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 th e receptor/G protein interface. To investigate this relationship, we compar ed the ability of a series of hydroxyl-substituted phenethylamine and imida zoline agonists to stimulate [S-35] guanosine 5'-O-(3-thio)triphosphate ([S -35]GTP gamma S) binding in membranes from alpha(2A/D)-adrenergic receptor- transfected PC12 cells with the magnitude of the GTP-induced reduction in a gonist affinity in [H-3]rauwolscine-binding studies. Agents previously desc ribed as full and partial agonists in functional studies showed similar rel ative efficacies in promoting GTP binding (r = 0.97) as well as similar rel ative potencies (r = 0.94). Efficacy among agonists for promotion of [S-35] GTP gamma S binding was closely correlated with the relative influence of G TP gamma S on agonist binding (r = 0.97), consistent with a bidirectional a llosteric influence by agonists and GTP on receptor/G protein complexation. In an additional series of tolazoline derivatives, a range in efficacy fro m full agonism to strong inverse agonism was observed, depending on the pre sence or absence of hydroxyl substituents. Together these results suggest t hat agonist-induced repositioning of transmembrane helices via their hydrox yl interactions is a critical determinant of the stability of the receptor/ G protein complex and therefore of agonist efficacy.