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PubMed2004

Modeling color percepts of dichromats.

Protanopes and deuteranopes, despite lacking a chromatic dimension at the receptor level, use the color terms "red" and "green", together with "blue" and "yellow", to describe their color percepts. Color vision models proposed so far fail to account for these findings in dichromats. We confirmed, by the method of hue scaling, the consistent use of these color terms, as well as their dependence on intensity, in subjects shown to have only a single X-chromosomal opsin gene each. We present a model for the processing of photoreceptor signals which, under physiologically plausible assumptions, achieves a trichromat-like representation of dichromatic receptor signals. Key feature of the dichromat model is the processing of the photoreceptor signals in parallel channels with different gains and nonlinearities. In this way, the two-dimensional receptor signals are represented on a manifold in a higher-dimensional space, supporting categorization for efficient image segmentation. Introducing a third cone opsin yields a model that explains normal, trichromat hue scaling.

Base Sequence↗
PubMed1997

Rayleigh match ranges of red/green color-deficient observers: psychophysical and molecular studies.

Large-field Rayleigh match ranges were measured in 27 red/green color-deficient male observers, using bright, temporally alternating, 3-9 deg annular test fields. The observers' X-linked opsin gene arrays were characterized by molecular genetic techniques, and used to infer the absorption maxima of each observer's L and/or M cone photopigment(s). Measured match ranges decreased rapidly as the inferred separation in pigment absorption maxima increased from 0 to 2-3 nm, and varied irregularly thereafter. Following He & Shevell [(1995) Vision Research, 35, 2579-2588] predicted match ranges were calculated for various pigment separations and assumed values of pigment optical density. The predicted variations in match range encompassed the measured match ranges of most (but not all) of the color-deficient observers. The calculations also showed that differences in pigment optical density, in two cone types containing the same pigment, are sufficient to allow a moderate degree of chromatic discrimination. Such models thus provide a possible account of the fact that some color-deficient observers, with only a single X-linked opsin gene, can make red/green chromatic discriminations.

Adolescent↗
PubMed2002

Opsin cDNA sequences of a UV and green rhodopsin of the satyrine butterfly Bicyclus anynana.

The cDNAs of an ultraviolet (UV) and long-wavelength (LW) (green) absorbing rhodopsin of the bush brown Bicyclus anynana were partially identified. The UV sequence, encoding 377 amino acids, is 76-79% identical to the UV sequences of the papilionids Papilio glaucus and Papilio xuthus and the moth Manduca sexta. A dendrogram derived from aligning the amino acid sequences reveals an equidistant position of Bicyclus between Papilio and Manduca. The sequence of the green opsin cDNA fragment, which encodes 242 amino acids, represents six of the seven transmembrane regions. At the amino acid level, this fragment is more than 80% identical to the corresponding LW opsin sequences of Dryas, Heliconius, Papilio (rhodopsin 2) and Manduca. Whereas three LW absorbing rhodopsins were identified in the papilionid butterflies, only one green opsin was found in B. anynana.

Amino Acid Sequence↗
PubMed1987

A second opsin gene expressed in the ultraviolet-sensitive R7 photoreceptor cells of Drosophila melanogaster.

We have identified a second Drosophila opsin gene, Rh4, which is expressed specifically in the ultraviolet-sensitive R7 photoreceptor cells. The two R7 opsins, Rh3 and Rh4, are expressed in nonoverlapping subsets of R7 cells. The Rh4 opsin shares 72% amino acid identity with Rh3, but is only about 35% homologous with the other 2 Drosophila opsins, ninaE and Rh2. Twenty-eight amino acids are conserved among all 4 invertebrate and 5 vertebrate opsin genes thus far sequenced, suggesting that these residues play an important role in rhodopsin function.

Amino Acid Sequence↗
PubMed1997

Parapinopsin, a novel catfish opsin localized to the parapineal organ, defines a new gene family.

