A conserved arginine in the distal third intracellular loop of the mu-opioid receptor is required for G protein

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Updated: Oct 12, 2026

Quantifying Agonist Activity at G Protein-coupled Receptors
11:45

Quantifying Agonist Activity at G Protein-coupled Receptors

Published on: December 26, 2011

H L Wang1

  • 1Department of Physiology, Chang Gung University School of Medicine, Tao-Yuan, Taiwan, Republic of China.

Journal of Neurochemistry
|February 26, 1999
PubMed

Insights

The C-terminal tail of the mu-opioid receptor is not crucial for Gi protein activation. However, arginine 280 in the third intracellular loop is essential for mu-opioid receptor signaling.

Area of Science:

  • Pharmacology
  • Molecular Biology
  • Neuroscience

Background:

  • The mu-opioid receptor (MOR) is a G protein-coupled receptor involved in pain modulation.
  • Intracellular loops of G protein-coupled receptors are critical for signal transduction.
  • The role of the MOR's C-terminal tail and third intracellular loop in Gi protein activation requires further elucidation.

Purpose of the Study:

  • To investigate the functional significance of the MOR's intracellular C-terminal loop in Gi protein activation.
  • To determine the specific residues within the third intracellular loop critical for MOR-mediated signaling.

Main Methods:

  • Construction and stable expression of C-terminal deletion mutant (MOR(CΔ45)) and point mutant (R280L) MOR in HEK 293 cells.
  • Assessment of agonist ([D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin - DAMGO) potency and efficacy in inhibiting forskolin-stimulated adenylate cyclase activity.
  • Site-directed mutagenesis of conserved basic residues (R276/R277/R280) in the third intracellular loop.

Main Results:

  • Deletion of the MOR C-terminal 45 amino acids did not significantly alter DAMGO's potency or efficacy in inhibiting adenylate cyclase.
  • Mutating basic residues R276/R277/R280 to neutral amino acids (L276/M277/L280) significantly impaired DAMGO-induced inhibition of cyclic AMP production.
  • Specifically, the R280L mutation markedly reduced the ability of DAMGO to inhibit adenylate cyclase activity.

Conclusions:

  • The intracellular C-terminal tail of the mu-opioid receptor plays a minimal role in activating Gi proteins.
  • The arginine residue at position 280 (R280) within the third intracellular loop is essential for Gi protein activation by the mu-opioid receptor.

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