The G protein-first activation mechanism of opioid receptors by Gi protein and agonists

Abstract

We report the G-Protein-First mechanism for activation of G protein-coupled receptors (GPCR) for the three closely subtypes of the opioid receptors (OR), h μOR, OR, and OR. We find that they couple to the inactive Gi protein-bound guanosine diphosphate (GDP) prior to agonist binding. The inactive Gi protein forms anchors to the intracellular loops of the inactive apo-μOR, apo-OR, and apo-OR, inducing opening of the cytoplasmic region to form a pre-activated state that holds Gi protein in place until agonist binds. Then, agonist binds to μOR, OR, and OR already complexed with Gi protein, to trigger the Gi to open up the tightly coupled GDP binding site, making GDP accessible for GTP exchange, an essential step for Gi signaling. We show that the agonist alone cannot open the intracellular region of μOR and OR, requiring Gi protein to open the cytoplasmic region by itself. We consider that this G Protein-First mechanism may apply to activation of other Class A GPCRs. However, for OR, agonist binding can open up the intracellular region to encourage Gi protein recruitment. Thus, activation of Gi protein mediated by OR favorably may proceed with either Ligand-First or G Protein-First activation mechanisms.

Predicted Structure of Fully Activated Tas1R3/1R3′ Homodimer Bound to G Protein and Natural Sugars: Structural Insights into G Protein Activation by a Class C Sweet Taste Homodimer with Natural Sugars

“…Overall, these two salt bridges serve as anchors that orient and position the G Gust so that the Gα–α5 helix is lined up for insertion into the cytoplasmic region of Tas1R3 (Figure and Figure S2). Strikingly, these ionic anchors are similar to ones that we found to play a crucial role in recruitment both of the Gi protein and of β-arrestin2 by opioid receptors − and also for the G Gust protein by the bitter taste Tas2R4 receptor …”

“…To model αβγ subunits of nucleotide-free heterotrimeric G Gust , we used our optimized Gi protein complexed with μ-opioid receptor − as a template to perform homology modeling using Prime (Schrödinger). , We chose the Gi protein because there is a high sequence similarity (>∼63%) between these two heterotrimeric G protein structures.…”

G protein coupling and activation of the metabotropic GABAB heterodimer

“…2 ). In previous studies for class A GPCRs, we reported that GPCRs bind strongly to their cognate GP via SB and HB interactions at all three ICLs 25 – 28 . However, the G i P binding mode for GABA B R is significantly different than typical class A GPCRs (Supplementary Fig.…”

Synergic Effects in the Activation of the Sweet Receptor GPCR Heterodimer for Various Sweeteners Predicted Using Molecular Metadynamics Simulations

“…Applying this procedure to the antagonist JDTic ligand in the X-ray structure leads to a structure deviating only 0.23 Å RMSD from the X-ray . This procedure led to successful ligand binding site predictions for CCR5, , GLP1R, CB1, AA3, TAS2R38, DP prostaglandin, TAS1R2/1R3 heterodimer, TAS2R4, κ-opioid receptor, − and μ-opioid receptor . Based on these predictions, we identified new candidates for experimental validation.…”

“…31 This procedure led to successful ligand binding site predictions for CCR5, 32,33 GLP1R, 34 CB1, 35 AA3, 36 TAS2R38, 37 DP prostaglandin, 38 TAS1R2/1R3 heterodimer, 39 TAS2R4, 40 κ-opioid receptor, 41−43 and μ-opioid receptor. 43 Based on these predictions, we identified new candidates for experimental validation. 2.2.…”