Structural snapshots capture nucleotide release at the μ-opioid receptor.

Area of Science:

• Pharmacology
• Structural Biology
• Biochemistry

Background:

• The μ-opioid receptor (MOR) is a key G protein-coupled receptor involved in pain signaling.
• MOR activation of heterotrimeric G proteins involves GDP-GTP exchange, with GDP release being the rate-limiting step.
• Ligand efficacy at MOR influences the rate of G protein activation.

Purpose of the Study:

• To elucidate the structural mechanisms underlying MOR activation and ligand efficacy.
• To characterize intermediate conformations of MOR during G protein activation.
• To understand how agonists and antagonists modulate GDP affinity and nucleotide exchange.

Main Methods:

• Pharmacological assays to assess agonist and antagonist effects on GDP affinity.
• Cryogenic electron microscopy (cryo-EM) to determine structural models of MOR-G protein complexes.
• Molecular dynamics simulations to investigate nucleotide exchange dynamics.

Main Results:

• Agonist efficacy correlates with decreased GDP affinity, facilitating GTP exchange.
• Antagonists increase GDP affinity, inhibiting G protein activation.
• Structural models revealed novel GDP-bound states and intermediate conformations of the Gα α-helical domain (AHD).
• Naloxone stabilizes a 'latent' state, while loperamide promotes an 'engaged' state poised for activation.

Conclusions:

• GDP-bound intermediates and AHD conformations are critical determinants of nucleotide exchange rates.
• Structural insights into MOR activation provide a mechanistic basis for ligand efficacy.
• Findings have broad implications for the development of G protein-coupled receptor-targeted therapeutics.