Abstract
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 [35S]guanosine 5′-O-(3-thio)triphosphate ([35S]GTPγS) binding in membranes from α2A/D-adrenergic receptor-transfected PC12 cells with the magnitude of the GTP-induced reduction in agonist affinity in [3H]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 [35S]GTPγS binding was closely correlated with the relative influence of GTPγS 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.
Footnotes
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Send reprint requests to: Dr. Richard C. Deth, Department of Pharmaceutical Sciences, 312 Mugar Hall, Northeastern University, 360 Huntington Ave., Boston, MA 02115. E-mail: r.deth{at}nunet.neu.edu
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↵1 This work was supported by U.S. Public Health Service Research Grant NIH-HL29847 (to R.C.D.).
- Abbreviations:
- R*
- active conformation of the receptor
- GTPγS
- guanosine-5′-O-(3-thio)triphosphate
- EPI
- epinephrine
- UK14304
- 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine
- BHT-933
- 2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxalo[5,4-d]azepin dihydrochloride
- Received May 28, 1999.
- Accepted October 26, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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