Influence of G Protein Type on Agonist Efficacy

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Abstract
An agonist at a specific G protein-coupled receptor may exhibit a range of efficacies for any given response in a cell-specific manner. We report that the relationship between different states of agonism is regulated by the type of G protein expressed in the cell. In NIH-3T3 α2-adrenergic receptor (AR) transfectants, the α2-AR agonists clonidine, oxymetazoline, UK-14304, and epinephrine increased [35S]guanosine-5′-O-(3-thio)triphosphate binding in a dose-dependent manner from a basal value of 101.2 ± 6.5 fmol/mg to a maximal response (100 μM) of 196.6 ± 9.8, 182.3 ± 2, 328.1 ± 11.2, and 340.6 ± 3 fmol/mg, respectively. Thus, clonidine and oxymetazoline behaved as partial agonists. Receptor-mediated activation of G proteins in membrane preparations was blocked by cell pretreatment with pertussis toxin, indicating receptor coupling to the subgroup of pertussis toxin-sensitive G protein (Giα2,3) expressed in NIH-3T3 cells. Ectopic expression of Goα1 but not Giα1 increased the relative efficacy of clonidine and oxymetazoline such that the two ligands now behaved as close to full agonists in this assay system. The relationship between full and partial agonists in the different genetic backgrounds was not altered by progressive reduction in the amount of G protein available for coupling to receptor. The increased efficacy observed for clonidine in the Goα1 transfectants was not due to changes in the relative affinities or amounts of high-affinity, Gpp(NH)p-sensitive binding of agonist. These data suggest that there is little difference in the ability of clonidine to interact with or stabilize α2-AR–Giα2/Giα3 versus α2-AR–Goα1 complexes, but that the subsequent step of signal transfer from receptor to G protein is more readily achieved for the clonidine/α2–AR/Goα1 complex. Such observations have important implications for receptor theory and drug development.