Regulation of G protein activation and effector modulation by fusion proteins between the human 5-hydroxytryptamine(1A) receptor and the alpha subunit of G(i1): differences in receptor-constitutive activity imparted by single amino acid substitutions in G(i1)alpha.
Fusion proteins were generated between the human 5-hydroxytryptamine (5-HT)(1A) receptor and both wild-type (Cys(351)) and pertussis toxin-resistant (Gly(351) and Ile(351)) forms of G(i1). These were expressed stably. Pertussis toxin treatment substantially reduced basal high-affinity GTPase activity in clones expressing the 5-HT(1A) receptor wild-type G(i1)alpha construct but not in clones expressing 5-HT(1A) receptor (Gly(351))G(i1)alpha or (Ile(351))G(i1)alpha. Spiperone functioned as an inverse agonist in membranes expressing the 5-HT(1A) receptor wild-type G(i1)alpha fusion protein and in those expressing 5-HT(1A) receptor (Ile(351))G(i1)alpha but not the 5-HT(1A) receptor (Gly(351))G(i1)alpha fusion protein. The effect of spiperone at the 5-HT(1A) receptor wild-type G(i1)alpha construct but not the 5-HT(1A) receptor (Ile(351))G(i1)alpha construct was blocked by pertussis toxin treatment. By contrast, agonists functioned with equal effectiveness at the three fusion proteins and were unaffected by pertussis toxin treatment of the (Ile(351))G(i1)alpha- and (Gly(351))G(i1)alpha-containing constructs. 5-HT resulted in strong inhibition of forskolin-amplified adenylyl cyclase in intact cells expressing the isolated 5-HT(1A) receptor. In fusion protein-expressing cells, 5-HT-mediated inhibition of adenylyl cyclase was also observed. Pertussis toxin treatment obliterated 5-HT-mediated inhibition in cells expressing the isolated receptor and the 5-HT(1A) receptor wild-type G(i1)alpha fusion protein but not in those expressing the 5-HT(1A) receptor (Ile(351)) or (Gly(351))G(i1)alpha fusion proteins. These studies demonstrate that alteration of a single amino acid in G(i1)alpha located at a key contact site between the G protein and a G protein-coupled receptor can regulate agonist-independent constitutive activity of the G protein-coupled receptor and that fusion proteins can directly regulate adenylyl cyclase.
Kellett
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Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom.
Carr
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Milligan
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D000262:Adenylyl Cyclases / Q000378:metabolism; D019943:Amino Acid Substitution; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D016133:Polymerase Chain Reaction; D011985:Receptors, Serotonin / Q000235:genetics / Q000378:metabolism*; D044263:Receptors, Serotonin, 5-HT1; D011993:Recombinant Fusion Proteins / Q000378:metabolism
D011985:Receptors, Serotonin; D044263:Receptors, Serotonin, 5-HT1; D011993:Recombinant Fusion Proteins; D005576:Colforsin; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases