Ligand-dependent G protein dynamics underlying opioid signaling efficacy

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins via conformational changes.
• G protein activation involves opening of the Gα α-helical domain (AHD) to enable GDP-GTP exchange.
• The precise mechanism by which GPCR ligands allosterically control G protein dynamics is not fully understood.

Purpose of the Study:

• To investigate how GPCR ligands modulate the conformational dynamics of the G protein Gi.
• To elucidate the relationship between ligand efficacy and G protein activation at the μ-opioid receptor (μOR).

Main Methods:

• Single-molecule fluorescence resonance energy transfer (smFRET) imaging was employed.
• AHD motions in Gi proteins coupled to μOR were monitored.
• Experiments spanned various ligand- and nucleotide-bound states.

Main Results:

• GPCR ligands differentially modulated Gi AHD dynamics from >70 Å away.
• Higher-efficacy agonists more effectively promoted transitions to an open, low-nucleotide-affinity conformation.
• Transient μOR-Gi intermediates during nucleotide binding were observed.

Conclusions:

• μ-opioid ligand efficacy is partly mediated by allosteric control over G protein conformational equilibria.
• Ligand-induced modulation of G protein dynamics kinetically gates activation.
• This study provides insights into the allosteric signaling mechanisms between GPCRs and G proteins.