Activator of G-protein Signaling 1 Blocks GIRK Channel Activation by a G-protein-coupled Receptor

The Ras-related protein, activator ofG-protein signaling 1 (AGS1) or Dexras1, interacts with G_i/G_oα and activates heterotrimeric G-protein signaling systems independent of a G-protein-coupled receptor (GPCR). As an initial approach to further define the cellular role of AGS1 in GPCR signaling, we determined the influence of AGS1 on the regulation of G_βγ-regulated inwardly rectifying K⁺ channel (GIRK) current (I_ACh) by M₂-muscarinic receptor (M₂-MR) in Xenopus oocytes. AGS1 expression inhibited receptor-mediated current activation by >80%. Mutation of a key residue (G31V) within the G₁ domain involved in nucleotide binding for Ras-related proteins eliminated the action of AGS1. The inhibition of I_ACh was not overcome by increasing concentrations of the muscarinic agonist acetylcholine but was progressively lost upon injection of increasing amounts of M₂-MR cRNA. These data suggest that AGS1 may antagonize GPCR signaling by altering the pool of heterotrimeric G-proteins available for receptor coupling and/or disruption of a preformed signaling complex. Such regulation would be of particular importance for those receptors that exist precoupled to heterotrimeric G-protein and for receptors operating within signaling complexes.