Low-temperature spectrophotometry of intermediates of rhodopsin

Ultrafast structural changes direct the first molecular events of vision

Vision is initiated by the rhodopsin family of light-sensitive G protein-coupled receptors (GPCRs)1. A photon is absorbed by the 11-cis retinal chromophore of rhodopsin, which isomerizes within 200 femtoseconds to the all-trans conformation2, thereby initiating the cellular signal transduction processes that ultimately lead to vision. However, the intramolecular mechanism by which the photoactivated retinal induces the activation events inside rhodopsin remains experimentally unclear. Here we use ultrafast time-resolved crystallography at room temperature3 to determine how an isomerized twisted all-trans retinal stores the photon energy that is required to initiate the protein conformational changes associated with the formation of the G protein-binding signalling state. The distorted retinal at a 1-ps time delay after photoactivation has pulled away from half of its numerous interactions with its binding pocket, and the excess of the photon energy is released through an anisotropic protein breathing motion in the direction of the extracellular space. Notably, the very early structural motions in the protein side chains of rhodopsin appear in regions that are involved in later stages of the conserved class A GPCR activation mechanism. Our study sheds light on the earliest stages of vision in vertebrates and points to fundamental aspects of the molecular mechanisms of agonist-mediated GPCR activation.

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.

Molecular basis of rod and cone differences

In the vertebrate retina, rods and cones both detect light, but they are different in functional aspects such as light sensitivity and time resolution, for example, and in some of cell biological aspects. For functional aspects, both photoreceptors are known to share a common mechanism, phototransduction cascade, consisting of a series of enzyme reactions to convert a photon-capture signal to an electrical signal. To understand the mechanisms of the functional differences between rods and cones at the molecular level, we compared biochemically each of the reactions in the phototransduction cascade between rods and cones using the cells isolated and purified from carp retina. Although proteins in the cascade are functionally similar between rods and cones, their activities together with their expression levels are mostly different between these photoreceptors. In general, reactions to generate a response are slightly less effective, as a total, in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca²⁺-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses. With comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins would be of interest for future studies on the difference between rods and cones.

Comparative Mutagenesis Studies of Retinal Release in Light-Activated Zebrafish Rhodopsin Using Fluorescence Spectroscopy

Rhodopsin is the visual pigment responsible for initiating scotopic (dim-light) vision in vetebrates. Once activated by light, release of all-trans-retinal from rhodopsin involves hydrolysis of the Schiff base linkage, followed by dissociation of retinal from the protein moiety. This kinetic process has been well studied in model systems such as bovine rhodopsin, but not in rhodopsins from cold-blooded animals, where physiological temperatures can vary considerably. Here, we characterize the rate of retinal release from light-activated rhodopsin in an ectotherm, zebrafish (Danio rerio), demonstrating in a fluorescence assay that this process occurs more than twice as fast as bovine rhodopsin at similar temperatures in 0.1% dodecyl maltoside. Using site-directed mutagenesis, we found that differences in retinal release rates can be attributed to a series of variable residues lining the retinal channel in three key structural motifs: an opening in metarhodopsin II between transmembrane helix 5 (TM5) and TM6, in TM3 near E122, and in the "retinal plug" formed by extracellular loop 2 (EL2). The majority of these sites are more proximal to the β-ionone ring of retinal than the Schiff base, indicating their influence on retinal release is more likely due to steric effects during retinal dissociation, rather than alterations to Schiff base stability. An Arrhenius plot of zebrafish rhodopsin was consistent with this model, inferring that the activation energy for Schiff base hydrolysis is similar to that of bovine rhodopsin. Functional variation at key sites identified in this study is consistent with the idea that retinal release might be an adaptive property of rhodopsin in vertebrates. Our study is one of the few investigating a nonmammalian rhodopsin, which will help establish a better understanding of the molecular mechanisms contributing to vision in cold-blooded vertebrates.

