the concentration of the nucleotide; (3) isoprenaline-activated steady-state adenylate cyclase can be inactivated by propranolol (a competitive antagonist that relaxes the receptor activation), in the presence of Gpp(NH)p (which provides a virtual absence of GTPase) and millimolar concentrations of Mg2+ (the rate of this inactivation is relatively fast); (4) increasing the concentration of Gpp(NH)p can saturate the steady-state activity of adenylate cyclase. The saturated enzyme activity was lower than that induced by isoprenaline under the same conditions. This additional agonist-induced activation was reversible. In the light of these results, we conclude that agonist can also activate the guanine nucleotide-saturated system in the absence of GTPase by a mechanism other than guanine nucleotide exchange. We explain these phenomena in the framework of a quinternary complex model as an agonist-induced and receptor-mediated dissociation of guanine nucleotide-saturated residual heterotrimer, the equilibrium concentration of which is not necessarily zero. These results, which suggest a continuous interaction between receptor and G-protein, can hardly be accommodated by the collision coupling model that was originally suggested for the present experimental system and then applied to many other G-protein systems. Therefore we attempt to unify the equilibrium and collision coupling approaches to provide a consistent theoretical basis for the G-protein-mediated beta-adrenergic receptor signalling in turkey erythrocyte membranes.
Ugur
O
O
Department of Pharmacology, Medical Faculty of Ankara University, Sihhiye 06100, Ankara, Turkey.
Onaran
H O
HO
D000262:Adenylyl Cyclases / Q000097:blood*; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000494:Allosteric Regulation; D000818:Animals; D003198:Computer Simulation; D000242:Cyclic AMP / Q000378:metabolism; D004789:Enzyme Activation; D004910:Erythrocyte Membrane / Q000201:enzymology*; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D046911:Macromolecular Substances; D008956:Models, Chemical; D011433:Propranolol / Q000494:pharmacology; D011485:Protein Binding; D011943:Receptors, Adrenergic, beta / Q000097:blood* / Q000187:drug effects; D015398:Signal Transduction / Q000502:physiology*; D014422:Turkeys / Q000097:blood
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D046911:Macromolecular Substances; D011943:Receptors, Adrenergic, beta; D006165:Guanylyl Imidodiphosphate; D011433:Propranolol; D000242:Cyclic AMP; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D007545:Isoproterenol