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At least 649 records · Page 36Linked to original sources
PubMed2007

Molecular dynamics of rhodopsin and free opsin: computer simulation.

Computer simulation was used to perform a comparative study of the molecular dynamics of rhodopsin containing the chromophore group (11-cis-retinal) and free opsin. The molecular dynamics were followed over a time interval of 3000 psec; a total of 3 x 10(6) discrete conformational states of rhodopsin and opsin. The presence of the chromophore group in the chromophore center of opsin was shown to have significant effects on the immediate protein environment of the chromophore and the conformational state of the cytoplasmic domain, but to have virtually no effect on the conformational state of the intradisk domain. The simulation results are used to discuss the possible intramolecular mechanism by which rhodopsin is maintained as a G-protein-coupled receptor in the inactive state, i.e., the function of the chromophore as an effective antagonist ligand.

Animals↗
PubMed1988

Opsin, G-protein and 48-kDa protein in normal and rd mouse retinas: developmental expression of mRNAs and proteins and light/dark cycling of mRNAs.

Retinal degeneration in rd mice is manifested during the most rapid period of postnatal photoreceptor differentiation and is hypothesized to be caused by a lesion in cGMP metabolism. We have studied the sequence of developmental expression of three proteins involved in the cGMP cascade and the mRNAs from which they are translated, in rd and control mouse retinas. Slot blot analysis of retinal RNAs indicates that the mRNAs coding for opsin, the alpha, beta and gamma subunits of G-protein and 48-kDa protein each has the same time for onset of expression in normal and diseased retinas. G beta and 48-kDa protein mRNAs are already detectable at birth, opsin mRNA appears by postnatal day 5 (P5), G gamma mRNA at P6 and G alpha mRNA by P8. The levels of all these mRNAs decrease in the diseased retinas after P11-P12, correlating with the reduction in photoreceptor cell number that characterizes the rd disease. Immunocytochemistry indicates that the 48-kDa protein is present at birth, G gamma and opsin are detectable at P4 and G alpha at P7. After P7, opsin and G-protein immunoreactivity are localized throughout the photoreceptor cell in the rd retinas but they are found only in the outer segment in control retinas. The 48-kDa protein immunoreactivity, which is observed in the whole photoreceptor layer both in rd and control retinas throughout development, is the only one of all immunoreactivities analysed that remains at 2 months of age in the rd retina and is probably localized in cones. However, at 6 months of age, 48-kDa protein immunoreactive cells are no longer present in the rd retina. We have also investigated whether there is a daily rhythm for the levels of mRNA present at different times during the light/dark periods in developing rd/rd and rd/+ retinas and in adult normal (+/+) retinas. We find that the levels of each mRNA analysed appear to cycle in the +/+ adult retina, with the greatest amount of opsin and the three subunits of G-protein mRNAs occurring just before light onset and the greatest amount of 48-kDa protein mRNA occurring just before lights off. Cycling in the developing diseased or control retinas (P0-P12) is not observed and may be masked by the pronounced cell growth that occurs during this period.

Aging↗
PubMed2002

Environmental stimulus perception and control of circadian clocks.

Circadian rhythms are regulated by clocks located in specific structures of the central nervous system, such as the suprachiasmatic nucleus (SCN) in mammals, and by peripheral oscillators present in various other tissues. Recent discoveries have elucidated the control of central and peripheral clocks by environmental signals. The major synchroniser in animals is light. In mammals, a subset of retinal ganglion cells receive light signals that are transmitted to the SCN via the retinohypothalamic tract. Photoreception is probably elicited by a novel opsin, melanopsin, although cryptochromes may also play a role. These signals feed directly to the SCN master clock, which then provides timing cues to peripheral clocks. In contrast to mammals, peripheral tissues in the fly and in the fish are directly photoreceptive. However, alternative routes exist. Some peripheral clocks in mammals can be specifically entrained in an SCN-independent manner by restricting food during the light period.

