Opsin

Source: Wikipedia, the free encyclopedia.
This article is about animal opsins. For microbial opsins, see microbial rhodopsin.
Three-dimensional structure of cattle 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.

Animal opsins are

chemicals
.

Structure and function

Animal opsins detect light and are the molecules that allow us to see. Opsins are

phototransduction cascade.[20] Thus, a chemoreceptor is converted to a light or photo(n)receptor.[21]

In the vertebrate photoreceptor cells, all-trans-retinal is released and replaced by a newly synthesized 11-cis-retinal provided from the retinal epithelial cells. Beside 11-

cis-retinal (A1), 11-cis-3,4-didehydroretinal (A2) is also found in vertebrates as ligand such as in freshwater fishes.[19] A2-bound opsins have a shifted λmax and absorption spectrum compared to A1-bound opsins.[22]

Functionally conserved residues and motifs

The seven transmembrane α-helical domains in opsins are connected by three extra-cellular and three cytoplasmic loops. Along the α-helices and the loops, many amino acid residues are highly conserved between all opsin groups, indicating that they serve important functions and thus are called functionally conserved residues. Actually, insertions and deletions in the α-helices are very rare and should preferentially occur in the loops. Therefore, different G-protein-coupled receptors have different length and homologous residues may be in different positions. To make such positions comparable between different receptors, Ballesteros and Weinstein introduced a common numbering scheme for G-protein-coupled receptors.[23] The number before the period is the number of the transmembrane domain. The number after the period is set arbitrarily to 50 for the most conserved residue in that transmembrane domain among GPCRs known in 1995. For instance in the seventh transmembrane domain, the proline in the highly conserved NPxxY7.53 motif is Pro7.50, the asparagine before is then Asp7.49, and the tyrosine three residues after is then Tyr7.53.[21] Another numbering scheme is based on cattle rhodopsin. Cattle rhodopsin has 348 amino acids and is the first opsin whose amino acid sequence[24] and whose 3D-structure were determined.[12] The cattle rhodopsin numbering scheme is widespread in the opsin literature.[21] Therefore, it is useful to use both schemes.

The retinal binding lysine

Opsins without the retinal binding lysine are not light sensitive.

tobacco hawk moth. However, the gluopsins have no known function.[21]

Such function does not need to be light detection, as some opsins are also involved in

mechanoreception such as hearing[32] detecting phospholipids, chemosensation, and other functions.[33][34] In particular, the Drosophila rhabdomeric opsins (rhabopsins, r-opsins) Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for aristolochic acid. These opsins still have Lys2967.43 like other opsins. However, if this lysine is replaced by an arginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid. Thus, Lys2967.43 is not needed for Rh1 to function as chemoreceptor.[26] Also the Drosophila rhabopsins Rh1 and Rh6 are involved in mechanoreception, again for mechanoreception Lys2967.43 is not needed, but needed for proper function in the photoreceptor cells.[25]

Beside these functions, an opsin without Lys2967.43, such as a gluopsin, could still be light sensitive, since in cattle rhodopsin, the retinal binding lysine can be shifted from position 296 to other positions, even into other transmembrane domains, without changing light sensitivity.[35]

Opsins have the retinal binding lysine, except the nemopsins and gluopsins[21]

In the

cnidarians. The branches to the clades have pie charts, which give support values for the branches. The values are from right to left SH-aLRT/aBayes/UFBoot. The branches are considered supported when SH-aLRT ≥ 80%, aBayes ≥ 0.95, and UFBoot ≥ 95%. If a support value is above its threshold the pie chart is black otherwise gray.[21]

The NPxxY motif

The

loss of function disease mutations are known for Tyr7.53⁠.[38]

Also mutations of

G-protein and has an NAxxY7.53 motif natively. If this motif is mutated to NPxxY7.53 (Ala7.50 → Pro7.50), the receptor cannot be activated, but can be rescued partially if the motif is mutated to NVxxY7.53 (Ala7.50 → Val7.50).[48] Furthermore, when the motif is mutated to NAxxY7.53 (Pro7.50 → Ala7.50) in cattle rhodopsin, the mutant has 141% of wild type activity.[37] This evidence shows that a GPCR does not need a standard NPxxY7.53 motif for signaling.[21]

