Zebrafish melanopsin: isolation, tissue localisation and phylogenetic position

Photoreception is best understood in retinal rods and cones, but it is not confined to these cells. In non-mammals. intrinsically photosensitive cells have been identified within several structures including the pineal, hypothalamus and,,kill. More recently novel light sensitive cells have been identified in the inner/basal retina of both teleosts and rodents. Melanopsin,, been proposed as the photopigment mediating many of these non-rod, non-cone responses to light. However, much about file melanopsin gene family remains to be clarified including their potential role as photopigments. and taxonomic distribution, We have isolated the first orthologue of melanopsin from a teleost fish and show expression of this gene in a sub-set of retinal horizontal :elk (type B). Zebrafish melanopsin, and orthologues of this gene, differ markedly from the vertebrate photopigment opsins. The putative counterion is not a glutamate but a tyrosine, the putative G-protein binding domain in the third cytoplasmic loop is not conserved, zinc they show low levels of amino acid identity (similar to27%) to both the known photopigment opsins and to other members of the melanopsin family. Mouse melanopsin is only 58% identical to Xenopus, and 68%, identical to zebrafish. By contrast, the photosensory opsin families show 75% conservation, On the basis of their structure, genomic organisation, discrete evolutionary lineage, and their co-expression with other opsins, the melanopins are not obvious photosensory opsins. They might represent a separate branch of photopigment evolution in the vertebrates or they may have a non-direct photosensory function, perhaps as a photoisomerase, in non-rod, non-cone light detection. (C) 2002 Elsevier Science B.V. All rights reserved.