Animals↗
PubMed1994

Retinal GlcNAc-transferases and the glycosylation of rhodopsin.

The major oligosaccharide chains of bovine, frog and human rhodopsins are identical in structure. These molecules are unique in terms of their abridged size as compared to other asparagine-linked glycoproteins, a property which must be reflected in the activities and properties of the glycosyltransferases of the retina that are concerned with their biosynthesis. One of the unusual characteristics of the major oligosaccharide chain is its lack of branching. A key enzymatic step required for branching to occur would be the transfer of a residue of N-acetylglucosamine to the terminal unsubstituted mannose residue of the major rhodopsin oligosaccharide. We have investigated the kinetic properties of enzymes that would catalyse this process. The N-acetylglucosaminyl-transferases (GlcNAc-transferases) of Golgi-enriched fractions of bovine retina, human retina, rat liver Golgi and a partially purified GlcNAc-transferase II from rat liver were examined using as acceptors rhodopsin, opsin and the oligosaccharide isolated from rhodopsin. The identification of the substrates and products of the reactions was carried out by chromatographic means. From an evaluation of the Vmax/K(m) ratios it was observed that bovine and human retinas have very limited abilities compared to the rat liver enzymes to carry out the transfer of GlcNAc to these acceptors, showing from 100- to about 800-fold lower efficiency in this regard. A comparison of the activities obtained with intact rhodopsin or opsin and the rhodopsin oligosaccharide indicated that the activity of GlcNAc-transferase II was not appreciably influenced by the polypeptide portion of the molecule. It was also observed that prior galactosylation of rhodopsin blocked the addition of GlcNAc to rhodopsin. It is suggested that these properties contribute to the assembly of the abridged structures that have been observed in the oligosaccharides of rhodopsins of all species thus far examined.

Acetylglucosamine↗
PubMed1996

Chromophore configuration of iodopsin and its photoproducts formed at low temperatures.

The photochemical reactions of iodopsin at low temperatures were investigated by a combination of absorption spectroscopy and chromophore extraction to show the formation of isomeric photoproducts other than the all-trans intermediates. We first confirmed that the chromophore in iodopsin is an 11-cis-retinal. Next, iodopsin samples were irradiated with light of different wavelengths at selected temperatures ranging from -190 to 0 degrees C, and their retinylidene chromophores were extracted as oximes after warming the sample to 0 degree C. The isomeric composition of the extracted chromophores was analyzed by high-performance liquid chromatography. It was confirmed that bathoiodopsin produced at -190 degrees C has an all-trans chromophore, but a considerable amount of its chromophore thermally reisomerizes to the 11-cis form upon warming. Photoproducts formerly assigned as lumi- and metaiodopsins [Yoshizawa, T., & Wald, G. (1967) Nature 214, 566-571; Hubbard, R., & Kropf, A. (1959) Nature 183, 448-450] were produced by extensive irradiation of iodopsin at -80 and -40 degrees C, respectively, with red light, but their chromophores were identified to be 7-cis-retinals instead of all-trans-retinals. Thus these photoproducts are artificial byproducts formed as a result of photon absorption by all-trans intermediates. The absorption spectrum of the 7-cis product formed from bovine rhodopsin shows no spectral shift when it is warmed from -80 to 0 degrees C, but the spectrum of 7-cis species formed from iodopsin shifted about 40 nm to the blue at a transition temperature of -60 degrees C. This result indicates a unique chromophore-opsin interaction in iodopsin. Four all-trans intermediates of iodopsin were identified above -80 degrees C under irradiation conditions in which no 7-cis products accumulated. Their absorption maxima were estimated to be approximately 570, approximately 530, approximately 470, and approximately 380 nm. These species should correspond to BL-iodopsin, lumiiodopsin, metaiodopsin I, and metaiodopsin II, respectively, assigned by room temperature laser photolysis [Shichida, Y., Okada, T., Kandori, H., Fukada, Y., & Yoshizawa, T. (1993) Biochemistry 32, 10832-10838].

