Wavelength modulation by molecular environment in visual pigments

The absorption and regenerability characteristics are compared for rhodopsin contained in rod outer segment membranes and purified in a series of alkyl sucrose esters. It is found that membrane-bound rhodopsin has maximum absorbance from 504 to 500 nm between 1.5 and 40 degrees C. After purification, rhodopsin absorbance can be blue-shifted by up to 6 nm, depending on the detergent species used. Only the longest chain sucrose esters give purified rhodopsin with maximum absorbance comparable to that of the native pigment. In the same manner, detergent-purified rhodopsin will be easily regenerated as long as its native spectral characteristics are maintained. Sucrose esters thus prove to be mild enough to maintain rhodopsin functionality with respect to these two properties and could probably be used successfully to maintain other membrane proteins' integrity.     

Rhodopsin's carboxyl-terminal threonines are required for wild-type arrestin-mediated quench of transducin activation in vitro

Many recent reports have demonstrated that rhodopsin's carboxyl-terminal serine residues are the main targets for phosphorylation by rhodopsin kinase. Phosphorylation at the serines would therefore be expected to promote high-affinity arrestin binding. We have examined the roles of the carboxyl serine and threonine residues during arrestin-mediated deactivation of rhodopsin using an in vitro transducin activation assay. Mutations were introduced into a synthetic bovine rhodopsin gene and expressed in COS-7 cells. Individual serine and threonine residues were substituted with neutral amino acids. The ability of the mutants to act as substrates for rhodopsin kinase was analyzed. The effect of arrestin on the activities of the phosphorylated mutant rhodopsins was measured in a GTPgammaS binding assay involving purified bovine arrestin, rhodopsin kinase, and transducin. A rhodopsin mutant lacking the carboxyl serine and threonine residues was not phosphorylated by rhodopsin kinase, demonstrating that phosphorylation is restricted to the seven putative phosphorylation sites. A rhodopsin mutant possessing a single phosphorylatable serine at 338 demonstrated no phosphorylation-dependent quench by arrestin. These results suggest that singly phosphorylated rhodopsin is deactivated through a mechanism that does not involve arrestin. Analysis of additional mutants revealed that the presence of threonine in the carboxyl tail of rhodopsin provides for greater arrestin-mediated quench than does serine. These results suggest that phosphorylation site selection could serve as a mechanism to modulate the ability of arrestin to quench rhodopsin.     

Isolation and recombination of bovine rod outer segment cGMP phosphodiesterase and its regulators

cGMP phosphodiesterase extracted from rod outer segments can be activated by GTP in the presence of phospholipid vesicles containing bleached rhodopsin. I have separated the phosphodiesterase from a phosphodiesterase inhibitory protein and a GTPase also present in the crude extracts from rods. The GTPase can be activated by bleached rhodopsin. However, in the absence of the GTPase and inhibitor, the phosphodiesterase was not activated by GTP in the presence of bleached rhodopsin. Recombination with these proteins partially restored the activation by GTP and bleached rhodopsin.     

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.     

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 (T_m) 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 T_m 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 T_ms of rhodopsin and opsin are expected to be perturbed if mixed oligomers are present. The T_m remained constant for rhodopsin and opsin in native disks regardless of the level of bleaching. In contrast, the T_m 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.     

Isolation and identification of the phosphorylated species of rhodopsin

Rhodopsin is phosphorylated in a light-dependent manner by a kinase intrinsic to the rod outer segment. We have used chromatofocusing to separate six phosphorylated species of rhodopsin and have recovered in the pH gradient fractions 60-80% of the initial phosphorylated sample loaded on the column. The isolated species of rhodopsin coincide with the species that are observed in isoelectric focusing gels in the pH range 6.1-4.7. Unphosphorylated rhodopsin focuses at a pI of 6.0. Two species having two phosphates per rhodopsin with isoelectric points of 5.45 and 5.40 have been isolated. The phosphate to rhodopsin ratios for the remaining species are 3.8, 5.0, 6.1, and 8.2 with isoelectric points of 5.16, 4.99, 4.85, and 4.73, respectively. The chromatofocusing profile suggests that there may be multiple forms of rhodopsin with the same number of phosphates among some of the other phosphorylated forms of rhodopsin.     

