title: Stoichiometry of receptor-Gs-adenylate cyclase interactions.
abstract: Little is known about the relative stoichiometry of guanine nucleotide-binding (G) proteins relative to the effector systems to which they link. We addressed this question for the stimulatory G protein (Gs) linked to adenylate cyclase. Forskolin stimulates the catalytic subunit of adenylate cyclase (C), but it has a higher efficacy and potency when C also interacts with the G protein Gs. Accordingly, we measured high-affinity [3H]forskolin binding to intact cells to assay alpha s-C complexes. No high-affinity specific binding occurred with unstimulated cells. The beta-adrenergic agonist isoproterenol promoted the binding of [3H]forskolin to about 3000 sites per cell, suggesting that each receptor on average activates at least several Gs molecules. Activating Gs directly with cholera toxin maximally promoted [3H]forskolin binding to a similar number of sites, suggesting that this is the maximal number of alpha s-C complexes formed per cell. We conclude that each cell likely contains only a few thousand functional copies of C, and that the availability of C (rather than Gs, which exists in more than 100,000 copies per cell) is likely to be limiting for agonist stimulation of adenylate cyclase activity.
authors.affiliation: Department of Pharmacology, University of California, San Diego, La Jolla 92093.
authors.forename: A A
authors.initials: AA
authors.lastname: Alousi
authors.forename: J R
authors.initials: JR
authors.lastname: Jasper
authors.forename: P A
authors.initials: PA
authors.lastname: Insel
authors.forename: H J
authors.initials: HJ
authors.lastname: Motulsky
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D002772:Cholera Toxin / Q000494:pharmacology; D005576:Colforsin / Q000378:metabolism / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D008223:Lymphoma / Q000378:metabolism; D051379:Mice; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D014407:Tumor Cells, Cultured
chemical_list: D011943:Receptors, Adrenergic, beta; D005576:Colforsin; D002772:Cholera Toxin; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D007545:Isoproterenol