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In silico characterisation and chromosomal localisation of human RRH (peropsin)--implications for opsin evolution
James Bellingham1, Dominic J Wells, Russell G Foster
1Gene Targeting Unit, Department of Neuromuscular Diseases, Division of Neuroscience and Psychological Medicine, Faculty of Medicine, Imperial College London, Charing Cross Hospital, St. Dunstan's Road, London, W6 8RP, UK. j.bellingham@imperial.ac.uk
Background:
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. The gene structure of another opsin, peropsin (retinal pigment epithelium-derived rhodopsin homologue, RRH) is unknown.
Results:
By 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.
Conclusion:
The 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.