opsin

3-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red.

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. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming.

Classification

There are two types of opsins, type 1 and type 2. Though similar in structure and function, evidence suggests that they evolved independently of one another in bacteria and animals.^[1]

Prokaryotic opsins (type 1)

Like eukaryotic opsins, prokaryotic opsins have a seven transmembrane domain structure similar to that found in eukaryotic G-protein coupled receptors. Despite this similarity, there is no evidence that they are evolutionarily related, suggesting that they evolved independently of one another.^[1]

Several type 1 opsins, such as proteo-, halo- and bacteriorhodopsin, are used by various bacterial groups to harvest energy from light to fix carbon using a non-chlorophyll-based pathway. Additionally, sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with transducer membrane-embedded proteins that have no relation to G proteins. ^[2]

Classical type 2 opsin groups

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 (λ_max), 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:^[3] For example, humans have the following set of photoreceptor proteins responsible for vision:

• Rhodopsin (Rh1, OPN2, RHO) – expressed in rod cells, used in night vision
• Four cone opsins (also known as photopsins) – expressed in cone cells, used in color vision
  • Long Wavelength Sensitive (LWS, OPN1LW) Opsin – λ_max in the red region of the electromagnetic spectrum
  • Middle Wavelength Sensitive (RH2 or MWS) Opsin – λ_max in the green region of the electromagnetic spectrum
  • Short Wavelength Sensitive 2 (SWS2) Opsin – λ_max in the blue region of the electromagnetic spectrum
  • Short Wavelength Sensitive 1 (SWS1) Opsin – λ_max in the violet/UV region of the electromagnetic spectrum

The human genes for these last three are OPN1MW, OPN1MW2, and OPN1SW, with the first two being "medium-wave" and the third being "short-wave".

Other animals may have different number of photoreceptor proteins, whose optical absorption spectra are shifted compared to those in human photoreceptors. This leads to a different perception of light and visual images in humans and other 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 type 2 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^[4].

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 λ_max 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.^[10]. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromophore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabilizing the protonation of the Schiff linkage between Lys296 and the chromophore. The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propagation of the transduction signal once a photon has been absorbed.

Certain amino acid residues, termed spectral tuning sites, have a strong effect on λ_max 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^[11] and Deeb^[12]. The impact of spectral tuning sites on λ_max differs between different opsin groups and between opsin groups of different species.

External links

• Illustration at Baldwin-Wallace College
• MeSH Opsin

References