Animals↗
PubMed2002

The molecular mechanism for the spectral shifts between vertebrate ultraviolet- and violet-sensitive cone visual pigments.

The short-wave-sensitive (SWS) visual pigments of vertebrate cone photoreceptors are divided into two classes on the basis of molecular identity, SWS1 and SWS2. Only the SWS1 class are present in mammals. The SWS1 pigments can be further subdivided into violet-sensitive (VS), with lambda(max) (the peak of maximal absorbance) values generally between 400 and 430 nm, and ultraviolet-sensitive (UVS), with a lambda(max)<380 nm. Phylogenetic evidence indicates that the ancestral pigment was UVS and that VS pigments have evolved separately from UVS pigments in the different vertebrate lineages. In this study, we have examined the mechanism of evolution of VS pigments in the mammalian lineage leading to present day ungulates (cow and pig). Amino acid sequence comparisons of the UVS pigments of teleost fish, amphibia, reptiles and rodents show that site 86 is invariably occupied by Phe but is replaced in bovine and porcine VS pigments by Tyr. Using site-directed mutagenesis of goldfish UVS opsin, we have shown that a Phe-86-->Tyr substitution is sufficient by itself to shift the lambda(max) of the goldfish pigment from a wild-type value of 360 nm to around 420 nm, and the reverse substitution of Tyr-86-Phe into bovine VS opsin produces a similar shift in the opposite direction. The substitution of this single amino acid is sufficient to account therefore for the evolution of bovine and porcine VS pigments. The replacement of Phe with polar Tyr at site 86 is consistent with the stabilization of Schiff-base protonation in VS pigments and the absence of protonation in UVS pigments.

Animals↗
PubMed1998

Contrasting levels of DNA polymorphism at the autosomal and X-linked visual color pigment loci in humans and squirrel monkeys.

The X-linked color pigment (opsin) locus is known to be highly polymorphic in the squirrel monkey and other New World monkeys. To see whether this is also the case for the autosomal (blue) opsin locus, we obtained 32 squirrel monkey and 30 human blue opsin gene sequences. No amino acid polymorphism was found in either the squirrel monkey sample or the human sample, contrary to the situation at the X-linked opsin locus. This sharp contrast in the level of polymorphism might be due to differences in gene expression between the autosomal and the X-linked loci. At the X-linked locus, heterozygote advantage can occur because, owing to X-inactivation, the two alleles in a heterozygote are expressed in different cone cells, producing two types of cone cell, whereas at the autosomal locus, heterozygote advantage cannot occur because the two alleles in a heterozygote are expressed in the same cone cells, producing only one type of cone cell (i.e., phenotypically a homozygote). From the sequence data, the levels of nucleotide diversity (pi, i.e., the number of nucleotide differences per site) are estimated: for the human sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.04% per fourfold degenerate site in the coding regions and 0.01% per site in intron 4; for the squirrel monkey sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.15% per fourfold degenerate site in the coding regions and 0.17% per site in intron 4. The blue opsin genes from the common and pygmy chimpanzees, the gorilla, the capuchin, and the howler monkey were also sequenced. Features critical to the function of the opsin are well conserved in all known mammalian sequences. However, the interhelical loops are, on average, actually more conservative than the transmembrane helical regions. In addition, these sequence data and those from some other genes indicate that the common and pygmy chimpanzees are not closely related, their divergence data being from one third to one half the date of the human-chimpanzee divergence.

Amino Acid Sequence↗
PubMed2006

Immunohistochemical evidence of a melanopsin cone in human retina.

