Rhodopsin In Disk Membranes Research Articles

Rhodopsin Forms a Dimer with Cytoplasmic Helix 8 Contacts in Native Membranes

G protein-coupled receptors form dimers and higher-order oligomers in membranes, but the precise mode of receptor-receptor interaction remains unknown. To probe the intradimeric proximity of helix 8 (H8), we conducted chemical cross-linking of endogenous cysteines in rhodopsin in disk membranes. We identified a Cys316-Cys316 cross-link using partial proteolysis and liquid chromatography with mass spectrometry. These results show that a symmetric dimer interface mediated by H1 and H8 contacts is present in native membranes.

Lipid and Detergent Effects on the Kinetic Stability of the GPCR, Rhodopsin

The photoreceptor, rhodopsin is a kinetically stable G-Protein Coupled Receptor that is located in rod outer segment disk membranes. Differential scanning calorimetry (DSC) studies have shown that rhodopsin exhibits an irreversible scan rate dependent endothermic transition (Tm) at approximately 72 oC. The activation energy for thermal denaturation (Eact) calculated from the scan rate dependence of the Tm is sensitive to the integrity of the lipid bilayer. It is also sensitive to proteolytic cleavage of the extramembraneous loops of rhodopsin. Here we investigate the influence of palmitoylation, membrane cholesterol and digitonin solubilization on rhodopsin kinetic stability. DSC experiments were performed using a MicroCal VP-DSC microcalorimeter. Samples were scanned at 15, 30, 60 and 90o/hr. Because the protein transitions are irreversible, a second scan was used to determine the baseline. Rhodopsin palmitoylation at cys 322 and cys 323 anchors the C-terminus to the membrane. The Eact for rhodopsin in disk membranes treated with hydroxylamine to remove these palmitate groups was approximately 35 Kcal/mole less than that of native rhodopsin. The Eact for rhodopsin was also determined at different membrane cholesterol levels. These data indicated a decrease in stability both above and below native membrane cholesterol levels. Finally, unlike solubilization in other detergents, solubilization in digitonin had little effect on rhodopsin kinetic stability.

Calorimetric Studies of Bovine Rod Outer Segment Disk Membranes Support a Monomeric Unit for Both Rhodopsin and Opsin

The photoreceptor rhodopsin is a G-protein coupled receptor that has recently been proposed to exist as a dimer or higher order oligomer, in contrast to the previously described monomer, in retinal rod outer segment disk membranes. Rhodopsin exhibits considerably greater thermal stability than opsin (the bleached form of the receptor), which is reflected in an ∼15°C difference in the thermal denaturation temperatures (Tm) of rhodopsin and opsin as measured by differential scanning calorimetry. Here we use differential scanning calorimetry to investigate the effect of partial bleaching of disk membranes on the Tm of rhodopsin and of opsin in native disk membranes, as well as in cross-linked disk membranes in which rhodopsin dimers are known to be present. The Tms of rhodopsin and opsin are expected to be perturbed if mixed oligomers are present. The Tm remained constant for rhodopsin and opsin in native disks regardless of the level of bleaching. In contrast, the Tm of cross-linked rhodopsin in disk membranes was dependent on the extent of bleaching. The energy of activation for denaturation of rhodopsin and cross-linked rhodopsin was calculated. Cross-linking rhodopsin significantly decreased the energy of activation. We conclude that in native disk membranes, rhodopsin behaves predominantly as a monomer.

Effect of Packing Density on Rhodopsin Stability and Function in Polyunsaturated Membranes

Rod outer segment disk membranes are densely packed with rhodopsin. The recent notion of raft or microdomain structures in disk membranes suggests that the local density of rhodopsin in disk membranes could be much higher than the average density corresponding to the lipid/protein ratio. Little is known about the effect of high packing density of rhodopsin on the structure and function of rhodopsin and lipid membranes. Here we examined the role of rhodopsin packing density on membrane dynamic properties, membrane acyl chain packing, and the structural stability and function of rhodopsin using a combination of biophysical and biochemical techniques. We reconstituted rhodopsin into large unilamellar vesicles consisting of polyunsaturated 18:0,22:6n3PC, which approximates the polyunsaturated nature of phospholipids in disk membranes, with rhodopsin/lipid ratios ranging from 1:422 to 1:40. Our results showed that increased rhodopsin packing density led to reduced membrane dynamics revealed by the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, increased phospholipid acyl chain packing, and reduced rhodopsin activation, yet it had minimal impact on the structural stability of rhodopsin. These observations imply that densely packed rhodopsin may impede the diffusion and conformational changes of rhodopsin, which could reduce the speed of visual transduction.

