Effects of Guanine, Inosine, and Xanthine Nucleotides on β2-Adrenergic Receptor/GsInteractions: Evidence for Multiple Receptor Conformations

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Abstract
The aim of our study was to examine the effects of different purine nucleotides [GTP, ITP, and xanthosine 5′-triphosphate (XTP)] on receptor/G protein coupling. As a model system, we used a fusion protein of the β2-adrenergic receptor and the α subunit of the G protein Gs. GTP was more potent and efficient than ITP and XTP at inhibiting ternary complex formation and supporting adenylyl cyclase (AC) activation. We also studied the effects of several β2-adrenergic receptor ligands on nucleotide hydrolysis and on AC activity in the presence of GTP, ITP, and XTP. The efficacy of agonists at promoting GTP hydrolysis correlated well with the efficacy of agonists for stimulating AC in the presence of GTP. This was, however, not the case for ITP hydrolysis and AC activity in the presence of ITP. The efficacy of ligands at stimulating AC in the presence of XTP differed considerably from the efficacies of ligands in the presence of GTP and ITP, and there was no evidence for receptor-regulated XTP hydrolysis. Our findings support the concept of multiple ligand-specific receptor conformations and demonstrate the usefulness of purine nucleotides as tools to study conformational states of receptors.