Photochemical and biochemical properties of chicken blue-sensitive cone visual pigment

Deprotonation of retinal Schiff base and structural dynamics in the early photoreaction of primate blue cone visual pigment

Animal rhodopsin is a photoreceptive protein crucial for vision, with activation triggered by the cis-trans isomerization of a retinal chromophore upon light absorption. This activation involves a series of thermal intermediates, ultimately leading to G protein-mediated signal transduction. The retinal chromophore is covalently bound to the protein through a protonated Schiff base, and its deprotonation during the formation of the active intermediate is believed to induce structural changes in α-helices that facilitate G-protein interactions. Using low-temperature UV-visible absorption and Fourier transform infrared spectroscopy, we investigated the early photoreaction of the primate blue cone visual pigment (MB). Our results demonstrate that Schiff base deprotonation in the early photoreaction is coupled with local perturbations in α-helices, promoting the formation of the Lumi intermediate. Using site-directed mutagenesis, we identified the proton acceptor involved in Schiff base deprotonation and mapped the regions of α-helical structural changes during the formation of the Lumi intermediate. We discovered that the proton released from the Schiff base is transferred to the counterion Glu113. Systematic mutagenesis revealed that structural perturbations in transmembrane helix 7 bring Glu113 and the lysine residue forming the Schiff base into proximity, facilitating efficient proton transfer during the early photoreaction. Additionally, the Lumi intermediate formed at low temperatures was found to revert to the original state through thermally driven reverse proton transfer, coupled with retinal reisomerization. From an evolutionary perspective, MB is part of a group of UV-sensitive cone visual pigments characterized by a deprotonated retinal Schiff base in the ground state. The observed propensity for MB to undergo Schiff base deprotonation is consistent with this evolutionary trait.

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Glu1022.53-Mediated Early Conformational Changes in the Process of Light-Induced Green Cone Pigment Activation

Ligand-triggered activation of G protein-coupled receptors (GPCRs) relies on the phenomenon of loose allosteric coupling, which involves conformational alterations spanning from the extracellular ligand-binding domain to the cytoplasmic region, where interactions with G proteins occur. During the GPCR activation process, several intermediate and equilibrium states orchestrate the movement of the flexible and rigid transmembrane (TM) segments of the GPCR. Monitoring early conformational changes is important in unraveling the structural intricacies of the loose allosteric coupling. Here, we focus on the lumi intermediate formed by thermal relaxation from the initial photointermediate, batho in primate green cone pigment (MG), a light-sensitive GPCR responsible for color vision. Our findings from light-induced Fourier transform infrared difference spectroscopy reveal its similarity with rhodopsin, which mediates twilight vision, specifically involving the flip motion of the β-ionone ring, the relaxation of the torsional structure of the retinal, and local perturbations in the α-helix upon lumi intermediate formation. Conversely, we observe a hydrogen bond modification specific to MG's protonated carboxylic acid, identifying its origin as Glu1022.53 situated in TM2. The weakening of the hydrogen bond strength at Glu1022.53 during the transition from the batho to the lumi intermediates corresponds to a slight outward movement of TM2. Additionally, within the X-ray crystal structure of the rhodopsin lumi intermediate, we note the relocation of the Met862.53 side chain in TM2, expanding the volume of the retinal binding pocket. Consequently, the position of 2.53 emerges as the early step in the conformational shift toward light-induced activation. Moreover, given the prevalence of IR-insensitive hydrophobic amino acids at position 2.53 in many rhodopsin-like GPCRs, including rhodopsin, the hydrogen bond alteration in the C═O stretching band at Glu1022.53 of MG can be used as a probe for tracing conformational changes during the GPCR activation process.

Ultrafast Transient Absorption Spectra and Kinetics of Rod and Cone Visual Pigments

Rods and cones are the photoreceptor cells containing the visual pigment proteins that initiate visual phototransduction following the absorption of a photon. Photon absorption induces the photochemical transformation of a visual pigment, which results in the sequential formation of distinct photo-intermediate species on the femtosecond to millisecond timescales, whereupon a visual electrical signal is generated and transmitted to the brain. Time-resolved spectroscopic studies of the rod and cone photo-intermediaries enable the detailed understanding of initial events in vision, namely the key differences that underlie the functionally distinct scotopic (rod) and photopic (cone) visual systems. In this paper, we review our recent ultrafast (picoseconds to milliseconds) transient absorption studies of rod and cone visual pigments with a detailed comparison of the transient molecular spectra and kinetics of their respective photo-intermediaries. Key results include the characterization of the porphyropsin (carp fish rhodopsin) and human green-cone opsin photobleaching sequences, which show significant spectral and kinetic differences when compared against that of bovine rhodopsin. These results altogether reveal a rather strong interplay between the visual pigment structure and its corresponding photobleaching sequence, and relevant outstanding questions that will be further investigated through a forthcoming study of the human blue-cone visual pigment are discussed.