Cyclic nucleotides and GTP analogues stimulate light-induced phosphorylation of octopus rhodopsin

Light-induced phosphorylation of octopus rhodopsin in microvillar membrane was shown to be stimulated by cyclic nucleotides in contrast to vertebrate rhodopsin kinase. Non-hydrolyzable GTP analogues, GTP lambda S and GppNHp, greatly enhanced the light-induced phosphorylation of octopus rhodopsin, but the non-hydrolyzable GDP analogue, GDP beta S, was not effective. These results suggest that rhodopsin A-kinase is involved in regulating the interaction between rhodopsin and G-protein in octopus photoreceptors.     

On a soluble system for studying light activation of rod outer segment cyclic GMP phosphodiesterase

Bovine rod outer segment (ROS) cyclic GMP phosphodiesterase (PDE) could be activated about 6-fold by light, an effect that could be simulated by isolated bleached rhodopsin. About 90% of PDE activity in ROS could be extracted with 10 mM Tris-HCl, pH 7.5, but light is ineffective in activating the soluble enzyme. However, bleached rhodopsin could activate it in the presence of a very low concentration of ATP, strongly suggesting the mediation of rhodopsin in the light activation of the enzyme in ROS. Direct evidence is presented to suggest that the phosphorylation of opsin (bleached rhodopsin) is unrelated to the activation of PDE by bleached rhodopsin and ATP. The reconstitution of the light activation of PDE in a soluble system presented here opens up a new direction to future investigations on the mechanism of light regulation of cyclic GMP levels in retina and its implication in the photoreceptor function.     

Production of antibodies against rhodopsin after immunization with beta gamma-subunits of transducin: evidence for interaction of beta gamma-subunits of guanosine 5'-triphosphate binding proteins with receptor

The light-detecting system of retinal rod outer segments is regulated by a guanyl nucleotide binding (G) protein, transducin, which is composed of alpha-, beta-, and gamma-subunits. Transducin couples rhodopsin to the intracellular effector enzyme, a cGMP phosphodiesterase. The beta gamma complex (T beta gamma) is required for the alpha-subunit (T alpha) to interact effectively with the photon receptor rhodopsin. It is not clear, however, whether T beta gamma binds directly to rhodopsin or promotes T alpha binding to rhodopsin only by binding to T alpha. We have found that serum from rabbits immunized with T beta gamma contained a population of antibodies that were reactive against rhodopsin. These antibodies could be separated from T beta gamma antibodies by absorbing the latter on immobilized transducin. Binding of purified rhodopsin antibodies was inhibited by T beta gamma, suggesting that the rhodopsin antibodies and T beta gamma bound to the same site on rhodopsin. We propose that the rhodopsin antibodies act both as antiidiotypic antibodies against the idiotypic T beta gamma antibodies and as antibodies against rhodopsin. This hypothesis is consistent with the conclusion that T beta gamma interacts directly with the receptor. It is probable that in an analogous way, G beta gamma interacts directly with receptors of the adenylate cyclase system.     

The nature of dominant mutations of rhodopsin and implications for gene therapy

Mutations in the rhodopsin gene are the most common cause of retinitis pigmentosa (RP) among human patients. The nature of the rhodopsin mutations has critical implications for the design of strategies for gene therapy. Nearly all rhodopsin mutations are dominant. Although dominance does not arise because of haploinsufficiency, it is unclear whether it is caused by gain-of-function or dominant-negative mutations. Current strategies for gene therapy have been devised to deal with toxic, gain-of-function mutations. However, analysis of results of transgenic and targeted expression of various rhodopsin genes in mice suggests that dominance may arise as a result of dominant-negative mutations. This has important consequences for gene therapy. The effects of dominant-negative mutations can be alleviated, in principle, by supplementation with additional wild-type rhodopsin. If added wild-type rhodopsin could slow retinal degeneration in human patients, as it does in mice, it would represent a valuable new strategy for gene therapy of RP caused by dominant rhodopsin mutations.     