Low-Temperature Trapping of Photointermediates of the Rhodopsin E181Q Mutant

Three active-site components in rhodopsin play a key role in the stability and function of the protein: 1) the counter-ion residues which stabilize the protonated Schiff base, 2) water molecules, and 3) the hydrogen-bonding network. The ionizable residue Glu-181, which is involved in an extended hydrogen-bonding network with Ser-186, Tyr-268, Tyr-192, and key water molecules within the active site of rhodopsin, has been shown to be involved in a complex counter-ion switch mechanism with Glu-113 during the photobleaching sequence of the protein. Herein, we examine the photobleaching sequence of the E181Q rhodopsin mutant by using cryogenic UV-visible spectroscopy to further elucidate the role of Glu-181 during photoactivation of the protein. We find that lower temperatures are required to trap the early photostationary states of the E181Q mutant compared to native rhodopsin. Additionally, a Blue Shifted Intermediate (BSI, λ_max = 498 nm, 100 K) is observed after the formation of E181Q Bathorhodopsin (Batho, λ_max = 556 nm, 10 K) but prior to formation of E181Q Lumirhodopsin (Lumi, λ_max = 506 nm, 220 K). A potential energy diagram of the observed photointermediates suggests the E181Q Batho intermediate has an enthalpy value 7.99 KJ/mol higher than E181Q BSI, whereas in rhodopsin, the BSI is 10.02 KJ/mol higher in enthalpy than Batho. Thus, the Batho to BSI transition is enthalpically driven in E181Q and entropically driven in native rhodopsin. We conclude that the substitution of Glu-181 with Gln-181 results in a significant perturbation of the hydrogen-bonding network within the active site of rhodopsin. In addition, the removal of a key electrostatic interaction between the chromophore and the protein destabilizes the protein in both the dark state and Batho intermediate conformations while having a stabilizing effect on the BSI conformation. The observed destabilization upon this substitution further supports that Glu-181 is negatively charged in the early intermediates of the photobleaching sequence of rhodopsin.

Photochemical nature of parietopsin

Parietopsin is a nonvisual green light-sensitive opsin closely related to vertebrate visual opsins and was originally identified in lizard parietal eye photoreceptor cells. To obtain insight into the functional diversity of opsins, we investigated by UV-visible absorption spectroscopy the molecular properties of parietopsin and its mutants exogenously expressed in cultured cells and compared the properties to those of vertebrate and invertebrate visual opsins. Our mutational analysis revealed that the counterion in parietopsin is the glutamic acid (Glu) in the second extracellular loop, corresponding to Glu181 in bovine rhodopsin. This arrangement is characteristic of invertebrate rather than vertebrate visual opsins. The photosensitivity and the molar extinction coefficient of parietopsin were also lower than those of vertebrate visual opsins, features likewise characteristic of invertebrate visual opsins. On the other hand, irradiation of parietopsin yielded meta-I, meta-II, and meta-III intermediates after batho and lumi intermediates, similar to vertebrate visual opsins. The pH-dependent equilibrium profile between meta-I and meta-II intermediates was, however, similar to that between acid and alkaline metarhodopsins in invertebrate visual opsins. Thus, parietopsin behaves as an "evolutionary intermediate" between invertebrate and vertebrate visual opsins.

Photochemical reaction dynamics of the primary event of vision studied by means of a hybrid molecular simulation

The photoisomerization reaction dynamics of a retinal chromophore in the visual receptor rhodopsin was investigated by means of hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations. The photoisomerization reaction of retinal constitutes the primary step of vision and is known as one of the fastest reactions in nature. To elucidate the molecular mechanism of the high efficiency of the reaction, we carried out hybrid ab initio QM/MM MD simulations of the complete reaction process from the vertically excited state to the photoproduct via electronic transition in the entire chromophore-protein complex. An ensemble of reaction trajectories reveal that the excited-state dynamics is dynamically homogeneous and synchronous even in the presence of thermal fluctuation of the protein, giving rise to the very fast formation of the photoproduct. The synchronous nature of the reaction dynamics in rhodopsin is found to originate from weak perturbation of the protein surroundings and from dynamic regulation of volume-conserving motions of the chromophore. The simulations also provide a detailed view of time-dependent modulations of hydrogen-out-of-plane vibrations during the reaction process, and identify molecular motions underlying the experimentally observed dynamic spectral modulations.

Photochemistry of retinal chromophore in mouse melanopsin

In mammals, melanopsin is exclusively expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs), which play an important role in circadian photoentrainment and other nonimage-forming functions. These ipRGCs reside in the inner retina, far removed from the pigment epithelium, which synthesizes the 11-cis retinal chromophore used by rod and cone photoreceptors to regenerate opsin for light detection. There has been considerable interest in the identification of the melanopsin chromophore and in understanding the process of photopigment regeneration in photoreceptors that are not in proximity to the classical visual cycle. We have devised an immuno-magnetic purification protocol that allows melanopsin-expressing retinal ganglion cells to be isolated and collected from multiple mouse retinas. Using this technique, we have demonstrated that native melanopsin in vivo exclusively binds 11-cis retinal in the dark and that illumination causes isomerization to the all-trans isoform. Furthermore, spectral analysis of the melanopsin photoproduct shows the formation of a protonated metarhodopsin with a maximum absorbance between 520 and 540 nm. These results indicate that even if melanopsin functions as a bistable photopigment with photo-regenerative activity native melanopsin must also use some other light-independent retinoid regeneration mechanism to return to the dark state, where all of the retinal is observed to be in the 11-cis form.