Early Proton Transfer Reaction in a Primate Blue-Sensitive Visual Pigment

The proton transfer reaction belongs to one of the key triggers for the functional expression of membrane proteins. Rod and cone opsins are light-sensitive G-protein-coupled receptors (GPCRs) that undergo the cis-trans isomerization of the retinal chromophore in response to light. The isomerization event initiates a conformational change in the opsin protein moiety, which propagates the downstream effector signaling. The final step of receptor activation is the deprotonation of the retinal Schiff base, a proton transfer reaction which has been believed to be identical among the cone opsins. Here, we report an unexpected proton transfer reaction occurring in the early photoreaction process of primate blue-sensitive pigment (MB). By using low-temperature UV-visible spectroscopy, we found that the Lumi intermediate of MB formed in transition from the BL intermediate shows an absorption maximum in the UV region, indicating the deprotonation of the retinal Schiff base. Comparison of the light-induced difference FTIR spectra of Batho, BL, and Lumi showed significant α-helical backbone C=O stretching and protonated carboxylate C=O stretching vibrations only in the Lumi intermediate. The transition from BL to Lumi thus involves dramatic changes in protein environment with a proton transfer reaction between the Schiff base and the counterion resulting in an absorption maximum in the UV region.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Comparison of Bovine and Carp Fish Visual Pigment Photo-Intermediates at Room Temperature

This paper presents room temperature nanoseconds to milliseconds time-resolved spectra and kinetics of the intermediate states and species of bovine and carp fish rhodopsin visual pigments, which also contained ~5% cone pigments. The nanoseconds to milliseconds range cover all the major intermediates in the visual phototransduction process except the formation of bathorhodopsin intermediate which occurs at the femtosecond time scale. The dynamics of these visual pigment intermediates are initiated by excitation with a 532 nm nanosecond laser pulse. The recorded differences between bovine and carp rhodopsin time-resolved spectra of the formation and decay kinetics of their intermediates are presented and discussed. The data show that the carp samples batho intermediate decays faster, nearly by a factor of three, compared to the bovine samples. The formation and decay spectra and kinetics of rhodopsin outer segments and extracted rhodopsin inserted in buffer solution were found to be identical, with very small differences between them in the decay lifetimes of bathorhodopsin and formation of lumirhodopsin.

Light-induced difference FTIR spectroscopy of primate blue-sensitive visual pigment at 163 K

Structural studies of color visual pigments lag far behind those of rhodopsin for scotopic vision. Using difference FTIR spectroscopy at 77 K, we report the first structural data of three primate color visual pig­ments, monkey red (MR), green (MG), and blue (MB), where the batho-intermediate (Batho) exhibits photo­equilibrium with the unphotolyzed state. This photo­chromic property is highly advantageous for limited samples since the signal-to-noise ratio is improved, but may not be applicable to late intermediates, because of large structural changes to proteins. Here we report the photochromic property of MB at 163 K, where the BL intermediate, formed by the relaxation of Batho, is in photoequilibrium with the initial MB state. A comparison of the difference FTIR spectra at 77 and 163 K provided information on what happens in the process of transition from Batho to BL in MB. The coupled C₁₁=C₁₂ HOOP vibration in the planer structure in MB is decoupled by distortion in Batho after retinal photoisomerization, but returns to the coupled C₁₁=C₁₂ HOOP vibration in the all-trans chromophore in BL. The Batho formation accompanies helical structural perturbation, which is relaxed in BL. Protein-bound water molecules that form an extended water cluster near the retinal chromophore change hydrogen bonds differently for Batho and BL, being stronger in the latter than in the initial state. In addition to structural dynamics, the present FTIR spectra show no signals of protonated carboxylic acids at 77 and 163 K, suggesting that E181 is deprotonated in MB, Batho and BL.

Unique Retinal Binding Pocket of Primate Blue-Sensitive Visual Pigment

The visual pigments of humans contain 11-cis retinal as the chromophore of light perception, and its photoisomerization to the all-trans form initiates visual excitation in our eyes. It is well-known that three isomeric states of retinal (11-cis, all-trans, and 9-cis) are in photoequilibrium at very low temperatures such as 77 K. Here we report the lack of formation of the 9-cis form in monkey blue (MB) at 77 K, as revealed by light-induced difference Fourier transform infrared spectroscopy. This indicates that the chromophore binding pocket of MB does not accommodate the 9-cis form, even though it accommodates the all-trans form by twisting the chromophore. Mutation of the blue-specific tyrosine at position 265 to tryptophan, which is highly conserved in other animal rhodopsins, led to formation of the 9-cis form in MB, suggesting that Y265 is one of the determinants of the unique photochemistry in blue pigments. We also found that 9-cis retinal does not bind to MB opsin, implying that the chromophore binding pocket does not accommodate the 9-cis form at physiological temperature. The unique property of MB is discussed on the basis of the results presented here.

Phosphorylation at Serine 21 in G protein-coupled receptor kinase 1 (GRK1) is required for normal kinetics of dark adaption in rod but not cone photoreceptors

Timely recovery of the light response in photoreceptors requires efficient inactivation of photoactivated rhodopsin. This process is initiated by phosphorylation of its carboxyl terminus by G protein-coupled receptor kinase 1 (GRK1). Previously, we showed that GRK1 is phosphorylated in the dark at Ser21 in a cAMP-dependent manner and dephosphorylated in the light. Results in vitro indicate that dephosphorylation of Ser21 increases GRK1 activity, leading to increased phosphorylation of rhodopsin. This creates the possibility of light-dependent regulation of GRK1 activity and its efficiency in inactivating the visual pigment. To address the functional role of GRK1 phosphorylation in rods and cones in vivo, we generated mutant mice in which Ser21 is substituted with alanine (GRK1-S21A), preventing dark-dependent phosphorylation of GRK1. GRK1-S21A mice had normal retinal morphology, without evidence of degeneration. The function of dark-adapted GRK1-S21A rods and cones was also unaffected, as demonstrated by the normal amplitude and kinetics of their responses obtained by ex vivo and in vivo ERG recordings. In contrast, rod dark adaptation following exposure to bright bleaching light was significantly delayed in GRK1-S21A mice, suggesting that the higher activity of this kinase results in enhanced rhodopsin phosphorylation and therefore delays its regeneration. In contrast, dark adaptation of cones was unaffected by the S21A mutation. Taken together, these data suggest that rhodopsin phosphorylation/dephosphorylation modulates the recovery of rhodopsin to the ground state and rod dark adaptation. They also reveal a novel role for cAMP-dependent phosphorylation of GRK1 in regulating the dark adaptation of rod but not cone photoreceptors.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.

