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

The evolutionary history and spectral tuning of vertebrate visual opsins

Many animals depend on the sense of vision for survival. In eumetazoans, vision requires specialized, light-sensitive cells called photoreceptors. Light reaches the photoreceptors and triggers the excitation of light-detecting proteins called opsins. Here, we describe the story of visual opsin evolution from the ancestral bilaterian to the extant vertebrate lineages. We explain the mechanisms determining color vision of extant vertebrates, focusing on opsin gene losses, duplications, and the expression regulation of vertebrate opsins. We describe the sequence variation both within and between species that has tweaked the sensitivities of opsin proteins towards different wavelengths of light. We provide an extensive resource of wavelength sensitivities and mutations that have diverged light sensitivity in many vertebrate species and predict how these mutations were accumulated in each lineage based on parsimony. We suggest possible natural and sexual selection mechanisms underlying these spectral differences. Understanding how molecular changes allow for functional adaptation of animals to different environments is a major goal in the field, and therefore identifying mutations affecting vision and their relationship to photic selection pressures is imperative. The goal of this review is to provide a comprehensive overview of our current understanding of opsin evolution in vertebrates.

Ancient whale rhodopsin reconstructs dim-light vision over a major evolutionary transition: Implications for ancestral diving behavior

Cetaceans are fully aquatic mammals that descended from terrestrial ancestors, an iconic evolutionary transition characterized by adaptations for underwater foraging via breath-hold diving. Although the evolutionary history of this specialized behavior is challenging to reconstruct, coevolving sensory systems may offer valuable clues. The dim-light visual pigment, rhodopsin, which initiates phototransduction in the rod photoreceptors of the eye, has provided insight into the visual ecology of depth in several aquatic vertebrate lineages. Here, we use ancestral sequence reconstruction and protein resurrection experiments to quantify light-activation metrics in rhodopsin pigments from ancestors bracketing the cetacean terrestrial-to-aquatic transition. By comparing multiple reconstruction methods on a broadly sampled cetartiodactyl species tree, we generated highly robust ancestral sequence estimates. Our experimental results provide direct support for a blue-shift in spectral sensitivity along the branch separating cetaceans from terrestrial relatives. This blue-shift was 14 nm, resulting in a deep-sea signature (λ_max = 486 nm) similar to many mesopelagic-dwelling fish. We also discovered that the decay rates of light-activated rhodopsin increased in ancestral cetaceans, which may indicate an accelerated dark adaptation response typical of deeper-diving mammals. Because slow decay rates are thought to help sequester cytotoxic photoproducts, this surprising result could reflect an ecological trade-off between rod photoprotection and dark adaptation. Taken together, these ancestral shifts in rhodopsin function suggest that some of the first fully aquatic cetaceans could dive into the mesopelagic zone (>200 m). Moreover, our reconstructions indicate that this behavior arose before the divergence of toothed and baleen whales.

Visual cells and visual pigments of the river lamprey revisited

Retinas of the river lamprey Lampetra fluviatilis were studied by microspectrophotometry, electroretinography and single-photoreceptor electrophysiology to reconcile the apparently contradictory conclusions on the nature of lamprey photoreceptor cells drawn in the early work by Govardovskii and Lychakov (J Comp Physiology A 154:279-286, 1984) and in recent studies. In agreement with recent works, we confirmed former identification of short photoreceptors as rods and of long photoreceptors as cones. In line with the results of 1984, we show that within a certain range of light intensities the lamprey retina exhibits "color discrimination". We found that the overlap of working intensity ranges of rods and cones is not a unique feature of lamprey short receptors, and suggest that rod-cone (possibly color) vision may be common among vertebrates. We show that the decay of meta-intermediates in lamprey cones occurs almost 100 times faster than in typical rod metarhodopsins. Rate of decay of metarhodopsins of lamprey rods take an intermediate position between typical rods and cones. This makes lamprey rhodopsin similar to transmuted cone visual pigment in "rods" of nocturnal geckos. We argue that defining various types of photoreceptors as simply "rods" and "cones" may be functionally correct, but neglects their genetic, biochemical and morphological features and evolutionary history.

