HOME

TheInfoList



OR:

Animal opsins are
G-protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...
s and a group of proteins made light-sensitive via a
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
, typically
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
. When bound to retinal, opsins become
retinylidene protein Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. R ...
s, but are usually still called opsins regardless. Most prominently, they are found in
photoreceptor cell 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 (visible electromagnetic radiation ...
s of the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
. Five classical groups of opsins are involved in
vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...
, mediating the conversion of a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina,
melanopsin Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinylidene protein, retinal proteins called opsins and encoded by the gene ''Opn4''. In the mammalian retina, there are two additional categories of opsins, b ...
, is involved in
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s and
pupillary reflex Pupillary reflex refers to one of the reflexes associated with pupillary function. These include the pupillary light reflex and accommodation reflex. Although the pupillary response, in which the pupil dilates or constricts due to light is not us ...
but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
,
sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...
, or
chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
s.


Structure and function

Animal opsins detect light and are the molecules that allow us to see. Opsins are
G-protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...
s (GPCRs), which are
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...
s and have seven transmembrane domains forming a
binding pocket In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding si ...
for a ligand. The
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
for opsins is the
vitamin A Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most not ...
-based
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
11-''cis''-retinal, which is covalently bound to a
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. Lysine contains an α-amino group (which is in the protonated form when the lysine is dissolved in water at physiological pH), an α-carboxylic acid group ( ...
residue in the seventh transmembrane domain through a Schiff-base. 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 a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
of light and isomerizes to all-''trans''-retinal, the receptor activating form, causing conformal changes in the opsin, which activate a
phototransduction cascade Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells ( rods and cones) in the vertebrate retina. A photon is absorbed by a retinal chromophore (each bound to an opsin), ...
. Thus, a chemoreceptor is converted to a light or photo(n)receptor. Material was copied and adapted from this source, which is available under
Creative Commons Attribution 4.0 International License
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
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s as ligand such as in freshwater fishes. A2-bound opsins have a shifted ''λ''max and absorption spectrum compared to A1-bound opsins.


Functionally conserved residues and motifs

The seven transmembrane α-helical domains in opsins are connected by three extra-cellular and three
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
ic loops. Along the α-helices and the loops, many
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
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. 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 Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the p ...
in the highly conserved NPxxY7.53 motif is Pro7.50, the
asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
before is then Asp7.49, and the
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
three residues after is then Tyr7.53. Another numbering scheme is based on
cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...
rhodopsin. Cattle
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
has 348
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s and is the first opsin whose
amino acid sequence Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthe ...
and whose 3D-structure were determined. The cattle rhodopsin numbering scheme is widespread in the opsin literature. Therefore, it is useful to use both schemes.