Impact of motion-associated noise on intrinsic optical signal imaging in humans with optical coherence tomography

A growing body of evidence suggests that phototransduction can be studied in the human eye in vivo by imaging of fast intrinsic optical signals (IOS). There is consensus concerning the limiting influence of motion-associated imaging noise on the reproducibility of IOS-measurements, especially in those employing spectral-domain optical coherence tomography (SD-OCT). However, no study to date has conducted a comprehensive analysis of this noise in the context of IOS-imaging. In this study, we discuss biophysical correlates of IOS, and we address motion-associated imaging noise by providing correctional post-processing methods. In order to avoid cross-talk of adjacent IOS of opposite signal polarity, cellular resolution and stability of imaging to the level of individual cones is likely needed. The optical Stiles-Crawford effect can be a source of significant IOS-imaging noise if alignment with the peak of the Stiles-Crawford function cannot be maintained. Therefore, complete head stabilization by implementation of a bite-bar may be critical to maintain a constant pupil entry position of the OCT beam. Due to depth-dependent sensitivity fall-off, heartbeat and breathing associated axial movements can cause tissue reflectivity to vary by 29% over time, although known methods can be implemented to null these effects. Substantial variations in reflectivity can be caused by variable illumination due to changes in the beam pupil entry position and angle, which can be reduced by an adaptive algorithm based on slope-fitting of optical attenuation in the choriocapillary lamina.

Binding specificity of retinal analogs to photoactivated visual pigments suggest mechanism for fine-tuning GPCR-ligand interactions

11-cis-retinal acts as an inverse agonist stabilizing the inactive conformation of visual pigments, and upon photoactivation, it isomerizes to all-trans-retinal, initiating signal transduction. We have analyzed opsin regeneration with retinal analogs for rhodopsin and red cone opsin. We find differential binding of the analogs to the receptors after photobleaching and a dependence of the binding kinetics on the oligomerization state of the protein. The results outline the sensitivity of retinal entry to the binding pocket of visual receptors to the specific conformation adopted by the receptor and by the molecular architecture defined by specific amino acids in the binding pocket and the retinal entry site, as well as the topology of the retinal analog. Overall, our findings highlight the specificity of the ligand-opsin interactions, a feature that can be shared by other G-protein-coupled receptors.

Rod visual pigment optimizes active state to achieve efficient G protein activation as compared with cone visual pigments

Most vertebrate retinas contain two types of photoreceptor cells, rods and cones, which show different photoresponses to mediate scotopic and photopic vision, respectively. These cells contain different types of visual pigments, rhodopsin and cone visual pigments, respectively, but little is known about the molecular properties of cone visual pigments under physiological conditions, making it difficult to link the molecular properties of rhodopsin and cone visual pigments with the differences in photoresponse between rods and cones. Here we prepared bovine and mouse rhodopsin (bvRh and mRh) and chicken and mouse green-sensitive cone visual pigments (cG and mG) embedded in nanodiscs and applied time-resolved fluorescence spectroscopy to compare their Gt activation efficiencies. Rhodopsin exhibited greater Gt activation efficiencies than cone visual pigments. Especially, the Gt activation efficiency of mRh was about 2.5-fold greater than that of mG at 37 °C, which is consistent with our previous electrophysiological data of knock-in mice. Although the active state (Meta-II) was in equilibrium with inactive states (Meta-I and Meta-III), quantitative determination of Meta-II in the equilibrium showed that the Gt activation efficiency per Meta-II of bvRh was also greater than those of cG and mG. These results indicated that efficient Gt activation by rhodopsin, resulting from an optimized active state of rhodopsin, is one of the causes of the high amplification efficiency of rods.

Cone visual pigments

Cone visual pigments are visual opsins that are present in vertebrate cone photoreceptor cells and act as photoreceptor molecules responsible for photopic vision. Like the rod visual pigment rhodopsin, which is responsible for scotopic vision, cone visual pigments contain the chromophore 11-cis-retinal, which undergoes cis-trans isomerization resulting in the induction of conformational changes of the protein moiety to form a G protein-activating state. There are multiple types of cone visual pigments with different absorption maxima, which are the molecular basis of color discrimination in animals. Cone visual pigments form a phylogenetic sister group with non-visual opsin groups such as pinopsin, VA opsin, parapinopsin and parietopsin groups. Cone visual pigments diverged into four groups with different absorption maxima, and the rhodopsin group diverged from one of the four groups of cone visual pigments. The photochemical behavior of cone visual pigments is similar to that of pinopsin but considerably different from those of other non-visual opsins. G protein activation efficiency of cone visual pigments is also comparable to that of pinopsin but higher than that of the other non-visual opsins. Recent measurements with sufficient time-resolution demonstrated that G protein activation efficiency of cone visual pigments is lower than that of rhodopsin, which is one of the molecular bases for the lower amplification of cones compared to rods. In this review, the uniqueness of cone visual pigments is shown by comparison of their molecular properties with those of non-visual opsins and rhodopsin. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Efficiencies of activation of transducin by cone and rod visual pigments

