Mechanisms of opsin activation

G protein-coupled receptor rhodopsin: a prospectus

Rhodopsin is a retinal photoreceptor protein of bipartite structure consisting of the transmembrane protein opsin and a light-sensitive chromophore 11-cis-retinal, linked to opsin via a protonated Schiff base. Studies on rhodopsin have unveiled many structural and functional features that are common to a large and pharmacologically important group of proteins from the G protein-coupled receptor (GPCR) superfamily, of which rhodopsin is the best-studied member. In this work, we focus on structural features of rhodopsin as revealed by many biochemical and structural investigations. In particular, the high-resolution structure of bovine rhodopsin provides a template for understanding how GPCRs work. We describe the sensitivity and complexity of rhodopsin that lead to its important role in vision.

11-cis-retinal reduces constitutive opsin phosphorylation and improves quantum catch in retinoid-deficient mouse rod photoreceptors

Rpe65(-/-) mice produce minimal amounts of 11-cis-retinal, the ligand necessary for the formation of photosensitive visual pigments. Therefore, the apoprotein opsin in these animals has not been exposed to its normal ligand. The Rpe65(-/-) mice contain less than 0.1% of wild type levels of rhodopsin. Mass spectrometric analysis of opsin from Rpe65(-/-) mice revealed unusually high levels of phosphorylation in dark-adapted mice but no other structural alterations. Single flash and flicker electroretinograms (ERGs) from 1-month-old animals showed trace rod function but no cone response. B-wave kinetics of the single-flash ERG are comparable with those of dark-adapted wild type mice containing a full compliment of rhodopsin. Application (intraperitoneal injection) of 11-cis-retinal to Rpe65(-/-) mice increased the rod ERG signal, increased levels of rhodopsin, and decreased opsin phosphorylation. Therefore, exogenous 11-cis-retinal improves photoreceptor function by regenerating rhodopsin and removes constitutive opsin phosphorylation. Our results indicate that opsin, which has not been exposed to 11-cis-retinal, does not generate the activity generally associated with the bleached apoprotein.

Rhodopsin: insights from recent structural studies

The recent report of the crystal structure of rhodopsin provides insights concerning structure-activity relationships in visual pigments and related G protein-coupled receptors (GPCRs). The seven transmembrane helices of rhodopsin are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The ligand-binding pocket of rhodopsin is remarkably compact, and several chromophore-protein interactions were not predicted from mutagenesis or spectroscopic studies. The helix movement model of receptor activation, which likely applies to all GPCRs of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor includes a helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. The cytoplasmic surface appears to be approximately large enough to bind to the transducin heterotrimer in a one-to-one complex. The structural basis for several unique biophysical properties of rhodopsin, including its extremely low dark noise level and high quantum efficiency, can now be addressed using a combination of structural biology and various spectroscopic methods. Future high-resolution structural studies of rhodopsin and other GPCRs will form the basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Mechanistic studies of ABCR, the ABC transporter in photoreceptor outer segments responsible for autosomal recessive Stargardt disease

ABCR is an ABC transporter that is found exclusively in vertebrate photoreceptor outer segments. Mutations in the human ABCR gene are responsible for autosomal recessive Stargardt disease, the most common cause of early onset macular degeneration. In this paper we review our recent work with purified and reconstituted ABCR derived from bovine retina and from cultured cells expressing wild type or site-directed mutants of human ABCR. These experiments implicate all-trans-retinal (or Schiff base adducts between all-trans-retinal and phosphatidylethanolamine) as the transport substrate, and they reveal asymmetric roles for the two nucleotide binding domains in the transport reaction. A model for the retinal transport reaction is presented which accounts for these experimental observations.