PURPOSE: Melanopsin, expressed in a subset of intrinsically photosensitive ganglion cells that project to the suprachiasmatic nucleus (SCN), is involved in the photic entrainment of circadian rhythms and other non-image-forming functions (pupil light reflex, masking, acute heart rate response, and alertness). Melanopsin has recently been shown to be a "bireactive" photopigment that functions as a photosensory opsin using 11-cis retinaldehyde as a chromophore and has intrinsic photoisomerase activity. Melanopsin is widely distributed in the retina of vertebrates and, depending on the species, is expressed in ganglion, amacrine, horizontal, and photoreceptor cells. The present study was conducted to determine the distribution of this opsin in the human retina. METHODS: Human donor eyes were obtained from donors and fixed shortly after death. Immunohistochemistry was used to determine melanopsin expression in the retinas of three donors. The possible coexpression of this photopigment with other opsins was studied by double-labeling immunocytochemistry and confocal analysis. RESULTS: In addition to the expected labeling in ganglion cells of the inner retinal layers, an unexpected finding showed melanopsin-immunopositive label in the outer segments of cones that did not coexpress other known opsins. These melanopsin-expressing cones are extremely sparse (5-25 cones/mm2; 0.1%-0.5% of the entire cone population) and are located in the peripheral retina. CONCLUSIONS: The presence of melanopsin in human cones suggests image and non-image-forming roles in visual responses at both the cone input and ganglion cell output stages and their involvement in a broad spectrum of irradiance detection functions in the visual system.

Adult↗
PubMed1994

Light deprivation profoundly affects gene expression of interphotoreceptor retinoid-binding protein in the mouse eye.

Ambient light appears to play a role in regulating gene and protein expression of interphotoreceptor retinoid-binding protein (IRBP), a protein that facilitates the transport of retinoids between the neural retina and pigment epithelium in the visual cycle. Pregnant CD-1 mice were placed in the dark approximately 48 hr before parturition, and the pups were reared for 14 days under these conditions. Control animals were reared on a 12 hr light/12 hr dark cycle. Northern blotting of total RNA isolated from whole mouse eyes at post-natal days 7-14 (P7-P14) showed a marked reduction in IRBP message in the light-deprived animals to 10-20% of levels in control animals. Reprobing of the blots for opsin and S-antigen message showed a significant decrease of about 80-90% in opsin message at 5.1 kb but no change in S-antigen message in the eyes of light-deprived mice. Light microscopic examination of the light-deprived mouse retinas showed no apparent abnormalities in morphological development and immunocytochemistry demonstrated normal distribution and levels of IRBP protein. Immunochemical quantitation of IRBP protein confirmed that there was no reduction in light-deprived as compared to normal mouse eyes. Similarly, when adult mice were light-deprived for 14 days, a marked reduction in IRBP message was also observed with no decrease in the amount of IRBP protein. Thus, light deprivation causes a large decrease in IRBP message in the mouse eye, but IRBP protein is not decreased. The dramatic effect of light deprivation on IRBP mRNA and some opsin mRNAs, but not on S-antigen message and the fact that IRBP protein levels are relatively unaffected, suggest a complex pathway of light regulation of photoreceptor function previously not encountered. This may involve regulatory controls at levels including gene transcription, mRNA stability or protein degradation that may make use of a feedback control mechanism involving light- or dark-dependent signal transduction.

Animals↗
PubMed2003

Genetic evidence for the ancestral loss of short-wavelength-sensitive cone pigments in mysticete and odontocete cetaceans.

All mammals ancestrally possessed two types of cone pigments, an arrangement that persists in nearly all contemporary species. However, the absence of one of these cone types, the short-wavelength-sensitive (SWS) cone, has recently been established in several delphinoid cetacean species, indicating that the loss of this pigment type may be widespread among cetaceans. To evaluate the functional condition of SWS cones in cetaceans, partial SWS cone-opsin gene sequences were obtained from nuclear DNA for 16 species representing 12 out of the 14 extant mysticete (baleen) and odontocete (toothed) families. For all these species one or more mutations were identified that indicate that their SWS cone-opsin genes are pseudogenes and thus do not code for functional visual pigment proteins. Parsimonious interpretation of the distribution of some of these mis-sense mutations indicates that the conversion of cetacean SWS coneopsin genes to pseudogenes probably occurred before the divergences of the mysticete and odontocete suborders. Thus, in the absence of dramatic homoplasy, all modern cetaceans lack functional SWS cone visual pigments and, by extension, the visual capacities that such pigments typically support.