Multiple sites of extraretinal photoreception are present in vertebrates, but the molecular basis of extraretinal phototransduction is poorly understood. This study reports the cloning of the first opsin specifically expressed in the directly photosensitive pineal and parapineal of cold-blooded vertebrates. This opsin, identified in channel catfish and termed parapinopsin, defines a new gene family of vertebrate photopigments and is expressed in a majority of parapinealocytes and a subset of pineal photoreceptor cells. Parapinopsin shows a caudal-rostral gradient of expression within the pineal organ. This study also reports the cloning of partial cDNAs encoding the channel catfish orthologues of rhodopsin and the red cone pigment-the full complement of retinal opsins in the species. In situ hybridization studies using probes derived from these retinal opsins, together with parapinopsin, reveal no expression of retinal opsins in pineal and parapineal organ and no expression of any opsin tested in the "deep brain," iris, or dermal melanophores. These data imply that phototransduction in these sites of extraretinal photoreception must be mediated by novel opsins.

Amino Acid Sequence↗
PubMed1998

Glycosylation and palmitoylation are not required for the formation of the X-linked cone opsin visual pigments.

PURPOSE: This study was designed to test whether palmitoylation and glycosylation are required for the formation of the green opsin visual pigment. METHODS: Stable cell lines were established by transfecting EBNA-293 cells with a pMEP4ss recombinant plasmid containing wild-type bovine rhodopsin or wild-type or mutant (N32S) green opsin cDNA molecules that included a tag for the eight amino acid residues located at the C-terminus of rhodopsin. The opsins were induced by addition of CdCl2 into the medium and then reconstituted with 11-cis-retinal. The reconstituted opsins were purified by immunoaffinity chromatography, then analyzed by difference spectra, and by binding 35S-GTP in the presence of bovine transducin. Non-reconstituted opsins were analyzed by Western blotting and by pulse-labeling with 3H-palmitic acid followed by immunoprecipitation. RESULTS: Elimination of glycosylation by mutagenesis of the N-linked glycosylation site did not impair the ability of the resulting cone opsin to absorb light at the appropriate wavelength nor to activate transducin. Furthermore, as judged by pulse-labeling with 3H-palmitic acid and immunoprecipitation and by gas chromatography-mass spectroscopy, the wild type green opsin differs from rhodopsin by not being palmitoylated. CONCLUSIONS: Glycosylation and palmitoylation are not required for the formation of cone opsin visual pigments. For the previously described green opsin C203R mutation, disruption of folding and transport, rather than altered glycosylation is sufficient to explain the associated color vision deficiency.

Blotting, Western↗
PubMed2005

Physiological characterization of the compound eye in monarch butterflies with focus on the dorsal rim area.

The spectral, angular and polarization sensitivities of photoreceptors in the compound eye of the monarch butterfly (Danaus plexippus) are examined using electrophysiological methods. Intracellular recordings reveal a spectrally homogenous population of UV receptors with optical axes directed upwards and >or=10 degrees to the contralateral side. Based on optical considerations and on the opsin expression pattern (Sauman et al. 2005), we conclude that these UV receptors belong to the anatomically specialized dorsal rim area (DRA) of the eye. Photoreceptors in the main retina with optical axes <10 degrees contralateral or ipsilateral have maximal sensitivities in the UV (lambda(max)<or=340 nm), the blue (lambda(max)=435 nm) or in the long-wave range (green, lambda(max)=540 nm). The polarization sensitivity (PS) of the UV receptors in the DRA is much higher (PS=9.4) than in the UV cells (PS=2.9) or green cells (PS=2.8) of the main retina. The physiological properties of the photoreceptors in the DRA and in the main retina fit closely with the anatomy and the opsin expression patterns described in these eye regions. The data are discussed in the light of present knowledge about polarized skylight navigation in Lepidopterans.

Animals↗
PubMed1993

Haplotype diversity in the human red and green opsin genes: evidence for frequent sequence exchange in exon 3.

We studied polymorphisms in the coding sequences of the human red and green opsin genes of 133 Caucasian males. Eleven polymorphic sites were discovered in the red opsin gene, seven of which were in exon 3, three in exon 4 and one in exon 5. Polymorphisms at 8 of these sites resulted in amino acid substitutions which generated a total of 18 unique red opsins in the population. The substitutions at three (S180A, I230T, and A233S) of the 8 sites involve hydroxyl-bearing to non-polar amino acid residues, and are therefore likely to alter spectral characteristics of the red pigment. Eight polymorphic sites were observed in the green opsin coding sequences, six of which were in exon 3, one in exon 2 and one in exon 5. Five of the eight involved amino acid substitutions which generated 15 unique green opsins in the population. Substitutions at two of these sites involve hydroxyl-bearing vs. non-polar residues. Six polymorphisms, all of which are located in exon 3, are shared between the red and green opsin genes, essentially making it difficult to assign this exon to either of these genes. Markers in exon 3 are in partial linkage disequilibrium with those in exons 4 and 5, whereas the latter two are in strong linkage disequilibrium with each other. Furthermore, markers in the 5' region of exon 3 are also in only partial (54%) disequilibrium with those in the 3' region. The above results strongly suggest a history of frequent gene conversion, mainly localized to exon 3, in the lineages leading to the human red and green opsin genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed2002

