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Key residues defining the mu-opioid receptor binding pocket: a site-directed mutagenesis study
A Mansour1, L P Taylor, J L Fine
1Mental Health Research Institute, University of Michigan, Ann Arbor 48109-0720, USA.
Journal of Neurochemistry
|January 1, 1997
Summary
Site-directed mutagenesis of the rat mu-opioid receptor revealed key amino acid residues critical for ligand binding and selectivity. His297 is essential for binding, while Asn150 and Ile196 significantly impact mu-opioid agonist affinity.
Area of Science:
- Pharmacology
- Molecular Biology
- Neuroscience
Background:
- The mu-opioid receptor (MOR) is a G protein-coupled receptor crucial for pain modulation and reward pathways.
- Understanding the structural basis of ligand binding and selectivity is vital for developing targeted therapeutics.
- Site-directed mutagenesis is a powerful tool to probe protein function and identify critical residues.
Purpose of the Study:
- To investigate the role of specific amino acid residues in the rat mu-opioid receptor in determining ligand binding and selectivity.
- To elucidate the structural determinants of agonist and antagonist interactions with the MOR.
- To explore the contribution of conserved and selectivity-determining residues to receptor function.
Main Methods:
- Site-directed mutagenesis was employed to generate five single amino acid mutations in the rat mu-opioid receptor.
- Mutated receptors were expressed and analyzed for their binding affinities with various radiolabeled opioid agonists and antagonists.
- Specific mutations targeted conserved residues (Asn150, His297, Tyr326) and residues involved in mu/delta selectivity (Ile196, Val202).
Main Results:
- Mutation of His297 in transmembrane domain 6 (TM6) abolished detectable binding for all tested opioid ligands.
- Mutation of Asn150 in TM3 significantly increased affinity (3- to 20-fold) for multiple mu-opioid agonists, without affecting antagonist binding.
- Ile196 to Val mutation decreased affinity for mu-opioid agonists (morphine, DAMGO) by four- to fivefold, while Val202 to Ile mutation had no effect.
- Tyr326 mutation in TM7 resulted in reduced affinity for a broad range of mu, delta, and kappa opioid agonists and antagonists.
Conclusions:
- His297 is an indispensable residue for ligand binding to the mu-opioid receptor.
- Asn150 plays a critical role in modulating the affinity of mu-opioid agonists.
- Ile196 contributes to the affinity of specific mu-opioid agonists, highlighting its role in selectivity.
- Tyr326 is involved in the binding of a wide spectrum of opioid ligands, suggesting a broader structural role.