Opsin

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Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red. Rhodopsin 3D.jpeg
Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red.
The retinal molecule inside an opsin protein absorbs a photon of light. Absorption of the photon causes retinal to change from its 11-cis-retinal isomer into its all-trans-retinal isomer. This change in shape of retinal pushes against the outer opsin protein to begin a signal cascade, which may eventually result in chemical signaling being sent to the brain as visual perception. The retinal is re-loaded by the body so that signaling can happen again. 1415 Retinal Isomers.jpg
The retinal molecule inside an opsin protein absorbs a photon of light. Absorption of the photon causes retinal to change from its 11-cis-retinal isomer into its all-trans-retinal isomer. This change in shape of retinal pushes against the outer opsin protein to begin a signal cascade, which may eventually result in chemical signaling being sent to the brain as visual perception. The retinal is re-loaded by the body so that signaling can happen again.

Opsins are a group of proteins, made light-sensitive, via the chromophore retinal found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision.

Chromophore the part of a molecule responsible for its color

A chromophore is the part of a molecule responsible for its color. The color that is seen by our eyes is the one not absorbed within a certain wavelength spectrum of visible light. The chromophore is a region in the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state. In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.

Retinal chemical compound

Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde. Retinal is one of the many forms of vitamin A. Retinal is a polyene chromophore, bound to proteins called opsins, and is the chemical basis of animal vision. Retinal allows certain microorganisms to convert light into metabolic energy.

Photoreceptor cell specialized type of cell found in the retina that is capable of visual phototransduction

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

Contents

Opsin classification

Opsins can be classified several ways, including function (vision, phototaxis, photoperiodism, etc.), type of chromophore (retinal, flavine, bilin), molecular structure (tertiary, quaternary), signal output (phosphorylation, reduction, oxidation), etc. [1]

Bilin (biochemistry) class of chemical compound

Bilins, bilanes or bile pigments are biological pigments formed in many organisms as a metabolic product of certain porphyrins. Bilin was named as a bile pigment of mammals, but can also be found in lower vertebrates, invertebrates, as well as red algae, green plants and cyanobacteria. Bilins can range in color from red, orange, yellow or brown to blue or green.

There are two groups of protein termed opsins. [2] [3] Type I opsins are employed by prokaryotes and by some algae (as a component of channelrhodopsins) and fungi, [4] whereas animals use type II opsins. No opsins have been found outside these groups (for instance in plants, or placozoans). [2]

Prokaryote group of organisms whose cells lack a cell nucleus

A prokaryote is a unicellular organism that lacks a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek πρό (pro) "before" and κάρυον (karyon) "nut or kernel". Prokaryotes are divided into two domains, Archaea and Bacteria. Species with nuclei and organelles are placed in the third domain, Eukaryota. Prokaryotes reproduce without fusion of gametes. The first living organisms are thought to have been prokaryotes.

Algae Group of eukaryotic organisms

Algae is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not necessarily closely related, and is thus polyphyletic. Including organisms ranging from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem, and phloem, which are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and the stoneworts.

At one time it was thought that type I and type II were related because of structural and functional similarities. With the advent of genetic sequencing it became apparent that sequence identity was no greater than could be accounted for by random chance. However, in recent years new methods have been developed specific to deep phylogeny . As a result, several studies have found evidence of a possible phylogenetic relationship between the two. [5] [6] . [7] However, this does not necessarily mean that the last common ancestor of type I and II opsins was itself an opsin, a light sensitive receptor. According to one hypothesis, both type-I and type-II opsins belong to the transporter-opsin-G protein-coupled receptor (TOG) superfamily , a proposed clade that includes G protein-coupled receptor (GPCR), Ion-translocating microbial rhodopsin (MR), and seven others. [8]

G protein-coupled receptor a large protein family of receptors that detect molecules outside the cell and activate internal signal transduction pathways and cellular responses

G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein–linked receptors (GPLR), constitute a large protein family of receptors that detect molecules outside the cell and activate internal signal transduction pathways and, ultimately, cellular responses. Coupling with G proteins, they are called seven-transmembrane receptors because they pass through the cell membrane seven times.

Microbial rhodopsin

Microbial rhodopsins, also known as bacterial rhodopsins are retinal-binding proteins that provide light-dependent ion transport and sensory functions in halophilic and other bacteria. They are integral membrane proteins with seven transmembrane helices, the last of which contains the attachment point for retinal.

