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Phosphatidylethanolamine Enhances Rhodopsin Photoactivation and Transducin Binding in a Solid Supported Lipid Bilayer as Determined Using Plasmon-Waveguide Resonance Spectroscopy

Isabel D. Alves ^*, Gilmar F. J. Salgado ^*, Zdzislaw Salamon ^*, Michael F. Brown ^* , Gordon Tollin ^*  and Victor J. Hruby ^* 

^* Department of Biochemistry and Molecular Biophysics, and Department of Chemistry, University of Arizona, Tucson, Arizona

Correspondence: Address reprint requests to Victor J. Hruby, Dept. of Chemistry, University of Arizona, Tucson, AZ 85721. Tel.: 520-621-6332; E-mail: hruby{at}email.arizona.edu.

Flash photolysis studies have shown that the membrane lipid environment strongly influences the ability of rhodopsin to form the key metarhodopsin II intermediate. Here we have used plasmon-waveguide resonance (PWR) spectroscopy, an optical method sensitive to both mass and conformation, to probe the effects of lipid composition on conformational changes of rhodopsin induced by light and due to binding and activation of transducin (G_t). Octylglucoside-solubilized rhodopsin was incorporated by detergent dilution into solid-supported bilayers composed either of egg phosphatidylcholine or various mixtures of a nonlamellar-forming lipid (dioleoylphosphatidylethanolamine; DOPE) together with a lamellar-forming lipid (dioleoylphosphatidylcholine; DOPC). Light-induced proteolipid conformational changes as a function of pH correlated well with previous flash photolysis studies, indicating that the PWR spectral shifts monitored metarhodopsin II formation. The magnitude of these effects, and hence the extent of the conformational transition, was found to be proportional to the DOPE content. Our data are consistent with previous suggestions that lipids having a negative spontaneous curvature favor elongation of rhodopsin during the activation process. In addition, measurements of the G_t/rhodopsin interaction in a DOPC/DOPE (25:75) bilayer at pH 5 demonstrated that light activation increased the affinity for G_t from 64 nM to 0.7 nM, whereas G_t affinity for dark-adapted rhodopsin was unchanged. By contrast, in DOPC bilayers the affinity of G_t for light-activated rhodopsin was only 18 nM at pH 5. Moreover exchange of GDP for GTPS was also monitored by PWR spectroscopy. Only the light-activated receptor was able to induce this exchange which was unaffected by DOPE incorporation. These findings demonstrate that nonbilayer-forming lipids can alter functionally linked conformational changes of G-protein-coupled receptors in membranes, as well as their interactions with downstream effector proteins.

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