Lumi I --> Lumi II: the last detergent independent process in rhodopsin photoexcitationt

Time-resolved absorbance difference spectra were collected at delays from 1 to 128 micros after photolysis of membrane and detergent suspensions of rhodopsin at 20 degrees C. Fitting both sets of data with two exponential decays plus a constant showed a similar fast process (lifetime 11 micros in membrane, 12 micros in 5% dodecyl maltoside) with a small but similar spectral change. This demonstrates that the Lumi I - Lumi II process, previously characterized in detergent suspensions, has similar properties in membrane without significant effect of detergent. The slower exponential process detected in the data is quite different in membrane compared to detergent solubilized samples, showing that the pronounced effect of detergent on the later rhodopsin photointermediates begins fairly abruptly near 20 micros. Besides affecting the late processes, the data collected here shows that detergent induces a small blue shift in the 1 micros difference spectrum (the Lumi I minus rhodopsin difference spectrum). The blue shift is similar to one induced by chloride ion in the E181Q rhodopsin mutant and may indicate that the ionization state of Glu181 in rhodopsin is affected by detergent.

A cryogenic optical waveguide spectrometer for the measurement of low-temperature absorption spectra of dilute biological samples

A cryogenic optical waveguide spectrometer that uses a Teflon-AF 2400 liquid core waveguide is described. In comparison to standard low-temperature absorption techniques, the liquid core waveguide approach not only affords the use of microliter samples but also provides significant improvements in sensitivity. Here we show low-temperature absorption spectra of various flavoproteins, including DNA photolyase, measured using this new technique. The technique has high reproducibility and can afford the detection of 15 ng of flavoprotein. In addition, the technique requires several hundredfold less protein than standard low-temperature techniques for the same sensitivity. The performance of the spectrometer in the ultraviolet (UV) region is investigated experimentally and compared with standard UV absorption techniques. Results indicate that, below 300 nm, the observed absorbances deviate from the Beer-Lambert law.

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.

Functional role of internal water molecules in rhodopsin revealed by X-ray crystallography

Activation of G protein-coupled receptors (GPCRs) is triggered and regulated by structural rearrangement of the transmembrane heptahelical bundle containing a number of highly conserved residues. In rhodopsin, a prototypical GPCR, the helical bundle accommodates an intrinsic inverse-agonist 11-cis-retinal, which undergoes photo-isomerization to the all-trans form upon light absorption. Such a trigger by the chromophore corresponds to binding of a diffusible ligand to other GPCRs. Here we have explored the functional role of water molecules in the transmembrane region of bovine rhodopsin by using x-ray diffraction to 2.6 A. The structural model suggests that water molecules, which were observed in the vicinity of highly conserved residues and in the retinal pocket, regulate the activity of rhodopsin-like GPCRs and spectral tuning in visual pigments, respectively. To confirm the physiological relevance of the structural findings, we conducted single-crystal microspectrophotometry on rhodopsin packed in our three-dimensional crystals and show that its spectroscopic properties are similar to those previously found by using bovine rhodopsin in suspension or membrane environment.

Serine 85 in transmembrane helix 2 of short-wavelength visual pigments interacts with the retinylidene Schiff base counterion

Short-wavelength cone visual pigments (SWS1) are responsible for detecting light from 350 to 430 nm. Models of this class of pigment suggest that TM2 has extensive contacts with the retinal binding pocket and stabilizes interhelical interactions. The role of TM2 in the structure-function of the Xenopus SWS1 (VCOP, lambda(max) = 427 nm) pigment was studied by replacement of the helix with that of bovine rhodopsin and also by mutagenesis of highly conserved residues. The TM2 chimera and G78D, F79L, M81E, P88T, V89S, and F90V mutants did not produce any significant spectral shift of the dark state or their primary photointermediate formed upon illumination at cryogenic temperatures. The mutant G77R (responsible for human tritanopia) was completely defective in folding, while C82A and F87T bound retinal at reduced levels. The position S85 was crucial for obtaining the appropriate spectroscopic properties of VCOP. S85A and S85T did not bind retinal. S85D bound retinal and had a wild-type dark state at room temperature and a red-shifted dark state at 45 K and formed an altered primary photointermediate. S85C absorbed maximally at 390 nm at neutral pH and at 365 nm at pH >7.5. The S85C dark state was red shifted by 20 nm at 45 K and formed an altered primary photointermediate. These data suggest that S85 is involved in a hydrogen bond with the protonated retinylidene Schiff base counterion in both the dark state and the primary photointermediate.