Type I opsins

Type I opsins are seven-transmembrane-domain proteins belonging to the G protein-coupled receptor (GPCR) superfamily. Type I opsins (also known as microbial opsins) are found in all three domains of life: Archaea, Bacteria, and Eukaryota. In Eukaryota, type I opsins are found mainly in unicellular organisms such as green algae, and in fungi. In most complex multicellular eukaryotes, type I opsins have been replaced with other light-sensitive molecules such as cryptochrome and phytochrome in plants, and type II opsins in Metazoa (animals). [9]

Archaea A domain of single-celled prokaryotic microorganisms

Archaea constitute a domain of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this classification is outdated.

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Cryptochrome protein-coding gene in the species Homo sapiens

Cryptochromes are a class of flavoproteins that are sensitive to blue light. They are found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and possibly also in the sensing of magnetic fields in a number of species. The name cryptochrome was proposed as a portmanteau combining the cryptic nature of the photoreceptor, and the cryptogamic organisms on which many blue-light studies were carried out.

Microbial opsins are often known by the rhodopsin form of the molecule, i.e., rhodopsin (in the broad sense) = opsin + chromophore. Among the many kinds of microbial opsins are the proton pumps bacteriorhodopsin (BR) and xanthorhodopsin (xR), the chloride pump halorhodopsin (HR) the photosensors sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII), as well as proteorhodopsin (PR), Neurospora opsin I (NOPI), Chlamydomonas sensory rhodopsins A (CSRA), Chlamydomonas sensory rhodopsins B (CSRB), channelrhodopsin (ChR), and archaerhodopsin (Arch). [10]

Bacteriorhodopsin is a protein used by Archaea, most notably by halobacteria, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.

Halorhodopsin is a light-gated ion pump, specific for chloride ions, found in archaea, known as halobacteria. It is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is similar in tertiary structure to vertebrate rhodopsins, the pigments that sense light in the retina. Halorhodopsin also shares sequence similarity to channelrhodopsin, another light-driven ion channel. Halorhodopsin contains the essential light-isomerizable vitamin A derivative all-trans-retinal. Due to the intense attention on solving the structure and function of this molecule, halorhodopsin is one of the few membrane proteins whose crystal structure is known.

Sensory rhodopsin II chemical compound

Sensory rhodopsin II (SRII), also known as pharaonis phoborhodopsin (ppR), is a membrane protein of archaea, responsible generating the phototaxis signal. Sensory rhodopsin II is found in Halobacterium salinarum and Natronomonas pharaonis.

Several type I opsins, such as proteo- and bacteriorhodopsin, are used by various bacterial groups to harvest energy from light to carry out metabolic processes using a non-chlorophyll-based pathway. Beside that, halorhodopsins of Halobacteria and channelrhodopsins of some algae, e.g. Volvox, serve them as light-gated ion channels, amongst others also for phototactic purposes. Sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with transducer membrane-embedded proteins that have no relation to G proteins. [11]

Type I opsins (like channelrhodopsin, halorhodopsin, and archaerhodopsin) are used in optogenetics to switch on or off neuronal activity. Type I opsins are preferred if the neuronal activity should be modulated at higher frequency, because they respond faster than type II opsins. This is because type I opsins are ion channels or proton/ion pumps and thus are activated by light directly, while type II opsins activate G-proteins, which then activate effector enzymes that produce metabolites to open ion channels. [12]

Type II opsins

Type II opsins (or animal opsins) are, like Type I opsins, members of the seven-transmembrane-domain proteins (35–55 kDa) of the GPCR superfamily. [13]

Type II opsins fall phylogenetically into four groups: C-opsins (Ciliary), Cnidops (cnidarian opsins), R-opsins (rhabdomeric), and Go/RGR opsins (also known as RGR/Go or Group 4 opsins). The Go/RGR opsins are divided into four sub-clades: Go-opsins, RGR, Peropsins, and Neuropsins. C-opsins, R-opsins, and the Go/RGR opsins are found only in Bilateria. [14] [15]

Type II visual opsins are traditionally classified as either ciliary or rhabdomeric. Ciliary opsins, found in vertebrates and cnidarians, attach to ciliary structures such as rods and cones. Rhabdomeric opsins are attached to light-gathering organelles called rhabdomeres. This classification cuts across phylogenetic categories (clades) so that both the terms "ciliary" and "rhabdomeric" can be ambiguous. Here, "C-opsins (ciliary)" refers to a clade found exclusively in Bilateria and excludes cnidarian ciliary opsins such as those found in the box jellyfish. Similarly, "R-opsin (rhabdomeric)" includes melanopsin even though it does not occur on rhabdomeres in vertebrates. [14]