Conformations of the active and inactive states of opsin

The signaling state metarhodopsin II of the visual pigment rhodopsin decays to the apoprotein opsin and all-trans retinal, which are then regenerated to rhodopsin by the visual cycle. Opsin is known to have at neutral pH only a small residual constitutive activity toward its G protein transducin, which is thought to play a considerable role in light adaptation (bleaching desensitization). In this study we show with Fourier-transform infrared spectroscopy that after metarhodopsin II decay, opsin exists in two conformational states that are in a pH-dependent equilibrium at 30 degrees C with a pK of 4.1 in the presence of hydroxylamine scavenging the endogenous all-trans retinal. Despite the lack of the native agonist in its binding pocket, the low pH opsin conformation is very similar to that of metarhodopsin II and is likewise stabilized by peptides derived from rhodopsin's cognate G protein, transducin. The high pH form, on the other hand, has some conformational similarity to the inactive metarhodopsin I state. We therefore conclude that the opsin apoprotein displays intrinsic conformational states that are merely modulated by bound all-trans retinal.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Visual cycle impairment in cellular retinaldehyde binding protein (CRALBP) knockout mice results in delayed dark adaptation

Mutations in the human CRALBP gene cause retinal pathology and delayed dark adaptation. Biochemical studies have not identified the primary physiological function of CRALBP. To resolve this, we generated and characterized mice with a non-functional CRALBP gene (Rlbp1(-/-) mice). The photosensitivity of Rlbp1(-/-) mice is normal but rhodopsin regeneration, 11-cis-retinal production, and dark adaptation after illumination are delayed by >10-fold. All-trans-retinyl esters accumulate during the delay indicating that isomerization of all-trans- to 11-cis-retinol is impaired. No evidence of photoreceptor degeneration was observed in animals raised in cyclic light/dark conditions for up to 1 year. Albino Rlbp(-/-) mice are protected from light damage relative to the wild type. These findings support a role for CRALBP as an acceptor of 11-cis-retinol in the isomerization reaction of the visual cycle.

Role of noncovalent binding of 11-cis-retinal to opsin in dark adaptation of rod and cone photoreceptors

Regeneration of visual pigments of vertebrate rod and cone photoreceptors occurs by the initial noncovalent binding of 11-cis-retinal to opsin, followed by the formation of a covalent bond between the ligand and the protein. Here, we show that the noncovalent interaction between 11-cis-retinal and opsin affects the rate of dark adaptation. In rods, 11-cis-retinal produces a transient activation of the phototransduction cascade that precedes sensitivity recovery, thus slowing dark adaptation. In cones, 11-cis-retinal immediately deactivates phototransduction. Thus, the initial binding of the same ligand to two very similar G protein receptors, the rod and cone opsins, activates one and deactivates the other, contributing to the remarkable difference in the rates of rod and cone dark adaptation.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Effect of 11-cis 13-demethylretinal on phototransduction in bleach-adapted rod and cone photoreceptors

We used 11-cis 13-demethylretinal to examine the physiological consequences of retinal's noncovalent interaction with opsin in intact rod and cone photoreceptors during visual pigment regeneration. 11-Cis 13-demethylretinal is an analog of 11-cis retinal in which the 13 position methyl group has been removed. Biochemical experiments have shown that it is capable of binding in the chromophore pocket of opsin, forming a Schiff-base linkage with the protein to produce a pigment, but at a much slower rate than the native 11-cis retinal (Nelson, R., J. Kim deReil, and A. Kropf. 1970. Proc. Nat. Acad. Sci. USA. 66:531-538). Experimentally, this slow rate of pigment formation should allow separate physiological examination of the effects of the initial binding of retinal in the pocket and the subsequent formation of the protonated Schiff-base linkage. Currents from solitary rods and cones from the tiger salamander were recorded in darkness before and after bleaching and then after exposure to 11-cis 13-demethylretinal. In bleach-adapted rods, 11-cis 13-demethylretinal caused transient activation of phototransduction, as evidenced by a decrease of the dark current and sensitivity, acceleration of the dim flash responses, and activation of cGMP phosphodiesterase and guanylyl cyclase. The steady state of phototransduction activity was still higher than that of the bleach-adapted rod. In contrast, exposure of bleach-adapted cones to 11-cis 13-demethylretinal resulted in an immediate deactivation of transduction as measured by the same parameters. These results extend the validity of a model for the effects of the noncovalent binding of a retinoid in the chromophore pockets of rod and cone opsins to analogs capable of forming a Schiff-base and imply that the noncovalent binding by itself may play a role for the dark adaptation of photoreceptors.