Functional trade-offs and environmental variation shaped ancient trajectories in the evolution of dim-light vision

Trade-offs between protein stability and activity can restrict access to evolutionary trajectories, but widespread epistasis may facilitate indirect routes to adaptation. This may be enhanced by natural environmental variation, but in multicellular organisms this process is poorly understood. We investigated a paradoxical trajectory taken during the evolution of tetrapod dim-light vision, where in the rod visual pigment rhodopsin, E122 was fixed 350 million years ago, a residue associated with increased active-state (MII) stability but greatly diminished rod photosensitivity. Here, we demonstrate that high MII stability could have likely evolved without E122, but instead, selection appears to have entrenched E122 in tetrapods via epistatic interactions with nearby coevolving sites. In fishes by contrast, selection may have exploited these epistatic effects to explore alternative trajectories, but via indirect routes with low MII stability. Our results suggest that within tetrapods, E122 and high MII stability cannot be sacrificed-not even for improvements to rod photosensitivity.

Evolution of nonspectral rhodopsin function at high altitudes

High-altitude environments present a range of biochemical and physiological challenges for organisms through decreases in oxygen, pressure, and temperature relative to lowland habitats. Protein-level adaptations to hypoxic high-altitude conditions have been identified in multiple terrestrial endotherms; however, comparable adaptations in aquatic ectotherms, such as fishes, have not been as extensively characterized. In enzyme proteins, cold adaptation is attained through functional trade-offs between stability and activity, often mediated by substitutions outside the active site. Little is known whether signaling proteins [e.g., G protein-coupled receptors (GPCRs)] exhibit natural variation in response to cold temperatures. Rhodopsin (RH1), the temperature-sensitive visual pigment mediating dim-light vision, offers an opportunity to enhance our understanding of thermal adaptation in a model GPCR. Here, we investigate the evolution of rhodopsin function in an Andean mountain catfish system spanning a range of elevations. Using molecular evolutionary analyses and site-directed mutagenesis experiments, we provide evidence for cold adaptation in RH1. We find that unique amino acid substitutions occur at sites under positive selection in high-altitude catfishes, located at opposite ends of the RH1 intramolecular hydrogen-bonding network. Natural high-altitude variants introduced into these sites via mutagenesis have limited effects on spectral tuning, yet decrease the stability of dark-state and light-activated rhodopsin, accelerating the decay of ligand-bound forms. As found in cold-adapted enzymes, this phenotype likely compensates for a cold-induced decrease in kinetic rates-properties of rhodopsin that mediate rod sensitivity and visual performance. Our results support a role for natural variation in enhancing the performance of GPCRs in response to cold temperatures.

Cone-like rhodopsin expressed in the all cone retina of the colubrid pine snake as a potential adaptation to diurnality

Colubridae is the largest and most diverse family of snakes, with visual systems that reflect this diversity, encompassing a variety of retinal photoreceptor organizations. The transmutation theory proposed by Walls postulates that photoreceptors could evolutionarily transition between cell types in squamates, but few studies have tested this theory. Recently, evidence for transmutation and rod-like machinery in an all cone retina has been identified in a diurnal garter snake (Thamnophis), and it appears that the rhodopsin gene at least may be widespread among colubrid snakes. However, functional evidence supporting transmutation beyond the existence of the rhodopsin gene remains rare. We examined the all cone retina of another colubrid, Pituophis melanoleucus, thought to be more secretive/burrowing than Thamnophis. We found that P. melanoleucus expresses two cone opsins (SWS1, LWS) and rhodopsin (RH1) within the eye. Immunohistochemistry localized rhodopsin to the outer segment of photoreceptors in the all-cone retina of the snake and all opsin genes produced functional visual pigments when expressed in vitro. Consistent with other studies, we found that P. melanoleucus rhodopsin is extremely blue-shifted. Surprisingly, P. melanoleucus rhodopsin reacted with hydroxylamine, a typical cone opsin characteristic. These results support the idea that the rhodopsin-containing photoreceptors of P. melanoleucus are the products of evolutionary transmutation from rod ancestors, and suggests that this phenomenon may be widespread in colubrid snakes. We hypothesize that transmutation may be an adaptation for diurnal, brighter-light vision, which could result in increased spectral sensitivity and chromatic discrimination with the potential for colour vision.

