RT Journal Article SR Electronic T1 Similar Apparent Constitutive Activity of Human Histamine H2-Receptor Fused to Long and Short Splice Variants of G JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 1013 OP 1020 VO 299 IS 3 A1 Katharina Wenzel-Seifert A1 Melissa T. Kelley A1 Armin Buschauer A1 Roland Seifert YR 2001 UL http://jpet.aspetjournals.org/content/299/3/1013.abstract AB Fusion proteins allow for the analysis of receptor/G protein coupling under defined conditions. The β2-adrenoceptor (β2AR) fused to the long splice variant of Gsα (GsαL) exhibits a higher apparent constitutive activity than the β2-adrenoceptor fused to the short splice variant of Gsα (GsαS). Experimentally, this results in higher efficacy and potency of partial agonists and in higher efficacy of inverse agonists at the β2AR fused to GsαL relative to the β2AR fused to GsαS, indicating that the agonist-free β2AR and the β2AR occupied by partial agonists promote GDP dissociation from GsαL more efficiently than from GsαS. In fact, the GDP affinity of GsαS fused to the β2AR is higher than the GDP affinity of GsαL fused to the β2AR. We asked the question whether the histamine H2-receptor (H2R) exhibits similar coupling to Gsα splice variants as the β2AR. To address this question, we studied H2R-Gsα fusion proteins expressed in Sf9 cells. In contrast to β2AR-Gsα fusion proteins, the potencies and efficacies of partial agonists and the efficacies of inverse agonists were similar at the H2R fused to GsαL and GsαS as assessed by guanosine-5′-O-(3-thio)triphosphate binding and/or steady-state GTPase activity. However, the time course analysis of guanosine-5′-O-(3-thio)triphosphate binding indicated that GsαS fused to the H2R possesses a higher GDP-affinity than GsαL fused to the H2R. Our data show that the H2R fused to GsαL and GsαS possesses similar constitutive activity and is insensitive to differences in GDP affinity of Gsα splice variants. Thus, GDP affinity of G proteins does not generally determine constitutive activity of receptors. The American Society for Pharmacology and Experimental Therapeutics