Assays for activation of opsin by all-trans-retinal

The data collected with the techniques discussed in this chapter suggest significant differences between the active conformation(s) of the opsin/atr complex, which are reversibly formed in the dark, and the active conformation (R*) of the meta-II photoproduct. First, there is good evidence for noncovalent opsin/atr complexes with considerable activity (although covalent binding of atr is found in mutant opsins. Even more intriguing, all-trans-retinal in an amount that saturates the activity of the opsin/atr complex toward Gt does not measurably inhibit the access of 11-cis-retinal to the light-sensitive binding site during regeneration (Fig. 2C). On the other hand, forced protonation at or near Glu-134 appears to be an integral mechanism for both the meta-II and the opsin-like activities (Fig. 4). Thus, it is not inconceivable that these two activities of the receptor arise from two fundamentally different conformations, one meta-II-like and one opsin-like. They would be similar with respect to the Gt (or RK) protein-protein interaction but different in their mode of retinal-protein interaction.

Intrahelical arrangement in the integral membrane protein rhodopsin investigated by site-specific chemical cleavage and mass spectrometry

Site-specific cleavage on the interhelical loop I on the cytoplasmic face of rhodopsin has been observed after activation of a Cu-phenanthroline tethered cleavage reagent attached on the cytoplasmic loop IV. The characterization of the reaction products by mass spectrometry, both of the membrane-bound protein and of the CNBr-cleaved peptides, allows the site of cleavage to be determined precisely. The specific cleavage of the peptide bond between Q64 and H65 on loop I leaves the N-terminal peptide (M1-Q64) intact, confirmed by MALDI-MS detection of the two N-linked glycosyl groups near the N-terminus of rhodopsin. The limited extension of the tether side chain requires a interresidue distance between the cleavage site, Q64, and the site of ligand attachment, C316, of less than 12 A. Upon photoactivation of the receptor, no change in the cleavage pattern is observed; however, a simulated Meta II intermediate activation state indicates a much more complex cleavage pattern. The development of this cleavage method, previously used primarily as a "chemical nuclease", in combination with mass spectrometry, may provide a powerful method on membrane protein conformation studies that can be used to complement other biophysical characterizations.

Diffusible ligand all-trans-retinal activates opsin via a palmitoylation-dependent mechanism

In rhodopsin's function as a photoreceptor, 11-cis-retinal is covalently bound to Lys(296) via a protonated Schiff base. 11-cis/all-trans photoisomerization and relaxation through intermediates lead to the metarhodopsin II photoproduct, which couples to transducin (G(t)). Here we have analyzed a different signaling state that arises from noncovalent binding of all-trans-retinal (atr) to the aporeceptor opsin and enhances the very low opsin activity by several orders of magnitude. Like with metarhodopsin II, coupling of G(t) to opsin-atr is sensitive to competition by synthetic peptides from the COOH termini of both G(t)alpha and G(t)gamma. However, atr does not compete with 11-cis-retinal incorporation into the Lys(296) binding site and formation of the light-sensitive pigment. Blue light illumination fails to photorevert opsin-atr to the ground state. Thus noncovalently bound atr has no access to the light-dependent binding site and reaction pathway. Moreover, in contrast to light-dependent signaling, removal of the palmitoyl anchors at Cys(322) and Cys(323) in the rhodopsin COOH terminus impairs the atr-stimulated activity. Repalmitoylation by autoacylation with palmitoyl-coenzyme A restores most of the original activity. We hypothesize that the palmitoyl moieties are part of a second binding pocket for the chromophore, mediating hydrophobic interactions that can activate a large part of the catalytic receptor/G-protein interface.