Consensus sequences of the different chromopsins: The first column contains a number for each chromopsin group for easy reference. The second column shows the names for each group. The third contains the number of sequences in each group. And the fourth column contains the sequence logo, the height of the letters indicates the percentage of that amino acid given at that position. The x-axis gives the position of the amino acid corresponding to cattle rhodopsin. Positions 2927.39 and 3147.64 are highlighted in gray. Lysine (K) 2967.43 is highlighted with a gray background, which is replaced in the nemopsins by arginine (R) and in the gluopsins by glutamic acid (E). The NPxxY7.53 motif is highlighted with a gray background. It is conserved in most opsins and G-protein-coupled receptors, however it is derived in the retinochromes, RGR-opsins, and Gluopsins.[21]

Other residues and motifs

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 ligand retinal. The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propagation of the transduction signal once a photon has been absorbed.

Spectral tuning sites

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[49] and Deeb.[50] 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. Name λmax Color Eye Brain Skin Chromosomal location[44]
OPN1LW L-cone (red-cone) opsin 557 nm Yellow Cone — — Xq28[44]
OPN1MW M-cone (green-cone) opsin 527 nm Green Cone — — Xq28[44]
OPN1SW S-cone (blue-cone) opsin 420 nm Violet Cone — Melanocytes, keratinocytes[51] 7q32.1[44]
OPN2 (RHO) Rhodopsin 505 nm Blue–green Rod — Melanocytes, keratinocytes[51] 3q22.1[44]
OPN3 Encephalopsin, panopsin S-M Blue–green Rod, cone, OPL, IPL, GCL[52] Cerebral cortex, cerebellum, striatum, thalamus, hypothalamus[53][54] Melanocytes, keratinocytes[51] 1q43[44]
OPN4
 Melanopsin 480 nm[55] Sky blue ipRGC[55] — — 10q23.2[44]
OPN5 Neuropsin 380 nm[56] Ultraviolet[56] Neural retina, RPE[57] Anterior hypothalamus[58] Melanocytes, keratinocytes[51] 6p12.3[44]
RRH Peropsin RPE cells - microvilli — — 4q25[44]
RGR Retinal G protein coupled receptor RPE cells — — 10q23.1[44]

RPE,

intrinsically photosensitive retinal ganglion cells; OPL, outer plexiform layer; IPL, inner plexiform layer; GCL, ganglion cell layer

Cuttlefish

Cuttlefish and octopuses contain opsin in their skin as part of the chromophores. The opsin is part of the sensing network detecting the colour and shape of the cuttlefish's surroundings.[59][60][61]

Phylogeny

Animal opsins (also known as type 2 opsins) are members of the seven-transmembrane-domain proteins of the G protein-coupled receptor (GPCR) superfamily.[1][2]

Animal opsins fall phylogenetically into five groups: The ciliary opsins (cilopsins, c-opsins), the

peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins.[21]

Animal visual opsins are traditionally classified as either ciliary or rhabdomeric. Ciliary opsins, found in

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.[27]

Ciliary opsins

Ciliary opsins (cilopsins, c-opsins) are expressed in ciliary photoreceptor cells, and include the vertebrate visual opsins and encephalopsins.[66] 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.[67])

Vertebrate visual opsins

Main article: Vertebrate visual opsin

Vertebrate visual opsins are a subclass of ciliary opsins that express in the vertebrate retina and mediate vision. They are further subdivided into:

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.[71] [72]

Pinopsins

The first Pineal Opsin (Pinopsin) was found in the chicken pineal gland. It is a blue sensitive opsin (λmax = 470 nm).[73][74]

Pineal opsins have a 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

habenular region of the brain.[75] It is sensitive to approximately 500 nm [14], found in most vertebrate classes, but not in mammals.[76]

Parapinopsins

The first parapinopsin (PP) was found in the

parapineal organ of the catfish.[77] The parapinopsin of lamprey is a UV-sensitive opsin (λmax = 370 nm).[78] 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.[79]

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.[80][81]

Encephalopsin or Panopsin

The panopsins are found in many tissues (skin,[51] brain,[53][82] testes,[53] heart, liver,[82] kidney, skeletal muscle, lung, pancreas and retina[82]). They were originally found in the human and mouse brain and thus called encephalopsin.[53]

