Membrane partitioning of various δ-opioid receptor forms before and after agonist activations: The effect of cholesterol

Lipid rafts depicted as densely packed and thicker membrane microdomains, based on the dynamic clustering of cholesterol and sphingolipids, may help as platforms involved in a wide variety of cellular processes. The reasons why proteins segregate into rafts are yet to be clarified. The human delta opioid receptor (hDOR) reconstituted in a model system has been characterised after ligand binding by an elongation of its transmembrane part, inducing rearrangement of its lipid microenvironment [Alves, Salamon, Hruby, and Tollin (2005) Biochemistry 44, 9168–9178]. We used hDOR to understand better the correlation between its function and its membrane microdomain localisation. A fusion protein of hDOR with the Green Fluorescent Protein (DOR⁎) allows precise receptor membrane quantification. Here we report that (i) a fraction of the total receptor pool requires cholesterol for binding activity, (ii) G-proteins stabilize a high affinity state conformation which does not seem modulated by cholesterol. In relation to its distribution, and (iii) a fraction of DOR⁎ is constitutively associated with detergent-resistant membranes (DRM) characterised by an enrichment in lipids and proteins raft markers. (iv) An increase in the quantity of DOR⁎ was observed upon agonist addition. (v) This DRM relocation is prevented by uncoupling the receptor–G-protein interaction.