Signalling states of photoactivated rhodopsin

In microseconds after photoexcitation, rhodopsin forms the Meta I intermediate from lumirhodopsin. In this conversion, contacts between retinal and the apoprotein are formed, which result in a defined arrangement of donor and acceptor groups for proton translocations. A system of protonation-dependent coupled equilibria is now adopted, comprising Meta intermediates I, II and III, and their isospectral subforms. Some Meta states were identified as signalling states, in which the receptor interacts with transducin (Gt), rhodopsin kinase (RK) and arrestin. The binding of Gt or arrestin shifts the equilibrium to Meta II, while RK does not, indicating exposure of the RK binding site(s) before Meta II is formed. On contact with the activated receptor, each signalling protein responds with a conformational change, which transforms it into a functionally active state. The bell-shaped pH/rate profiles which are seen for the activation of both the G protein and the receptor kinase, indicate the necessary protonation and deprotonation of groups with different pKa. The right wing of the profile reflects the formation of the protonated subconformation (termed MIIb) of Meta II. For the interaction with Gt, recent work suggests a 'sequential fit' mechanism, involving the recognition of the C-terminal peptide of the Gt alpha subunit and of the farnesylated C-terminus of the gamma subunit. Isolated peptides derived from these portions of the G protein mimic the left wing of the pH/rate profile. We discuss the sequential fit as a time-ordered sequence of microscopic recognition and conformational interlocking in the interaction with the G protein.

Activation of the rod G-protein Gt by the thrombin receptor (PAR1) expressed in Sf9 cells

Functional coupling of the human thrombin receptor PAR1 (protease-activated receptor 1) with the retinal rod G-protein transducin (Gt, a member of the Gi family) was studied in a reconstituted system of membranes from Sf9 cells expressing the thrombin receptor and purified Gt from bovine rod outer segments. TRAP6-agonist-activated PAR1 interacts productively with the distant G-protein. Agonist-dependent Gt activation was measured using a real-time fluorimetric GTP[S]-binding assay and membranes from Sf9 cells. To characterize nucleotide-exchange catalysis by PAR1, we analyzed dependence on nucleotides, temperature and pH. Activation was inhibited by low GDP concentrations (IC50 = 5.2 +/- 1.5 microM at 5 microM GTP[S]), indicating that receptor-Gt coupling, followed by instantaneous GDP release, is rate limiting under the conditions (25 degrees C). Arrhenius plots of the temperature dependence reflect an apparent Ea of 60 +/- 3.5 kJ.mol-1. Evaluation of the pH/rate profiles of Gt activation indicates that the activating conformation of the receptor is determined by protonation of a titratable group with an apparent pKa of 6.4. This supports the idea that the active state of agonist-bound PAR1 depends on forced protonation, indicating possible analogies to the scheme established for rhodopsin.

Phosphorylation alters the pH-dependent active state equilibrium of rhodopsin by modulating the membrane surface potential

Phosphorylation reduces the lifetime and activity of activated G protein-coupled receptors, yet paradoxically shifts the metarhodopsin I-II (MI-MII) equilibrium (K(eq)) of light-activated rhodopsin toward MII, the conformation that activates G protein. In this report, we show that phosphorylation increases the apparent pK for MII formation in proportion to phosphorylation stoichiometry. Decreasing ionic strength enhances this effect. Gouy-Chapman theory shows that the change in pK is quantitatively explained by the membrane surface potential, which becomes more negative with increasing phosphorylation stoichiometry and decreasing ionic strength. This lowers the membrane surface pH compared to the bulk pH, increasing K(eq) and the rate of MII formation (k(1)) while decreasing the back rate constant (k(-)(1)) of the MI-MII relaxation. MII formation has been observed to depend on bulk pH with a fractional stoichiometry of 0.6-0.7 H(+)/MII. We find that the apparent fractional H(+) dependence is an artifact of altering the membrane surface charge during a titration, resulting in a fractional change in membrane surface pH compared to bulk pH. Gouy-Chapman calculations of membrane pH at various phosphorylation levels and ionic strengths suggest MII formation behavior consistent with titration of a single H(+) binding site with 1:1 stoichiometry and an intrinsic pK of 6.3 at 0.5 degrees C. We show evidence that suggests this same site has an intrinsic pK of 5.0 prior to light activation and its protonation before activation greatly enhances the rate of MII formation.

