Wikipedia

Opsin

This article is about animal opsins. For microbial opsins, seemicrobial rhodopsin.

Animal opsinsareG-protein-coupled receptorsand a group of proteins made light-sensitive via achromophore,typicallyretinal.When bound to retinal, opsins becomeretinylidene proteins,but are usually still called opsins regardless. Most prominently, they are found inphotoreceptor cellsof theretina.Five classical groups of opsins are involved invision,mediating the conversion of aphotonof light into an electrochemical signal, the first step in thevisual transduction cascade.Another opsin found in the mammalian retina,melanopsin,is involved incircadian rhythmsandpupillary reflexbut not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sensetemperature,sound,orchemicals.

Three-dimensional structure of cattle rhodopsin. The seven transmembrane domains are shown in varying colors. Thechromophoreis 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.

Structure and function

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Animal opsins detect light and are the molecules that allow us to see. Opsins areG-protein-coupled receptors(GPCRs),[1][2]which arechemoreceptorsand have seven transmembranedomainsforming abinding pocketfor a ligand.[3][4]Theligandfor opsins is thevitamin A-basedchromophore11-cis-retinal,[5][6][7][8][9]which is covalently bound to alysineresidue[10]in the seventh transmembrane domain[11][12][13]through aSchiff-base.[14][15]However, 11-cis-retinal only blocks the binding pocket and does not activate the opsin. The opsin is only activated when 11-cis-retinal absorbs aphotonof light andisomerizesto all-trans-retinal,[16][17]the receptor activating form,[18][19]causing conformal changes in the opsin,[18]which activate aphototransduction cascade.[20]Thus, a chemoreceptor is converted to alight 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 invertebratesas ligand such as in freshwater fishes.[19]A2-bound opsins have a shiftedλmaxand absorption spectrum compared to A1-bound opsins.[22]

Functionally conserved residues and motifs

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The seven transmembrane α-helical domains in opsins are connected by three extra-cellular and threecytoplasmicloops. Along the α-helices and the loops, manyamino acidresidues are highly conserved between all opsin groups, indicating that they serve important functions and thus are calledfunctionally 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 andWeinsteinintroduced 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, theprolinein the highly conserved NPxxY7.53motifis Pro7.50,theasparaginebefore is then Asp7.49,and thetyrosinethree residues after is then Tyr7.53.[21]Another numbering scheme is based oncattlerhodopsin. Cattlerhodopsinhas 348amino acidsand is the first opsin whoseamino acid sequence[24]and whose3D-structurewere 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

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Opsins without the retinal binding lysine are not light sensitive.[25][26]Incattlerhodopsin,this lysine is the 296th amino acid[12][24]and thus according to both numbering schemes Lys2967.43.It is well conserved among opsins, so well conserved that sequences without it were not even considered opsins and thus excluded from large scalephylogenetic reconstructions.[27][28]Even so, most opsins have Lys2967.43,some have lost it during evolution: In the nemopsins fromnematodes,Lys2967.43is replaced byArginine.[29][21]In the astropsins fromsea urchins[30][21]and in the gluopsins, Lys2967.43is replaced byglutamic acid.[21]A nemopsin is expressed in chemosensory cells inCaenorhabditis elegans.Therefore, the nemopsins are thought to bechemoreceptors.[29]The gluopsins are found ininsectssuch asbeetles,scorpionflies,dragonflies,andbutterfliesandmothsincludingmodel organismssuch as thesilk mothand thetobacco 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 inthermosensation,[31]mechanoreceptionsuch ashearing[32]detectingphospholipids,chemosensation,and other functions.[33][34]In particular, theDrosophilarhabdomeric opsins (rhabopsins, r-opsins) Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors foraristolochic acid.These opsins still have Lys2967.43like 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, Lys2967.43is not needed for Rh1 to function as chemoreceptor.[26]Also the Drosophila rhabopsins Rh1 and Rh6 are involved in mechanoreception, again for mechanoreception Lys2967.43is 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]
  • Most known opsins have the retinal binding lysine except some among the tetraopins, The outgroup contains otherG protein-coupled receptors.
  • Most tetraopsins have also the retinal binding lysine except some of the chromopsins, which are highlighted by the frame and expanded in the next image. The outgroup contains other G protein-coupled receptors including the other opsins.
  • Most chromopsins have also the retinal binding lysine except the nemopsins, where it is replaced byargenine(R), and the gluopsins, where it is replaced byglutamic acid(E). The astropsins, the nemopsins and the gluopsins are highlighted by the frames. The outgroup contains other G protein-coupled receptors including the other opsins.