Molecular evolution of proteins involved in vertebrate phototransduction.

Vision is one of the most important senses for vertebrates. As a result, vertebrates have evolved a highly organized system of retinal photoreceptors. Light triggers an enzymatic cascade, called the phototransduction cascade, that leads to the hyperpolarization of photoreceptors. It is expected that a systematic comparison of phototransduction cascades of various vertebrates can provide insights into the diversity of vertebrate photoreceptors and into the evolution of vertebrate vision. However, only a few attempts have been made to compare each phototransduction protein participating in this cascade. Here, we determine phylogenetic trees of the vertebrate phototransduction proteins and compare them. It is demonstrated that vertebrate opsin sequences fall into five fundamental subfamilies. It is speculated that this is crucial for the diversity of the spectral sensitivity observed in vertebrate photoreceptors and provides the vertebrates with the molecular tools to discriminate the color of incident light. Other phototransduction proteins can be classified into only a few subfamilies. Cones generally share isoforms of phototransduction proteins that are different from those found in rods. The difference in sensitivity to light between rods and cones is likely due to the difference in the molecular properties of these isoforms. The phototransduction proteins seem to have co-evolved as a system. Switching the expression of these isoforms may characterize individual vertebrate photoreceptors.

Amino Acid Sequence↗
PubMed1997

Opsin-like immunoreactivity in the circadian pacemaker neurons and photoreceptors of the eye of the opisthobranch mollusc Bulla gouldiana.

Circadian pacemaker cells in the eyes of the opisthobranch mollusc Bulla gouldiana generate a near 24-h rhythm in the frequency of optic nerve impulses. Previous electrophysiological studies suggest that these basal retinal neurons are intrinsically photosensitive and transduce light signals that shift the phase of their pacemaker mechanism. To test whether the pacemaker neurons contain opsin-like proteins, several polyclonal antibodies that recognize opsins of vertebrate photoreceptors have been tested on histological sections of the eye and on the neurons in primary cell culture. The antibodies label both the pacemaker cells and the large distal photoreceptors that surround the lens. Immunoblot analyses of the proteins of the eye have identified a single band at 62+/-4 kDa. These opsin antibodies may label the photopigment used in the entrainment of the circadian pacemaker.

Animals↗
PubMed2006

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate.

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.

Animals↗
PubMed2002

Y-chromosomal red-green opsin genes of nocturnal New World monkey.

The X-chromosomal locality of the red-green-sensitive opsin genes has been the norm for all mammals and is essential for color vision of higher primates. Owl monkeys (Aotus), a genus of New World monkeys, are the only nocturnal higher primates and are severely color-blind. We demonstrate that the owl monkeys possess extra red-green opsin genes on the Y-chromosome. The Y-linked opsin genes were found to be extremely varied, in one male appearing to be a functional gene and in other males to be multicopy pseudogenes. These Y-linked opsin genes should offer a rare opportunity to study the evolutionary fate of genes translocated to the Y chromosome.

Animals↗
PubMed1998

Numbers and ratios of X-chromosomal-linked opsin genes.

Quantitative Southern blotting and PCR/RFLP analysis were used to determine the number and ratio of long-wave-sensitive (L-) and mid-wave-sensitive (M-) opsin genes in 25 colour-normal caucasian males. The average observed ratio was 1:2.8 +/- 1.2 for Southern blot analysis and 1:3.0 +/- 1.7 for PCR/RFLP analysis. Thus, the two techniques yielded similar results for the ratio of L- to M-opsin genes (Wilcoxon t-test, P < 0.01). PCR/RFLP analysis of a Sma I polymorphism specific for the most proximal opsin gene suggested an average gene number of 6.0 +/- 2.1, with a range from 4 to 12 in individual subjects. In contrast, Southern blot analysis suggested an average number of 3.8 +/- 1.2, with a range from 2 to 7 (on the assumption that only one L-opsin gene is ever present). Differences between the L- to M-opsin gene ratio and the total gene number in some subjects may result from the presence of multiple L-opsin genes and/or hybrid opsin genes in colour-normal males. An exact determination of the total gene number will require employing other molecular techniques.

