Opsins are a group of light-sensitive 35-55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. They are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade.
Two families of vertebrate opsins are generally recognized due to different spatial expression and evolutionary histories. Rhodopsins, which are used in night vision, are high-sensitivity, low-acuity opsins found in the rod photoreceptor cells. Cone opsins, employed in color vision, are low-sensitivity, high-acuity opsins located in the cone photoreceptor cells. Cone opsins are further subdivided according to their absorption maxima (λmax), the wavelength at which the highest light absorption is observed. Evolutionary relationships, deduced using the amino acid sequence of the opsins, are also frequently used to categorize cone opsins into their respective group. Both methods predict four general cone opsin groups in addition to rhodopsin: For example, humans have the following set of photoreceptor proteins responsible for vision:
- Rhodopsin (Rh1, RHO) – expressed in rod cells, used in night vision
- Four cone opsins (also knows as photopsins) – expressed in cone cells, used in color vision
- Long Wavelength Sensitive (LWS, OPN1LW) Opsin – λmax in the red region of the electromagnetic spectrum
- Middle Wavelength Sensitive (RH2 or MWS) Opsin – λmax in the green region of the electromagnetic spectrum
- Short Wavelength Sensitive 2 (SWS2) Opsin – λmax in the blue region of the electromagnetic spectrum
- Short Wavelength Sensitive 1 (SWS1) Opsin – λmax in the violet/UV region of the electromagnetic spectrum
Other animals may have different number of photoreceptor proteins, whose optical adsorption spectra are shifted compare to that in human photoreceptors. This leads to differen perception of light and visual images in humans and animals. Some insects, for example, can see ultraviolet light, whereas animals with only one type of opsins can see the world only in black and white colors.
Novel opsin groups
Over the last decade, several novel opsin groups have been discovered that are not involved in vision and that do not group with the five classical groups described above. Much of the research is still ongoing, with the function of many novel opsins unknown. The five classical opsins above are expressed solely in the retina, whereas the new novel opsins have a wide range of expression patterns. Phylogenetic studies have been undertaken to categorize these new opsins and determine their evolutionary relationship to the classical opsins.
- Pineal Opsin (Pinopsin) – wide range of expression in the brain, most notably in the pineal region
- Vertebrate Ancient (VA) opsin – has three isoforms VA short (VAS), VA medium (VAM), and VA long (VAL). It is expressed in the inner retina, within the horizontal and amacrine cells, as well as the pineal organ and habenular region of the brain
- Parapinopsin (PP) Opsin 
- Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh) – Rhodopsin-like protein expressed in the pineal region
- Encephalopsin or Panopsin (OPN3) – originally found in human and mice tissue with a very wide range of expression (brain, testes, heart, liver, kidney, skeletal muscle, lung, pancreas and retina)
Structure and Function
Opsin proteins covalently bind to a vitamin A-based retinaldehyde chromophore through a Schiff base linkage to a lysine residue in the seventh transmembrane alpha helix. In vertebrates, the chromophore is either 11-cis-retinal (A1) or 11-cis-3,4-didehydroretinal (A2) and is found in the retinal binding pocket of the opsin. The absorption of a photon of light results in the photoisomerisation of the chromophore from the 11-cis to an all-trans conformation. The photoisomerization induces a conformational change in the opsin protein, causing the activation of the phototransduction cascade. The opsin remains insensitive to light in the trans form. It is regenerated by the replacement of the all-trans retinal by a newly synthesized 11-cis-retinal provided from the retinal epithelial cells. Opsins are functional while bound to either chromophore, with A2-bound opsin λmax being at a longer wavelength than A1-bound opsin.
Opsins contain seven transmembrane α-helical domains connected by three extra-cellular and three cytoplasmic loops. Many amino acid residues, termed functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. All residue positions discussed henceforth are relative to the 348 amino acid bovine rhodopsin crystallized by Palczewski et al. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromaphore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabalizing the Lys296/chromaphore interaction by neutralizing the positive charge that builds up when the Schiff linkage occurs, The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propegation of the transduction signal once a photon has been absorbed.
Certain amino acid residues, termed spectral tuning sites, have a strong effect on λmax values. Using site-directed mutagenesis, it is possible to selectively mutate these residues and investigate the resulting changes in light absorption properties of the opsin. It is important to differentiate spectral tuning sites, residues that affect the wavelength at which the opsin absorbs light, from functionally conserved sites, residues important for the proper functioning of the opsin. They are not mutually exclusive, but, for practical reasons, it is easier to investigate spectral tuning sites that do not affect opsin functionality. For a comprehensive review of spectral tuning sites see Yokoyama S and Deeb SS . The impact of spectral tuning sites on λmax differs between different opsin groups and between opsin groups of different species.
- Illustration at Baldwin-Wallace College
- Overview at NASA
- Tree of vertebrate opsin at University of Tokyo
- Opsin at the US National Library of Medicine Medical Subject Headings (MeSH)
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