Retinal stimulates ATP hydrolysis by purified and reconstituted ABCR, the photoreceptor-specific ATP-binding cassette transporter responsible for Stargardt disease

Many substrates for P-glycoprotein, an ABC transporter that mediates multidrug resistance in mammalian cells, have been shown to stimulate its ATPase activity in vitro. In the present study, we used this property as a criterion to search for natural and artificial substrates and/or allosteric regulators of ABCR, the rod photoreceptor-specific ABC transporter responsible for Stargardt disease, an early onset macular degeneration. ABCR was immunoaffinity purified to apparent homogeneity from bovine rod outer segments and reconstituted into liposomes. All-trans-retinal, a candidate ligand, stimulates the ATPase activity of ABCR 3-4-fold, with a half-maximal effect at 10-15 microM. 11-cis- and 13-cis-retinal show similar activity. All-trans-retinal stimulates the ATPase activity of ABCR with Michaelis-Menten behavior indicative of simple noncooperative binding that is associated with a rate-limiting enzyme-substrate intermediate in the pathway of ATP hydrolysis. Among 37 structurally diverse non-retinoid compounds, including nine previously characterized substrates or sensitizers of P-glycoprotein, only four show significant ATPase stimulation when tested at 20 microM. The dose-response curves of these four compounds are indicative of multiple binding sites and/or modes of interaction with ABCR. Two of these compounds, amiodarone and digitonin, can act synergistically with all-trans-retinal, implying that they interact with a site or sites on ABCR different from the one with which all-trans-retinal interacts. Unlike retinal, amiodarone appears to interact with both free and ATP-bound ABCR. Together with clinical observations on Stargardt disease and the localization of ABCR to rod outer segment disc membranes, these data suggest that retinoids, and most likely retinal, are the natural substrates for transport by ABCR in rod outer segments. These observations have significant implications for understanding the visual cycle and the pathogenesis of Stargardt disease and for the identification of compounds that could modify the natural history of Stargardt disease or other retinopathies associated with impaired ABCR function.

Occupancy of the chromophore binding site of opsin activates visual transduction in rod photoreceptors

The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.

Enhancement of opsin activity by all-trans-retinal

The rod cell photoreceptor apoprotein, opsin, activates the G-protein, transducin, although at a much reduced level than light-activated rhodopsin. The ability of all-trans-retinal to enhance opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. All-trans-retinal enhanced opsin activity in a concentration-dependent manner. At high concentrations of all-trans-retinal, the activity of the all-trans-retinal-opsin complex was comparable to that from an equimolar amount of metarhodopsin(II). However, in contrast to metarhodopsin(II), the active all-trans-retinalopsin complex did not require a stable Schiff base linkage between opsin and all-trans-retinal. The lack of a stable Schiff base and differences in activity at high pH imply that opsin and all-trans-retinal form a complex that is distinct from metarhodopsin(II). The ability of all-trans-retinal to stimulate the transduction cascade may be a source of post-bleach noise in photoreceptors.

Rhodopsin phosphorylation and its role in photoreceptor function

Light-stimulated phosphorylation of rhodopsin was first described 25 years ago. This paper reviews the progress that has been made towards (i) understanding the nature of the enzymes that phosphorylate and dephosphorylate rhodopsin (ii) identifying the sites of phosphorylation on rhodopsin and (iii) understanding the physiological importance of rhodopsin phosphorylation. Many important questions related to rhodopsin phosphorylation remain unanswered and new strategies and methods are needed to address issues such as the roles of Ca2+ and recoverin. We present one such method that uses mass spectrometry to quantitate rhodopsin phosphorylation in intact mouse retinas.