The retinal binding lysine

Opsins without the retinal binding lysine are not light sensitive. In
cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
, this lysine is the 296th amino acid 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 scale phylogenetic reconstructions. Even so, most opsins have Lys2967.43, some have lost it during evolution: In the nemopsins from
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
s, Lys2967.43 is replaced by
Arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...
. In the astropsins from
sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...
s and in the gluopsins, Lys2967.43 is replaced by
glutamic acid Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α- amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can ...
. A nemopsin is expressed in chemosensory cells in ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''r ...
''. Therefore, the nemopsins are thought to be
chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...
s. The gluopsins are found in
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s such as
beetle Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 40 ...
s,
scorpionflies Mecoptera (from the Greek language, Greek: ''mecos'' = "long", ''ptera'' = "wings") is an Order (biology), order of insects in the superorder Holometabola with about six hundred species in nine Family (biology), families worldwide. Mecopterans a ...
,
dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of dragonflies are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threate ...
, and
butterflies Butterflies are winged insects from the lepidopteran superfamily Papilionoidea, characterized by large, often brightly coloured wings that often fold together when at rest, and a conspicuous, fluttering flight. The oldest butterfly fossi ...
and
moth Moths are a group of insects that includes all members of the order Lepidoptera that are not Butterfly, butterflies. They were previously classified as suborder Heterocera, but the group is Paraphyly, paraphyletic with respect to butterflies (s ...
s including
model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
s such as the silk moth and the
tobacco hawk moth ''Manduca sexta'' is a moth of the family Sphingidae present through much of the Americas. The species was first described by Carl Linnaeus in his 1763 ''Centuria Insectorum''. Commonly known as the Carolina sphinx moth and the tobacco hawk mo ...
. However, the gluopsins have no known function. Such function does not need to be light detection, as some opsins are also involved in thermosensation,
mechanoreception A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into electrical signals that, in animals, ar ...
such as
hearing Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory sci ...
detecting
phospholipid Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s, chemosensation, and other functions. In particular, the
Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...
rhabdomeric opsins (rhabopsins, r-opsins) Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for
aristolochic acid Aristolochic acids () are a family of carcinogenic, mutagenic, and nephrotoxic phytochemicals commonly found in the flowering plant family Aristolochiaceae (birthworts). Aristolochic acid (AA) I is the most abundant one. The family Aristolochiace ...
. 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. 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. 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. In the
phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...
above, each
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
contains 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 certain
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
at the position in the sequences corresponding Lys2967.43 in cattle rhodopsin. The amino acids are color-coded. The colors are red for
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. Lysine contains an α-amino group (which is in the protonated form when the lysine is dissolved in water at physiological pH), an α-carboxylic acid group ( ...
(K), purple for
glutamic acid Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α- amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can ...
(E), orange for argenine (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 for
craniate A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the mu ...
s, dark green for
cephalochordate The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets div ...
s, mid green for
echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s, brown for
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
s, pale pink for
annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
s, dark blue for
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s, light blue for
mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000  extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...
s, and purple for
cnidarian Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
s. 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.


The NPxxY motif

The NPxxY7.53 motif is well-conserved among opsins and G-protein-coupled receptors. This motif is important for G-protein binding and receptor activation. For instance, if it is mutated to DPxxY7.53 ( Asn7.49Asp7.49) in the
human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
m3 muscarinic receptor The muscarinic acetylcholine receptor, also known as cholinergic/acetylcholine receptor M3, or the muscarinic 3, is a muscarinic acetylcholine receptor encoded by the human gene CHRM3. The M3 muscarinic receptors are located at many places in ...
, activation is not affected, but it is abolished if it is mutated to APxxY7.53 ( Asn7.49Ala7.49). 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 ( Tyr7.53Ala7.53), cattle rhodopsin does not activate the G-protein. Such a mutation also reduces the activation of the vasopressin V2 receptor. In fact in G-protein-coupled receptors, only loss of function disease mutations are known for Tyr7.53⁠. Also mutations of
Pro Pro is an abbreviation meaning "professional". Pro, PRO or variants thereof might also refer to: People * Miguel Pro (1891–1927), Mexican priest * Pro Hart (1928–2006), Australian painter * Mlungisi Mdluli (born 1980), South African ret ...
7.50 influence G-protein activation, if the motif is mutated to NAxxY7.53 (
Pro Pro is an abbreviation meaning "professional". Pro, PRO or variants thereof might also refer to: People * Miguel Pro (1891–1927), Mexican priest * Pro Hart (1928–2006), Australian painter * Mlungisi Mdluli (born 1980), South African ret ...
7.50Ala7.50) in the
rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include '' Neotoma'' (pack rats), '' Bandicota'' (bandicoo ...
m3 muscarinic receptor The muscarinic acetylcholine receptor, also known as cholinergic/acetylcholine receptor M3, or the muscarinic 3, is a muscarinic acetylcholine receptor encoded by the human gene CHRM3. The M3 muscarinic receptors are located at many places in ...
, the receptor can still be activated but less efficiently, this mutation even abolishes activation in the
cholecystokinin B receptor The cholecystokinin B receptor also known as CCKBR or CCK2 is a protein that in humans is encoded by the ''CCKBR'' gene. This gene encodes a G protein-coupled receptor for gastrin and cholecystokinin (CCK), regulatory peptides of the brain and g ...
completely.⁠ In fact, the RGR-opsins have NAxxY7.53 and retinochromes have VPxxY7.53 for annelids or YPxxY7.53 for mollusks, natively. Both RGR-opsins and retinochromes, belong to the chromopsins. RGR-opsins and retinochromes 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 as photoisomerases to produce 11-''cis''-retinal for other opsins. This view is considered established in the opsin literature, even so it has not been shown, conclusively. In fact, the human MT2
melatonin receptor Melatonin receptors are G protein-coupled receptors (GPCR) which bind melatonin. Three types of melatonin receptors have been cloned. The MT1 (or Mel1A or MTNR1A) and MT2 (or Mel1B or MTNR1B) receptor subtypes are present in humans and other ...
signals via a
G-protein G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their ...
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 (Ala7.50Val7.50). Furthermore, when the motif is mutated to NAxxY7.53 (Pro7.50 → Ala7.50) in cattle rhodopsin, the mutant has 141% of wild type activity. This evidence shows that a GPCR does not need a standard NPxxY7.53 motif for signaling.