Blotting, Southern↗
PubMed1984

Specific reaction of 9-cis-retinoyl fluoride with bovine opsin.

Opsin readily undergoes Schiff base formation between an active site lysine and 9-cis- or 11-cis-retinaldehyde to form the visual pigments isorhodopsin (lambda max = 487 nm) and rhodopsin (lambda max = 500 nm), respectively (Dratz, 1977). It would be predicted that 9-cis-retinoyl fluoride (1), an isostere of 9-cis-retinal, should be an active site directed, mechanism-based labeling agent of opsin, since a stable peptide bond should be formed instead of a Schiff base. It is shown here that 9-cis-retinoyl fluoride (1) reacts with opsin in a time-dependent fashion (t1/2 = 9 min at 25 microM 1) to form a new, nonbleachable pigment with a lambda max of approximately 365 nm. beta-Ionone competitively slows down the rate of the reaction. The absorbance of the new pigment at approximately 365 nm is similar to that of model amide compounds. This result is consistent in a general and qualitative way with the Nakanishi-Honig point-charge model for visual pigments which requires that the chromophore be charged, a situation not possible when the retinoid is linked to opsin via a peptide bond rather than a protonated Schiff base [Honig, B., Dinur, U., Nakanishi, K., Balogh-Nair, V., Gawinowicz, M.A., Arnabaldi, M., & Motto, M.G. (1979) J. Am. Chem. Soc. 101, 7084-7086]. 9-cis-Retinoyl fluoride (1) is approximately 4-fold more potent than all-trans-retinoyl fluoride (2) as an inactivator of bovine opsin. Importantly, 13-cis-retinoyl fluoride (3) is inactive, and no new absorption band at 365 nm is observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

Molecular characterization and expression of the UV opsin in bumblebees: three ommatidial subtypes in the retina and a new photoreceptor organ in the lamina.

Ultraviolet-sensitive photoreceptors have been shown to be important for a variety of visual tasks performed by bees, such as orientation, color and polarization vision, yet little is known about their spatial distribution in the compound eye or optic lobe. We cloned and sequenced a UV opsin mRNA transcript from Bombus impatiens head-specific cDNA and, using western blot analysis, detected an eye protein band of approximately 41 kDa, corresponding to the predicted molecular mass of the encoded opsin. We then characterized UV opsin expression in the retina, ocelli and brain using immunocytochemistry. In the main retina, we found three different ommatidial types with respect to the number of UV opsin-expressing photoreceptor cells, namely ommatidia containing two, one or no UV opsin-immunoreactive cells. We also observed UV opsin expression in the ocelli. These results indicate that the cloned opsin probably encodes the P350 nm pigment, which was previously characterized by physiological recordings. Surprisingly, in addition to expression in the retina and ocelli, we found opsin expression in different parts of the brain. UV opsin immunoreactivity was detected in the proximal rim of the lamina adjacent to the first optic chiasm, which is where studies in other insects have found expression of proteins involved in the circadian clock, period and cryptochrome. We also found UV opsin immunoreactivity in the core region of the antennal lobe glomeruli and different clusters of perikarya within the protocerebrum, indicating a putative function of these brain regions, together with the lamina organ, in the entrainment of circadian rhythms. In order to test for a possible overlap of clock protein and UV opsin spatial expression, we also examined the expression of the period protein in these regions.

Amino Acid Sequence↗
PubMed1990

Opsin synthesis and mRNA levels in dystrophic retinas devoid of outer segments in retinal degeneration slow (rds) mice.

