The rhodopsin-retinochrome system elements in a chiton

The process of photoreception in most animals depends on the light induced isomerization of the chromophore retinal, bound to rhodopsin. To re-use retinal, the all-trans-retinal form needs to be re-isomerized to 11-cis-retinal, which can be achieved in different ways. In vertebrates, this mostly includes a stepwise enzymatic process called the visual cycle. The best studied re-isomerization system in protostomes is the rhodopsin-retinochrome system of cephalopods, which consists of rhodopsin, the photoisomerase retinochrome and the protein RALBP functioning as shuttle for retinal. Here, we study the presence of the rhodopsin-retinochrome system in the polyplacophoran mollusk Leptochiton asellus. Our analyses revealed that several protostomes exhibit clear orthologs of cephalopod retinochrome and RALBP. We show that L. asellus retinochrome is coexpressed with the visual rhodopsin and RALBP in the larval chiton photoreceptor cells (PRCs). Surprisingly, we found another retinoid binding protein ortholog of the vertebrate visual cycle protein, RLBP1, as well as its regulators in vertebrates, sox9, otx2 lhx2/9 and mitf, all coexpressed in the PRCs. In addition, we found multiple retinal dehydrogenases expressed in the PRCs, which might also contribute to the rhodopsin-retinochrome system. The recent finding of a xenopsin expressed in L.asellus photoreceptors may further add complexity. We conclude that the rhodopsin-retinochrome system is a common feature of mollusk PRCs, likely sharing elements with the vertebrate visual cycle. Hence, the retinal re-isomerization system of mollusks might be more complex than previously assumed.