The role of the non-covalent β-ionone-ring binding site in rhodopsin: historical and physiological perspective

Bleached rhodopsin regenerates by way of the Schiff base formation between the 11-cis retinal and opsin. Recovery of human vision from light adapted states follows biphasic kinetics and each adaptive phase is assigned to two distinct classes of visual pigments in cones and rods, respectively, suggesting that the speed of Schiff base formation differs between iodopsin and rhodopsin. Matsumoto and Yoshizawa predicted the existence of a β-ionone ring-binding site in rhodopsin, which has been proven by structural studies. They postulated that rhodopsin regeneration starts with a non-covalent binding of the β-ionone ring moiety of 11-cis-retinal, followed by the Schiff base formation. Recent physiological investigation revealed that non-covalent occupation of the β-ionone ring binding site transiently activates the visual transduction cascade in the dark. In order to understand the role of non-covalent binding of 11-cis-retinal to opsin during regeneration, we studied the kinetics of rhodopsin regeneration from opsin and 11-cis-retinal and found that the Schiff base formation is accelerated ∼10(7) times compared to that between retinal and free amine. According to Cordes and Jencks, Schiff base formation in solution exhibits a bell-shaped pH dependence. However, we discovered that the rhodopsin formation is independent of pH over a wide pH range, suggesting that aqueous solvents do not have access to the Schiff base milieu during its formation. According to Hecht et al. the regeneration of iodopsin must be significantly faster than that of rhodopsin. Does this suggest that the Schiff base formation in iodopsin is favored due to its structural architecture? The iodopsin structure once solved would answer such a question as how molecular fine-tuning of retinal proteins realizes their dark adaptive functions. In contrast, bacteriorhodopsin does not require occupancy of a distinct β-ionone ring-binding site, enabling an aldehyde without the cyclohexene ring to form a pigment. Studies of regeneration reaction of other retinal proteins, which are scarcely available, would clarify the molecular structure-phenotype relationships and their physiological roles.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

Kinetics of thermal activation of an ultraviolet cone pigment

Visual pigments can be thermally activated via isomerization of the retinyl chromophore and hydrolysis of the Schiff base (SB) through which the retinyl chromophore is bound to the opsin protein. Here, we present the first combined experimental and theoretical study of the thermal activation of a Siberian hamster ultraviolet (SHUV) pigment. We measured the rates of thermal isomerization and hydrolysis in the SHUV pigment and bovine rhodopsin. We found that these rates were significantly faster in the UV pigment than in rhodopsin due to the difference in the structural and electrostatic effects surrounding the unprotonated Schiff base (USB) retinyl chromophore in the UV pigment. Theoretical (DFT-QM/MM) calculations of the cis-trans thermal isomerization revealed a barrier of ∼23 kcal/mol for the USB retinyl chromophore in SHUV compared to ∼40 kcal/mol for protonated Schiff base (PSB) chromophore in rhodopsin. The lower barrier for thermal isomerization in the SHUV pigment is attributed to the (i) lessening of the steric restraints near the β-ionone ring and SB ends of the chromophore, (ii) displacement of the transmembrane helix 6 (TM6) away from the binding pocket toward TM5 due to absence of the salt bridge between the USB and the protonated E113 residue, and (iii) change in orientation of the hydrogen-bonding networks (HBNs) in the extracellular loop 2 (EII). The results in comparing thermal stability of UV cone pigment and rhodopsin provide insight into molecular evolution of vertebrate visual pigments in achieving low discrete dark noise and high photosensitivity in rod pigments for dim-light vision.

Different phosphorylation rates among vertebrate cone visual pigments with different spectral sensitivities

Cone photoreceptor subtypes having different spectral sensitivities exhibit different recovery kinetics in their photoresponses in some vertebrates. Phosphorylation by G protein-coupled receptor kinase (GRK) is essential for the rapid inactivation of light-activated visual pigment, which is the rate-limiting step of the cone photoresponse recovery in salamander. In this study we compared the rate of light-dependent phosphorylation by GRK7 of carp green- and blue-sensitive cone visual pigments. Blue pigment was phosphorylated significantly less effectively than green pigment, suggesting that the difference in the pigment phosphorylation rate is responsible for the difference in photoresponse kinetics among cone photoreceptor subtypes.