C-opsins (ciliary)

Ciliary opsins (or c-opsins) are expressed in ciliary photoreceptor cells, and include the vertebrate visual opsins and encephalopsins. [16] They convert light signals to nerve impulses via cyclic nucleotide gated ion channels, which work by increasing the charge differential across the cell membrane (i.e. hyperpolarization. [2] )

Vertebrate visual opsins

Vertebrate visual opsins are a subset of C-opsins (ciliary). They are expressed in the vertebrate retina and mediate vision. They can be further subdivided into rod opsins and four types of cone opsin. [16] Rod opsins (rhodopsins, usually denoted Rh), [17] are used in dim-light vision, are thermally stable, and are found in the rod photoreceptor cells. Cone opsins, employed in color vision, are less-stable opsins located in the cone photoreceptor cells. Cone opsins are further subdivided according to their absorption maxima (λmax), the wavelength at which the highest light absorption is observed. Evolutionary relationships, deduced using the amino acid sequence of the opsins, are also frequently used to categorize cone opsins into their respective group. Both methods predict four general cone opsin groups in addition to rhodopsin. [18]

LWS

SWS1

SWS2

Rh2

Rh1

Vertebrates typically have four cone opsins (LWS, SWS1, SWS2, and Rh2) inherited from the first vertebrate (and thus predating the first vertebrate), as well as the rod opsin, rhodopsin (Rh1), which emerged after the first vertebrate but before the first Gnathostome (jawed vertebrate). These five opsins emerged through a series of gene duplications beginning with LWS and ending with Rh1. Each one has since evolved into numerous variants and thus constitutes an opsin family or subtype. [19] [20]

NameAbbr.Photo
receptor
λmaxColorHuman variant
Long-wave sensitiveLWSCone500–570 nmGreen, yellow, redOPN1LW "red" / OPN1MW "green"
Short-wave sensitive 1SWS1Cone355–445 nmUltraviolet, violetOPN1SW "blue" (extinct in monotremes)
Short-wave sensitive 2SWS2Cone400–470 nmViolet, blue(Extinct in therian mammals)
Rhodopsin-like 2Rh2Cone480–530 nmGreen(Extinct in mammals)
Rhodopsin-like 1
(vertebrate rhodopsin)
Rh1Rod~500 nmBlue–greenOPN2/Rho, human rhodopsin

Humans have the following set of photoreceptor proteins responsible for vision:

  • Rhodopsin (Rh1, OPN2, RHO) expressed in rod cells, used in night vision
  • Three cone opsins (also known as photopsins) expressed in cone cells, used in color vision
    • Long-wavelength sensitive (OPN1LW) Opsin λmax of 560 nm, in the yellow-green region of the electromagnetic spectrum. [21] May be called the "red opsin," "erythrolabe," "L opsin" or "LWS opsin." Note that despite its common name as the "red" opsin, this opsin's peak sensitivity is not in the red region of the spectrum. However, it is more sensitive to red than the other two human opsins. [22] This receptor also has a secondary response in the violet high frequencies [23] [24]
    • Middle-wavelength sensitive (OPN1MW) Opsin λmax of 530 nm, in the green region of the electromagnetic spectrum. [21] May be called the "green opsin," "chlorolabe," "M opsin" or "MWS opsin."
    • Short-wavelength sensitive (OPN1SW) Opsin λmax of 430 nm, in the blue region of the electromagnetic spectrum. [21] May be called the "blue opsin," "cyanolabe," "S opsin" or "SWS opsin."

Pinopsins

The first Pineal Opsin (Pinopsin) was found in the chicken pineal gland. It is a blue sensitive opsin (λmax = 470 nm). [25]

wide range of expression in the brain, most notably in the pineal region

Vertebrate Ancient (VA) opsin

Vertebrate Ancient (VA) opsin has three isoforms VA short (VAS), VA medium (VAM), and VA long (VAL). It is expressed in the inner retina, within the horizontal and amacrine cells, as well as the pineal organ and habenular region of the brain. [26] It is sensitive to approximately 500 nm [14], found in most vertebrate classes, but not in mammals. [27]

Parapinopsins

The first parapinopsin (PP) opsin was found in the parapineal organ of the catfish. [28] The parapinopsin of lamprey is a UV-sensitive opsin (λmax = 370 nm). [29] The teleosts have two groups of parapinopsins, one is sensitive to UV (λmax = 360-370 nm), the other is sensitive to blue (λmax = 460-480 nm) light. [30]