The photobleaching sequence of a short-wavelength visual pigment

The photobleaching pathway of a short-wavelength cone opsin purified in delipidated form (lambda(max) = 425 nm) is reported. The batho intermediate of the violet cone opsin generated at 45 K has an absorption maximum at 450 nm. The batho intermediate thermally decays to the lumi intermediate (lambda(max) = 435 nm) at 200 K. The lumi intermediate decays to the meta I (lambda(max) = 420 nm) and meta II (lambda(max) = 388 nm) intermediates at 258 and 263 K, respectively. The meta II intermediate decays to free retinal and opsin at >270 K. At 45, 75, and 140 K, the photochemical excitation of the violet cone opsin at 425 nm generates the batho intermediate at high concentrations under moderate illumination. The batho intermediate spectra, generated via decomposing the photostationary state spectra at 45 and 140 K, are identical and have properties typical of batho intermediates of other visual pigments. Extended illumination of the violet cone opsin at 75 K, however, generates a red-shifted photostationary state (relative to both the dark and the batho intermediates) that has as absorption maximum at approximately 470 nm, and thermally reverts to form the normal batho intermediate when warmed to 140 K. We conclude that this red-shifted photostationary state is a metastable state, characterized by a higher-energy protein conformation that allows relaxation of the all-trans chromophore into a more planar conformation. FTIR spectroscopy of violet cone opsin indicates conclusively that the chromophore is protonated. A similar transformation of the rhodopsin binding site generates a model for the VCOP binding site that predicts roughly 75% of the observed blue shift of the violet cone pigment relative to rhodopsin. MNDO-PSDCI calculations indicate that secondary interactions involving the binding site residues are as important as the first-order chromophore protein interactions in mediating the wavelength maximum.

Characterization of the primary photointermediates of Drosophila rhodopsin

Invertebrate opsins are unique among the visual pigments because the light-activated conformation, metarhodopsin, is stable following exposure to light in vivo. Recovery of the light-activated pigment to the dark conformation (or resting state) occurs either thermally or photochemically. There is no evidence to suggest that the chromophore becomes detached from the protein during any stage in the formation or recovery processes. Biochemical and structural studies of invertebrate opsins have been limited by the inability to express and purify rhodopsins for structure-function studies. In this study, we used Drosophila to produce an epitope-tagged opsin, Rh1-1D4, in quantities suitable for spectroscopic and photochemical characterization. When expressed in Drosophila, Rh1-1D4 is localized to the rhabdomere membranes, has the same spectral properties in vivo as wild-type Rh1, and activates the phototransduction cascade in a normal manner. Purified Rh1-1D4 visual pigment has an absorption maximum of the dark-adapted state of 474 nm, while the metarhodopsin absorption maximum is 572 nm. However, the metarhodopsin state is not stable as purified in dodecyl maltoside but decays with kinetics that require a double-exponential fit having lifetimes of 280 and 2700 s. We investigated the primary properties of the pigment at low temperature. At 70 K, the pigment undergoes a temperature-induced red shift to 486 nm. Upon illumination with 435 nm light, a photostationary state mixture is formed consisting of bathorhodopsin (lambda(max) = 545 nm) and isorhodopsin (lambda(max) = 462 nm). We also compared the spectroscopic and photochemical properties of this pigment with other vertebrate pigments. We conclude that the binding site of Drosophila rhodopsin is similar to that of bovine rhodopsin and is characterized by a protonated Schiff base chromophore stabilized via a single negatively charged counterion.

