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 β-adrenergic receptor signalling in essentially different manners. Analysis of the steady-state coupling of β-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 β-adrenergic receptor signalling in turkey erythrocyte membranes.
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Research Article|
May 01 1997
Allosteric equilibrium model explains steady-state coupling of β-adrenergic receptors to adenylate cyclase in turkey erythrocyte membranes
Özlem UGUR;
Özlem UGUR
1Department of Pharmacology, Medical Faculty of Ankara University, Sıhhiye 06100, Ankara, Turkey
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H. Ongun ONARAN
H. Ongun ONARAN
*
1Department of Pharmacology, Medical Faculty of Ankara University, Sıhhiye 06100, Ankara, Turkey
*To whom correspondence should be addressed.
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Publisher: Portland Press Ltd
Received:
July 23 1996
Revision Received:
December 02 1996
Accepted:
January 06 1997
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London © 1997
1997
Biochem J (1997) 323 (3): 765–776.
Article history
Received:
July 23 1996
Revision Received:
December 02 1996
Accepted:
January 06 1997
Citation
Özlem UGUR, H. Ongun ONARAN; Allosteric equilibrium model explains steady-state coupling of β-adrenergic receptors to adenylate cyclase in turkey erythrocyte membranes. Biochem J 1 May 1997; 323 (3): 765–776. doi: https://doi.org/10.1042/bj3230765
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