Structural, energetic, and mechanical perturbations in rhodopsin mutant that causes congenital stationary night blindness

Several point mutations in rhodopsin cause retinal diseases including congenital stationary night blindness and retinitis pigmentosa. The mechanism by which a single amino acid residue substitution leads to dysfunction is poorly understood at the molecular level. A G90D point mutation in rhodopsin causes constitutive activity and leads to congenital stationary night blindness. It is unclear which perturbations the mutation introduces and how they can cause the receptor to be constitutively active. To reveal insight into these mechanisms, we characterized the perturbations introduced into dark state G90D rhodopsin from a transgenic mouse model expressing exclusively the mutant rhodopsin in rod photoreceptor cells. UV-visible absorbance spectroscopy revealed hydroxylamine accessibility to the chromophore-binding pocket of dark state G90D rhodopsin, which is not detected in dark state wild-type rhodopsin but is detected in light-activated wild-type rhodopsin. Single-molecule force spectroscopy suggested that the structural changes introduced by the mutation are small. Dynamic single-molecule force spectroscopy revealed that, compared with dark state wild-type rhodopsin, the G90D mutation decreased energetic stability and increased mechanical rigidity of most structural regions in the dark state mutant receptor. The observed structural, energetic, and mechanical changes in dark state G90D rhodopsin provide insights into the nature of perturbations caused by a pathological point mutation. Moreover, these changed properties observed for dark state G90D rhodopsin are consistent with properties expected for an active state.     

Target size analysis of rhodopsin in retinal rod disk membranes

Radiation inactivation of rhodopsin in situ using high-energy electrons gave a value for Mr of 20,200 by spectral assay, but 47,100 by assay of rhodopsin regeneration from opsin and 11-cis-retinal (sequence Mr = 38,840). No light/dark differences were seen. We conclude: (a) radiation inactivation measures the size of the functional unit, and the single hit hypothesis does not hold in our experiments; (b) 500 nm absorbance requires only about half the rhodopsin molecule to be intact, but reconstitution of rhodopsin from opsin requires the whole molecule; (c) we find no evidence for functional interactions between rhodopsin monomers in darkness or light.     

Mechanism and specificity of rhodopsin phosphorylation.

Partial separation of protein kinase activity from rhodopsin in isolated bovine retinal photoreceptor outer segments was accomplished by mild ultrasonic treatment followed by ultracentrifugation. Residual kinase activity in the rhodopsin-rich sediment was destroyed by chemical denaturation which did not affect the spectral properties of the rhodopsin. The retinal outer segment kinase was found to be specific for rhodopsin, since in these preparations it alone of several bovine protein kinases was capable of phosphorylating rhodopsin in the light. The phosphorylation reaction apparently requires a specific conformation of the rhodopsin molecule since it is abolished by heat denaturation of rhodopsin, and it is greatly reduced or abolished by treatment of the visual pigment protein with potassium alum after the rhodopsin has been "bleached" by light. When kinase and rhodopsin or opsin fractions were prepared from dark-adapted and bleached outer segments and the resultant fractions were mixed in various combinations of bleached and unbleached preparations, the observed pattern of light-activated phosphorylation was consistent only with the interpretation that a conformational change in the rhodopsin molecule in the light exposes a site on the visual pigment protein to the kinase and ATP. These results rule out the possibility of a direct or indirect (rhodopsin-mediated) light activation of the kinase. Finally, phosphorylation of retinal outer segment protein in monochromatic lights of various wavelengths followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that both rhodopsin and the higher molecular weight visual pigment protein reported by several laboratories have the same action spectrum for phosphorylation. This result is consistent with the suggestion that the higher molecular weight species is a rhodopsin dimer.     