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

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

Movement of retinal along the visual transduction path

Movement of the ligand/receptor complex in rhodopsin (Rh) has been traced. Bleaching of diazoketo rhodopsin (DK-Rh) containing 11-cis-3-diazo-4-oxo-retinal yields batho-, lumi-, meta-I-, and meta-II-Rh intermediates corresponding to those of native Rh but at lower temperatures. Photoaffinity labeling of DK-Rh and these bleaching intermediates shows that the ionone ring cross-links to tryptophan-265 on helix F in DK-Rh and batho-Rh, and to alanine-169 on helix D in lumi-, meta-I-, and meta-II-Rh intermediates. It is likely that these movements involving a flip-over of the chromophoric ring trigger changes in cytoplasmic membrane loops resulting in heterotrimeric guanine nucleotide-binding protein (G protein) activation.

Photochemistry of the primary event in short-wavelength visual opsins at low temperature

Two short-wavelength cone opsins, frog (Xenopus laevis) violet and mouse UV, were expressed in mammalian COS1 cells, purified in delipidated form, and studied using cryogenic UV-vis spectrophotometry. At room temperature, the X. laevis violet opsin has an absorption maximum at 426 nm when generated with 11-cis-retinal and an absorption maximum of 415 nm when generated with 9-cis-retinal. The frog short-wavelength opsin has two different batho intermediates, one stable at 30 K (lambda(max) approximately 446 nm) and the other at 70 K (lambda(max) approximately 475 nm). Chloride ions do not affect the absorption maximum of the violet opsin. At room temperature, mouse UV opsin has an absorption maximum of 357 nm, while at 70 K, the pigment exhibits a bathochromic shift to 403 nm with distinct vibronic structure and a strong secondary vibronic band at 380 nm. We have observed linear relationships when analyzing the energy difference between the initial and bathochromic intermediates and the normalized difference spectra of the batho-shifted intermediates of rod and cone opsins. We conclude that the binding sites of these pigments change from red to green to violet via systematic shifts in the position of the primary counterion relative to the protonated Schiff base. The mouse UV cone opsin does not fit this trend, and we conclude that wavelength selection in this pigment must operate via a different molecular mechanism. We discuss the possibility that the mouse UV chromophore is initially unprotonated.

Difference in molecular properties between chicken green and rhodopsin as related to the functional difference between cone and rod photoreceptor cells

Using low-temperature spectroscopy, we have investigated the photobleaching process of chicken green, a green-sensitive cone visual pigment present in chicken retina, and compared it to that of rhodopsin, a rod visual pigment. Like rhodopsin, chicken green converts to all-trans-retinal and opsin through batho, lumi, and meta I, II, and III intermediates. However, all of the intermediates of chicken green except lumi, are less stable than the corresponding intermediates of rhodopsin. While early intermediates, batho and lumi are similar in absorption maxima between chicken green and rhodopsin, the meta intermediates of chicken green are about 20 nm blue shifted from those of rhodopsin. Low-temperature time-resolved spectroscopy was applied to estimate the thermodynamic properties of meta intermediates, and it indicated that the less stable properties of meta II and III intermediates of chicken green originate from the smaller activation enthalpies. The decay of the meta II intermediate of chicken green is greatly suppressed when a chicken green sample is irradiated at alkaline conditions while the net charge becomes similar to that of rhodopsin at neutral conditions. These results strongly suggest that the functional properties of chicken green that are different from those of rhodopsin are regulated by the dissociative amino acid residue(s).

Photostationary state compositions of retinal and related compounds included in beta-lactoglobulin. Effects of protein host on isomer distribution of polyene substrates

The UV-visible absorption spectra and photostationary state compositions of retinal (or the related C-18 ketone, 3-dehydroretinal and the C-22 aldehyde) imbedded in the binding cavity of beta-lactoglobulin (BLG) are consistent with the view that the carbonyl group of these polyenes are hydrogen-bonded with the protein host, most likely with the lone protonated lysine residue in the binding pocket. Patterns of variation in photochemical behavior of the imbedded chromophore versus that in solution are discussed in terms of possible specific protein-substrate interactions. The results are also compared with that of the methyl ether of retinol where similar hydrogen bonding is not possible.

