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.     

Photointermediates of visual pigments

Much progress has been made in recent years toward understanding the interactions between various proteins responsible for visual transduction which are initiated by an activated state of visual pigments. However, the changes which take place in the visual pigments themselves to convert them to the activated state are more poorly understood. Many spectroscopic techniques have been applied to this problem in recent years and considerable progress has been made. A major goal of these efforts is to understand at which stages protein change occurs and to characterize its structural features. In the visual system evidence is accumulating, for example, that chromophore independent protein change begins immediately prior to lumirhodopsin formation. Considerable insight has been gained recently into the early intermediates of visual transduction and the stage is set to achieve similar understanding of the later intermediates leading to rhodopsin's activated state.     

Microspectrophotometry of the visual pigment of the spider crab Libinia emarginata

Summary Isolated dark-adapted rhabdoms from the spider crab Libinia emarginata were examined by microspectrophotometry to determine the visual pigments present and their light-sensitive characteristics. The rhabdoms contain a single pigment with _max=493 nm. Upon one minute irradiation with bright orange light this pigment forms a light-stable photoproduct with nearly the same _max as the parent pigment but with slightly greater absorption to the long wavelength side of the absorption peak. On exposure to orange or yellow light in the presence of 5% glutaraldehyde, however, the pigment of Libinia rhabdoms bleaches slowly.The photosensitive pigment of properly oriented, transversely illuminated rhabdoms shows isotropic and dichroic regions, corresponding to layers of the rhabdom in which the microvilli are respectively parallel and perpendicular to the direction of propagation of the measuring beam. The maximum dichroic ratio is about 2, with most absorption when the plane of polarization is parallel to the microvillar axes.

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Zusammenfassung Die isolierten, dunkeladaptierten Rhabdome von Libinia emarginata wurden mikrospektrophotometrisch untersucht, um die Sehfarbstoffe und ihre lichtempfindlichen Eigenschaften zu entdecken. Die Rhabdome enthalten ein einziges Pigment (_max 493 nm). Nach einer Bestrahlungszeit von 1 min mit hellem orangem Licht entsteht ein lichtfestes Photoprodukt mit beinahe dem gleichen _max wie das ursprüngliche Pigment, aber mit etwas größerer Absorption gegen die langwellige Seite des Absorptionsmaximums hin. Das Pigment von Libinia-Rhabdomen bleicht langsam aus, wenn man es orangem oder gelbem Licht in Gegenwart von 5% Glutaraldehyd aussetzt.Werden richtig orientierte Rhabdome von der Seite belichtet, so zeigt das lichtempfindliche Pigment teils Isotropismus teils Dichroismus. Im ersten Fall sind die Mikrovilli parallel zur Richtung des Meßstrahles orientiert, im zweiten Fall stehen sie senkrecht dazu. Das größte Dichroismus-Verhältnis liegt bei 2. Man erhält die größte Absorption, wenn die Polarisationsrichtung parallel zur Achse der Mikrovilli steht.

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This work was supported by U.S.P.H.S. grant NB-03333. Some of the experiments were done at The Marine Biological Laboratory, Woods Hole, Mass.     

Fly visual pigments difference in visual pigments of blowfly and dronefly peripheral retinula cells

Journal of Comparative Physiology A - The visual pigments of peripheral retinula cells in fly eyes have been investigated by microspectrophotometry in vivo. Since flies have a pupil mechanism...     

Evolution of visual pigments in geckos

Most geckos are nocturnal forms and possess rod retinas, but some diurnal genera have pure-cone retinas. We isolated cDNAs encoding the diurnal gecko opsins, dg1 and dg2, similar to nocturnal gecko P521 and P467, respectively. Despite the large morphological differences between the diurnal and nocturnal gecko photoreceptor types, they express phylogenetically closely related opsins. These results provide molecular evidence for the reverse transmutation, that is, rods of an ancestral nocturnal gecko have backed into cones of diurnal geckos. The amino acid substitution rates of dgl and dg2 are higher than those of P521 and P467, respectively. Changes of behavior regarding photic environment may have contributed to acceleration of amino acid substitutions in the diurnal gecko opsins.     

On the visual pigments of deep-sea fish

The retinal visual pigments of 52 species of deep-sea fish were measured by partial bleaching of detergent extracts. The retinae of 45 species contained only a single rhodopsin with maximum absorbance (λ_max) at a wavelength between 474 and 490 nm, matching both the region of highest intensity downwelling sunlight and the maximum emission of most deep-sea bioluminescence. Seven species were shown to have more than one visual pigment within their retinae and these had λ_max values that generally fell outside the usual range. One of these, Bonapartia pedaliota , was particularly interesting as, unlike most such multipigment species, it had one rhodopsin and one porphyropsin pigment, apparently based on different opsins. The relative proportions of the visual pigments in the seven multipigment species are presented.     