Rapid release of retinal from a cone visual pigment following photoactivation

As part of the visual cycle, the retinal chromophore in both rod and cone visual pigments undergoes reversible Schiff base hydrolysis and dissociation following photobleaching. We characterized light-activated release of retinal from a short-wavelength-sensitive cone pigment (VCOP) in 0.1% dodecyl maltoside using fluorescence spectroscopy. The half-time (t(1/2)) of release of retinal from VCOP was 7.1 s, 250-fold faster than that of rhodopsin. VCOP exhibited pH-dependent release kinetics, with the t(1/2) decreasing from 23 to 4 s with the pH decreasing from 4.1 to 8, respectively. However, the Arrhenius activation energy (E(a)) for VCOP derived from kinetic measurements between 4 and 20 °C was 17.4 kcal/mol, similar to the value of 18.5 kcal/mol for rhodopsin. There was a small kinetic isotope (D(2)O) effect in VCOP, but this effect was smaller than that observed in rhodopsin. Mutation of the primary Schiff base counterion (VCOP(D108A)) produced a pigment with an unprotonated chromophore (λ(max) = 360 nm) and dramatically slowed (t(1/2) ~ 6.8 min) light-dependent retinal release. Using homology modeling, a VCOP mutant with two substitutions (S85D and D108A) was designed to move the counterion one α-helical turn into the transmembrane region from the native position. This double mutant had a UV-visible absorption spectrum consistent with a protonated Schiff base (λ(max) = 420 nm). Moreover, the VCOP(S85D/D108A) mutant had retinal release kinetics (t(1/2) = 7 s) and an E(a) (18 kcal/mol) similar to those of the native pigment exhibiting no pH dependence. By contrast, the single mutant VCOP(S85D) had an ~3-fold decreased retinal release rate compared to that of the native pigment. Photoactivated VCOP(D108A) had kinetics comparable to those of a rhodopsin counterion mutant, Rho(E113Q), both requiring hydroxylamine to fully release retinal. These results demonstrate that the primary counterion of cone visual pigments is necessary for efficient Schiff base hydrolysis. We discuss how the large differences in retinal release rates between rod and cone visual pigments arise, not from inherent differences in the rate of Schiff base hydrolysis but rather from differences in the properties of noncovalent binding of the retinal chromophore to the protein.

Rod outer segment retinol formation is independent of Abca4, arrestin, rhodopsin kinase, and rhodopsin palmitylation

PURPOSE

The reactive aldehyde all-trans retinal is released in rod photoreceptor outer segments by photoactivated rhodopsin and is eliminated through reduction to all-trans retinol. This study was undertaken to determine whether all-trans retinol formation depends on Abca4, arrestin, rhodopsin kinase, and the palmitylation of rhodopsin, all of which are factors that affect the release and sequestration of all-trans retinal.

METHODS

Experiments were performed in isolated retinas and single living rods derived from 129/sv wild-type mice and Abca4-, arrestin-, and rhodopsin kinase-deficient mice and in genetically modified mice containing unpalmitylated rhodopsin. Formation of all-trans retinol was measured by imaging its fluorescence and by HPLC of retina extracts. The release of all-trans retinal from photoactivated rhodopsin was measured in purified rod outer segment membranes according to the increase in tryptophan fluorescence. All experiments were performed at 37°C.

RESULTS

The kinetics of all-trans retinol formation in the different types of genetically modified mice are in reasonable agreement with those in wild-type animals. The kinetics of all-trans retinol formation in 129/sv mice are similar to those in C57BL/6, although the latter are known to regenerate rhodopsin much more slowly. The release of all-trans retinal from rhodopsin in purified membranes is significantly faster than the formation of all-trans retinol in intact cells and is independent of the presence of the palmitate groups.

CONCLUSIONS

The regeneration of rhodopsin and the recycling of its chromophore are not strongly coupled. Neither the activities of Abca4, rhodopsin kinase, and arrestin, nor the palmitylation of rhodopsin affects the formation of all-trans retinol.