Low-temperature trapping of early photointermediates of alpha-isorhodopsin

Alpha-Isorhodopsin, an artificial visual pigment with a 9-cis-4,5-dehydro-5,6-dihydro(alpha)retinal chromophore, was photolyzed at low temperatures and absorption difference spectra were collected as the sample was warmed. A bathorhodopsin (Batho)-like intermediate absorbing at ca 495 nm was detected below 55 K,a blue-shifted intermediate (BSI)-like intermediate absorbing at ca 453 nm was observed when the temperature was raised to 60 K and a lumirhodopsin (Lumi)-like intermediate absorbing at ca 470 nm was found when the sample was warmed to 115 K. Photointermediates from this pigment were compared to those of native rhodopsin and 5,6-dihydroisorhodopsin. As in native rhodopsin, Batho is the first intermediate detected in alpha-isorhodopsin, though unlike native rhodopsin at low temperatures BSI is observed prior to Lumi formation. Alpha-Isohodopsin behaves similarly to 5,6-dihydroisorhodopsin, with the same early intermediates observed in both artificial visual pigments lacking the C5-C6 double bond. The transition temperature for BSI formation is higher in alpha-isorhodopsin, suggesting an interaction involving the chromophore ring in BSI formation. The transition temperature for Lumi formation is similar for these two pigments as well as for native rhodopsin, suggesting comparable changes in the protein environment in that transition.

Direct observation of the thermal equilibria among lumirhodopsin, metarhodopsin I, and metarhodopsin II in chicken rhodopsin

Using low-temperature time-resolved spectroscopy, we have directly observed thermal back reaction of metarhodopsin I (meta I) to lumirhodopsin (lumi) and that of metarhodopsin II (meta II) to meta I in chicken rhodopsin to demonstrate the presence of thermal equilibria among lumi, meta I, and meta II. The back reaction from meta I to lumi was observed when the rhodopsin sample irradiated at -35 degrees C was warmed to -20 degrees C, while that from meta II to meta I was observed when the sample irradiated at -10 degrees C was cooled to -20 degrees C. Thermodynamic parameters of lumi, meta I, and meta II were calculated from the equilibrium constants estimated by analyzing the spectra of the equilibrium states at temperatures ranging from -30 to -10 degrees C. The results showed that meta I has an enthalpy and an entropy considerably smaller than those of lumi and meta II, while the difference in thermodynamic parameters between lumi and meta II is not so large. These results suggest that meta I is a crucial stage of conversion of the light energy captured by the chromophore into restricted conformations of the chromophore and/or protein, from which a large conformational change of the protein starts to form meta II.

Is chicken green-sensitive cone visual pigment a rhodopsin-like pigment? A comparative study of the molecular properties between chicken green and rhodopsin

Chicken green is a visual pigment present in chicken green-sensitive cones and has an amino acid sequence more similar than any other cone visual pigments to the rod visual pigments, rhodopsins. Here we have investigated the molecular properties of chicken green and compared them with those of rhodopsin to elucidate whether or not chicken green is a rhodopsin-like pigment. While chicken green has a molecular extinction coefficient and a photosensitivity very similar to those of rhodopsin, it displays faster regeneration from 11-cis-retinal and opsin and faster formation and decay of the physiologically active meta II intermediate than rhodopsin. These differences correlate with the physiological difference between cones and rods. Thus in spite of the similarity in amino acid sequence, chicken green displays molecular properties required for a cone visual pigment that are clearly different from those of rhodopsin.

Room temperature trapping of rhodopsin photointermediates

By suspending bovine rhodopsin in trehalose-water glass films, it is possible to trap photostates in the light-activation process. Because of the unusually high vitrification temperature of trehalose-water mixtures, this trapping can be accomplished at room temperature. This allows for a facile investigation of the spectroscopic properties of rhodopsin's photointermediates. Depending on experimental conditions, it is possible to trap photolysis products that have visible absorbance spectra closely resembling the two different photointermediates, metarhodopsin I and metarhodopsin II. When rhodopsin is maintained in the native rod outer segment membrane, the photolysis product has the spectral properties of metarhodopsin I. Upon detergent solubilization, the photolysis product closely resembles metarhodopsin II. Ultraviolet circular dichroism spectra show that the metarhodopsin I product had no change in secondary structure compared with unbleached rhodopsin. The metarhodopsin II product did show a significant decrease in alpha-helical content. Resonance energy transfer was measured from extrinsic probes located on each of the cytoplasmic cysteine residues to the retinal in the trapped photoproducts. It is seen that these distances are the same for rhodopsin and metarhodopsin I while metarhodopsin II shows considerably shorter distances. Metarhodopsin II is intimately associated with the signal transduction process, and the present results suggest that large structural changes have occurred in the transition to this state. These results demonstrate the utility of room temperature trapping of photostates in trehalose-water glasses.

