Coupling of a mutated form of the human beta 2-adrenergic receptor to Gi and Gs. Requirement for multiple cytoplasmic domains in the coupling process.
We constructed five genes encoding mutant human beta 2-adrenergic receptor sequence (beta 2AR) which contained 12-22 amino acid substitutions with corresponding sequence from the human alpha 2AAR in order to assess the receptor domains involved in Gs versus Gi recognition and coupling. Mutant beta 2AR with substitutions in the N (S1)- and C-terminal (S2) portions of the third intracellular loop, the proximal cytoplasmic tail (S3), and two combinations thereof (S2,3 and S1,2,3), were stably expressed in Chinese hamster fibrobasts (CHW-1102), as were the human beta 2AR and alpha 2AAR at comparable receptor levels. All mutant receptors with S2 substitutions (i.e. S2, S2,3, S1,2,3) were significantly (approximately 85%) uncoupled from Gs. Upon exposure to pertussis toxin, which uncouples receptors from Gi, S1,2,3 exhibited a 526 +/- 99% increase in agonist-stimulated adenylylcyclase activity compared with a 59 +/- 13% increase with the wild type receptor. This enhanced ability of S1,2,3 to interact with Gs following pertussis toxin treatment indicates that, in the absence of toxin exposure, substantial coupling occurs between the mutant receptor and Gi. Mutant beta 2AR bearing only one or two alpha 2AAR-substituted sequences showed no such enhancement. Forskolin-stimulated enzyme activities were increased by pertussis toxin treatment to similar degrees in all clones examined, indicating that the observed effects are confined to the receptor-mediated pathway. In the absence of GTP, competition binding experiments with S1,2,3, beta 2AR and alpha 2AAR revealed that approximately 40-50% of the receptors formed a high affinity binding state for agonist. Pertussis toxin treatment markedly reduced this to approximately 19% with S1,2,3, while having no effect on beta 2AR and completely eliminating high affinity agonist binding to alpha 2AAR. These results suggest that S1,2,3 interacts with Gi as well as Gs, and that receptor:G protein coupling requires the concerted participation of multiple cytoplasmic receptor domains.
Liggett, SB; Caron, MG; Lefkowitz, RJ; Hnatowich, M
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