title: Dimers of class A G protein-coupled receptors function via agonist-mediated trans-activation of associated G proteins.
abstract: The histamine H1 receptor and the alpha1b-adrenoreceptor are G protein-coupled receptors that elevate intracellular [Ca2+] via activation of Gq/G11. Assessed by co-immunoprecipitation and time-resolved fluorescence resonance energy transfer they both exist as homo-dimers. The addition of the G protein G11alpha to the C terminus of these receptors did not prevent dimerization. Agonists produced a large stimulation of guanosine 5'-3-O-([35S]thio)triphosphate ([35S]GTPgammaS) binding to receptor-G protein fusions containing wild type forms of both polypeptides. For both receptors this was abolished by incorporation of G208AG11alpha into the fusions. Mutation of a highly conserved leucine in intracellular loop 2 of each receptor also eliminated agonist function but not binding. Co-expression of the two non-functional but complementary fusion constructs reconstituted agonist-mediated binding of [35S]GTPgammaS in membranes of HEK293 cells and elevation of [Ca2+]i in mouse embryo fibroblasts lacking both Gq and G11. Co-expression of the histamine H1 receptor- and the alpha1b-adrenoreceptor-G11alpha fusions allowed detection of functional hetero-dimeric complexes, whereas co-expression of histamine H1 receptor-G11alpha with increasing amounts of L151Dalpha1b-adrenoreceptor resulted in decreasing levels of histamine-stimulated [35S]GTPgammaS binding. Co-expression of the alpha1b-adrenoreceptor with a fusion protein incorporating the N-terminal domain and transmembrane helix 1 of the alpha1b-adrenoreceptor and G11alpha did not result in agonist activation of the G protein but did indicate a role for transmembrane helix 1 in dimerization. These data demonstrate that dimers of these class A receptors function via trans-activation of associated G proteins.
authors.affiliation: Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Carrillo
authors.forename: John
authors.initials: J
authors.lastname: Pediani
authors.forename: Graeme
authors.initials: G
authors.lastname: Milligan
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D002460:Cell Line; D019281:Dimerization; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000235:genetics / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D051379:Mice; D018340:Receptors, Adrenergic, alpha-1 / Q000235:genetics / Q000502:physiology; D043562:Receptors, G-Protein-Coupled / Q000819:agonists / Q000235:genetics / Q000502:physiology*; D011969:Receptors, Histamine H1; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: C547716:ADRA1B protein, human; C547717:Adra1b protein, mouse; D018340:Receptors, Adrenergic, alpha-1; D043562:Receptors, G-Protein-Coupled; D011969:Receptors, Histamine H1; D011993:Recombinant Fusion Proteins; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D043802:GTP-Binding Protein alpha Subunits, Gq-G11