An inactive receptor-G protein complex maintains the dynamic range of agonist-induced signaling

Abstract

Agonist binding promotes activation of G protein-coupled receptors (GPCRs) and association of active receptors with G protein heterotrimers. The resulting active-state ternary complex is the basis for conventional stimulus-response coupling. Although GPCRs can also associate with G proteins before agonist binding, the impact of such preassociated complexes on agonist-induced signaling is poorly understood. Here we show that preassociation of 5-HT7serotonin receptors with Gsheterotrimers is necessary for agonist-induced signaling. 5-HT7receptors in their inactive state associate with Gs, as these complexes are stabilized by inverse agonists and receptor mutations that favor the inactive state. Inactive-state 5-HT7–Gscomplexes dissociate in response to agonists, allowing the formation of conventional agonist–5-HT7–Gsternary complexes and subsequent Gsactivation. Inactive-state 5-HT7–Gscomplexes are required for the full dynamic range of agonist-induced signaling, as 5-HT7receptors spontaneously activate Gsvariants that cannot form inactive-state complexes. Therefore, agonist-induced signaling in this system involves two distinct receptor-G protein complexes, a conventional ternary complex that activates G proteins and an inverse-coupled binary complex that maintains the inactive state when agonist is not present.