Other residues and motifs

Cys138 and Cys110 form a highly conserved
disulfide bridge In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...
. 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 Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
residues, termed ''spectral tuning sites'', have a strong effect on ''λ''max values. Using
site-directed mutagenesis Site-directed mutagenesis is a molecular biology method that is used to make specific and intentional mutating changes to the DNA sequence of a gene and any gene products. Also called site-specific mutagenesis or oligonucleotide-directed mutagenes ...
, 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 and Deeb. The impact of spectral tuning sites on ''λ''max differs between different opsin groups and between opsin groups of different species.


Opsins in the human eye, brain, and skin

RPE,
retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...
; ipRGC,
intrinsically photosensitive retinal ganglion cells Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence ...
; OPL, outer plexiform layer; IPL, inner plexiform layer; GCL, ganglion cell layer


Cuttlefish

Cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...
and
octopus An octopus (: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like oth ...
es 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.


Frogs (order Anura)

Frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...
s have evolved unique visual systems to adapt to their diverse habitats, from brightly lit forests to dimly lit ponds. Frogs are distinct among
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s because they lack the RH2 opsin, typically used for detecting middle wavelengths of light in other species. This loss likely reflects their evolutionary focus on low-light vision, with RH1, a rod-specific opsin, taking the lead in supporting
nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...
and
crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal (active during dawn), vespertine (biology), vespertine/vespertinal (active during dusk), or both. This is distinguished from diurnalit ...
(dawn and dusk) activity. Despite the loss of RH2, frogs retain three cone opsins—SWS1, SWS2, and LWS—that allow for color vision during daylight. The SWS2 opsin, for instance, is tuned to detect blue and green light, which is especially useful in aquatic environments or shaded areas. This tuning is enhanced by specific
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
s which increases sensitivity to low-light conditions and stabilizes the protein for better performance in dim environments. However, some frog species, such as
poison dart frog Poison dart frog (also known as dart-poison frog, poison frog or formerly known as poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to tropical Central and South America. These species are ...
s in the family Dendrobatidae, have lost the SWS2 opsin entirely. This change aligns with their reliance on longer wavelengths, like red and yellow, for tasks such as mate selection and predator deterrence, often linked to their vibrant
aposematic Aposematism is the Advertising in biology, advertising by an animal, whether terrestrial or marine, to potential predation, predators that it is not worth attacking or eating. This unprofitability may consist of any defenses which make the pr ...
(warning) coloration.