The Temperature Dependence of Internal Molecular Motions in Hydrated and Dry α-Amylase: The Role of Hydration Water in the Dynamical Transition of Proteins

Internal molecular motions of proteins are strongly affected by environmental conditions, like temperature and hydration. As known from numerous studies, the dynamical behavior of hydrated proteins on the picosecond time scale is characterized by vibrational motions in the low-temperature regime and by an onset of stochastic large-amplitude fluctuations at a transition temperature of 180–230 K. The present study reports on the temperature dependence of internal molecular motions as measured with incoherent neutron scattering from the globular water-soluble protein α-amylase and from a protein-lipid complex of rhodopsin in disk membranes. Samples of α-amylase have been measured in a hydrated and dehydrated state. In contrast to the hydrated sample, which exhibits a pronounced dynamical transition near 200 K, the dehydrated α-amylase does not show an appreciable proportion of stochastic large-amplitude fluctuations and no dynamical transition in the measured temperature range of 140–300 K. The obtained results, which are compared to the dynamical behavior of protein-lipid complexes, are discussed with respect to the influence of hydration on the dynamical transition and in the framework of the glass transition.

Differential rhodopsin regeneration in photoreceptor membranes is correlated with variations in membrane properties.

Rhodopsin, the major transmembrane protein in both the plasma membrane and the disk membranes of photoreceptor rod outer segments (ROS) forms the apo-protein opsin upon the absorption of light. In vivo the regeneration of rhodopsin is necessary for subsequent receptor activation and for adaptation, in vitro this regeneration can be followed after the addition of 11-cis retinal. In this study we investigated the ability of bleached rhodopsin to regenerate in the compositionally different membrane environments found in photoreceptor rod cells. When 11-cis retinal was added to bleached ROS plasma membrane preparations, rhodopsin did not regenerate within the same time course or to the same extent as bleached rhodopsin in disk membranes. Over 80% of the rhodopsin in newly formed disks regenerated within 90 minutes while only 40% regenerated in older disks. Since disk membrane cholesterol content increases as disks are displaced from the base to the apical tip of the outer segment, we looked at the affect of membrane cholesterol content on the regeneration process. Enrichment or depletion of disk membrane cholesterol did not alter the % rhodopsin that regenerated. Bulk membrane properties measured with a sterol analog, cholestatrienol and a fatty acid analog, cis parinaric acid, showed a more ordered, less "fluid", lipid environment within plasma membrane relative to the disks. Collectively these results show that the same membrane receptor, rhodopsin, functions differently as monitored by regeneration in the different lipid environments within photoreceptor rod cells. These differences may be due to the bulk properties of the various membranes.

Synthetic phosphopeptide from rhodopsin sequence induces retinal arrestin binding to photoactivated unphosphorylated rhodopsin

A synthetic heptaphosphopeptide comprising the fully phosphorylated carboxyl terminal phosphorylation region of bovine rhodopsin, residues 330–348, was found to induce a conformational change in bovine arrestin. This caused an alteration of the pattern of limited proteolysis of arrestin similar to that induced by binding phosphorylated rhodopsin or heparin. Unlike heparin, the phosphopeptide also induced light-activated binding of arrestin to both unphosphorylated rhodopsin in disk membranes as well as to endoproteinase Asp-N-treated rhodopsin (des 330–348). These findings suggest that one function of phosphorylation of rhodopsin is to activate arrestin which can then bind to other regions of the surface of the photoactivated rhodopsin.

Rhodopsin mobility, structure, and lipid-protein interaction in squid photoreceptor membranes.