Nontransducing rhodopsin

Rhodopsin is converted by light to an active photoproduct that triggers the transduction cascade. The active photoproduct must then be inactivated by some kind of chemical modification. The question addressed here is whether photoconversion of the inactive photoproduct to rhodopsin creates a modified form of rhodopsin that is unable to support transduction. This question was investigated in ultraviolet receptors of Limulus median eye by measuring the relative quantum efficiency of excitation after photoregeneration of rhodopsin from the inactive photoproduct. The results show that when this newly created rhodopsin absorbs a photon, no receptor potential is generated; i.e., the pigment is nontransducing. A dark process requiring 30-60 min returns rhodopsin to its transducing form.     

In vivo farnesylation of rat rhodopsin kinase.

We have shown by intravitreal injection of [3H]mevalonolactone that a 65 kDa protein in rat photoreceptors is posttranslationally modified by farnesylation. We further identified this 65 kDa prenylated protein as rhodopsin kinase based on its affinity for photolyzed rhodopsin and its ability to autophosphorylate in the presence of [gamma-32P]ATP. The farnesylation of rhodopsin kinase may be important for correctly targeting this enzyme to the photoreceptor outer segments, allowing it to phosphorylate photolyzed rhodopsin efficiently.     

Protein-lipid interactions at membrane surfaces: a deuterium and phosphorus nuclear magnetic resonance study of the interaction between bovine rhodopsin and the bilayer head groups of dimyristoylphosphatidylcholine

Rhodopsin, isolated from bovine retinal rod outer segment disk membranes, has been reconstituted into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine which was deuterated in the terminal methyl groups of the choline polar head group. By use of a mixed detergent system of cholate and octyl glucoside to solubilize the phospholipid and rhodopsin, 15 membrane complexes of predetermined phospholipid to rhodopsin mole ratios of between 350:1 and 65:1 have been produced by exhaustive dialysis and studied by a variety of techniques. Electron micrographs of replicas from freeze-fractured membrane complexes showed that the majority of the lipid, for all rhodopsin:phospholipid ratios, was contained in large bilayer vesicles with diameters in excess of 400 nm. Complexes produced with rhodopsin from frozen retina produced an absorption maximum at 478 nm after photobleaching whereas rhodopsin from fresh retina could be bleached more completely to an absorption maximum at 380 nm. Deuterium nuclear magnetic resonance (NMR) spectra from the lipid head groups of bilayers above the gel to liquid-crystalline phase transition temperature were shown to be sensitive in a systematic way to the presence of rhodopsin which could be bleached to 380 nm. The measured quadrupole splittings, taken as the separation of the turning points of the recorded NMR spectra, decreased from a value of 1.28 kHz for protein-free bilayers to approximately 0.40 kHz for bilayers containing 65 molecules of phospholipid for each rhodopsin at 32 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inactivation of photoexcited rhodopsin in retinal rods: the roles of rhodopsin kinase and 48-kDa protein (arrestin)

The inactivation of excited rhodopsin in the presence of ATP, rhodopsin kinase, and/or arrestin has been studied from its effect on the two subsequent steps in the light-induced enzymatic cascade: metarhodopsin II catalyzed activation of G-protein and G-protein-dependent activation of cGMP phosphodiesterase. The inactivation of G-protein (from light-scattering measurements) and that of phosphodiesterase (from measurements of cGMP hydrolysis) have been studied and compared in reconstituted systems containing various combinations of the proteins involved (rhodopsin, G-protein, phosphodiesterase, kinase, and arrestin). Our results show that rhodopsin kinase alone can terminate the activation of G-protein and that arrestin speeds up the process at a relative concentration similar to that reported in the rod (half-maximal effect at 50 nM for 4.4 microM rhodopsin). Measurements of rhodopsin phosphorylation under identical conditions show that in the presence of arrestin total metarhodopsin II inactivation is achieved when only 0.5-1.4 phosphates are bound per bleached rhodopsin, whereas in the absence of arrestin it requires binding of 12-16 phosphates per bleached rhodopsin. Phosphodiesterase activity can similarly be turned off by kinase, and the process is similarly accelerated by arrestin.     