Thermal activation and photoactivation of visual pigments

A visual pigment molecule in a retinal photoreceptor cell can be activated not only by absorption of a photon but also "spontaneously" by thermal energy. Current estimates of the activation energies for these two processes in vertebrate rod and cone pigments are on the order of 40-50 kcal/mol for activation by light and 20-25 kcal/mol for activation by heat, which has forced the conclusion that the two follow quite different molecular routes. It is shown here that the latter estimates, derived from the temperature dependence of the rate of pigment-initiated "dark events" in rods, depend on the unrealistic assumption that thermal activation of a complex molecule like rhodopsin (or even its 11-cis retinaldehyde chromophore) happens through a simple process, somewhat like the collision of gas molecules. When the internal energy present in the many vibrational modes of the molecule is taken into account, the thermal energy distribution of the molecules cannot be described by Boltzmann statistics, and conventional Arrhenius analysis gives incorrect estimates for the energy barrier. When the Boltzmann distribution is replaced by one derived by Hinshelwood for complex molecules with many vibrational modes, the same experimental data become consistent with thermal activation energies that are close to or even equal to the photoactivation energies. Thus activation by light and by heat may in fact follow the same molecular route, starting with 11-cis to all-trans isomerization of the chromophore in the native (resting) configuration of the opsin. Most importantly, the same model correctly predicts the empirical correlation between the wavelength of maximum absorbance and the rate of thermal activation in the whole set of visual pigments studied.     

Photoregeneration of visual pigments in a moth

Summary The spectral absorbance by the visual pigments in the compound eye of the mothDeilephila elpenor was determined by microphotometry. Two visual pigments and their photoproducts were demonstrated. The photoproducts are thermostable and are reconverted to the visual pigments by light. The concentrations of the visual pigments and the photoproducts at each wavelength are determined by their absorbance coefficients at this wavelength. P 525: The experimental recordings (difference spectra and spectral absorbance changes after exposure to monochromatic lights) were completely reproduced by calculations using nomograms for vertebrate rhodopsin. The identity between experimental recordings and calculations show: One visual pigment absorbs maximally at 525 nm (P 525). The resonance spectrum of the visual pigment is identical to that for a vertebrate rhodopsin (_max at 525 nm). The photoproduct of this pigment absorbs maximally at 480 nm (M 480). It is similar to the acid metarhodopsin in cephalopods. The relative absorbance of P 525 to that of M 480 is 11.75. The quantum efficiency for photoconversion of P 525 to M 480 is nearly equal to that for reconversion of M 480 to P 525. Wavelengths exceeding about 570 nm are absorbed only by P 525, i. e. P 525 is completely converted to M 480. Shorter wavelengths are absorbed both by P 525 and M 480. At these wavelengths a photoequilibrium between the two pigments is formed. Maximal concentration of P 525 is obtained at about 450 nm. P 350: A second visual pigment absorbs maximally at about 350 nm (P 350), and its photoproduct at 450 to 460 nm. In the region of spectral overlap a photoequilibrium between the two pigments is formed.The visual pigment and the photoproduct are similar to those in the neuropteran insectAscalaphus.The work reported in this article was supported by Deutsche Forschungsgemeinschaft, Schwerpunktsprogramm Rezeptorphysiologie Ha 258-10, and SFB 114, by the Swedish Medical Research Council (grant no B 73-04X-104-02B), by Karolinska Institutet, and by a grant (to G. Höglund) from Deutscher Akademischer Austauschdienst.     

How color visual pigments are tuned.

The absorption maximum of the retinal chromophore in color visual pigments is tuned by interactions with the protein (opsin) to which it is bound. Recent advances in the expression of rhodopsin-like transmembrane receptors and in spectroscopic techniques have allowed us to measure resonance Raman vibrational spectra of the retinal chromophore in recombinant visual pigments to examine the molecular basis of this spectral tuning. The dominant physical mechanism responsible for the opsin shift in color vision is the interaction of dipolar amino acid residues with the ground- and excited-state charge distributions of the chromophore.     