Treatment of outer segment membranes from Loligo forbesi with endoprotease-V8 from Staphylococcus aureus results in cleavage of the C-terminal extension of the squid rhodopsin, with accompanying reduction of the apparent molecular weight from 47,000 to 36,000 on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Negative-stain electron microscopy of the intact membranes shows that small clusters of the rhodopsin C-termini form structures extending from the membrane surface and that these are absent after protease treatment. Fourier transform infrared spectra of the amide I band of the protein indicate that removal of the C-terminal extension increases the relative alpha-helical content of squid rhodopsin to a level comparable to that for bovine rhodopsin in disk membranes, and to an extent which suggests that the alpha-helical structure lies mainly in the M(r) 36,000 (transmembrane) section of the protein. Saturation-transfer electron spin resonance (ESR) spectroscopy of the spin-labeled protein reveals that the rotational diffusion of squid rhodopsin in outer segment membranes that have been extensively washed with urea to remove peripheral proteins is much slower than that of bovine rhodopsin in rod outer segment disk membranes. This reduction in rotational mobility is also found with purified squid rhodopsin reconstituted in egg phosphatidylcholine and in urea-washed outer segment membranes which have been treated with endoprotease-V8 to remove the C-terminal extension of squid rhodopsin. In the latter case, the saturation-transfer ESR spectra are virtually identical to those of the non-proteolyzed membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative characterization of the structure of rhodopsin in disc membrane by means of Fourier transform infrared spectroscopy.

Fourier transform infrared (FTIR) spectroscopy has been used for the detailed characterization and quantification of the secondary structure of bovine rhodopsin in native disc membranes. FTIR spectra were obtained in aqueous media, both in 1H2O and in 2H2O. Analysis of spectra by means of Fourier self-deconvolution, complemented with maximum likelihood restoration and Fourier derivative, has allowed the characterization of major amide I secondary structure-sensitive component bands of structural relevance which had not been detected before. In consequence, we show a richer secondary structure for rhodopsin than previously described. Our results indicate a total regular helix content around 51%, which would include not only the main alpha 1-type helix but also 3(10)-like helix. The presence of distorted helicoid sequences might furthermore increase to a certain extent the total helix amount. It is also indicated that a significant proportion of the amino acid residues are involved in extended/beta-structures and in reverse turns, as well as in "random" segments, which had not been directly demonstrated before. 61 +/- 4% of rhodopsin is determined to be solvent-accessible, which is a substantially higher value than previously reported. Helices account for most of the inaccessible moiety.

Structural and functional properties of rhodopsin from rod outer segment disk and plasma membrane

The structural and functional properties of bovine rhodopsin from rod outer segment disk and plasma membranes were compared by high performance liquid chromatography (HPLC), mass spectrometric analyses, and in vitro rhodopsin phosphorylation assays. Disk and plasma membranes separated by a ricin gold-dextran affinity perturbation method were treated with trypsin or cynogen bromide, and the N-terminal and C-terminal rhodopsin peptides were isolated by immunoaffinity chromatography using antirhodopsin monoclonal antibodies coupled to Sepharose. Reverse phase HPLC chromatograms of the C-terminal and N-terminal peptides from disk and plasma membrane rhodopsin were found to be similar. Mass spectrometric, PicoTag, and hexose analyses of the tryptic 1–16 N-terminal peptides further indicated that the post-translational glycosylation of plasma membrane rhodopsin is identical to that of disk membrane rhodopsin. HPLC analysis of soluble peptides obtained from cyanogen bromide and tryptic digestion of immunoaffinity purified rhodopsin also indicated that no significant differences exist between disk and plasma membrane rhodopsin. Light-induced phosphorylation of rhodopsin in disk and plasma membranes were also compared using in vitro phosphorylation assays. Plasma membrane rhodopsin was found to undergo light-dependent, rhodopsin kinase catalyzed phosphorylation to the same extent as disk membrane rhodopsin. These results indicate that the bulk rhodopsin in rod outer segment plasma membranes appears to be identical to rhodopsin in disk membranes in regard to primary structure, post-translational glycosylation and light-dependent phosphorylation. On this basis, it is unlikely that the sorting of rhodopsin between disk and plasma membranes occurs by a mechanism based on differences in structural properties of rhodopsin.

Palmitylation of a G-protein coupled receptor. Direct analysis by tandem mass spectrometry.