Multiple functions of Schiff base counterion in rhodopsins

In rhodopsins, visible-light absorption is achieved by the protonation of the chromophore Schiff base. The Schiff base proton is stabilized by the negative charge of an amino acid residue called the Schiff base counterion. Since E113 was identified as the counterion in bovine rhodopsin, there has been growing evidence that the counterion has multiple functions besides proton stabilization. Here, we first introduce generally accepted findings as well as some controversial theories about the identity of the Schiff base counterion in the dark and in intermediate states and then review multiple functions of the counterion in vertebrate visual pigments. Special focus is placed on the recently demonstrated role in photoisomerization efficiency. Finally, differences in the position of the counterion between vertebrate visual pigments and other opsins and its relevance to the molecular evolution of opsins are discussed.

Evolution of vertebrate retinal photoreception

Recent findings shed light on the steps underlying the evolution of vertebrate photoreceptors and retina. Vertebrate ciliary photoreceptors are not as wholly distinct from invertebrate rhabdomeric photoreceptors as is sometimes thought. Recent information on the phylogenies of ciliary and rhabdomeric opsins has helped in constructing the likely routes followed during evolution. Clues to the factors that led the early vertebrate retina to become invaginated can be obtained by combining recent knowledge about the origin of the pathway for dark re-isomerization of retinoids with knowledge of the inability of ciliary opsins to undergo photoreversal, along with consideration of the constraints imposed under the very low light levels in the deep ocean. Investigation of the origin of cell classes in the vertebrate retina provides support for the notion that cones, rods and bipolar cells all originated from a primordial ciliary photoreceptor, whereas ganglion cells, amacrine cells and horizontal cells all originated from rhabdomeric photoreceptors. Knowledge of the molecular differences between cones and rods, together with knowledge of the scotopic signalling pathway, provides an understanding of the evolution of rods and of the rods' retinal circuitry. Accordingly, it has been possible to propose a plausible scenario for the sequence of evolutionary steps that led to the emergence of vertebrate photoreceptors and retina.

Vertebrate rhodopsin adaptation to dim light via rapid meta-II intermediate formation

Rhodopsin is a photoreceptive protein present in vertebrate rod photoreceptor cells, which are responsible for scotopic vision. Recent molecular studies have shown that several aquatic vertebrate species have independently acquired rhodopsin containing Asp83Asn, Glu122Gln, and Ala292Ser substitutions, causing a blue shift in the rhodopsin absorption spectra for adaptation to the blue-green photic environment in deep water. Here, we provide new evidence for the evolutionary and functional relevance of the Asp83Asn substitution. Spectroscopic and kinetic analyses of rhodopsins in six cichlid fishes from the East African Great Lakes using charge-coupled device spectrophotometer revealed that the Asp83Asn substitution accelerated the formation of meta-II, a rhodopsin intermediate crucial for activation of the G-protein transducin. Because rapid formation of meta-II likely results in effective transduction of photic signals, it is reasonable to assume that deep-water cichlid species have acquired rhodopsin containing Asn83 to adapt to dim lighting. Remarkably, rhodopsin containing Asn83 has been identified in terrestrial vertebrates such as bats, and these rhodopsin variants also exhibit accelerated meta-II formation. Our results indicated that the Asp83Asn substitution observed in a variety of animal species was acquired independently in many different lineages during vertebrate evolution for adaptation to dimly lit environments.

The evolution of early vertebrate photoreceptors

Meeting the challenge of sampling an ancient aquatic landscape by the early vertebrates was crucial to their survival and would establish a retinal bauplan to be used by all subsequent vertebrate descendents. Image-forming eyes were under tremendous selection pressure and the ability to identify suitable prey and detect potential predators was thought to be one of the major drivers of speciation in the Early Cambrian. Based on the fossil record, we know that hagfishes, lampreys, holocephalans, elasmobranchs and lungfishes occupy critical stages in vertebrate evolution, having remained relatively unchanged over hundreds of millions of years. Now using extant representatives of these 'living fossils', we are able to piece together the evolution of vertebrate photoreception. While photoreception in hagfishes appears to be based on light detection and controlling circadian rhythms, rather than image formation, the photoreceptors of lampreys fall into five distinct classes and represent a critical stage in the dichotomy of rods and cones. At least four types of retinal cones sample the visual environment in lampreys mediating photopic (and potentially colour) vision, a sampling strategy retained by lungfishes, some modern teleosts, reptiles and birds. Trichromacy is retained in cartilaginous fishes (at least in batoids and holocephalans), where it is predicted that true scotopic (dim light) vision evolved in the common ancestor of all living gnathostomes. The capacity to discriminate colour and balance the tradeoff between resolution and sensitivity in the early vertebrates was an important driver of eye evolution, where many of the ocular features evolved were retained as vertebrates progressed on to land.