Parietopsins

The first parietopsin was found in the photoreceptor cells of the lizard parietal eye. The lizard parietopsin is green-sensitive (λmax = 522 nm), and despite it is a c-opsin, like the vertebrate visual opsins, it does not induce hyperpolarization via a Gt-protein, but induces depolarization via a Go-protein. [31] [32]

OPN3 (Encephalopsin or Panopsin)

Panopsins are found in many tissues (skin, [33] brain, [34] [35] testes, [34] heart, liver, [35] kidney, skeletal muscle, lung, pancreas and retina [35] ). They were originally found in the human and mouse brain and thus called encephalopsin. [34]

The first invertebrate panopsin was found in the ciliary photoreceptor cells of the annelid Platynereis dumerilii and is called c(iliary)-opsin. [36] This c-opsin is UV-sensitive (λmax = 383 nm) and can be tuned by 125 nm at a single amino-acid (range λmax = 377 - 502 nm). [37] Thus, not unsurprisingly, a second but cyan sensitive c-opsin (λmax = 490 nm) exists in Platynereis dumerilii. [38] The first c-opsin mediates in the larva UV induced gravitaxis. The gravitaxis forms with phototaxis a ratio-chromatic depth-gauge. [39] In different depths, the light in water is composed of different wavelengths: First the red (> 600 nm) and the UV and violet (< 420 nm) wavelengths disappear. The higher the depth the narower the spectrum so that only cyan light (480 nm) is left. [40] Thus, the larvae can determine their depth by color. The color unlike brightness stays almost constant independent of time of day or the weather, for instance if it is cloudy. [41] [42]

Panopsins are also expressed in the brains of some insects. [16] The panopsins of mosquito and pufferfish absorb maximally at 500 nm and 460 nm, respectively. Both activate in vitro Gi and Go proteins. [43]

The panopsins of teleost fish are called: Teleost multiple tissue (TMT) opsins.

Teleost Multiple Tissue (TMT) Opsin

Teleost fish opsins are expressed in many tissues and therefore called Teleost Multiple Tissue (TMT) opsins. [44] TMT opsins form three groups which are most closely related to a fourth groups the panopsins. [45] [46] In fact, TMT opsins in teleost fish are orthologous to the panopsins in the other vertebrates. They also have the same introns and the same place, which confirms that they belong together. [44]

Cnidarian opsins

Cnidaria, which include jellyfish, corals, and sea anemones, are the most basal animals to possess complex eyes. Jellyfish opsins in the rhopalia couple to Gs-proteins raising the intracellular cAMP level. [47] [48] Coral opsins can couple to Gq-proteins and Gc-proteins. Gc-proteins are a subtype of G-proteins specific to cnidarians. [49] The cnidarian opsins have been identified as one group and so called cnidops, [14] however at least some of them belong to the c-opsins, r-opsins, and Go/RGR-opsins found in bilaterians. [13] [50] [51]

r-opsins (rhabdomeric) / Gq-coupled

Rhabdomeric opsins (or r-opsins) are also known as Gq-opsins, because they couple to a Gq-protein. R-opsins are used by molluscs and arthropods. Arthropods appear to attain colour vision in a similar fashion to the vertebrates, by using three (or more) distinct groups of opsins, distinct both in terms of phylogeny and spectral sensitivity. [16] The r-opsin melanopsin is also expressed in vertebrates, where it regulates circadian rhythms and mediates the pupillary reflex. [16]

Unlike c-opsins, r-opsins are associated with canonical transient receptor potential ion channels; these lead to the electric potential difference across a cell membrane being eradicated (i.e. depolarization). [2]

The identification of the crystal structure of squid rhodopsin [52] is likely to further our understanding of its function in this group.

Arthropods use different opsins in their different eye types, but at least in Limulus the opsins expressed in the lateral and the compound eyes are 99% identical and presumably diverged recently. [53]

Melanopsin OPN4

Involved in circadian rhythms, pupillary reflex, and color correction in high-brightness situations. Phylogenetically a member of the R-opsin (rhabdomeric) group, functionally and structurally an r-opsin, but does not occur in rhabdomeres.

Go/RGR (Group 4) opsins

Go/RGR opsins include Go-opsins, RGR-opsins, neuropsins, and peropsins.