Bovine rhodopsin has been reported to be S-palmitylated at cysteines 322 and 323 (Ovchinnikov, Y. A., Abdulaev, N. G., and Bogachuk, A.S. (1988) FEBS Lett. 230, 1-5). Using a combination of enzymatic and chemical cleavage techniques in conjunction with tandem mass spectrometry, the sites of incorporation of the palmityl groups are shown. Bovine rhodopsin in disc membranes was digested with thermolysin to generate the C-terminal fragment (241-327), which was subsequently cleaved with cyanogen bromide to generate the peptide Val-Thr-Thr-Leu-Cys-Cys-Gly-Lys-Asn-Pro (318-327). A bis-S-palmitylated synthetic standard had the same retention time by reversed-phase high performance liquid chromatography as the isolated peptide and the same molecular weight (MH+1511.7) by liquid secondary ion mass spectrometry. Dithiothreitol reduction of both the isolated and the synthetic peptide cleaved the two thioester-linked palmityl groups to produce reduction products of the same appropriately decreased molecular weight (MH+1035.5). Tandem mass spectrometry of the isolated and the synthetic peptide identified the sites of attachment of the palmityl groups on cysteines 322 and 323. These results prove the modification of cysteines 322 and 323 with palmitic acid in bovine rhodopsin, and illustrate the utility of mass spectrometry to characterize the post-translational modifications in G-protein coupled receptors.

Dual effect of local anesthetics on the function of excitable rod outer segment disk membrane.

The effects of local anesthetics and a divalent cation, Ca2+, on the function of rhodopsin were estimated from the measurements of light-induced proton uptake. The light-induced proton uptake by rhodopsin in the rod outer segment disk membrane was enhanced at lower pH (4) but depressed at higher pHs (6 to 8) by the tertiary amine local anesthetics lidocaine, bupivacaine, tetracaine, and dibucaine. The order of local anesthetic-induced depression of the proton uptake followed that of their clinical anesthetic potencies. The depression of the proton uptake versus the concentration of the uncharged form of local anesthetic nearly describes the same curve for small and large dose of added anesthetic. Furthermore, a neutral local anesthetic, benzocaine, depressed the proton uptake at all pHs between 4 and 7. These results indicate that the depression of the proton uptake is due to the effect of only the uncharged form. It is hypothesized that the uncharged form of local anesthetics interacts hydrophobically with the rhodopsin in the disk membrane. The dual effect of local anesthetics on the proton uptake, on the other hand, suggests that the activation of the function of rhodopsin may be caused by the charged form. There was no significant change in the light-induced proton uptake by rhodopsin when 1 mM of Ca2+ was introduced into the disk membrane at varying pHs in the absence or presence of local anesthetics. This fact indicates that Ca2+ ion does not influence the diprotonating process of metarhodopsin; neither does it interfere with the local anesthetic-induced changes in the rhodopsin molecule.

Saturation transfer dispersion electron paramagnetic resonance using a balanced cavity: Dynamical calibration and biological applications

Saturation transfer dispersion electron paramagnetic resonance spectra of biological samples were recorded using a newly developed balanced cavity. With this configuration the U′ 1 spectrum (first harmonic dispersion 90° out of phase) has a signal-to-noise ratio comparable to that of the conventionally employed first harmonic-in-phase absorption signal ( V 1). Spin-labeled bovine serum albumin in glycerol/water mixtures was used to establish calibration curves of U′ 1 spectra. It is shown that the sensitivity of U′ 1; to isotropic motions extends from 5.10 −7 to 10 −4 sec. The potential of U′ 1 combined with the balanced cavity scheme for the study of the dynamics of membrane bound proteins is also investigated. Cytochrome c oxidase and rhodopsin in disk membranes were used as model systems. The U′ 1 spectra of satisfactory signal-to-noise ratio can be obtained with one 8-minute scan and a spin concentration as small as 50 μM in 10 μ1 It is shown that the calibration curves obtained can be utilized as a first approximation for these anisotropic systems.

