Coupling of β2-Adrenoceptors to XLαs and Gαs: A New Insight into Ligand-Induced G Protein Activation

• Published in: The Journal of pharmacology and experimental therapeutics 2009-04, Vol.329 (1), p.350
• Main Authors: A. I. Kaya, Ö. Uǧur, S. S. Öner, M. Bastepe, H. O. Onaran
• Format: Article
• Language: English
• ISSN: 0022-3565; 1521-0103
• DOI: 10.1124/jpet.108.149989

Gα s and e x tra-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.