A growing body of evidence supports the idea that the plasma membrane bilayer is characterized by a laterally inhomogeneous mixture of lipids, having an organized structure in which lipid molecules segregate into small domains or patches. Such microdomains are characterized by high contents of sphingolipids that form thicker liquid-ordered regions that are resistant to extraction with nonionic detergents. The existence of lipid lateral segregation has been demonstrated in both model and biological membranes, although its role in protein sorting and membrane function still remains unclear. In these studies, plasmon-waveguide resonance (PWR) spectroscopy was employed to investigate the properties of microdomains in a model system consisting of a solid-supported lipid bilayer composed of a 1:1 mixture of palmitoyloleoylphosphatidylcholine (POPC) and brain sphingomyelin (SM), and their influence on the partitioning and functioning of the human delta opioid receptor (hDOR), a G-protein coupled receptor (GPCR). Resonance signals corresponding to two microdomains (POPC-rich and SM-rich) were observed in such bilayers, and the sorting of the receptor into each domain was highly dependent on the type of ligand that was bound. When no ligand was bound, the receptor was incorporated preferentially into the POPC-rich domain; when an agonist or antagonist was bound, the receptor was incorporated preferentially into the SM-rich component, although with a 2-fold greater propensity for this microdomain in the case of the agonist. Binding of G-protein to the agonist-bound receptor in the SM-rich domain occurred with a 30-fold higher affinity than binding to the receptor in the PC-rich domain. The binding of the agonist to an unliganded receptor in the bilayer produced receptor trafficking from the PC-rich to the SM-rich component. Since the SM-rich domain is thicker than the PC-rich domain, and previous studies with the hDOR have shown that the receptor is elongated upon agonist activation, we propose that hydrophobic matching between the receptor and the lipid is a driving force for receptor trafficking to the SM-rich component.
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