34] Proteolysis of rhodopsin

This chapter discusses the proteolysis of rhodopsin. Proteases have been valuable probes of the structure of many membrane proteins. Limited proteolysis of bovine rhodopsin has revealed information about the location of specific sites along the polypeptide sequence, about the organization of rhodopsin in natural and reconstituted membranes, and about conformational changes induced by light. Proteolysis of rhodopsin in disk membranes may be used to help localize any newly described marker in the rhodopsin polypeptide. It has been observed that at higher ratios of subtilisin to rhodopsin, a third discrete cleavage site may be exposed within F1, permitting higher resolution mapping. Most exciting is the possibility that excision of extradiskal portions of rhodopsin can be used to localize regions of interaction between rhodopsin and the light-activated enzymes of the rod outer segment.

84] Ultraviolet circular dichroism of rhodopsin in disk membranes and detergent solution

Publisher Summary This chapter discusses the ultraviolet circular dichroism (CD) of rhodopsin in disk membranes and detergent solution. The ultraviolet CD of proteins is dominated by the contributions of the peptide group. The magnitude, sign, and position of the peptide circular dichroic bands are sensitive to the secondary folding of the polypeptide backbone. Analysis of this region of the CD spectrum is the most direct method of estimating the secondary structure of proteins in solution and has been used successfully with soluble globular proteins. However, the structural analysis of membrane proteins has produced ambiguous results, because of certain anomalies that are observed in the CD spectra of such samples; these are characterized by a red shift and loss of intensity in the peptide cotton effects, which is attributable to the particulate nature of the membrane suspensions.

Rhodopsin-phospholipid complexes in apolar solvents: Formation and properties

The extraction of rhodopsin-phospholipid complexes into lipid solvents is described. Two general procedures were developed: (i) the extraction of detergent-solubilized rhodopsin which involved: (a) solubilization of rhodopsin from bovine retinal rod disc membranes in detergent solutions; (b) partial removal of detergent from solubilized rhodopsin; (c) recombination with a defined lipid mixture by sonication of lipid and protein in salt media; and (d) sequential extraction into organic solvents. (ii) The direct transfer of rhodopsin from purified retinal rod disc membranes into apolar solvents which involved: (a) resuspension of membranes in hexane containing phospholipids; (b) sonication; (c) charge neutralization with salts; and (d) sequential extraction into organic solvents. The extraction yield was dependent on several parameters such as: phospholipid to rhodopsin ratio, sonication, type of solvent, and others. The composition of the extracts regarding the molar ratio of phospholipid, detergent, and rhodopsin varied according to the extraction protocol. The spectral characteristics of rhodopsin in the organic phase were similar to those of native rhodopsin in disc membranes. The capacity to regenerate the characteristic dark absorbance maximum at ∼485 nm after incubation of dark chromophore (9-cis retinal) with bleached rhodopsin was preserved to about 70%. The general applicability of these techniques in functional reconstitution studies of membrane systems is considered.

Hydrogen exchange study of membrane-bound rhodopsin. II. Light-induced protein structure change.

Hydrogen exchange studies of rhodopsin in disc membranes demonstrated that photolysis induces changes in the protein itself. Two different altered forms were detected. A late photointermediate in the bleaching sequence, which can be identified with metarhodopsin II, displays accelerated exchange. Subsequently, at the stage of fully bleached opsin, exchange becomes even slower than in rhodopsin. These changes involve only a small fraction of the protein's internally hydrogen-bonded peptide groups. The unusually large fraction of exposed peptide hydrogens observed previously for rhodopsin is unaltered in the photolyzed forms.

Conformational aspects of rhodopsin and retinal disc membranes.

AbstractWe have used two experimental approaches – fluorescence spectroscopy and lectin binding – to obtain information about the structure of rhodopsin and its position in retinal disc membranes. Energy transfer was used as a spectroscopic ruler to deduce proximity relationships in rhodopsin and in disc membranes. Concanavalin A and wheat germ agglutinin served as specific macromolecular probes of the accessibility of the carbohydrate unit of rhodopsin in disc membranes. We infer from these studies that: (1) The rhodopsin molecule is at least 75 Å long and has an elongated shape. (2) There may be distinct hydrophobic and hydrophobic and hydrophilic domains in rhodopsin. (3) The carbohydrate moiety of rhodopsin is on the surface of the disc membrane, probably only on the external face. (4) The disc membrane has an asymmetric structure. (5) The external surface of the disc membrane is closer to site A (a sulfhydryl residue on rhodopsin) than to 11–cis retinal.