| G-protein coupled receptors (GPCRs) represent one of the largest families of proteins in mammals, with 1--5% of the total cell proteins belonging to this group. Together they constitute one of the principal targets of the drugs currently being used in the market, especially those acting in the central nervous system. A better understanding of the mechanisms of activation and signal transduction of these proteins seems imperative. Despite their crucial interest, however, very little is known about their structure/function relationships, mainly as a consequence of their rather low natural abundance and their integral membrane nature. Due to the low stability of membrane proteins when outside of the cell membrane, methodologies that allow direct studies to be done while they are incorporated into artificial lipid bilayers are advantageous. One such technique is plasmon-waveguide resonance (PWR) spectroscopy, which was developed in our laboratories and has been successfully used to investigate several aspects of signal transduction. Because of its ability to obtain resonances with light polarized both perpendicular ( p-polarization) and parallel (s-polarization) to the resonator surface, PWR can follow changes in conformation accompanying protein-ligand, protein-protein and protein-lipid interactions occurring in GPCRs in real time with high sensitivity and without the need for molecular labeling. In the present work we have used two GPCRs, namely rhodopsin (a GPCR prototype) and the human delta opioid receptor (hDOR), which our laboratory has been actively investigating from the drug design point of view for 25 years. For these studies we have incorporated the detergent-solubilized receptor into a solid-supported lipid bilayer and monitored PWR changes upon activation by ligand (hDOR) or light (rhodopsin), as well as the interaction of these receptors with their G-proteins, one of the first steps in the signaling cascade. We have investigated the lipid modulation of rhodopsin activation and have found that lipids can greatly affect not only the activation process, but the G-protein interaction as well. In addition, we have found that the hDOR adopts different conformations upon binding of different classes of ligands, and that its affinity for G-proteins is highly dependent on the nature of the ligand pre-bound to the receptor. Significant selectivity towards the different G-protein subtypes and subunits was also observed. The results have yielded new insights into GPCR function and have demonstrated that PWR provides a new and direct approach to investigate transmembrane signaling. |