Characterization of photoreceptor cell types in the little brown bat Myotis lucifugus (Vespertilionidae)

We report the expression of three visual opsins in the retina of the little brown bat (Myotis lucifugus, Vespertilionidae). Gene sequences for a rod-specific opsin and two cone-specific opsins were cloned from cDNA derived from bat eyes. Comparative sequence analyses indicate that the two cone opsins correspond to an ultraviolet short-wavelength opsin (SWS1) and a long-wavelength opsin (LWS). Immunocytochemistry using antisera to visual opsins revealed that the little brown bat retina contains two types of cone photoreceptors within a rod-dominated background. However, unlike other mammalian photoreceptors, M. lucifugus cones and rods are morphologically indistinguishable by light microscopy. Both photoreceptor types have a thin, elongated outer segment. Using microspectrophotometry we classified the absorption spectrum for the ubiquitous rods. Similar to other mammals, bat rhodopsin has an absorption peak near 500 nm. Although we were unable to confirm a spectral range, cellular and molecular analyses indicate that M. lucifugus expresses two types of cone visual pigments located within the photoreceptor layer. This study provides important insights into the visual capacity of a nocturnal microchiropteran species.

Structure of cone photoreceptors

Although outnumbered more than 20:1 by rod photoreceptors, cone cells in the human retina mediate daylight vision and are critical for visual acuity and color discrimination. A variety of human diseases are characterized by a progressive loss of cone photoreceptors but the low abundance of cones and the absence of a macula in non-primate mammalian retinas have made it difficult to investigate cones directly. Conventional rodents (laboratory mice and rats) are nocturnal rod-dominated species with few cones in the retina, and studying other animals with cone-rich retinas presents various logistic and technical difficulties. Originating in the early 1900s, past research has begun to provide insights into cone ultrastructure but has yet to afford an overall perspective of cone cell organization. This review summarizes our past progress and focuses on the recent introduction of special mammalian models (transgenic mice and diurnal rats rich in cones) that together with new investigative techniques such as atomic force microscopy and cryo-electron tomography promise to reveal a more unified concept of cone photoreceptor organization and its role in retinal diseases.

Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup

Charles Darwin appreciated the conceptual difficulty in accepting that an organ as wonderful as the vertebrate eye could have evolved through natural selection. He reasoned that if appropriate gradations could be found that were useful to the animal and were inherited, then the apparent difficulty would be overcome. Here, we review a wide range of findings that capture glimpses of the gradations that appear to have occurred during eye evolution, and provide a scenario for the unseen steps that have led to the emergence of the vertebrate eye.

Structure and photoreaction of photoactive yellow protein, a structural prototype of the PAS domain superfamily

Photoactive yellow protein (PYP) is a water-soluble photosensor protein found in purple photosynthetic bacteria. Unlike bacterial rhodopsins, photosensor proteins composed of seven transmembrane helices and a retinal chromophore in halophilic archaebacteria, PYP is a highly soluble globular protein. The alpha/beta fold structure of PYP is a structural prototype of the PAS domain superfamily, many members of which function as sensors for various kinds of stimuli. To absorb a photon in the visible region, PYP has a p-coumaric acid chromophore binding to the cysteine residue via a thioester bond. It exists in a deprotonated trans form in the dark. The primary photochemical event is photo-isomerization of the chromophore from trans to cis form. The twisted cis chromophore in early intermediates is relaxed and finally protonated. Consequently, the chromophore becomes electrostatically neutral and rearrangement of the hydrogen-bonding network triggers overall structural change of the protein moiety, in which local conformational change around the chromophore is propagated to the N-terminal region. Thus, it is an ideal model for protein conformational changes that result in functional change, responding to stimuli and expressing physiological activity. In this paper, recent progress in investigation of the photoresponse of PYP is reviewed.

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.