Go-opsins

Go-opsins are absent from higher vertebrates [54] and ecdysozoans. [55] They are found in the ciliary photoreceptor cells of the scallop eye [56] and the basal chordate amphioxus. [57] In Platynereis dumerilii however, a Go-opsin is expressed in the rhabdomeric photoreceptor cells of the eyes. [40]

RGR opsins

RGR opsins, also known as Retinal G protein coupled receptors are expressed in the retinal pigment epithelium (RPE) and Müller cells. [58] They preferentially bind all-trans-retinal in the dark instead of 11-cis-retinal. [59] RGR opsins were thought to be photomerases. [18] But instead, they regulate retinoid traffic and production. [16] [60] In particular, they speed up light-independently the production of 11-cis-retinol (a precursor of 11-cis-retinal) from all-trans-retinyl-esters. [61] However, the all-trans-retinyl-esters are made available light-dependently by RGR-opsins. Whether RGR-opsins regulate this via a G-protein or another signaling mechanism is unknown. [62] The cattle RGR opsin absorbs maximally at different wavelengths depending on the pH-value. At high pH it absorbs maximally blue (469 nm) light and at low pH it absorbs maximally UV (370 nm) light. [63]

Peropsin

Peropsin, a visual pigment-like receptor, is a protein that in humans is encoded by the RRH gene. [64]

Neuropsins

Neuropsins are sensitive to UVA, typically at 380 nm. They are found in the brain, testes, skin, and retina of humans and rodents, as well as in the brain and retina of birds. In birds and rodents they mediate ultraviolet vision. [33] [65] [66] They couple to Gi-proteins. [65] [66] In humans, Neuropsin is encoded by the OPN5 gene. In the human retina, its function is unknown. In the mouse, it photo-entrains the retina and cornea at least ex vivo. [67]

Unclassified

Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh)

These pineal opsins, found in the Actinopterygii (ray-finned fish) apparently arose as a result of gene duplication from Rh1 (rhodopsin). These opsins appear to serve functions similar to those of pinopsin found in birds and reptiles. [68] [69]

Structure and function

Opsin proteins covalently bind to a vitamin A-based retinaldehyde chromophore through a Schiff base linkage to a lysine residue in the seventh transmembrane alpha helix. In vertebrates, the chromophore is either 11- cis -retinal (A1) or 11-cis-3,4-didehydroretinal (A2) and is found in the retinal binding pocket of the opsin. The absorption of a photon of light results in the photoisomerization of the chromophore from the 11-cis to an all-trans conformation. The photoisomerization induces a conformational change in the opsin protein, causing the activation of the phototransduction cascade. The opsin remains insensitive to light in the trans form. It is regenerated by the replacement of the all-trans retinal by a newly synthesized 11-cis-retinal provided from the retinal epithelial cells. Opsins are functional while bound to either chromophore, with A2-bound opsin λmax being at a longer wavelength than A1-bound opsin.

Opsins contain seven transmembrane α-helical domains connected by three extra-cellular and three cytoplasmic loops. Many amino acid residues, termed functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. All residue positions discussed henceforth are relative to the 348 amino acid bovine rhodopsin crystallized by Palczewski et al. [70] Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromophore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabilizing the protonation of the Schiff linkage between Lys296 and the chromophore. The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propagation of the transduction signal once a photon has been absorbed.

Certain amino acid residues, termed spectral tuning sites, have a strong effect on λmax values. Using site-directed mutagenesis, it is possible to selectively mutate these residues and investigate the resulting changes in light absorption properties of the opsin. It is important to differentiate spectral tuning sites, residues that affect the wavelength at which the opsin absorbs light, from functionally conserved sites, residues important for the proper functioning of the opsin. They are not mutually exclusive, but, for practical reasons, it is easier to investigate spectral tuning sites that do not affect opsin functionality. For a comprehensive review of spectral tuning sites see Yokoyama [71] and Deeb. [72] The impact of spectral tuning sites on λmax differs between different opsin groups and between opsin groups of different species.