The first invertebrate panopsin was found in the ciliary photoreceptor cells of the annelid

amino-acid (range λmax = 377 - 502 nm).[84] Thus, not unsurprisingly, a second but cyan sensitive c-opsin (λmax = 490 nm) exists in Platynereis dumerilii.[85] The first c-opsin mediates in the larva UV induced gravitaxis. The gravitaxis forms with phototaxis a ratio-chromatic depth-gauge.[86] 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 narrower the spectrum so that only cyan light (480 nm) is left.[87] 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.[88][89]

Panopsins are also expressed in the brains of some insects.[66] The panopsins of mosquito and pufferfish absorb maximally at 500 nm and 460 nm, respectively. Both activate in vitro Gi and Go proteins.[90]

The panopsins are sister to the TMT-opsins.[28][91][47][92]

Teleost Multiple Tissue (TMT) Opsin

The first TMT-opsin was found in many tissues in

paralogous to the TMT-opsins.[28][47][91][92] TMT-opsins and panopsins also share the same introns, which confirms that they belong together.[93]

Opsins in cnidarians

bilaterians.[65][96][97]

Rhabdomeric opsins

Rhabdomeric opsins (rhabopsins, r-opsins) are also known as Gq-opsins, because they couple to a Gq-protein. Rhabopsins 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.[66] The rhabopsin melanopsin is also expressed in vertebrates, where it regulates circadian rhythms and mediates the pupillary reflex.[66]

Unlike cilopsins, rhabopsins 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).[67]

The identification of the crystal structure of squid rhodopsin[13] 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.[98]

Melanopsin

Melanopsin (OPN4) is involved in circadian rhythms, the pupillary reflex, and color correction in high-brightness situations. Phylogenetically, it is a member of the rhabdomeric opsins (rhabopsins, r-opsins) and functionally and structurally a rhabopsin, but does not occur in rhabdomeres.

Tetraopsins

The tetraopsins include the

peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins.[21]

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.[51][56][99] They couple to Gi-proteins.[56][99] 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.[100]

Go-opsins

Go-opsins are absent from

amphioxus.[103] In Platynereis dumerilii however, a Go-opsin is expressed in the rhabdomeric photoreceptor cells of the eyes.[87]

RGR-opsins

RGR-opsins, also known as

Müller cells.[104] They preferentially bind all-trans-retinal in the dark instead of 11-cis-retinal.[41] RGR-opsins were thought to be photoisomerases[44] but instead, they regulate retinoid traffic and production.[66][105] In particular, they speed up light-independently the production of 11-cis-retinol (a precursor of 11-cis-retinal) from all-trans-retinyl-esters.[106] 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.[107] 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.[108]

Peropsin

Peropsin, a visual pigment-like receptor, is a protein that in humans is encoded by the RRH gene.[109]

Other proteins called opsins

Main article: Microbial rhodopsin

Photoreceptors can be classified several ways, including function (vision, phototaxis, photoperiodism, etc.), type of

oxidation), etc.[110]

Beside animal opsins, which are

placozoans).[67]

Microbial and animal opsins are also called type 1 and type 2 opsins respectively. Both types are called opsins, because at one time it was thought that they were related: Both are seven-transmembrane receptors and bind covalently retinal as chromophore, which turns them into photoreceptors sensing light. However, both types are not related on the sequence level.[113]

In fact, the sequence identity between animal and mirobial opsins is no greater than could be accounted for by random chance. However, in recent years new methods have been developed specific to deep

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.[117]

Most microbial opsins are

Eukaryota. In Eukaryota, microbial opsins are found mainly in unicellular organisms such as green algae, and in fungi. In most complex multicellular eukaryotes, microbial opsins have been replaced with other light-sensitive molecules such as cryptochrome and phytochrome in plants, and animal opsins in animals.[118]

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).[119]

Several microbal opsins, such as

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.[120]

Microbal opsins (like channelrhodopsin, halorhodopsin, and archaerhodopsin) are used in optogenetics to switch on or off neuronal activity. Microbal opsins are preferred if the neuronal activity should be modulated at higher frequency, because they respond faster than animal opsins. This is because microbal opsins are ion channels or proton/ion pumps and thus are activated by light directly, while animal opsins activate G-proteins, which then activate effector enzymes that produce metabolites to open ion channels.[121]

See also

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