Molecular properties of rhodopsin and rod function

Signal transduction in rod cells begins with photon absorption by rhodopsin and leads to the generation of an electrical response. The response profile is determined by the molecular properties of the phototransduction components. To examine how the molecular properties of rhodopsin correlate with the rod-response profile, we have generated a knock-in mouse with rhodopsin replaced by its E122Q mutant, which exhibits properties different from those of wild-type (WT) rhodopsin. Knock-in mouse rods with E122Q rhodopsin exhibited a photosensitivity about 70% of WT. Correspondingly, their single-photon response had an amplitude about 80% of WT, and a rate of decline from peak about 1.3 times of WT. The overall 30% lower photosensitivity of mutant rods can be explained by a lower pigment photosensitivity (0.9) and the smaller single-photon response (0.8). The slower decline of the response, however, did not correlate with the 10-fold shorter lifetime of the meta-II state of E122Q rhodopsin. This shorter lifetime became evident in the recovery phase of rod cells only when arrestin was absent. Simulation analysis of the photoresponse profile indicated that the slower decline and the smaller amplitude of the single-photon response can both be explained by the shift in the meta-I/meta-II equilibrium of E122Q rhodopsin toward meta-I. The difference in meta-III lifetime between WT and E122Q mutant became obvious in the recovery phase of the dark current after moderate photobleaching of rod cells. Thus, the present study clearly reveals how the molecular properties of rhodopsin affect the amplitude, shape, and kinetics of the rod response.     

Carboxyl terminal of rhodopsin kinase is required for the phosphorylation of photo—activated rhodopsin

Human rhodopsin kinase (RK) and a carboxyl terminus-truncated mutant RK lacking the last 59 amino acids (RKC) were expressed in human embryonic kidney 293 cells to investigate the role of the carboxyl terminus of RK in recognition and phosphorylation of rhodopsin.RKC,like the wild-type RK,was detected in both plasma membranes and cytosolic fractions.The Cterminal truncated rhodopsin kinase was unable to phosphorylate photo-activated rhodopsin,but possesses kinase activity similar to the wild-type RK in phosphorylation of small peptide substrate.It suggests that the truncation did not disturb the gross structures of RK catalytic domain.Our results also show that RKC failed to translocate to photo-activated rod out segments.Taken together,our study demonstrate the carboxyl terminus of RK is required for phosphorylation of photo-activated rhodopsin and strongly indicate that carboxyl-terminus of RK may be involved in interaction with photo-activated rhodopsin.     

Distinct roles of the second and third cytoplasmic loops of bovine rhodopsin in G protein activation

In contrast to the extensive studies of light-induced conformational changes in rhodopsin, the cytoplasmic architecture of rhodopsin related to the G protein activation and the selective recognition of G protein subtype is still unclear. Here, we prepared a set of bovine rhodopsin mutants whose cytoplasmic loops were replaced by those of other ligand-binding receptors, and we compared their ability for G protein activation in order to obtain a clue to the roles of the second and third cytoplasmic loops of rhodopsin. The mutants bearing the third loop of four other G(o)-coupled receptors belonging to the rhodopsin superfamily showed significant G(o) activation, indicating that the third loop of rhodopsin possibly has a putative site(s) related to the interaction of G protein and that it is simply exchangeable with those of other G(o)-coupled receptors. The mutants bearing the second loop of other receptors, however, had little ability for G protein activation, suggesting that the second loop of rhodopsin contains a specific region essential for rhodopsin to be a G protein-activating form. Systematic chimeric and point mutational studies indicate that three amino acids (Glu(134), Val(138), and Cys(140)) in the N-terminal region of the second loop of rhodopsin are crucial for efficient G protein activation. These results suggest that the second and third cytoplasmic loops of bovine rhodopsin have distinct roles in G protein activation and subtype specificity.