Phylogeny

Animal opsins (also known as type 2 opsins) are members of the seven-transmembrane-domain proteins of the
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily related ...
(GPCR) superfamily. Animal opsins fall phylogenetically into five groups: The ciliary opsins (cilopsins, c-opsins), the rhabdomeric opsins (r-opsins, rhabopsins), the xenopsins, the nessopsins, and the tetraopsins. Four of these subclades occur in
Bilateria Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...
(all but the nessopsins). However, the bilaterian clades constitute a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
taxon without the opsins from the
cnidarian Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
s. The nessopsins are also known as anthozoan opsins II or simply as the cnidarian opsins. The tetraopsins are also known as RGR/Go or Group 4 opsins and contain three subgroups: the neuropsins, the Go-opsins, and the chromopsins. The chromopsins have seven subgroups: the RGR-opsins, the retinochromes, the peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins. Animal visual opsins are traditionally classified as either ciliary or rhabdomeric. Ciliary opsins, found in
vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
and
cnidarians Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
, attach to ciliary structures such as rods and
cones In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the ''apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines, ...
. 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 Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...
and excludes cnidarian ciliary opsins such as those found in the
box jellyfish Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their box-like (i.e., cube-shaped) body. Some species of box jellyfish produce potent venom delivered by contact with their tentacles. Stings from some species, including ' ...
. Similarly, "R-opsin (rhabdomeric)" includes melanopsin even though it does not occur on rhabdomeres in vertebrates.


Ciliary opsins

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


Vertebrate visual opsins

Vertebrate visual opsins are a subclass of ciliary opsins that express in the vertebrate retina and mediate vision. They are further subdivided into: *
Photopsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still t ...
s - those responsible for
photopic vision Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108  cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher vis ...
(daylight), which are expressed in cone cells; hence also
cone opsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still t ...
s. Photopsins are further subdivided according to their
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characteristics ...
, namely the wavelength at which the highest light absorption is observed (''λ''max). Vertebrates generally have four (SWS1, SWS2, RH2, LWS) classes of photopsins. Mammals lost Rh2 and SWS2 classes during the
nocturnal bottleneck The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively ...
, so are generally dichromatic. Primate ancestors later developed two distinct LWS opsins (LWS and MWS), leaving humans with 3 photopsins in 2 classes: SWS1 (
OPN1SW Blue-sensitive opsin is a protein that in humans is encoded by the ''OPN1SW'' gene. The OPN1SW gene provides instructions for making a protein that is essential for normal color vision. This protein is found in the retina, which is the light-sensi ...
) and two forms of LWS (
OPN1LW OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. The OPN1LW gene provides instructions for making an opsin pigment that is more sensitive to light in the yellow/orange part of the vis ...
,
OPN1MW Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene. OPN1MW2 is a similar opsin. The OPN1MW gene provides instructions for making an opsin pigment that is more sensitive to light in the middle of the visible spect ...
). * Scotopsins - those responsible for
scotopic vision In the study of visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from the Greek ''skotos'', meaning 'darkness', and ''-opia'', meaning 'a condition of sight'. In the human eye, c ...
(dim light), which are expressed in rod cells; hence also rod opsins. The most common form of scotopsin is
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
, thus usually denoted Rh1.


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

These pineal opsins, found in the
Actinopterygii Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class (biology), class of Osteichthyes, bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built ...
(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.


Pinopsins

The first Pineal Opsin (Pinopsin) was found in the chicken
pineal gland The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep, sleep patterns following the diurnal c ...
. It is a blue sensitive opsin (''λ''max = 470 nm). 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
amacrine cells In the anatomy of the eye, amacrine cells are interneurons in the retina. They are named , because of their short neuronal processes. Amacrine cells are inhibitory neurons which project their dendritic arbors onto the inner plexiform layer ...
, as well as the pineal organ and habenular region of the brain. It is sensitive to approximately 500 nm 4 found in most vertebrate classes, but not in mammals.