Microspectrophotometry ofEuglena gracilis

The paraflagellar bodies (PFBs) of isolated flagella ofEuglena gracilis were investigated microspectrophotometrically using a visible- and infrared-light microscope with image analyzer and microspectrophotometer. Flagella with attached PFBs were separated from the cell bodies by a short exposure to near-UV light. Fluorescence-emission spectra (excitation at 365 nm) of single PFBs had maxima near 470 and 520 nm, indicating the presence of pterins and flavins. No fluorescence was associated with the flagella themselves. Pterin- and flavin-like fluorescence emission was also found in blue-fluorescing vesicles distributed throughout the entire cell body ofEuglena. Their characterization by microfluorimetry was greatly aided by the use of chlorophyll-free mutants in which the signal-to-noise ratio was distinctly enhanced because of the lack of chlorophyll fluorescence. Our finding of flavin-like fluorescence associated with PFB strengthens similar earlier reports in the literature. The occurrence of pterin-like fluorescence in the PFB lends further support to our earlier proposal that pterins as well as flavins may function as photoreceptor pigments for near-UV and blue light.     

The visual pigments of four deep-sea crustacean species

Summary The visual pigments of four mesopelagic crustacean species were studied at sea by means of microspectrophotometry. The absorbance maxima obtained for the visual pigments and their metarhodopsins, respectively, were: 493 nm and 481 nm (Systellaspis debilis), 485 nm and 480 nm (Acanthephyra curtirostris), 491 nm and 482 nm (A. smithi), and 495 nm and 487 nm (Sergestes tenuiremis). The spectral characteristics of the rhodopsins and metarhodopsins permit high photosensitivity and facilitate photoregeneration in a nearly monochromatic environment. Photic regeneration of rhodopsins from the deep-sea environment was demonstrated, and data were obtained which are consistent with the occurrence of dark regeneration. Specific optical density of the observed visual pigments was calculated for two species.     

The cone photoreceptors and visual pigments of chameleons

Visual pigments, oil droplets and photoreceptor types in the retinas of four species of true chameleons have been examined by microspectrophotometry. The species occupy different photic environments: two species of Chamaeleo are from Madagascar and two species of Furcifer are from Africa and the Arabian Peninsula. In addition to double cones, four spectrally distinct classes of single cone were identified. No rod photoreceptors were observed. The visual pigments appear to be mixtures of rhodopsins and porphyropsins. Double cones contained a pale oil droplet in the principle member and both outer segments contained a long-wave-sensitive visual pigment with a spectral maximum between about 555 nm and 610 nm, depending on the rhodopsin/porphyropsin mixture. Long-wave-sensitive single cones contained a visual pigment spectrally identical to the double cones, but combined with a yellow oil droplet. The other three classes of single cone contained visual pigments with maxima at about 480–505, 440–450 and 375–385 nm, combined with yellow, clear and transparent oil droplets respectively. The latter two classes were sparsely distributed. The transmission of the lens and cornea of C. dilepis was measured and found to be transparent throughout the visible and near ultraviolet, with a cut off at about 350 nm.     

Purification of cone visual pigments from chicken retina

A novel method for purification of chicken cone visual pigments was established by use of a 3-[(3-cholamidopropyl)dimethylammonio]-1- propanesulfonate-phosphatidylcholine (CHAPS-PC) mixture. Outer segment membranes isolated from chicken retinas were extracted with 0.75% CHAPS supplemented with 1.0 mg/mL phosphatidylcholine (CHAPS-PC system). After the extract was diluted to 0.6% CHAPS, it was loaded on a concanavalin A-Sepharose column. Elution from the column with different concentrations of methyl alpha-mannoside yielded three fractions: the first was composed of chicken violet, blue, and red in roughly equal amounts, the second predominantly contained chicken red, and the third was rhodopsin with a small amount of chicken green, which was separated from rhodopsin by DEAE-Sepharose column chromatography. Since CHAPS has little absorbance at both ultraviolet and visible regions, we could demonstrate the absolute absorption spectra of chicken red (92%) and rhodopsin (greater than 96%) in these regions. The maximum of the difference spectrum between either chicken red or rhodopsin and its photoproduct (all-trans-retinal oxime plus opsin) was determined to be 571 or 503 nm, respectively. Although chicken green was contaminated with a small amount of rhodopsin having a similar spectral shape, the maximum of its difference spectrum was located at 508 nm by taking advantage of the difference in susceptibility against hydroxylamine between these pigments. Although chicken blue and chicken violet were minor pigments present in the first fraction from the concanavalin A column, their maxima in the difference spectra were determined to be at 455 and 425 nm, respectively, by a partial bleaching method.(ABSTRACT TRUNCATED AT 250 WORDS)