Coupling of beta 2-Adrenoceptors to XL alpha(s) and G alpha(s): A New Insight into Ligand-Induced G Protein Activation

G alpha(s) and extra-large G alpha(s) (XL alpha(s)) can both transduce receptor activation into intracellular cAMP generation. It is unknown, however, whether these two GNAS-locus products display distinct properties with respect to receptor coupling. Here, we show that XL alpha(s) couples to the beta 2-adrenoceptor more efficiently than G alpha(s). In transfected human embryonic kidney 293 cells and mouse embryonic fibroblasts null for both G alpha(s) and XL alpha(s) (2B2 cells), basal cAMP accumulation mediated by XL alpha(s) was higher than that mediated by G alpha(s). Inverse agonist treatment reduced G alpha(s)-mediated basal activity, whereas its effect was markedly blunted on XL alpha(s)-mediated basal activity. Rank order of ligand efficacies regarding cAMP accumulation was the same when the receptor was coupled to XL alpha(s) or G alpha(s). However, ligand-induced and XL alpha(s)-mediated cAMP generation was higher than that mediated by G alpha(s). The relatively high efficiency of XL alpha(s)-mediated cAMP generation was conditional, disappearing with increased level of receptor expression or increased efficacy of ligand. Full-agonist responses in XL alpha(s)- and G alpha(s)-expressing cells were comparable even at low receptor levels, whereas partial agonist responses became comparable only when the receptor expression was increased (>3 pmol/mg). Radioligand binding studies showed that the high-affinity component in agonist binding to beta 2-adrenoceptor was more pronounced in cells expressing XL alpha(s) than those expressing G alpha(s). We discuss these findings in the framework of current receptor-G protein activation models and offer an extended ternary complex model that can fully explain our observations.