Amino Acid Sequence↗
PubMed2001

The abundant retinal protein of the Chlamydomonas eye is not the photoreceptor for phototaxis and photophobic responses.

The chlamyopsin gene (cop) encodes the most abundant eyespot protein in the unicellular green alga Chlamydomonas reinhardtii. This opsin-related protein (COP) binds retinal and was thought to be the photoreceptor controlling photomovement responses via a set of photoreceptor currents. Unfortunately, opsin-deficient mutants are not available and targeted disruption of non-selectable nuclear genes is not yet possible in any green alga. Here we show that intron-containing gene fragments directly linked to their intron-less antisense counterpart provide efficient post-transcriptional gene silencing (PTGS) in C. reinhardtii, thus allowing an efficient reduction of a specific gene product in a green alga. In opsin-deprived transformants, flash-induced photoreceptor currents (PC) are left unchanged. Moreover, photophobic responses as studied by motion analysis and phototaxis tested in a light-scattering assay were indistinguishable from the responses of untransformed wild-type cells. We conclude that phototaxis and photophobic responses in C. reinhardtii are triggered by an as yet unidentified rhodopsin species.

Algal Proteins↗
PubMed2006

Divergent selection on opsins drives incipient speciation in Lake Victoria cichlids.

Divergent natural selection acting on ecological traits, which also affect mate choice, is a key element of ecological speciation theory, but has not previously been demonstrated at the molecular gene level to our knowledge. Here we demonstrate parallel evolution in two cichlid genera under strong divergent selection in a gene that affects both. Strong divergent natural selection fixed opsin proteins with different predicted light absorbance properties at opposite ends of an environmental gradient. By expressing them and measuring absorbance, we show that the reciprocal fixation adapts populations to divergent light environments. The divergent evolution of the visual system coincides with divergence in male breeding coloration, consistent with incipient ecological by-product speciation.

Africa, Eastern↗
PubMed1990

seven in absentia, a gene required for specification of R7 cell fate in the Drosophila eye.

Signals from directly adjacent cells are critical for the development of the R7 photoreceptor cell in the Drosophila eye. We have identified a gene, seven in absentia (sina), for which loss of function results in the R7 precursor cell adopting the fate of a nonneuronal cell type. Function of the sina gene is required only in R7 for correct R7 cell development. The sina transcription unit is located within the intron of a gene that encodes an R7 cell-specific opsin. The sina protein, which has a potential metal binding domain, is localized in the nuclei of several ommatidial precursor cells including R7, and sina expression in R7 appears before R7 overtly begins to differentiate. These data indicate that the sina gene product is necessary at a stage in the determination of R7 cell fate when R7 receives and interprets developmental signals from neighboring cells, and possibly acts by regulating gene expression.

Amino Acid Sequence↗
PubMed2003

Ligand channeling within a G-protein-coupled receptor. The entry and exit of retinals in native opsin.

Deactivation of light-activated rhodopsin (metarhodopsin II) involves, after rhodopsin kinase and arrestin interactions, the hydrolysis of the covalent bond of all-trans-retinal to the apoprotein. Although the long-lived storage form metarhodopsin III is transiently formed, all-trans-retinal is eventually released from the active site. Here we address the question of whether the release results in a retinal that is freely diffusible in the lipid phase of the photoreceptor membrane. The release reaction is accompanied by an increase in intrinsic protein fluorescence (release signal), which arises from the relief of the fluorescence quenching imposed by the retinal in the active site. An analogous fluorescence decrease (uptake signal) was evoked by exogenous retinoids when they non-covalently bound to native opsin membranes. Uptake of 11-cis-retinal was faster than formation of the retinylidene linkage to the apoprotein. Endogenous all-trans-retinal released from the active site during metarhodopsin II decay did not generate the uptake signal. The data show that in addition to the retinylidene pocket (site I) there are two other retinoidbinding sites within opsin. Site II involved in the uptake signal is an entrance site, while the exit site (site III) is occupied when retinal remains bound after its release from site I. Support for a retinal channeling mechanism comes from the rhodopsin crystal structure, which unveiled two putative hydrophobic binding sites. This mechanism enables a unidirectional process for the release of photoisomerized chromophore and the uptake of newly synthesized 11-cis-retinal for the regeneration of rhodopsin.

