Stoichiometry of receptor‐G s ‐adenylate cyclase interactions

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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 Gg. Accordingly, we measured high-affinity [3H]forskolin binding to intact cells to assay α,-C complexes. No high-affinity specific binding occurred with unstimulated cells. The β-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 Gg molecules. Activating Gg directly with cholera toxin maximally promoted [3H]forskolin binding to a similar number of sites, suggesting that this is the maximal number of αg-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 Gg, which exists in more than 100,000 copies per cell) is likely to be limiting for agonist stimulation of adenylate cyclase activity.—Alousi, A.; Jasper, J. R.; Insel, P. A.; Motulsky, H. J. Stoichiometry of receptor-Gg-adenylate cyclase interactions. FASEB J. 5: 2300–2303; 1991.

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