Divergent pathways in photobleaching of 7,9-dicis-rhodopsin and 9,11-dicis-12-fluororhodopsin: one-photon-two-bond and one-photon-one-bond isomerization

Through low-temperature photochemistry, UV/vis spectroscopy, and chromophore extraction experiments, we have established that 7,9-dicis-rhodopsin undergoes one-photon-two-bond photoisomerization to a batho intermediate (its absorption maximum is slightly blue shifted from that of bathorhodopsin) containing the all-trans geometry, while 9,11-dicis-12-fluororhodopsin undergoes one-photon-one-bond isomerization to the corresponding 9-cis isomer and then the all-trans batho intermediate. The difference in the photochemical properties of the two dicis pigment analogs was rationalized by possible local protein perturbation, lability of the 11-cis geometry, and photochemical properties of the chromophores.

Oxygen diffusion-concentration product in rhodopsin as observed by a pulse ESR spin labeling method

Permeation of molecular oxygen in rhodopsin, an integral membrane protein, has been investigated by monitoring the bimolecular collision rate between molecular oxygen and the nitroxide spin label using a pulse electron spin resonance (ESR) T1 method. Rhodopsin was labeled by regeneration with the spin-labeled 9-cis retinal analogue in which the beta-ionone ring of retinal is replaced by the nitroxide tetramethyl-oxypyrrolidine ring. The bimolecular collision rate was evaluated in terms of an experimental parameter W(x), defined as T1(-1)(air,x)--T1(-1)(N2,x) where T1's are the spin-lattice relaxation times of the nitroxide in samples equilibrated with atmospheric air and nitrogen respectively, which is proportional to the product of local oxygen concentration and local diffusion coefficient (transport). W-values at the beta-ionone binding site in spin-labeled rhodopsin are in the range of 0.02-0.13 microseconds-1, which are 10-60 times smaller than W's in water and 1.1-20 times smaller than in model membranes in the gel phase, indicating that membrane proteins create significant permeation resistance to transport of molecular oxygen inside and across the membrane. W(thereby the oxygen diffusion-concentration product) is larger in the meta II-enriched sample than in rhodopsin, indicating light-induced conformational changes of opsin around the beta-ionone binding site. W decreases with increase of temperature for both rhodopsin and meta II-enriched samples, suggesting that temperature-induced conformational changes take place in both samples. These changes were not observable using conventional ESR spectroscopy. It is concluded that W is a sensitive monitor of conformational changes of proteins.

Photoreaction cycle of phoborhodopsin studied by low-temperature spectrophotometry

The photochemical and subsequent thermal reactions of phoborhodopsin (pR490), which mediates the negative phototaxis (phobic reaction) of Halobacterium halobium, were investigated by low-temperature spectrophotometry. At room temperature, the absorption spectrum of pR490 displayed vibrational structure with a maximum at 490 nm and a shoulder at 460 nm, which were remarkably sharpened by cooling, resulting in the appearance of two well-separated peaks. On irradiation of pR490 at -170 degrees C, a photo-steady-state mixture composed of pR490 and two photoproducts, P520 and P480, was formed. P480 had an absorption maximum at 480 nm and thermally converted to pR490 above -160 degrees C, while P520 had an absorption maximum at 515 nm and thermally converted to P350, the next intermediate, above -60 degrees C. Above -30 degrees C, P350 was converted to P530, and then reverted to pR490. P520, P350, and P530 may correspond to K, M, and O intermediates of bacteriorhodopsin, respectively, on the basis of their absorption spectra, but the intermediates corresponding to L and N intermediates were not observed. On the basis of these results, a new scheme of the photoreaction cycle of pR490 was presented.

Conformation changes of cuttlefish (Euprymna morsei) rhodopsin following photoconversion

Cuttlefish (Euprymna morsei) rhodopsin solubilized in lauryl ester of sucrose and its photoproduct, acid metarhodopsin, were examined by small-angle X-ray scattering and chromatofocusing to investigate the conformation changes of visual pigment following photoconversion. From spectroscopic studies, it was found that more than 93% of Euprymna rhodopsin could be converted to meta form under the condition of red light irradiation at neutral pH. Since almost pure acid metarhodopsin solution was prepared without changing the specimen concentration, the small-angle X-ray scattering intensities of both pigment-detergent complexes were directly compared. The radius of gyration increased on going from rhodopsin to acid metarhodopsin by approximately 1.5%. There were also discernible changes in the secondary peak intensities. The distribution function, derived by the Fourier transformation of intensity data, showed a significant change around 55 A. The maximum linear dimension of the rhodopsin-detergent complex was about 95 A and hardly changed after illumination. Intensity at zero angle did not change after illumination, suggesting that the aggregation did not occur. The change of the intensity profile could be due to the conformational change of the pigment-detergent monomers. The pI value of rhodopsin determined by chromatofocusing was 5.32 and that of acid metarhodopsin was 5.06, indicating that a few carboxyl groups are newly dissociated. The shift of the protein mass and the charge redistribution were observed following photoconversion.