How the light-induced transducin (Gt) activation process differs biochemically between cone visual pigments and rod visual pigment (rhodopsin) has remained unclear, because the Gt-activating state (Meta-II) of cone visual pigment decays too fast to precisely measure the activation efficiency by conventional biochemical methods such as the GTPγS binding assay. Here we measured the activation efficiencies of chicken green-sensitive cone visual pigment (cG) and bovine rhodopsin (bRh) in real time by monitoring the intrinsic fluorescence of tryptophan residues in the pigments and Gt. Michaelis-Menten analysis of Gt activation showed that the initial velocity for cG was approximately half that for bRh, while their Michaelis constants were comparable. Gt activation by cG was immediately slowed because of the fast hydrolysis of the retinal Schiff base in Meta-II, but this hydrolysis was suppressed by forming the complex with Gt. Using mutants of cG and bRh for positions 122 and 189, which exhibit altered rates of chromophore hydrolysis in Meta-II, we found that the initial velocity of Gt activation is negatively correlated with the rate of chromophore hydrolysis. These results suggest that the amino acid residues at positions 122 and 189 account for not only the resistance to the chromophore hydrolysis in Meta-II but also the conformation of Meta-II for efficient Gt activation. The substantially longer lifetime of the Gt activating state of Rh would be necessary to suppress the spontaneous quenching by the stochastic decay of the Gt-activating state when a rod responds to a single photon.

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.

Rhodopsin in nanodiscs has native membrane-like photointermediates

Time-dependent studies of membrane protein function are hindered by extensive light scattering that impedes application of fast optical absorbance methods. Detergent solubilization reduces light scattering but strongly perturbs rhodopsin activation kinetics. Nanodiscs may be a better alternative if they can be shown to be free from the serious kinetic perturbations associated with detergent solubilization. To resolve this, we monitored absorbance changes due to photointermediates formed on the microsecond to hundred millisecond time scale after excitation of bovine rhodopsin nanodiscs and compared them to photointermediates that form in hypotonically washed native membranes as well as to those that form in lauryl maltoside suspensions at 15 and 30 °C over a pH range from 6.5 to 8.7. Time-resolved difference spectra were collected from 300 to 700 nm at a series of time delays after photoexcitation and globally fit to a sum of time-decaying exponential terms, and the photointermediates present were determined from the spectral coefficients of the exponential terms. At the temperatures and pHs studied, photointermediates formed after photoexcitation of rhodopsin in nanodiscs are extremely similar to those that form in native membrane, in particular displaying the normal forward shift of the Meta I(480) ⇄ Meta II equilibrium with increased temperature and reduced pH which occurs in native membrane but which is not observed in lauryl maltoside detergent suspensions. These results were obtained using the amount of rhodopsin in nanodiscs which is required for optical experiments with rhodopsin mutants. This work demonstrates that late, physiologically important rhodopsin photointermediates can be characterized in nanodiscs, which provide the superior optical properties of detergent without perturbing the activation sequence.

Adaptation of pineal expressed teleost exo-rod opsin to non-image forming photoreception through enhanced Meta II decay

Photoreception by vertebrates enables both image-forming vision and non-image-forming responses such as circadian photoentrainment. Over the recent years, distinct non-rod non-cone photopigments have been found to support circadian photoreception in diverse species. By allowing specialization to this sensory task a selective advantage is implied, but the nature of that specialization remains elusive. We have used the presence of distinct rod opsin genes specialized to either image-forming (retinal rod opsin) or non-image-forming (pineal exo-rod opsin) photoreception in ray-finned fish (Actinopterygii) to gain a unique insight into this problem. A comparison of biochemical features for these paralogous opsins in two model teleosts, Fugu pufferfish (Takifugu rubripes) and zebrafish (Danio rerio), reveals striking differences. While spectral sensitivity is largely unaltered by specialization to the pineal environment, in other aspects exo-rod opsins exhibit a behavior that is quite distinct from the cardinal features of the rod opsin family. While they display a similar thermal stability, they show a greater than tenfold reduction in the lifetime of the signaling active Meta II photoproduct. We show that these features reflect structural changes in retinal association domains of helices 3 and 5 but, interestingly, not at either of the two residues known to define these characteristics in cone opsins. Our findings suggest that the requirements of non-image-forming photoreception have lead exo-rod opsin to adopt a characteristic that seemingly favors efficient bleach recovery but not at the expense of absolute sensitivity.

Cone arrestin confers cone vision of high temporal resolution in zebrafish larvae

Vision of high temporal resolution depends on careful regulation of photoresponse kinetics, beginning with the lifetime of activated photopigment. The activity of rhodopsin is quenched by high-affinity binding of arrestin to photoexcited phosphorylated photopigment, which effectively terminates the visual transduction cascade. This regulation mechanism is well established for rod photoreceptors, yet its role for cone vision is still controversial. In this study we therefore analyzed arrestin function in the cone-dominated vision of larval zebrafish. For both rod (arrS ) and cone (arr3 ) arrestin we isolated two paralogs, each expressed in the respective subset of photoreceptors. Labeling with paralog-specific antibodies revealed subfunctionalized expression of Arr3a in M- and L-cones, and Arr3b in S- and UV-cones. The inactivation of arr3a by morpholino knockdown technology resulted in a severe delay in photoresponse recovery which, under bright light conditions, was rate-limiting. Comparison to opsin phosphorylation-deficient animals confirmed the role of cone arrestin in late cone response recovery. Arr3a activity partially overlapped with the function of the cone-specific kinase Grk7a involved in initial response recovery. Behavioral measurements further revealed Arr3a deficiency to be sufficient to reduce temporal contrast sensitivity, providing evidence for the importance of arrestin in cone vision of high temporal resolution.

