Different Structural States of the Proteolipid Membrane Are Produced by Ligand Binding to the Human δ-Opioid Receptor as Shown by Plasmon-Waveguide Resonance Spectroscopy

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Abstract
Understanding structure-function relationships and mechanisms of signal transduction in G-protein-coupled receptors (GPCRs) is becoming increasingly important, both as a fundamental problem in membrane biology and as a consequence of their central role as pharmacological targets. Their integral membrane nature and rather low natural abundance present many challenging problems. Using a recently developed technique, plasmon-waveguide resonance (PWR) spectroscopy, we investigated the structural changes accompanying the binding of ligands to the human δ-opioid receptor (hDOR) immobilized in a solid-supported lipid bilayer. This highly sensitive technique can directly monitor changes in mass density, conformation, and orientation occurring in such thin proteolipid films. Without requiring labeling protocols, PWR allows the direct determination of binding constants in a system very close to the receptor's natural environment. In the present study, conformational changes of a proteolipid membrane containing the hDOR were investigated upon binding of a variety of peptide and nonpeptide agonists, partial agonists, antagonists, and inverse agonists. Distinctly different structural states of the membrane were observed upon binding of each of these classes of ligands, reflecting different receptor conformational states, and the formation of each state was characterized by different kinetic properties. Binding constants, obtained by quantifying the extent of conformational change as a function of the amount of ligand bound, were in good agreement with published values determined by radiolabeling methods. The results provide new insights into ligand-induced GPCR functioning and illustrate a powerful new protocol for drug development.