Animals↗
PubMed1998

Two visual pigments in a single photoreceptor cell: identification and histological localization of three mRNAs encoding visual pigment opsins in the retina of the butterfly Papilio xuthus.

This paper describes the localization of newly identified visual pigment opsins in the tiered retina of the Japanese yellow swallowtail Papilio xuthus. We first cloned three cDNAs encoding visual pigment opsins, PxRh1, PxRh2 and PxRh3, and then carried out histological in situ hybridization to localize their mRNAs in the retina. By combining the present data with our previous electrophysiological results, we concluded that both PxRh1 and PxRh2 correspond to visual pigments expressed in photoreceptor cells sensitive in the green wavelength region (green receptors), whereas PxRh3 corresponds to a pigment in red receptors. The in situ hybridization studies showed that some photoreceptor cells express two opsin mRNAs. In the ventral half of the eye, all green receptors in the distal tier were labelled by both PxRh1 and PxRh2 probes. The labelling by the PxRh2 and PxRh3 probes was detected throughout the eye in the proximal tier; in 18 % of ommatidia, the probes labelled the same photoreceptor cell. These results suggest that the possible co-localization of two different visual pigments will broaden the sensitivity spectrum of the photoreceptor cells.

Amino Acid Sequence↗
PubMed2001

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.

Animals↗
PubMed1992

Human tritanopia associated with two amino acid substitutions in the blue-sensitive opsin.

Tritanopia is an autosomal dominant genetic disorder of human vision characterize by a selective deficiency of blue spectral sensitivity. The defect is manifested within the retina and could be caused by a deficiency in function or numbers (or both) of blue-sensitive cone photoreceptors. We have used PCR, denaturing gradient gel electrophoresis, and DNA sequencing of amplified exons to detect in four of nine unrelated tritanopic subjects two different point mutations in the gene encoding the blue-sensitive opsin, each leading to an amino acid substitution. Segregation analysis within pedigrees and hybridization of oligonucleotides specific for each allele to DNA samples from control subjects support the hypothesis that these mutations cause tritanopia. These results complete the genetic evidence for the trichromatic theory of human color vision.

Arginine↗
PubMed1987

The pineal organ is the first differentiated light receptor in the embryonic salmon, Salmo salar L.

The initial appearance of S-antigen, alpha-transducin, opsin and 5-HT during embryogenesis of the pineal organ and retina was studied by means of immunocytochemistry in the Atlantic salmon, Salmo salar L. The presence of these substances may be taken as a good indication of photoreceptor differentiation; alpha-transducin and S-antigen are involved in the phototransduction process, opsin is the proteinaceous component of the photopigment rhodopsin, and 5-HT is a neurotransmitter or neurohormone produced by pineal photoreceptors. Two days after the retinal pigment layer became visible in the eggs, the outer segments of a few pineal photosensory cells showed immunoreactivity to opsin and alpha-transducin. At the same time S-antigen and serotonin were present in pineal cells of the photoreceptor type. The number of immunoreactive cells in the pineal organ increased up to hatching. In the differentiating retina of the salmon, no immunoreactivity to antibodies raised against the mentioned substances were detectable until after hatching. These results indicate that in ontogeny the developing pineal organ of the salmon embryo has the ability to perceive light information much earlier than the retina.

Animals↗