Evolution of opsins and phototransduction

Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom. They can change their conformation from a resting state to a signalling state upon light absorption, which activates the G protein, thereby resulting in a signalling cascade that produces physiological responses. This process of capturing a photon and transforming it into a physiological response is known as phototransduction. Recent cloning techniques have revealed the rich and diverse nature of these molecules, found in organisms ranging from jellyfish to humans, functioning in visual and non-visual phototransduction systems and photoisomerases. Here we describe the diversity of these proteins and their role in phototransduction. Then we explore the molecular properties of opsins, by analysing site-directed mutants, strategically designed by phylogenetic comparison. This site-directed mutant approach led us to identify many key features in the evolution of the photoreceptor molecules. In particular, we will discuss the evolution of the counterion, the reduction of agonist binding to the receptor, and the molecular properties that characterize rod opsins apart from cone opsins. We will show how the advances in molecular biology and biophysics have given us insights into how evolution works at the molecular level.

Signaling properties of a short-wave cone visual pigment and its role in phototransduction

Although visual pigments play key structural and functional roles in photoreceptors, the relationship between the properties of mammalian cone pigments and those of mammalian cones is not well understood. We generated transgenic mice with rods expressing mouse short-wave cone opsin (S-opsin) to test whether cone pigment can substitute for the structural and functional roles of rhodopsin and to investigate how the biophysical and signaling properties of the short-wave cone pigment (S-pigment) contribute to the specialized function of cones. The transgenic S-opsin was targeted to rod outer segments, and formed a pigment with peak absorption at 360 nm. Expression of S-opsin in rods lacking rhodopsin (rho-/-) promoted outer segment growth and cell survival and restored their ability to respond to light while shifting their action spectrum to 355 nm. Using the spectral separation between S-pigment and rhodopsin, we found that the two pigments produced similar photoresponses. Dark noise did not increase in transgenic rods, indicating that thermal activation of S-pigment might not contribute to the low sensitivity of mouse S-cones. Using rod arrestin knock-out animals (arr1-/-), we found that the physiologically active (meta II) state of S-pigment decays 40 times faster than that of rhodopsin. Interestingly, rod arrestin was efficient in deactivating S-pigment in rods, but its deletion did not have any obvious effect on dim-flash response shutoff in cones. Furthermore, transgenic cone arrestin was not able to rescue the slow shutoff of S-pigment dim-flash response in arr1-/- rods. Thus, the connection between rod/cone arrestins and S-pigment shutoff remains unclear.

Physiological properties of rod photoreceptor cells in green-sensitive cone pigment knock-in mice

Rod and cone photoreceptor cells that are responsible for scotopic and photopic vision, respectively, exhibit photoresponses different from each other and contain similar phototransduction proteins with distinctive molecular properties. To investigate the contribution of the different molecular properties of visual pigments to the responses of the photoreceptor cells, we have generated knock-in mice in which rod visual pigment (rhodopsin) was replaced with mouse green-sensitive cone visual pigment (mouse green). The mouse green was successfully transported to the rod outer segments, though the expression of mouse green in homozygous retina was approximately 11% of rhodopsin in wild-type retina. Single-cell recordings of wild-type and homozygous rods suggested that the flash sensitivity and the single-photon responses from mouse green were three to fourfold lower than those from rhodopsin after correction for the differences in cell volume and levels of several signal transduction proteins. Subsequent measurements using heterozygous rods expressing both mouse green and rhodopsin E122Q mutant, where these pigments in the same rod cells can be selectively irradiated due to their distinctive absorption maxima, clearly showed that the photoresponse of mouse green was threefold lower than that of rhodopsin. Noise analysis indicated that the rate of thermal activations of mouse green was 1.7 x 10(-7) s(-1), about 860-fold higher than that of rhodopsin. The increase in thermal activation of mouse green relative to that of rhodopsin results in only 4% reduction of rod photosensitivity for bright lights, but would instead be expected to severely affect the visual threshold under dim-light conditions. Therefore, the abilities of rhodopsin to generate a large single photon response and to retain high thermal stability in darkness are factors that have been necessary for the evolution of scotopic vision.

Phototransduction in mouse rods and cones

Phototransduction is the process by which light triggers an electrical signal in a photoreceptor cell. Image-forming vision in vertebrates is mediated by two types of photoreceptors: the rods and the cones. In this review, we provide a summary of the success in which the mouse has served as a vertebrate model for studying rod phototransduction, with respect to both the activation and termination steps. Cones are still not as well-understood as rods partly because it is difficult to work with mouse cones due to their scarcity and fragility. The situation may change, however.

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.

The vertebrate phototransduction cascade: amplification and termination mechanisms

The biochemical cascade which transduces light into a neuronal signal in retinal photoreceptors is a heterotrimeric GTP-binding protein (G protein) signaling pathway called phototransduction. Works from psychophysicists, electrophysiologists, biochemists, and geneticists over several decades have come together to shape our understanding of how photon absorption leads to photoreceptor membrane hyperpolarization. The insights of phototransduction provide the foundation for a mechanistic account of signaling from many other G protein-coupled receptors (GPCR) found throughout nature. The application of reverse genetic techniques has strengthened many historic findings and helped to describe this pathway at greater molecular details. However, many important questions remain to be answered.

Late stages of visual pigment photolysis in situ: cones vs. rods

Slow photolysis reactions and the regeneration of the dark pigment constitute the mechanisms of dark adaptation whereby photoreceptor cells restore their sensitivity after bright illumination. We present data on the kinetics of the late stages of the photolysis of the visual pigment in intact rods and red- and green-sensitive cones of the goldfish retina. Measurements were made on single photoreceptors by means of a fast-scanning dichroic microspectrophotometer. We show that in cones the hydrolysis of the opsin-all-trans 3-dehydroretinal linkage proceeds with a half-time of approximately 5s at 20 degrees C that is almost two orders of magnitude faster than in rods. 3-Dehydroretinol in cones is produced approximately 3-fold faster than retinol in amphibian rhodopsin rods; the rate of the reaction is limited by the speed of retinal reduction catalyzed by retinoldehydrogenase. The fast hydrolysis of the 3-dehydroretinal/opsin Schiff base and the correspondingly fast appearance of the substrates for dark visual pigment regeneration (free opsin and 3-dehydroretinol) provide essential conditions for faster dark adaptation of cone (diurnal) as compared to rod (nocturnal) vision.