Rhodopsin: a prototypical G protein-coupled receptor

A variety of spectroscopic and biochemical studies of recombinant site-directed mutants of rhodopsin and related visual pigments have been reported over the past 9 years. These studies have elucidated key structural elements common to visual pigments. In addition, systematic analysis of the chromophore-binding pocket in rhodopsin and cone pigments has led to an improved understanding of the mechanism of the opsin shift, and of particular molecular determinants underlying color vision in humans. Identification of the conformational changes that occur on rhodopsin photoactivation has been of particular recent concern. Assignments of light-dependent molecular alterations to specific regions of the chromophore have also been attempted by studying native opsins regenerated with synthetic retinal analogs. Site-directed mutagenesis of rhodopsin has also provided useful information about the retinal-binding pocket and the molecular mechanism of rhodopsin photoactivation. Individual molecular groups have been identified to undergo structural alterations or environmental changes during photoactivation. Analysis of particular mutant pigments in which specific groups are locked into their respective "off" or "on" states has provided a framework to identify determinants of the active conformation, as well as the minimal number of intramolecular transitions required to switch between inactive and active conformations. A simple model for the active state of rhodopsin can be compared to structural models of its ground state to localize chromophore-protein interactions that may be important in the photoactivation mechanism. This review focuses on the recent functional characterization of site-directed mutants of bovine rhodopsin and some cone pigments. In addition, an attempt is made to reconcile previous key findings and existing structural models with information gained from the analysis of site-directed mutant pigments.

Role of the C9 methyl group in rhodopsin activation: characterization of mutant opsins with the artificial chromophore 11-cis-9-demethylretinal

Activation of the visual pigment rhodopsin involves both steric and electrostatic interactions between the chromophore and opsin within the retinal-binding site. Removal of the C9 methyl group of 11-cis-retinal inhibits light-dependent activation of the G protein, transducin, suggesting a direct steric contact. More recently, we have shown that steric interactions lead to receptor activation when Gly121 in the middle of transmembrane helix 3 is replaced by larger hydrophobic residues. In order to understand in more detail the role of the C9 methyl group of retinal in the structure and function of rhodopsin, we first studied the properties of recombinant 9-dm-Rho (opsin reconstituted with 11-cis-9-demethylretinal). The 9-dm-Rho pigment displayed a blue-shifted lambdamax, increased hydroxylamine reactivity, and decreased ability to activate transducin. These properties are consistent with the hypothesis that the C9 methyl group is a crucial structural anchor for the correct docking of the chromophore in its binding site. Next, we investigated the possible interaction between Gly121 of opsin and the C9 methyl group of retinal by characterizing recombinant pigments produced by combining mutant opsins (G121A, -V, -I, -L, and -W) with 11-cis-9-demethylretinal. Mutant opsins G121I, -L, and -W failed to bind the chromophore. However, the double mutant G121L/F261A bound 11-cis-9-demethylretinal to form a stable pigment with a lambdamax of 451 nm. When activity was assayed in membranes, the reduction in transducin activation by 9-dm-Rho caused by the lack of a C9 methyl group on the chromophore could be partially restored by replacing Gly121 with a bulky residue (leucine, isoleucine, or tryptophan). These results support a model of receptor activation that involves steric interaction between the C9 methyl group of the chromophore and the opsin in the vicinity of Gly121 on transmembrane helix 3.

Null mutation in the rhodopsin kinase gene slows recovery kinetics of rod and cone phototransduction in man

Rhodopsin kinase (RK), a specialized G-protein-coupled receptor kinase expressed in retina, is involved in quenching of light-induced signal transduction in photoreceptors. The role of RK in recovery after photoactivation has been explored in vitro and in vivo experimentally but has not been specifically defined in humans. We investigated the effects on human vision of a mutation in the RK gene causing Oguchi disease, a recessively inherited retinopathy. In vitro experiments demonstrated that the mutation, a deletion of exon 5, abolishes the enzymatic activity of RK and is likely a null. Both a homozygote and heterozygote with this RK mutation had recovery phase abnormalities of rod-isolated photoresponses by electroretinography (ERG); photoactivation was normal. Kinetics of rod bleaching adaptation by psychophysics were dramatically slowed in the homozygote but normal final thresholds were attained. Light adaptation was normal at low backgrounds but became abnormal at higher backgrounds. A slight slowing of cone deactivation kinetics in the homozygote was detected by ERG. Cone bleaching adaptation and background adaptation were normal. In this human in vivo condition without a functional RK and probable lack of phosphorylation and arrestin binding to activated rhodopsin, reduction of photolyzed chromophore and regeneration processes with 11-cis-retinal probably constitute the sole pathway for recovery of rod sensitivity. The role of RK in rods would thus be to accelerate inactivation of activated rhodopsin molecules that in concert with regeneration leads to the normal rate of recovery of sensitivity. Cones may rely mainly on regeneration for the inactivation of photolyzed visual pigment, but RK also contributes to cone recovery.

