According to early models of GPCR signaling, G proteins only interact with activated receptors. However, some GPCRs were shown to assemble with G proteins before receptor activation, in accordance with more recent models. Previously, we found that the 5-HT7 receptor, as opposed to the 5-HT4 receptor, was preassociated with Gs, but the molecular determinants for this interaction are still elusive. In a series of chimeric 5-HT7 receptors with intracellular segments from 5-HT4, we determined the receptor-G protein interaction by performing antibody-immobilized fluorescence recovery after photobleaching and fluorescence resonance energy transfer. We identified the intracellular loop 3 and C-tail of the 5-HT7 receptor to be responsible for the preassociation with Gs, and we further delineated the TM5 extension in the intracellular loop 3 and helix 8 in the C-tail as the molecular determinants. These chimeric exchanges converted the 5-HT7 receptor into a collision-coupled receptor that recruited G proteins only upon agonist activation, whereas reciprocal exchanges converted 5-HT4 to a preassociated receptor. The 5-HT7 receptor displayed 2-component agonist-induced Gs signaling with high and low potency. In addition, the same segments were involved in low-potency signaling and preassociation. The correspondence between Gs preassociation and low-potency Gs signaling is a novel aspect of GPCR pharmacology.-Ulsund, A. H., Dahl, M., Frimurer, T. M., Manfra, O., Schwartz, T. W., Levy, F. O., Andressen, K. W. Preassociation between the 5-HT7 serotonin receptor and G protein Gs: molecular determinants and association with low potency activation of adenylyl cyclase.
Keywords: FRAP; FRET; collision coupling; preassembly; precoupling.