Crystal structure of cone arrestin at 2.3A: evolution of receptor specificity

Arrestins play a fundamental role in the regulation and signal transduction of G protein-coupled receptors. Here we describe the crystal structure of cone arrestin at 2.3A resolution. The overall structure of cone visual arrestin is similar to the crystal structures of rod visual and the non-visual arrestin-2, consisting of two domains, each containing ten beta-sheets. However, at the tertiary structure level, there are two major differences, in particular on the concave surfaces of the two domains implicated in receptor binding and in the loop between beta-strands I and II. Functional analysis shows that cone arrestin, in sharp contrast to its rod counterpart, bound cone pigments and non-visual receptors. Conversely, non-visual arrestin-2 bound cone pigments, suggesting that it may also regulate phototransduction and/or photopigment trafficking in cone photoreceptors. These findings indicate that cone arrestin displays structural and functional features intermediate between the specialized rod arrestin and the non-visual arrestins, which have broad receptor specificity. A unique functional feature of cone arrestin was the low affinity for its cognate receptor, resulting in an unusually rapid dissociation of the complex. Transient arrestin binding to the photopigment in cones may be responsible for the extremely rapid regeneration and reuse of the photopigment that is essential for cone function at high levels of illumination.

Knockdown of cone-specific kinase GRK7 in larval zebrafish leads to impaired cone response recovery and delayed dark adaptation

Phosphorylation of rhodopsin by rhodopsin kinase GRK1 is an important desensitization mechanism in scotopic vision. For cone vision GRK1 is not essential. However, cone opsin is phosphorylated following light stimulation. In cone-dominant animals as well as in humans, but not in rodents, GRK7, a cone-specific homolog of GRK1, has been identified in cone outer segments. To investigate the function of GRK7 in vivo, we cloned two orthologs of grk7 in zebrafish and knocked down gene expression of grk7a in zebrafish larvae by morpholino antisense nucleotides. Photoresponse recovery in Grk7a-deficient larvae was delayed in electroretinographic measurements, and temporal contrast sensitivity was reduced, particularly under bright-light conditions. These results show that function of a cone-specific kinase is essential for cone vision in the zebrafish retina and argue that pigment bleaching and spontaneous decay alone are not sufficient for light adaptation and rapid cone response inactivation.

Breaking the covalent bond--a pigment property that contributes to desensitization in cones

Retinal rod and cone pigments consist of an apoprotein, opsin, covalently linked to a chromophore, 11-cis retinal. Here we demonstrate that the formation of the covalent bond between opsin and 11-cis retinal is reversible in darkness in amphibian red cones, but essentially irreversible in red rods. This dissociation, apparently a general property of cone pigments, results in a surprisingly large amount of free opsin--about 10% of total opsin--in dark-adapted red cones. We attribute this significant level of free opsin to the low concentration of intracellular free 11-cis retinal, estimated to be only a tiny fraction (approximately 0.1 %) of the pigment content in red cones. With its constitutive transducin-stimulating activity, the free cone opsin produces an approximately 2-fold desensitization in red cones, equivalent to that produced by a steady light causing 500 photoisomerizations s-1. Cone pigment dissociation therefore contributes to the sensitivity difference between rods and cones.

Molecular properties of rod and cone visual pigments from purified chicken cone pigments to mouse rhodopsin in situ

We have investigated the molecular properties of rod and cone visual pigments to elucidate the differences in the molecular mechanism(s) of the photoresponses between rod and cone photoreceptor cells. We have found that the cone pigments exhibit a faster pigment regeneration and faster decay of meta-II and meta-III intermediates than the rod pigment, rhodopsin. Mutagenesis experiments have revealed that the amino acid residues at positions 122 and 189 in the opsins are the determinants for these differences. In order to study the relationship between the molecular properties of visual pigments and the physiology of rod photoreceptors, we used mouse rhodopsin as a model pigment because, by gene-targeting, the spectral properties of the pigment can be directly correlated to the physiology of the cells. In the present paper, we summarize the spectroscopic properties of cone pigments and describe our studies with mouse rhodopsin utilizing a high performance charge coupled device (CCD) spectrophotometer.

M opsin phosphorylation in intact mammalian retinas

The deactivation of visual pigments involved in phototransduction is critical for recovering sensitivity after exposure to light in rods and cones of the vertebrate retina. In rods, phosphorylation of rhodopsin by rhodopsin kinase (GRK1) and the subsequent binding of visual arrestin completely terminates phototransduction. Although signal termination in cones is predicted to occur via a similar mechanism as in rods, there may be differences due to the expression of related but distinct gene products. While rods only express GRK1, cones in some species express only GRK1 or GRK7 and others express both GRKs. In the mouse, cone opsin is phosphorylated by GRK1, but this has not been demonstrated in mammals that express GRK7 in cones. We compared cone opsin phosphorylation in intact retinas from the 13-lined ground squirrel (GS) and pig, cone- and rod-dominant mammals, respectively, which both express GRK7. M opsin phosphorylation increased during continuous exposure to light, then declined between 3 and 6 min. In contrast, rhodopsin phosphorylation continued to increase during this time period. In GS retina homogenates, anti-GS GRK7 antibody blocked M opsin phosphorylation by 73%. In pig retina homogenates, only 20% inhibition was observed, possibly due to phosphorylation by GRK1 released from rods during homogenization. Our results suggest that GRK7 phosphorylates M opsin in both of these mammals. Using an in vitro GTPgammaS binding assay, we also found that the ability of recombinant M opsin to activate G(t) was greatly reduced by phosphorylation. Therefore, phosphorylation may participate directly in the termination of phototransduction in cones by decreasing the activity of M opsin.

