Selective activation of a chimeric Gi1/Gs G protein alpha subunit by the human IP prostanoid receptor: analysis using agonist stimulation of high affinity GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding.
A FLAG-tagged form of the human IP prostanoid receptor was expressed stably in HEK 293 cells. This bound [3H]iloprost with high affinity and stimulated cAMP production when exposed to agonist. Iloprost produced weak stimulation of GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding in membranes of these cells. Pretreatment of cells with pertussis toxin did not modify iloprost-mediated stimulation, but this was blocked by cholera toxin. The effects of iloprost were not increased by coexpression of either Gsalpha or Gi1alpha. In contrast, coexpression of a chimeric G protein alpha subunit in which the carboxyl-terminal six amino acids of Gi1alpha were altered to those of Gsalpha resulted in robust stimulation by iloprost. Because the chimeric G protein alpha subunit (Gi1/Gs6alpha) is not a substrate for either pertussis or cholera toxin, pretreatment of cells coexpressing the IP prostanoid receptor and Gi1/Gs6alpha with a mixture of these toxins resulted in resolution of the signal derived from activation of the chimeric G protein. Agonist-stimulated [35S]guanosine-5'-O-(3-thio)triphosphate binding and GTPase activity assays are the most commonly used strategies to examine interactions between G protein-coupled receptors and G proteins. These usually are not appropriate for receptors such as the IP prostanoid receptor that interact with G proteins with low rates of guanine nucleotide exchange and hydrolysis. Chimeric G proteins such as Gi1/Gs6alpha that allow appropriate receptor contacts to be converted to the higher nucleotide turnover rates typical of the Gi family G proteins can overcome this and offer a novel means to examine agonist function at such receptors.
Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, Scotland.
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Fong
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Bahia
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Rees
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Milligan
D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism*; D006801:Humans; D016285:Iloprost / Q000378:metabolism*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D044006:Receptors, Epoprostenol; D011982:Receptors, Prostaglandin / Q000819:agonists / Q000378:metabolism*; D013462:Sulfur Radioisotopes
C113936:PTGIR protein, human; D010975:Platelet Aggregation Inhibitors; D044006:Receptors, Epoprostenol; D011982:Receptors, Prostaglandin; D013462:Sulfur Radioisotopes; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000242:Cyclic AMP; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D016285:Iloprost