Parapinopsins

The first parapinopsin (PP) was found in the parapineal organ of the
catfish Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...
. The parapinopsin of
lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...
is a UV-sensitive opsin (''λ''max = 370 nm). The teleosts have two groups of parapinopsins, one is sensitive to UV (''λ''max = 360-370 nm), the other is sensitive to blue (''λ''max = 460-480 nm) light.


Parietopsins

The first parietopsin was found in the photoreceptor cells of the lizard parietal eye. The lizard parietopsin is green-sensitive (''λ''max = 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.


Encephalopsin or Panopsin

The panopsins are found in many tissues (skin, brain, testes, heart, liver, kidney, skeletal muscle, lung, pancreas and retina). They were originally found in the human and
mouse brain A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
and thus called encephalopsin. The first invertebrate panopsin was found in the ciliary photoreceptor cells of the annelid ''
Platynereis dumerilii ''Platynereis dumerilii'' is a species of annelid polychaete worm. It was originally placed into the genus '' Nereis'' and later reassigned to the genus '' Platynereis''. ''Platynereis dumerilii'' lives in coastal marine waters from temperate ...
'' and is called c(iliary)-opsin. This c-opsin is UV-sensitive (''λ''max = 383 nm) and can be tuned by 125 nm at a single
amino-acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
(range ''λ''max = 377 - 502 nm). Thus, not unsurprisingly, a second but cyan sensitive c-opsin (''λ''max = 490 nm) exists in ''Platynereis dumerilii''. The first c-opsin mediates in the larva UV induced
gravitaxis Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. There are a few different causes for gravitaxis. Many microorganisms have receptors like statocysts that allow them to ...
. The gravitaxis forms with
phototaxis Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive ...
a ratio-chromatic depth-gauge. In different depths, the light in water is composed of different
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s: 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 Cyan () is the color between blue and green on the visible spectrum of light. It is evoked by light with a predominant wavelength between 500 and 520 nm, between the wavelengths of green and blue. In the subtractive color system, or CMYK c ...
light (480 nm) is left. 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. Panopsins are also expressed in the brains of some insects. The panopsins of mosquito and pufferfish absorb maximally at 500 nm and 460 nm, respectively. Both activate in vitro Gi and Go proteins. The panopsins are sister to the TMT-opsins.


Teleost Multiple Tissue (TMT) Opsin

The first TMT-opsin was found in many tissues in
Teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
fish and therefore they are called Teleost Multiple Tissue (TMT) opsins. TMT-opsins form three groups which are most closely related to a fourth group the panopsins, which thus are
paralogous Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speci ...
to the TMT-opsins. TMT-opsins and panopsins also share the same
intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...
s, which confirms that they belong together.


Opsins in cnidarians

Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
, which include jellyfish, corals, and sea anemones, are the most
basal animals Marine life, sea life or ocean life is the collective ecological communities that encompass all aquatic animals, aquatic plant, plants, algae, marine fungi, fungi, marine protists, protists, single-celled marine microorganisms, microorganisms ...
to possess complex eyes. Jellyfish opsins in the
rhopalia Rhopalia (singular: rhopalium) from Ancient Greek ῥόπαλον (''rhópalon'') 'club' are small sensory structures of certain Scyphozoan (true jellyfish) and Cubozoan (box jellyfish) species. Description The structures typically occur in mul ...
couple to Gs-proteins raising the intracellular cAMP level. Coral opsins can couple to Gq-proteins and Gc-proteins. Gc-proteins are a subtype of G-proteins specific to cnidarians. 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. However, earlier studies have found that some cnidarian opsins belong to the cilopsins, rhabopsins, and the tetraopsins of the
bilaterian Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...
s.


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. The rhabopsin melanopsin is also expressed in vertebrates, where it regulates
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s and mediates the pupillary reflex. 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 In biology, depolarization or hypopolarization is a change within a cell (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...
). The identification of the crystal structure of squid rhodopsin 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.