Diversity of visual pigments from the viewpoint of G protein activation--comparison with other G protein-coupled receptors

The visual pigment present in the photoreceptor cells of the retina is a member of the family of G protein-coupled receptors and contains an 11-cis-retinal as a light-absorbing chromophore. Light induces conformational changes in the protein moiety of the visual pigment through cis-trans isomerization of the chromophore, which leads to the activation of a G protein-mediated signal transduction cascade that eventually generates an electrical response of the photoreceptor cells. So far, various types of visual pigments have been identified from a variety of photoreceptor cells and the structure-function relationship of visual pigments has been widely investigated by means of biophysical, biochemical and molecular biological techniques. Recent identifications of visual pigment-like proteins in the extra-ocular cells emphasize the importance of the visual pigment family as the photoreceptive molecules in not only visual but also non-visual photoreception. This article reviews the functional diversity of visual pigments from the viewpoint of the molecular mechanisms of photoreception and G protein activation. In addition, the similarity and difference of G protein activation mechanism between visual pigment and other G protein-coupled receptors are discussed for furthering our understanding of the common mechanism of G protein activation by G protein-coupled receptors.

Visual pigment phosphorylation but not transducin translocation can contribute to light adaptation in zebrafish cones

The ability of cone photoreceptors to adapt to light is extraordinary. In this study we evaluated two biochemical processes, visual pigment phosphorylation and transducin translocation, for their ability to contribute to light adaptation in zebrafish cones. Since cytoplasmic Ca2+ regulates light adaptation, the sensitivities of these processes to both light and Ca2+ were examined. Cytoplasmic Ca2+ regulates the sites of light-stimulated phosphorylation. Unexpectedly, we found that Ca2+ also regulates the extent of phosphorylation of unbleached cone pigments. Immunocytochemical analyses revealed that neither light nor cytoplasmic Ca2+ influences the localization of transducin in zebrafish cones.

Bistable UV pigment in the lamprey pineal

Lower vertebrates can detect UV light with the pineal complex independently of eyes. Electrophysiological studies, together with chromophore extraction analysis, have suggested that the underlying pigment in the lamprey pineal exhibits a bistable nature, that is, reversible photoreaction by UV and visible light, which is never achieved by known UV pigments. Here we addressed the molecular identification of the pineal UV receptor. Our results showed that the long-hypothesized pigment is a lamprey homologue of parapinopsin, which exhibits an absorption maximum at 370 nm, in the UV region. UV light causes cis-trans isomerization of its retinal(2) chromophore, forming a stable photoproduct having an absorption maximum at 515 nm, in the green region. The photoproduct reverts to the original pigment upon visible light absorption, showing photoregeneration of the pigment. In situ hybridization showed that parapinopsin is selectively expressed in the cells located in the dorsal region of the pineal organ. We successfully obtained the hyperpolarizing responses with a maximum sensitivity of approximately 380 nm from the photoreceptor cells at the dorsal region, in which the outer segment was clearly stained with anti-parapinopsin antibody. These results demonstrated that parapinopsin is the pineal UV pigment having photointerconvertible two stable states. The bistable nature of the parapinopsin can account for the photorecovery of the pineal UV sensitivity by background green light in the lamprey. Furthermore, we isolated the parapinopsin homologues from fish and frog pineal complexes that exhibit UV sensitivity, suggesting that parapinopsin is a common molecular basis for pineal UV reception in the vertebrate.

Vertebrate ultraviolet visual pigments: protonation of the retinylidene Schiff base and a counterion switch during photoactivation

For visual pigments, a covalent bond between the ligand (11-cis-retinal) and receptor (opsin) is crucial to spectral tuning and photoactivation. All photoreceptors have retinal bound via a Schiff base (SB) linkage, but only UV-sensitive cone pigments have this moiety unprotonated in the dark. We investigated the dynamics of mouse UV (MUV) photoactivation, focusing on SB protonation and the functional role of a highly conserved acidic residue (E108) in the third transmembrane helix. On illumination, wild-type MUV undergoes a series of conformational changes, batho --> lumi --> meta I, finally forming the active intermediate meta II. During the dark reactions, the SB becomes protonated transiently. In contrast, the MUV-E108Q mutant formed significantly less batho that did not decay through a protonated lumi. Rather, a transition to meta I occurred above approximately 240 K, with a remarkable red shift (lambda(max) approximately 520 nm) accompanying SB protonation. The MUV-E108Q meta I --> meta II transition appeared normal but the MUV-E108Q meta II decay to opsin and free retinal was dramatically delayed, resulting in increased transducin activation. These results suggest that there are two proton donors during the activation of UV pigments, the primary counterion E108 necessary for protonation of the SB during lumi formation and a second one necessary for protonation of meta I. Inactivation of meta II in SWS1 cone pigments is regulated by the primary counterion. Computational studies suggest that UV pigments adopt a switch to a more distant counterion, E176, during the lumi to meta I transition. The findings with MUV are in close analogy to rhodopsin and provides further support for the importance of the counterion switch in the photoactivation of both rod and cone visual pigments.

