Allosteric communication across the μ-opioid receptor-Gi1 protein complex induced by ligands of varying efficacy

Some of the most effective medications for severe pain are high-efficacy μ-opioid receptor (MOR) agonists (e.g., morphine). However, these drugs can also cause adverse effects, including potentially fatal respiratory depression from overdose, and addiction. Interestingly, several low-efficacy MOR agonists, including clinically used opioid drugs (e.g., buprenorphine) and atypical opioid chemotypes (e.g., mitragynine pseudoindoxyl (MP)), cause fewer or weaker adverse effects compared to full agonists. Available MOR-Gi1 cryo-electron microscopy (cryo-EM) structures, bound to either the high-efficacy opioid lofentanil or the low-efficacy agonist MP, together with molecular dynamics (MD) simulation studies of the corresponding ligand-MOR systems (without Gi1), have suggested that the different efficacies of these opioids may result from variations in the binding site promoting distinct active-state conformations of the receptor at the interface with Gi1.In this work, we explore the possibility of variations within the Gi1, and offer atomic-level details of the ligand-specific allosteric communication across both the MOR and the Gi1 to deepen our understanding of the molecular determinants that influence the different safety profiles of opioid ligands. Specifically, we built complete structures of various ligand-MOR-Gi1 complexes by incorporating the absent Gi1 alpha helical domain into the existing cryo-EM structures of MOR-Gi1 complexes in a starting open conformation, using an available structural template. We then derived ligand-induced allosteric communication pathways across the entire ligand-MOR-Gi1 complexes, applying robust statistical analysis tools to trajectories from MD simulations to analyze the causality of correlated motions in these systems. Our analyses unveiled statistically significant differences among the MOR residues that were the primary contributors to the allosteric communication in each simulated ligand-MOR-Gi1 system. Some residues uniquely contributed to allosteric communication in certain systems, thereby offering testable hypotheses on the mechanistic basis of opioid ligand efficacy for Gi1 signaling.