Site of conversion of endogenous all-trans-retinoids to 11-cis-retinoids in the bovine eye

By use of a new high-resolution high-pressure liquid chromatographic method for the separation of isomeric forms of retinol, retinal, retinyl ester and retinal oxime, various retinoids were analyzed in separated retinal pigment epithelial tissue or neural retinal tissue from fresh bleached bovine eyes after incubation in the dark at either 30 or 4 degrees C for 90 min. 11-cis-Retinoids significantly increased during incubation at 30 degrees C, relative to those at 4 degrees C, in the retinal pigment epithelium, but not in the retina. The major forms of vitamin A in incubated retinal pigment epithelium and neural retina were retinyl esters (70%) and all-trans-retinol (69%), respectively. Thus, in keeping with observations on the isomerization of radioactive retinol in homogenates of eye tissues, the retinal pigment epithelium seems to be the primary site of 11-cis-retinoid formation from endogenous all-trans-retinoids in the bovine eye.

The primary process of vision and the structure of bathorhodopsin: a mechanism for photoisomerization of polyenes

A model for the primary process of vision is proposed, which involves a novel concerted-twist motion. Application of such motions to rhodopsin and bathorhodopsin successfully accounts for the properties of bathorhodopsin and related intermediates, including specific assignment of molecular structures to bathorhodopsin, to lumirhodopsin, and, less specifically, to hypsorhodopsin.

Formation of hypsorhodopsin at room temperature by picosecond green pulse

Excitation of squid rhodopsin with a single laser pulse (532 nm, 25 ps) at 18 degrees C yielded photorhodopsin, a precursor of bathorhodopsin. In the linear region, no relation between amount of photorhodopsin and excitation-energy hypsorhodopsin was detected, while in a photon saturation region this was observed. The time constant of hypsorhodopsin to bathorhodopsin decay was about 125 ps. Dependencies of formation of photorhodopsin and hypsorhodopsin on the excitation energy suggest that hypsorhodopsins of squid and octopus are formed by a two-photon reaction. No cattle hypsorhodopsin was detected in our experimental conditions.

Primary intermediates of rhodopsin studied by low temperature spectrophotometry and laser photolysis. Bathorhodopsin, hypsorhodopsin and photorhodopsin

The primary photochemical processes of rhodopsin studied by low temperature spectrophotometry and picosecond laser spectroscopy in our group was summarized. Low temperature spectroscopic experiments demonstrated that the retinylidene chromophores of hypso- and bathorhodopsins are in a twisted all-trans forms. Excitation of rhodopsin with 532 nm laser pulse (width: 25 psec) yielded a new bathochromic photoproduct "photorhodopsin"; its spectrum was located at longer wavelengths than that of bathorhodopsin. Photorhodopsin decays to bathorhodopsin with time constants of about 200 psec in squid and 40 psec in cattle. Squid and octopus hypsorhodopsins were produced within 25 psec by high energy pulse, but not by low energy pulse. Thus hypsorhodopsin is produced by two photon reactions (sequential two photochemical reactions) and decayed to bathorhodopsin with time constant of 125 psec.

Activation of phosphodiesterase by rhodopsin and its analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35.) in frog rod outer segment membrane by rhodopsin and its analogues was investigated. The Schiff-base linkage between opsin and retinal in rhodopsin was not always necessary for the phosphodiesterase activation. The binding of beta-ionone ring of retinal to a hydrophobic region of opsin was not enough to induce the enzyme activation. A striking photo-activation of the enzyme was induced by photo-isomerization of rhodopsin analogues from cis to trans form. It seems probable that an "expanded" conformation of opsin around the retinylidene chromophore induced by the cis to trans isomerization may be the trigger for the activation of phosphodiesterase. On the other hand, the phosphodiesterase in frog rod outer segment was activated by warming of bathorhodopsin to -12 degrees C and then incubating it at the same temperature. Thus, metarhodopsin II or an earlier intermediate than metarhodopsin II should be a direct intermediate for the enzyme activation.