Abstract

Gα_s and extra-large Gα_s (XLα_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 XLα_s couples to the β2-adrenoceptor more efficiently than Gα_s. In transfected human embryonic kidney 293 cells and mouse embryonic fibroblasts null for both Gα_s and XLα_s (2B2 cells), basal cAMP accumulation mediated by XLα_s was higher than that mediated by Gα_s. Inverse agonist treatment reduced Gα_s-mediated basal activity, whereas its effect was markedly blunted on XLα_s-mediated basal activity. Rank order of ligand efficacies regarding cAMP accumulation was the same when the receptor was coupled to XLα_s or Gα_s. However, ligand-induced and XLα_s-mediated cAMP generation was higher than that mediated by Gα_s. The relatively high efficiency of XLα_s-mediated cAMP generation was conditional, disappearing with increased level of receptor expression or increased efficacy of ligand. Full-agonist responses in XLα_s- and Gα_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 β2-adrenoceptor was more pronounced in cells expressing XLα_s than those expressing Gα_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.