Opsin gene regulation, as a function of outer segment structure, was studied in normal and mutant retinal degeneration slow (rds) mice. We investigated the level of expression of the opsin gene in the rds mutant to determine if the reduced opsin content observed in this mutation (around 3% of normal) is a consequence of lowered expression of its gene. Normal BALB/c and rds mice were analyzed for levels of opsin mRNA and opsin content by Northern and immunoblot analysis, respectively. The rate of opsin synthesis in isolated retinas was measured by 35S-methionine incorporation in vitro, followed by analysis of the radiolabeled opsin by SDS-gel electrophoresis and autoradiography. Photoreceptor cell loss at various stages of degeneration was determined by quantitation of surviving photoreceptor nuclei. Opsin was localized in the mutant photoreceptors by immunoelectron microscopy of LR gold-embedded retinas using anti-opsin and antibody gold conjugates. The results indicate that 11- and 30-d-old mutant mice have considerable levels of opsin mRNA (60-70% of normal) and opsin synthetic rates (76-92% of normal), after the data from mutant mice are corrected for photoreceptor cell loss. We conclude, therefore, that the very low level of opsin observed in rds mice (approximately 3%) is not a result of greatly reduced expression of the opsin gene. Rather, continuous turnover of newly synthesized opsin as a result of its failure to become sequestered into an intact outer segment appears to account for the low levels of opsin in the rds mutant.

Animals↗
PubMedDate not supplied

Cone visual pigments of aquatic mammals.

It has long been hypothesized that the visual systems of animals are evolutionarily adapted to their visual environment. The entrance many millions of years ago of mammals into the sea gave these new aquatic mammals completely novel visual surroundings with respect to light availability and predominant wavelengths. This study examines the cone opsins of marine mammals, hypothesizing, based on previous studies [Fasick et al. (1998) and Levenson & Dizon (2003)], that the deep-dwelling marine mammals would not have color vision because the pressure to maintain color vision in the dark monochromatic ocean environment has been relaxed. Short-wavelength-sensitive (SWS) and long-wavelength-sensitive (LWS) cone opsin genes from two orders (Cetacea and Sirenia) and an additional suborder (Pinnipedia) of aquatic mammals were amplified from genomic DNA (for SWS) and cDNA (for LWS) by PCR, cloned, and sequenced. All animals studied from the order Cetacea have SWS pseudogenes, whereas a representative from the order Sirenia has an intact SWS gene, for which the corresponding mRNA was found in the retina. One of the pinnipeds studied (harp seal) has an SWS pseudogene, while another species (harbor seal) appeared to have an intact SWS gene. However, no SWS cone opsin mRNA was found in the harbor seal retina, suggesting a promoter or splice site mutation preventing transcription of the gene. The LWS opsins from the different species were expressed in mammalian cells and reconstituted with the 11-cis-retinal chromophore in order to determine maximal absorption wavelengths (lambda(max)) for each. The deeper dwelling Cetacean species had blue shifted lambda(max) values compared to shallower-dwelling aquatic species. Taken together, these findings support the hypothesis that in the monochromatic oceanic habitat, the pressure to maintain color vision has been relaxed and mutations are retained in the SWS genes, resulting in pseudogenes. Additionally, LWS opsins are retained in the retina and, in deeper-dwelling animals, are blue shifted in lambda(max).

Amino Acid Sequence↗
PubMed1993

Constitutive activation of opsin: influence of charge at position 134 and size at position 296.

In previous studies, mutation of Lys296 or Glu113 in opsin has been shown to result in constitutive activation of the protein--that is, these mutants can activate the G protein transducin in the absence of chromophore and in the absence of light. These and other data have led to the suggestion that a salt bridge between Lys296 and Glu113 helps to constrain opsin to an inactive conformation. It is shown here that of 12 different amino acids substituted at position 296, all, except Arg and the wild-type Lys, are constitutively active at neutral pH, lending further support to this suggestion. However, activation of opsin appears also to be influenced significantly by the size of amino acid side chain at position 296. Thus, there are multiple effects of the mutations. Wild-type opsin is also shown to be weakly active at pH 6.1. Five other charged amino acids in the membrane-embedded region of the protein (Asp83, Glu122, Glu134, Arg135, and Glu201) were mutated to see if they affect constitutive activity. Of these amino acids, only mutation of Glu134 results in an increase in the activity of opsin. Changing Glu134 to Gln increases the activity of opsin, while changing Glu134 to Asp inhibits activity. These results suggest that a negative charge on Glu134 is important in stabilizing the inactive state of opsin. Glu134 is highly conserved in all visual pigments and most of the other G protein-linked receptors.

Animals↗