Bidirectional allosteric effects of agonists and GTP at alpha(2A/D)-adrenoceptors.
Agonists and GTP exert reciprocal effects on the stability of the G protein-coupled receptor/G protein complex, implying bidirectional control over the receptor/G protein interface. To investigate this relationship, we compared the ability of a series of hydroxyl-substituted phenethylamine and imidazoline agonists to stimulate [(35)S]guanosine 5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) binding in membranes from alpha(2A/D)-adrenergic receptor-transfected PC12 cells with the magnitude of the GTP-induced reduction in agonist affinity in [(3)H]rauwolscine-binding studies. Agents previously described as full and partial agonists in functional studies showed similar relative efficacies in promoting GTP binding (r = 0.97) as well as similar relative potencies (r = 0.94). Efficacy among agonists for promotion of [(35)S]GTPgammaS binding was closely correlated with the relative influence of GTPgammaS on agonist binding (r = 0.97), consistent with a bidirectional allosteric influence by agonists and GTP on receptor/G protein complexation. In an additional series of tolazoline derivatives, a range in efficacy from full agonism to strong inverse agonism was observed, depending on the presence or absence of hydroxyl substituents. Together these results suggest that agonist-induced repositioning of transmembrane helices via their hydroxyl interactions is a critical determinant of the stability of the receptor/G protein complex and therefore of agonist efficacy.
Tian
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Department of Pharmaceutical Sciences, Northeastern University, Boston, Massachusetts, USA.
Miller
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D000322:Adrenergic Agonists / Q000494:pharmacology*; D000494:Allosteric Regulation / Q000502:physiology; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D006160:Guanosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology*; D007093:Imidazoles / Q000494:pharmacology; D016716:PC12 Cells; D010627:Phenethylamines / Q000494:pharmacology; D011485:Protein Binding; D011869:Radioligand Assay; D051381:Rats; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects*; D014043:Tolazoline / Q000031:analogs & derivatives / Q000494:pharmacology; D014162:Transfection; D015016:Yohimbine / Q000378:metabolism
D000322:Adrenergic Agonists; D007093:Imidazoles; D010627:Phenethylamines; D018341:Receptors, Adrenergic, alpha-2; D015016:Yohimbine; D006160:Guanosine Triphosphate; D014043:Tolazoline