Melanopsin

Melanopsin Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinylidene protein, retinal proteins called opsins and encoded by the gene ''Opn4''. In the mammalian retina, there are two additional categories of opsins, b ...
(OPN4) is involved in
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s, the
pupillary reflex Pupillary reflex refers to one of the reflexes associated with pupillary function. These include the pupillary light reflex and accommodation reflex. Although the pupillary response, in which the pupil dilates or constricts due to light is not us ...
, 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 neuropsins, the Go-opsins, and the chromopsins. The chromopsins consist of seven subgroups: the RGR-opsins, the retinochromes, the peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins.


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. They couple to Gi-proteins. 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.


Go-opsins

Go-opsins are absent from higher vertebrates and ecdysozoans. They are found in the ciliary photoreceptor cells of the
scallop Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...
eye and the basal chordate
amphioxus The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets div ...
. In ''
Platynereis dumerilii ''Platynereis dumerilii'' is a species of annelid polychaete worm. It was originally placed into the genus '' Nereis'' and later reassigned to the genus '' Platynereis''. ''Platynereis dumerilii'' lives in coastal marine waters from temperate ...
'' however, a Go-opsin is expressed in the rhabdomeric photoreceptor cells of the eyes.


RGR-opsins

RGR-opsins, also known as Retinal G protein coupled receptors are expressed in the
retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...
(RPE) and
Müller cells Müller may refer to: Companies * Müller (company), a German multinational dairy company ** Müller Milk & Ingredients, a UK subsidiary of the German company * Müller (store), a German retail chain * GMD Müller, a Swiss aerial lift manufact ...
. They preferentially bind all-trans-retinal in the dark instead of 11-cis-retinal. RGR-opsins were thought to be photoisomerases but instead, they regulate retinoid traffic and production. In particular, they speed up light-independently the production of 11-cis-retinol (a precursor of 11-cis-retinal) from all-trans-retinyl-esters. 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. 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.


Peropsin

Peropsin, a visual pigment-like receptor, is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that in humans is encoded by the ''RRH''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
.