Opsins in the human eye, brain, and skin

Abbr.NameλmaxColorEyeBrainSkinChromosomal location a
OPN1LWL-cone (red-cone) opsin557 nmYellowConeN/AN/AXq28 [18]
OPN1MWM-cone (green-cone) opsin527 nmGreenConeN/AN/AXq28 [18]
OPN1SWS-cone (blue-cone) opsin420 nmVioletConeN/AMelanocytes, keratinocytes [33] 7q32.1 [18]
OPN2 (RHO)Rhodopsin505 nmBlue–greenRodN/AMelanocytes, keratinocytes [33] 3q22.1 [18]
OPN3Encephalopsin, panopsinS-MBlue–greenRod, cone, OPL, IPL, GCL [73] Cerebral cortex, cerebellum, striatum, thalamus, hypothalamus [74] [75] Melanocytes, keratinocytes [33] 1q43 [18]
OPN4Melanopsin480 nm [76] Sky blueipRGC [76] N/AN/A10q23.2 [18]
OPN5Neuropsin380 nm [65] Ultraviolet [65] Neural retina, RPE [77] Anterior hypothalamus [78] Melanocytes, keratinocytes [33] 6p12.3 [18]
RRHPeropsinRPE cells - microvilliN/AN/A4q25 [18]
RGRRetinal G protein coupled receptorRPE cellsN/AN/A10q23.1 [18]

RPE, retinal pigment epithelium; ipRGC, intrinsically photosensitive retinal ganglion cells; OPL, outer plexiform layer; IPL, inner plexiform layer; GCL, ganglion cell layer

See also

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Photopsin

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Melanopsin protein-coding gene in the species Homo sapiens

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.

Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and photoreception. In medical terminology, "photopigment" commonly refers to the photoreceptor proteins of the retina.

Evolution of the eye The origins and diversification of the organs of sight through geologic time

Many researchers have found the evolution of the eye attractive to study, because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms, plants and animals. Complex, image-forming eyes have evolved independently several times.

Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria. They mediate light responses as varied as visual perception, phototropism and phototaxis, as well as responses to light-dark cycles such as circadian rhythm and other photoperiodisms including control of flowering times in plants and mating seasons in animals.

Retinylidene protein, is a family of proteins that use retinal as a chromophore for light reception. It is the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin. While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, rhodopsin in the broad sense refers any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle.

The bleach and recycle process is used within the retina to ensure that the chromophore 11-cis retinal is present within opsin molecules in sufficient quantities to allow phototransduction to occur. It uses vitamin A (retinol) derivatives.

RRH protein-coding gene in the species Homo sapiens

Peropsin, a visual pigment-like receptor, is a protein that in humans is encoded by the RRH gene.

OPN5 protein-coding gene in the species Homo sapiens

Neuropsin is a protein that in humans is encoded by the OPN5 gene. It is a photoreceptor protein sensitive to ultraviolet (UV) light. The OPN5 gene was discovered in mouse and human genomes and its mRNA expression was also found in neural tissues. Neuropsin is bistable at 0 °C and activates a UV-sensitive, heterotrimeric G protein Gi-mediated pathway in mammalian and avian tissues.

OPN1LW protein-coding gene in the species Homo sapiens

OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum. The gene contains 6 exons with variability that induces shifts in the spectral range. OPN1LW is subject to homologous recombination with OPN1MW, as the two have very similar genomes. These recombinations can lead to various vision problems, such as red-green colourblindness and blue monochromacy. The protein encoded is a G-protein coupled receptor with embedded 11-cis-retinal, whose light excitation causes a cis-trans conformational change that begins the process of chemical signalling to the brain.

Retinal G protein coupled receptor protein-coding gene in the species Homo sapiens

RPE-retinal G protein-coupled receptor also known as RGR-opsin is a protein that in humans is encoded by the RGR gene.

<i>Platynereis dumerilii</i> species of annelid

Platynereis dumerilii is a species of annelid. It was originally placed into the genus Nereis and later reassigned to Platynereis. Platynereis dumerilii lives in coastal marine waters from temperate to tropical zones. It can be found in a wide range from the Azores, the Mediterranean, in the North Sea, the English Channel, and the Atlantic down to the Cape of Good Hope, in the Black Sea, the Red Sea, the Persian Gulf, the Sea of Japan, the Pacific, and the Kerguelen Islands. Platynereis dumerilii is today an important lab animal, it is considered as a living fossil, and it is used in many phylogenetic studies as a model organism. Platynereis dumerilii reaches an age of 3 to 18 month and males reach a length of 2 to 3 cm, while females reach a length of 3 to 4 cm.

Evolution of human colour vision over time: humans have developed a trichromatic view of the world in comparison to a majority of other mammals that only see the world from a dichromatic view. Early human ancestors are believed to have viewed the world using UV vision as far back as 90 million years ago. It is thought that the shift to trichromatic vision capabilities and the ability to see blue light have evolved as an adaptive trait over time.

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