In thephylogenyabove, eachcladecontains sequences from opsins and other G protein-coupled receptors. The number of sequences and two pie charts are shown next to the clade. The first pie chart shows the percentage of a certainamino acidat the position in the sequences corresponding Lys2967.43in cattle rhodopsin. The amino acids are color-coded. The colors are red forlysine(K), purple forglutamic acid(E), orange forargenine(R), dark and mid-gray for other amino acids, and light gray for sequences that have no data at that position. The second pie chart gives the taxon composition for each clade, green stands forcraniates,dark green forcephalochordates,mid green forechinoderms,brown fornematodes,pale pink forannelids,dark blue forarthropods,light blue formollusks,and purple forcnidarians.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

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TheNPxxY7.53motifis well-conserved among opsins and G-protein-coupled receptors. This motif is important for G-protein binding and receptor activation.[21]For instance, if it is mutated to DPxxY7.53(Asn7.49→Asp7.49) in thehumanm3 muscarinic receptor,activation is not affected, but it is abolished if it is mutated to APxxY7.53(Asn7.49→Ala7.49).[36]Such a mutation to APxxY7.53(Asn7.49→ Ala7.49) reduces the G-protein activation of cattle rhodopsin to 45% compared to wild type. Also in cattle rhodopsin, if the motif is mutated to NPxxA7.53(Tyr7.53→Ala7.53), cattle rhodopsin does not activate the G-protein.[37]Such a mutation also reduces the activation of thevasopressin V2 receptor.In fact in G-protein-coupled receptors, onlyloss of functiondisease mutations are known for Tyr7.53⁠.[38]

Also mutations ofPro7.50influence G-protein activation, if the motif is mutated to NAxxY7.53(Pro7.50→Ala7.50) in theratm3 muscarinic receptor,the receptor can still be activated but less efficiently,[39]this mutation even abolishes activation in thecholecystokinin B receptorcompletely.[40]⁠ In fact, theRGR-opsinshave NAxxY7.53andretinochromeshave VPxxY7.53 for annelids or YPxxY7.53 for mollusks, natively. Both RGR-opsins and retinochromes, belong to the chromopsins.[21]RGR-opsins[41]and retinochromes[42]also bind unlike most opsins all-trans-retinal in the dark and convert it to 11-cis-retinal when illuminated. Therefore, RGR-opsins and retinochromes are thought to neither signal nor activate a phototransduction cascade but to work asphotoisomerasesto produce 11-cis-retinal for other opsins.[43][44]This view is considered established in the opsin literature,[34][45][43][46][47]even so it has not been shown, conclusively.[21]In fact, the human MT2melatonin receptorsignals via aG-proteinand has an NAxxY7.53motif 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 NPxxY7.53motif 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 thesequence logo,the height of the letters indicates the percentage of thatamino acidgiven at that position. The x-axis gives the position of the amino acid corresponding to cattle rhodopsin. Positions 2927.39and 3147.64are highlighted in gray.Lysine(K) 2967.43is highlighted with a gray background, which is replaced in the nemopsins byarginine(R) and in the gluopsins byglutamic acid(E). The NPxxY7.53motif 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

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Cys138 and Cys110 form a highly conserveddisulfide 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

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Certainamino acidresidues, termedspectral tuning sites,have a strong effect onλmaxvalues. Usingsite-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 differentiatespectral tuning sites,residues that affect the wavelength at which the opsin absorbs light, fromfunctionally 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 onλmaxdiffers between different opsin groups and between opsin groups of different species.