Other proteins called opsins

Photoreceptors can be classified several ways, including function (vision, phototaxis, photoperiodism, etc.), type of
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
(
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
, flavine, bilin), molecular structure (
tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
,
quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
), signal output (
phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...
, reduction,
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
), etc. Beside animal opsins, which are
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily related ...
s, there is another group of
photoreceptor protein 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 bacteriorhod ...
s called opsins. These are the microbial opsin, they are used by
prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s and by some
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
(as a component of
channelrhodopsins Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed ...
) and
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
, whereas
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s use animal opsins, exclusively. No opsins have been found outside these groups (for instance in plants, or
placozoans Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, plac ...
). 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 Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
as chromophore, which turns them into photoreceptors sensing light. However, both types are not related on the sequence level. 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
phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...
''. As a result, several studies have found evidence of a possible phylogenetic relationship between the two. 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 large
G-protein coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...
(GPCR)
gene family A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on ...
, 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 represents
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
, not homology. Microbial rhodopsins are, by sequence, very different from any of the GPCR families. According to one hypothesis, both microbial and animal opsins belong to the '' transporter-opsin-G protein-coupled receptor (TOG) superfamily'', a proposed clade that includes
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily related ...
(GPCR), Ion-translocating
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 transmembr ...
(MR), and seven others. Most microbial opsins are
ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...
s or
pumps A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of applications such ...
instead of proper receptors and do not bind to a
G protein G proteins, also known as guanine nucleotide-binding proteins, are a Protein family, family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell (biology), ...
. Microbial opsins are found in all three domains of life:
Archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
,
Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
, and
Eukaryota The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
. 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 Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fiel ...
and
phytochrome Phytochromes are a class of photoreceptor proteins found in plants, bacteria and fungi. They respond to light in the red and far-red regions of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or ...
in plants, and animal opsins in
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s. 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 pump A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction: : n one side of a biological membrane/sub> + energy n the other side of the m ...
s
bacteriorhodopsin Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by Haloarchaea, 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 res ...
(BR) and xanthorhodopsin (xR), the chloride pump
halorhodopsin Halorhodopsin is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is a chloride-specific light-activated ion pump found in archaea known as halobacteria. It is activated by green light wavelengths of approximatel ...
(HR), the photosensors sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII), as well as
proteorhodopsin Proteorhodopsin (PR or pRhodopsin) belongs to the Protein family, family of Bacteria, bacterial Transmembrane protein, transmembrane Rhodopsin, rhodopsins (Retinylidene protein, retinylidene proteins). In 1971, the first Microorganism, microbial ...
(PR),
Neurospora ''Neurospora'' is a genus of Ascomycete fungi. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores that resemble axons. The best known species in this genus is '' Neurospora crassa'', a common model organ ...
opsin I (NOPI), Chlamydomonas sensory rhodopsins A (CSRA), Chlamydomonas sensory rhodopsins B (CSRB),
channelrhodopsin Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed ...
(ChR), and
archaerhodopsin Archaerhodopsin proteins are a Protein family, family of retinal-containing receptor protein, photoreceptors found in the archaea genera ''Halobacterium'' and ''Halorubrum''. Like the homologous bacteriorhodopsin (bR) protein, archaerhodopsins harv ...
(Arch). Several microbal opsins, such as proteo- and
bacteriorhodopsin Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by Haloarchaea, 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 res ...
, are used by various bacterial groups to harvest energy from light to carry out metabolic processes using a non-
chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
-based pathway. Beside that, halorhodopsins of
Halobacteria Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class (biology), class of prokaryotic archaea under the phylum Euryarchaeota, found in water Saturated and unsaturated compounds, saturated or nearly saturated with ...
and
channelrhodopsins Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed ...
of some algae, e.g.
Volvox ''Volvox'' is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. ''Volvox'' species form spherical colonies of up to 50,000 cells, and for this reason they are sometimes called globe algae. First reported by Antonie van L ...
, serve them as light-gated ion channels, amongst others also for
phototactic Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus (physiology), stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most e ...
purposes. Sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with
transducer A transducer is a device that Energy transformation, converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, M ...
membrane-embedded proteins that have no relation to G proteins. Microbal opsins (like
channelrhodopsin Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed ...
,
halorhodopsin Halorhodopsin is a seven-transmembrane retinylidene protein from microbial rhodopsin family. It is a chloride-specific light-activated ion pump found in archaea known as halobacteria. It is activated by green light wavelengths of approximatel ...
, and
archaerhodopsin Archaerhodopsin proteins are a Protein family, family of retinal-containing receptor protein, photoreceptors found in the archaea genera ''Halobacterium'' and ''Halorubrum''. Like the homologous bacteriorhodopsin (bR) protein, archaerhodopsins harv ...
) are used in
optogenetics Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by Gene expression, expression of Channelrhodopsin, light-sensitive ion channels, Halorhodopsin, pumps or Photoactivated ade ...
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 Effector may refer to: *Effector (biology), a molecule that binds to a protein and thereby alters the activity of that protein * ''Effector'' (album), a music album by the Experimental Techno group Download * ''EFFector'', a publication of the El ...
enzymes that produce metabolites to open ion channels.


See also

*
Retinylidene protein Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. R ...
* Visual cycle *
Visual phototransduction Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells ( rods and cones) in the vertebrate retina. A photon is absorbed by a retinal chromophore (each bound to an op ...
*
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 transmembr ...
*
Channelrhodopsins Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed ...


External links


Illustration
at
Baldwin-Wallace College Baldwin Wallace University (BW) is a private university in Berea, Ohio, United States. Established in 1845 as Baldwin Institute by Methodist businessman John Baldwin, it merged with nearby German Wallace College in 1913 to become Baldwin-Wallac ...
*


References

{{G protein-coupled receptors G protein-coupled receptors Vision