Role of visual pigment properties in rod and cone phototransduction

Retinal rods and cones share a phototransduction pathway involving cyclic GMP. Cones are typically 100 times less photosensitive than rods and their response kinetics are several times faster, but the underlying mechanisms remain largely unknown. Almost all proteins involved in phototransduction have distinct rod and cone variants. Differences in properties between rod and cone pigments have been described, such as a 10-fold shorter lifetime of the meta-II state (active conformation) of cone pigment and its higher rate of spontaneous isomerization, but their contributions to the functional differences between rods and cones remain speculative. We have addressed this question by expressing human or salamander red cone pigment in Xenopus rods, and human rod pigment in Xenopus cones. Here we show that rod and cone pigments when present in the same cell produce light responses with identical amplification and kinetics, thereby ruling out any difference in their signalling properties. However, red cone pigment isomerizes spontaneously 10,000 times more frequently than rod pigment. This high spontaneous activity adapts the native cones even in darkness, making them less sensitive and kinetically faster than rods. Nevertheless, additional factors are probably involved in these differences.

Distribution of rod- and cone-specific phosducins in retinas of non-mammalian vertebrates

In mammalian retinas, it has been believed that just one kind of phosducin (PD) commonly exists in both rods and cones. However, we have previously reported that there are rod- and cone-specific PDs (OlPD-R and OlPD-C) in medaka (Oryzias latipes) retina [FEBS Lett., 502, 117-121, 2001]. To clarify the distribution and evolution of these photoreceptor type-specific PDs, we investigated PDs of another teleost and a reptile. Immunohistochemical and Western blot analyses using anti-medaka PD antisera demonstrated that two kinds of PDs are expressed in zebrafish (Danio rerio) photoreceptor cells. Our study is suggestive that teleosts generally possess rod- and cone-specific PDs. We isolated a cDNA encoding putative PD (PmlPD) of a diurnal gecko (Phelsuma madagascariensis longinsulae). Because diurnal gecko possesses a pure-cone retina, it was expected that PmlPD would be expressed in cones. Molecular phylogenetic analysis demonstrated that PmlPD was more closely related to mammalian PDs than teleost cone-specific PDs, suggesting that the rod- and cone-specific subtype of teleost PDs have arisen after the teleost-tetrapod divergence.

Species-specific differences in expression of G-protein-coupled receptor kinase (GRK) 7 and GRK1 in mammalian cone photoreceptor cells: implications for cone cell phototransduction

Desensitization plays an important role in the rapid termination of G-protein signaling pathways. This process, which involves phosphorylation by a G-protein-coupled receptor kinase (GRK) followed by arrestin binding, has been studied extensively in the rod photoreceptor cell of the mammalian retina. In contrast, less is known regarding desensitization in cone photoreceptor cells, which occurs more rapidly than in rod cells. Recently, our laboratory has cloned a novel GRK family member, GRK7, from the retina of a cone-dominant mammal, the 13-lined ground squirrel. Here we report the cloning of GRK7 from rod-dominant pig and human retinas, suggesting that this kinase plays a role in human visual signaling. Because GRK1 (rhodopsin kinase), the GRK that mediates rhodopsin desensitization in the rod cell, is reportedly expressed in both rods and cones, a detailed comparison of the localization of the two kinases is a necessary step toward determining their potential roles in cone visual signaling. Immunocytochemical analysis using antibodies selective for these two GRKs unexpectedly demonstrated species-specific differences in GRK7 and GRK1 expression in cones. In pigs and dogs, cones express only GRK7, whereas in mice and rats, we detected only GRK1 in cones. These results suggest that either GRK7 or GRK1 may participate in cone opsin desensitization, depending on the expression pattern of the kinases in different species. In contrast, GRK7 and GRK1 are coexpressed in monkey and human cones, suggesting that coordinate regulation of desensitization by both kinases may occur in primates.

Identification of rod- and cone-specific phosducins in teleost retinas

Phosducin (PD) is a regulatory protein of vertebrate phototransduction cascades. In mammalian retina, it has been thought that only one kind of PD commonly exists in both rods and cones. However, we have found two kinds of PD (OIPD-R and OIPD-C) in the retina of a teleost, medaka (Oryzias latipes). In situ hybridization and immunohistochemical analysis demonstrated that OIPD-R and -C are selectively expressed in rods and cones, respectively. The antiserum against medaka PDs recognized two kinds of proteins in bluegill (Lepomis macrochirus) retina. These results suggest that rod- and cone-specific PDs exist in teleost retinas, probably creating differences in light adaptation between rods and cones.

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.

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.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Amino acid residues controlling the properties and functions of rod and cone visual pigments

The visual transduction processes in rod and cone photoreceptor cells are initiated by photon absorption by the different types of visual pigments. In relation to the functional difference between these cells, cone visual pigments in chicken retinas exhibit faster regeneration from 11-cis-retinal and opsin and faster decay of physiologically active intermediate (Meta II) than rod visual pigment, rhodopsin. Replacement of the amino acid residue at position 122 of chicken rhodopsin by the residues present in the respective cone pigments dramatically changes both the decay rate of Meta II and the rate of regeneration into those of the cone pigment-type, indicating that the residue at this position is a major determinant controlling these properties. Thus, the single replacement of amino acid residue at this position would be one of the key steps of the divergence into twilight and daylight vision.

Identification of a new intermediate state that binds but not activates transducin in the bleaching process of bovine rhodopsin

Using time-resolved low-temperature spectroscopy, we have examined whether or not bovine rhodopsin has a unique transducin-binding state, meta Ib, previously detected from chicken rhodopsin. Unlike chicken meta Ib, bovine meta Ib was detected only by detailed kinetics analysis of the bleaching process, but it was stabilized by transducin and visualized in the observed spectral changes. From the effect of GTPgammaS, it was revealed that meta Ib induced no GDP-GTP exchange reaction in transducin. Thus meta Ib is a common intermediate of vertebrate rhodopsin and transducin is activated in two steps by meta Ib and meta II.