Opsins in the human eye, brain, and skin

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

Cuttlefish

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Cuttlefishandoctopusescontain 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

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

Animal opsins fall phylogenetically into five groups: The ciliary opsins (cilopsins, c-opsins), therhabdomericopsins (r-opsins, rhabopsins), the xenopsins, the nessopsins, and the tetraopsins. Four of these subclades occur inBilateria(all but the nessopsins).[21][28]However, the bilaterian clades constitute aparaphyletictaxon without the opsins from thecnidarians.[21][28][27][62]The nessopsins are also known as anthozoan opsins II[63]or simply as the cnidarian opsins.[64]The tetraopsins are also known as RGR/Go[65]or Group 4 opsins[27]and contain three subgroups: theneuropsins,the Go-opsins, and the chromopsins.[21][28][64]The chromopsins have seven subgroups: theRGR-opsins,theretinochromes,theperopsins,the varropsins, the astropsins, the nemopsins, and the gluopsins.[21]

Animal visual opsins are traditionally classified as either ciliary or rhabdomeric. Ciliary opsins, found invertebratesandcnidarians,attach to ciliary structures such asrodsandcones.Rhabdomericopsins 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 inBilateriaand excludes cnidarian ciliary opsins such as those found in thebox jellyfish.Similarly, "R-opsin (rhabdomeric)" includes melanopsin even though it does not occur on rhabdomeres in vertebrates.[27]

Ciliary opsins

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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

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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)

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These pineal opsins, found in theActinopterygii(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

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The first Pineal Opsin (Pinopsin) was found in the chickenpineal 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 thepineal region.

Vertebrate Ancient (VA) opsin

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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 andamacrine cells,as well as the pineal organ andhabenularregion of the brain.[75]It is sensitive to approximately 500 nm [14], found in most vertebrate classes, but not in mammals.[76]

Parapinopsins

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The first parapinopsin (PP) was found in theparapineal organof thecatfish.[77]The parapinopsin oflampreyis 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

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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

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Thepanopsinsare 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 andmouse brainand thus called encephalopsin.[53]

The first invertebrate panopsin was found in the ciliary photoreceptor cells of the annelidPlatynereis dumeriliiand is called c(iliary)-opsin.[83]This c-opsin isUV-sensitive (λmax= 383 nm) and can be tuned by 125 nm at a singleamino-acid(rangeλmax= 377 - 502 nm).[84]Thus, not unsurprisingly, a second but cyan sensitive c-opsin (λmax= 490 nm) exists inPlatynereis dumerilii.[85]The first c-opsin mediates in the larva UV inducedgravitaxis.The gravitaxis forms withphototaxisa ratio-chromaticdepth-gauge.[86]In different depths, the light in water is composed of differentwavelengths:First the red (> 600 nm) and the UV and violet (< 420 nm) wavelengths disappear. The higher the depth the narrower the spectrum so that onlycyanlight (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

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The first TMT-opsin was found in many tissues inTeleostfish and therefore they are called Teleost Multiple Tissue (TMT) opsins.[93]TMT-opsins form three groups which are most closely related to a fourth group the panopsins, which thus areparalogousto the TMT-opsins.[28][47][91][92]TMT-opsins and panopsins also share the sameintrons,which confirms that they belong together.[93]

Opsins in cnidarians

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Cnidaria,which include jellyfish, corals, and seaanemones,are the mostbasal animalsto possess complex eyes. Jellyfish opsins in therhopaliacouple to Gs-proteins raising the intracellular cAMP level.[94][62]Coral opsins can couple to Gq-proteins and Gc-proteins. Gc-proteins are a subtype of G-proteins specific to cnidarians.[95]The cnidarian opsins belong to two groups the xenopsins and the nessopsins. The xenopsins contain also bilaterian opsins, while the nessopsins are restricted to the cnidarians.[21][28]However, earlier studies have found that some cnidarian opsins belong to the cilopsins, rhabopsins, and the tetraopsins of thebilaterians.[65][96][97]

Rhabdomeric opsins

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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 regulatescircadian rhythmsand 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 inLimulusthe opsins expressed in the lateral and the compound eyes are 99% identical and presumably diverged recently.[98]

Melanopsin

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Melanopsin(OPN4) is involved incircadian rhythms,thepupillary 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