A comparison of the efficiency of G protein activation by ligand-free and light-activated forms of rhodopsin

Activation of the photoreceptor G protein transducin (Gt) by opsin, the ligand-free form of rhodopsin, was measured using rod outer segment membranes with densities of opsin and Gt similar to those found in rod cells. When GTPgammaS was used as the activating nucleotide, opsin catalyzed transducin activation with an exponential time course with a rate constant k(act) on the order of 2 x 10(-3)s(-1). Comparison under these conditions to activation by flash-generated metarhodopsin II (MII) revealed that opsin- and R*-catalyzed activation showed similar kinetics when MII was present at a surface density approximately 10(-6) lower than that of opsin. Thus, in contrast to some previous reports, we find that the catalytic potency of opsin is only approximately 10(-6) that of MII. In the presence of residual retinaldehyde-derived species present in membranes treated with hydroxylamine after bleaching, the apparent k(act) observed was much higher than that for opsin, suggesting a possible explanation for previous reports of more efficient activation by opsin. These results are important for considering the possible role of opsin in the diverse phenomena in which it has been suggested to play a key role, such as bleaching desensitization and retinal degeneration induced by continuous light or vitamin A deprivation.

Partial agonist activity of 11-cis-retinal in rhodopsin mutants

Rhodopsin, the photoreceptor molecule of the vertebrate rod cell, is a G protein-coupled receptor. Rhodopsin consists of the opsin apoprotein and its 11-cis-retinal chromophore, which is covalently bound to a specific lysine residue by a stable protonated Schiff base linkage. Rhodopsin activation occurs when light causes photoisomerization of the 11-cis chromophore to its all-trans form. The all-trans chromophore is the receptor agonist. The 11-cis-retinylidene chromophore is analogous pharmacologically to a potent inverse agonist of the receptor. We report here that replacement of a highly conserved glycine residue (Gly121) causes 11-cis-retinal to become a pharmacologic partial agonist. Although the mutant apoproteins do not display constitutive activity, they are active in the dark when bound to an 11-cis-retinylidene chromophore, or to a "locked" chromophore analogue, Ret-7. The degree of partial agonism is directly related to the size of the amino acid replacement at position 121, and it can be reversed by a specific second-site replacement of Phe261. Thus, mutation of Gly121 in rhodopsin causes 11-cis-retinal to act as a partial agonist rather than an inverse agonist, allowing the mutant pigment to activate transducin in the dark.

GTP-binding-protein-coupled receptor kinases--two mechanistic models

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.

Modulation of opsin apoprotein activity by retinal. Dark activity of rhodopsin formed at low temperature

The bovine opsin apoprotein activates transducin, although at a much reduced level than light-activated rhodopsin (Surya, A., Foster, K., and Knox, B. (1995) J. Biol. Chem. 270, 5024-5031). The ability of retinal to modulate opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. 11-cis-Retinal reacted with opsin at 22 degrees C to (a) reform pigment having maximal absorbance at 500 nm and (b) reduce opsin activity by >80%. Pigment formation also occurred at 0 degrees C with a t1/2 of 260 min. However, unlike rhodopsin formed at 22 degrees C (R22), the rhodopsin formed at 0 degrees C (R0) activated transducin with the same half-saturating concentration as opsin in an exhaustive binding assay. Thus, the formation of a protonated Schiff base associated with 500 nm absorbance does not by itself lead to the inactivation of opsin. The R0 conformation was partially inactivated by incubation at 22 degrees C (t1/2 = 61 +/- 9 min), suggesting that it may be an intermediate conformation in the regeneration of rhodopsin.

Activation and inactivation steps in the visual transduction pathway

Recent genetic, biochemical and electrophysiological evidence has provided insights into the molecular identity of the substance responsible for bleaching desensitization in vision. Studies examining the molecular defects that cause delayed dark adaptation suggest that the desensitizing substance is recognized by rhodopsin kinase and/or arrestin and, therefore, is probably a complex comprising all-trans-retinal and opsin.