Agonist binding to peptide hormone receptors

Area of Science:

• Molecular Pharmacology
• Biochemistry
• G-protein-coupled receptor (GPCR) research

Background:

• Understanding the molecular mechanisms differentiating agonist and antagonist interactions with receptors is crucial.
• G-protein-coupled receptors (GPCRs), including the vasopressin V(1a) receptor (V(1a)R), are activated by peptide ligands like vasopressin and oxytocin (OT).

Purpose of the Study:

• To elucidate the distinct molecular interactions of agonists versus antagonists at the V(1a)R.
• To identify specific amino acid residues and receptor regions involved in ligand binding and activation.

Main Methods:

• Characterization of truncated V(1a)R constructs to map ligand-binding domains.
• Site-directed mutagenesis to investigate the role of specific amino acid residues.
• Construction and analysis of chimeric receptors (OTR(N)-V(1a)R) to assess functional conservation.

Main Results:

• A specific N-terminal segment was identified as critical for agonist binding, but not antagonist binding.
• A single residue, Arginine 46 (Arg46) in V(1a)R, was found to be essential for agonist binding and receptor activation.
• The N-terminus of the oxytocin receptor (OTR) could restore agonist binding to a chimeric receptor, suggesting conserved mechanisms.
• Arginine 34 (Arg34) in OTR, corresponding to Arg46 in V(1a)R, also mediated agonist-specific binding, indicating a conserved role across related GPCRs.

Conclusions:

• The N-terminus of V(1a)R plays a critical role in discriminating between agonist and antagonist binding.
• Arginine at position 46 in V(1a)R is indispensable for high-affinity agonist binding and subsequent receptor activation.
• This arginine residue and its role in agonist binding appear conserved within the oxytocin and vasopressin receptor subfamily.