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The tetraopsins include theneuropsins,the Go-opsins, and the chromopsins.[21][28][64]The chromopsins consist of seven subgroups: theRGR-opsins,theretinochromes,theperopsins,the varropsins, the astropsins, the nemopsins, and the gluopsins.[21]

Neuropsins

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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 theOPN5gene. 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

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Go-opsins are absent fromhigher vertebrates[27]andecdysozoans.[101]They are found in the ciliary photoreceptor cells of thescallopeye[102]and the basal chordateamphioxus.[103]InPlatynereis dumeriliihowever, a Go-opsin is expressed in the rhabdomeric photoreceptor cells of the eyes.[87]

RGR-opsins

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RGR-opsins, also known asRetinal G protein coupled receptorsare expressed in theretinal pigment epithelium(RPE) andMüller 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

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Peropsin,a visual pigment-like receptor, is aproteinthat in humans is encoded by theRRHgene.[109]

Other proteins called opsins

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Main article:Microbial rhodopsin

Photoreceptors can be classified several ways, including function (vision, phototaxis, photoperiodism, etc.), type ofchromophore(retinal,flavine,bilin), molecular structure (tertiary,quaternary), signal output (phosphorylation,reduction,oxidation), etc.[110]

Beside animal opsins, which areG protein-coupled receptors,there is another group ofphotoreceptor proteinscalled opsins.[67][111]These are themicrobial opsin,they are used byprokaryotesand by somealgae(as a component ofchannelrhodopsins) andfungi,[112]whereasanimalsuse animal opsins, exclusively. No opsins have been found outside these groups (for instance in plants, orplacozoans).[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 covalentlyretinalas chromophore, which turns them intophotoreceptorssensing 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 todeepphylogeny.As a result, several studies have found evidence of a possible phylogenetic relationship between the two.[114][35][115]However, this does not necessarily mean that the last common ancestor of microbial and animal opsins was itself light sensitive: All animal opsins arose (by gene duplication and divergence) late in the history of the largeG-protein coupled receptor(GPCR)gene family,which itself arose after the divergence of plants, fungi, choanflagellates and sponges from the earliest animals. The retinal chromophore is found solely in the opsin branch of this large gene family, meaning its occurrence elsewhere representsconvergent evolution,nothomology.Microbial rhodopsins are, by sequence, very different from any of the GPCR families.[116]According to one hypothesis, both microbial and animal opsins belong to thetransporter-opsin-G protein-coupled receptor (TOG) superfamily,a proposed clade that includesG protein-coupled receptor(GPCR), Ion-translocatingmicrobial rhodopsin(MR), and seven others.[117]

Most microbial opsins areion channelsorpumpsinstead of proper receptors and do not bind to aG protein.Microbal opsins are found in all three domains of life:Archaea,Bacteria,andEukaryota.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 ascryptochromeandphytochromein plants, and animal opsins inanimals.[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 theproton pumpsbacteriorhodopsin(BR) and xanthorhodopsin (xR), thechloride pumphalorhodopsin(HR), the photosensors sensory rhodopsin I (SRI) andsensory rhodopsin II(SRII), as well asproteorhodopsin(PR),Neurosporaopsin I (NOPI), Chlamydomonas sensory rhodopsins A (CSRA), Chlamydomonas sensory rhodopsins B (CSRB),channelrhodopsin(ChR), andarchaerhodopsin(Arch).[119]

Several microbal opsins, such asproteo-andbacteriorhodopsin,are used by various bacterial groups to harvest energy from light to carry out metabolic processes using a non-chlorophyll-based pathway. Beside that,halorhodopsinsofHalobacteriaandchannelrhodopsinsof some algae, e.g.Volvox,serve them aslight-gated ion channels,amongst others also forphototacticpurposes. Sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting withtransducermembrane-embedded proteins that have no relation to G proteins.[120]

Microbal opsins (likechannelrhodopsin,halorhodopsin,andarchaerhodopsin) are used inoptogeneticsto 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 pumpsand thus are activated by light directly, while animal opsins activate G-proteins, which then activateeffectorenzymes that produce metabolites to open ion channels.[121]

See also

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References

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