Regulator of G protein signaling 4 confers selectivity to specific G proteins to modulate μ- and δ-opioid receptor signaling

In vitro studies have shown that the Regulator of G protein Signaling 4 (RGS4) interacts with the C-termini of μ- and δ-opioid receptors (μ-OR, δ-OR) (Georgoussi et al., 2006, Cell. Signal.18, 771–782). Herein we demonstrate that RGS4 associates with these receptors in living cells and forms selective complexes with Gi/Go proteins in a receptor dependent manner. This interaction occurs within the predicted fourth intracellular loop of μ, δ-ORs as part of a signaling complex consisting of the opioid receptor, activated Gα and RGS4. RGS4 is recruited to the plasma membrane upon opioid receptor stimulation. Expression of RGS4 in HEK293 cells attenuated agonist-mediated extracellular signal regulated kinase (ERK1,2) phosphorylation for both receptors and accelerated agonist-induced internalization of the δ-OR. RGS4 lacking its N-terminal domain failed to interact with both opioid receptors and to modulate opioid receptor signaling. Our findings demonstrate that RGS4 plays a key role in G protein coupling selectivity and signaling of the μ- and δ-ΟRs.