Evidence for Specificity in Lipid-Rhodopsin Interactions

Abstract

The interaction of bovine rhodopsin with poly-and monounsaturated lipids was studied by 1 H MAS NMR with magnetization transfer from rhodopsin to lipid. Experiments were conducted on bovine rod outer segment (ROS) disks and on recombinant membranes containing lipids with polyunsaturated, docosahexaenoyl (DHA) chains. Poly-and monounsaturated lipids interact specifically with different sites on the rhodopsin surface. Rates of magnetization transfer from protein to DHA are lipid headgroup-dependent and increased in the sequence PC < PS < PE. Boundary lipids are in fast exchange with the lipid matrix on a time scale of milliseconds or shorter. All rhodopsin photointermediates transferred magnetization preferentially to DHA-containing lipids, but highest rates were observed for Meta-III rhodopsin. The experiments show clearly that the surface of rhodopsin has sites for specific interaction with lipids. Current theories of lipid-protein interaction do not account for such surface heterogeneity.Rhodopsin is the light receptor responsible for dim light vision in the rod photoreceptor cells of vertebrates. A large body of research demonstrated that efficiency of the rhodopsindependent steps of the visual process is exquisitely sensitive to membrane lipid composition, in particular to the content of -3 polyunsaturates (1-3). Retinal membranes of mammals, similar to synaptosomal membranes in brain, contain up to 50 mol % of docosahexaenoic acid (DHA, 2 22:6n3), a polyunsaturated fatty acid with 22 carbon atoms and six double bonds that are evenly distributed over the length of the chain (4).In earlier reconstitution experiments with bovine rhodopsin it was established that the equilibrium concentration of Meta-II rhodopsin increased with the concentration of DHA hydrocarbon chains in the lipid matrix (5-7). Also the headgroups of phospholipids had a significant effect on Meta-II formation (5, 7) and activation of G t (8, 9). It was observed that phosphatidylethanolamines (PE) and the negatively charged phosphatidylserines (PS) increased the amount of Meta-II.The influence of PS on the equilibrium was assigned to changes of the membrane electric surface potential (7). In contrast, the sensitivity of membranes to PE content correlated with an alteration of membrane curvature elasticity (10) as proposed for other membrane proteins by Navarro et al. (11), Jensen and Schutzbach (12), Gruner (13), Lindblom and coworkers (14), and Cantor (15). Litman and co-workers (6, 16) found a correlation between mobility and orientation of the fluorescence probe DPH in lipid bilayers, summarized as a membrane free volume parameter, and Meta-II formation. Furthermore, they observed a preference of rhodopsin to locate in domains rich in di-22:6n3-PC that formed in di-16:0-PC/ di-22:6-PC/cholesterol mixtures. Mouritsen proposed a link between hydrophobic thickness of lipid bilayers and activity of membrane proteins (18). Brown and co-workers (10) suggested that Meta-II has a greater hydrophobic thickness than Meta-I, and that the lipid bilayer...

Drastic neofunctionalization associated with evolution of the timezyme AANAT 500 Mya

“…Docosahexaenoic acid is essential to the structure and function of photoreceptor cells in both the retina and the pineal organ (43)(44)(45)(46). Accordingly, VT-AANAT might have functioned to enhance the efficiency of the primitive vertebrate's photodetector by preventing arylalkylamine adduct formation with several essential photodetection molecules.…”

Human milk: maternal dietary lipids and infant development

“…In vitro studies have provided evidence that DHA plays a role in photoreceptor signal transduction by influencing the ability of photons to transform rhodopsin to the activated meta-rhodopsin ll state and the efficiency of G-protein-coupled receptor signalling (Niu et al 2004). More recently, it has been shown (Soubias et al 2006) that there are specific binding sites for DHA in the rhodopsin molecules. In the brain DHA is enriched in synaptic terminal membranes and has diverse roles in brain growth and function and in protecting against oxidative stress (Innis, 2007).…”

Lipid second messengers and related enzymes in vertebrate rod outer segments

“…Results from several laboratories have reported the presence of lipid components involved in signaling functions in ROS and the modulation of these components by light ( 18,(46)(47)(48). Phototransduction in fl ies represents the fastest G-protein-signaling cascade known.…”

“…About half of the phospholipid acyl chains in rod disc membranes are made of docosahexaenoic acid (DHA, 22:6n3) ( 7, 10,12,[14][15][16][17][18][19]. These DHA chains provide either maximal fl uidity or maximal free volume of acyl chain packing in disc membranes, which is responsible for optimal visual function ( 2, [20][21][22][23][24].…”