Conformational Energetics of Rhodopsin Modulated by Nonlamellar-Forming Lipids

Abstract

Rhodopsin is an important example of a G protein-coupled receptor (GPCR) in which 11-cis-retinal is the ligand and acts as an inverse agonist. Photolysis of rhodopsin leads to formation of the activated meta II state from its precursor meta I. Various mechanisms have been proposed to explain how the membrane composition affects the meta I-meta II conformational equilibrium in the visual process. For rod disk membranes and recombinant membranes containing rhodopsin, the lipid properties have been discussed in terms of elastic deformation of the bilayer. Here we have investigated the relation of nonlamellar-forming lipids, such as dioleoylphosphatidylethanolamine (DOPE), together with dioleoylphosphatidylcholine (DOPC), to the photochemistry of membrane-bound rhodopsin. We conducted flash photolysis experiments for bovine rhodopsin recombined with DOPE/DOPC mixtures (0:100 to 75:25) as a function of pH to explore the dependence of the photochemical activity on the monolayer curvature free energy of the membrane. It is well-known that DOPC forms bilayers, whereas DOPE has a propensity to adopt the nonlamellar, reverse hexagonal (H(II)) phase. In the case of neutral DOPE/DOPC recombinants, calculations of the membrane surface pH confirmed that an increase in DOPE favored the meta II state. Moreover, doubling the PE headgroup content versus the native rod membranes substituted for the polyunsaturated, docosahexaenoic acyl chains (22:6 omega 3), suggesting rhodopsin function is associated with a balance of forces within the bilayer. The data are interpreted by applying a flexible surface model, in which the meta II state is stabilized by lipids tending to form the H(II) phase, with a negative spontaneous curvature. A simple theory, based on principles of surface chemistry, for coupling the energetics of membrane proteins to material properties of the bilayer lipids is described. For rhodopsin, the free energy balance of the receptor and the lipids is altered by photoisomerization of retinal and involves curvature stress/strain of the membrane (frustration). A new biophysical principle is introduced: matching of the spontaneous curvature of the lipid bilayer to the mean curvature of the lipid/water interface adjacent to the protein, which balances the lipid/protein solvation energy. In this manner, the thermodynamic driving force for the meta I-meta II conformational change of rhodopsin is tightly controlled by mixtures of nonlamellar-forming lipids having distinctive material properties.

Two protonation switches control rhodopsin activation in membranes

“…Rhodopsin in native hypotonically washed disk membranes was prepared from cattle retinae (Lawson) according to standard procedures (40). For experiments on recombinant membranes, rhodopsin was purified in dodecyltrimethylammonium bromide detergent and reconstituted into synthetic POPC lipid membranes according to published procedures (41). Experiments were performed with sandwich samples with 0.5 nmol of pigment (prepared as described in detail in ref.…”

Chemical–Physical Changes in Cell Membrane Microdomains of Breast Cancer Cells After Omega-3 PUFA Incorporation

“…The plasma membrane is involved in almost all aspects of cell biology, including morphogenesis, proliferation, migration, invasion, transformation, differentiation, secretion and apoptosis. Numerous experimental data indicate that the presence of PUFA in the membrane bilayer might determine dramatic changes in physicalchemical properties [4][5], a significant lowering of cholesterol solubility [6], and changes in the activity of transmembrane proteins such as the G-protein coupled membrane receptors [7][8]. In fact the structure and dynamics of these polyunsaturated chains at the molecular level are profoundly different from their saturated counterpart, with very short reorientational correlation times and extremely low chain order.…”

Importance of Direct Interactions with Lipids for the Function of the Mechanosensitive Channel MscL

“…A correlation between lipid spontaneous curvature and channel formation has been observed for alamethicin (8,12) and gramicidin (13), and effects of lipid structure and bilayer additives on the conformational equilibrium of rhodopsin (5) and on sodium channel function have also been correlated with changes in lipid spontaneous curvature (6). However, a study of the function of the multidrug transporter LmrP suggested that direct interactions between the lipid headgroup and LmrP were more important than changes in spontaneous curvature (14).…”