Abstract
BackgroundThe vertebrate opsins are proteins which utilise a retinaldehyde chromophore in their photosensory or photoisomerase roles in the visual/irradiance detection cycle. The majority of the opsins, such as rod and cone opsins, have a very highly conserved gene structure suggesting a common lineage. Exceptions to this are RGR-opsin and melanopsin, whose genes have very different intron insertion positions. The gene structure of another opsin, peropsin (retinal pigment epithelium-derived rhodopsin homologue, RRH) is unknown.ResultsBy in silico analysis of the GenBank database we have determined that the human RRH comprises 7 exons spanning approximately 16.5 kb and is localised to chromosome 4q25 in the following gene sequence: cen-EGF-RRH-IF-qter – a position that excludes this gene as a candidate for the RP29 autosomal recessive retinitis pigmentosa locus. A comparison of opsin gene structures reveals that RRH and RGR share two common intron (introns 1 and 4) insertion positions which may reflect a shared ancestral gene.ConclusionThe opsins comprise a diverse group of genes which appear to have arisen from three different lineages. These lineages comprise the "classical opsin superfamily" which includes the rod and cone opsins, pinopsin, VA-opsin, parapinopsin and encephalopsin; the RRH and RGR group; and the melanopsin line. A common lineage for RRH and RGR, together with their sites of expression in the RPE, indicates that peropsin may act as a retinal isomerase.
Highlights
The vertebrate opsins are proteins which utilise a retinaldehyde chromophore in their photosensory or photoisomerase roles in the visual/irradiance detection cycle
The photosensory opsins are a family of membrane bound, heptahelical G-protein coupled receptors (GPCRs) characterised by their ability to covalently bind a vitamin A-based retinaldehyde chromophore via a Schiff base to a lysine residue located in the 7th transmembrane α-helix [1,2]
We have recently shown that the non-rod, noncone opsins such as RGR-opsin, peropsin, melanopsin and encephalopsin are all expressed early in the embryonic development of the mammalian eye
Summary
The vertebrate opsins are proteins which utilise a retinaldehyde chromophore in their photosensory or photoisomerase roles in the visual/irradiance detection cycle. The majority of the opsins, such as rod and cone opsins, have a very highly conserved gene structure suggesting a common lineage. Exceptions to this are RGR-opsin and melanopsin, whose genes have very different intron insertion positions. Absorption of a photon of light by the chromophore located in the retinal binding pocket of an opsin causes its photoisomerisation from the 11-cis to an all-trans conformation. This change of the chromophore induces a conformational change of its surrounding opsin (page number not for citation purposes). The one key feature present in all the opsin classes is the presence of a lysine residue in the 7th transmembrane α-helix which is thought to enable retinal attachment in all cases [12]
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