Abstract
Opsins are light-sensor proteins, each absorbing a specific wavelength of light. This, in turn, drives a specific G protein-mediated phototransduction cascade, leading to a photoreceptor cell response. Recent genome projects have revealed an unexpectedly large number of opsin genes for vision and non-visual photoreception in various animals. However, the significance of this multiplicity of opsins remains largely unknown, except for that of cone visual opsins, which are diversified with respect to spectral sensitivity to achieve vertebrate color vision. Here, an implication of multiplicity is discussed, with focus on an ultraviolet-sensitive non-visual opsin—parapinopsin—that underlies pineal wavelength discrimination in lower vertebrates. Parapinopsins are phylogenetically close to vertebrate visual opsins, which have bleaching properties, but interestingly, parapinopsins are bleach-resistant bistable pigments, which photo-convert to stable photoproducts that revert to their original dark state by subsequent light absorption, similar to invertebrate rhodopsins. The unique characteristics of parapinopsin as an evolutionary intermediate between bistable and bleaching pigments provide insight into the evolutionary transition between signaling molecules that interact with two types of opsin-based pigments. Furthermore, the parapinopsin gene was duplicated in the teleost lineage and the spectral sensitivities of the duplicated parapinopsins were different from each other. On the basis of these results, together with the histochemical findings of parapinopsin, a plausible link between the diversification of a non-visual opsin parapinopsin and diverse pineal functions, wavelength discrimination, and melatonin secretion, implying why multiple opsins exist in animals, is proposed.
Highlights
Many animals utilize light information for vision and non-visual functions such as circadian photoentrainment
These facts suggest that opsinGt and opsin-visual arrestin interactions were established during the course of vertebrate visual opsin evolution but questions remain regarding the detailed timing of the establishment
Because it is well known that ß-arrestin mediates clathrin-mediated G protein-coupled receptors (GPCRs) internalization (Lohse et al, 1990; Ferguson et al, 1996; Goodman et al, 1996), the results suggest that the granules were generated in a lightdependent manner by ß-arrestin-mediated internalization of parapinopsins from the outer segments
Summary
Many animals utilize light information for vision and non-visual functions such as circadian photoentrainment. Lower vertebrates can discriminate wavelengths of light with pineals and related organs independently of image-forming color vision in the eyes (Dodt and Meissl, 1982). This so-called pineal “color discrimination” is achieved through antagonism primarily between ultraviolet (UV) and visible light reception, which cause inhibitory and excitatory effects, respectively, on neuronal firing of a specialized type of ganglion cell. Parapinopsin is considered to be a common molecular basis for UV reception in pineal wavelength discrimination This idea is supported by the observation that parapinopsin is expressed in photoreceptor cells of the iguana parietal eye, where the ratio of UV to visible light is detected (Wada et al, 2012). Parapinopsins could be an evolutionary intermediate between bistable pigment and bleaching pigment, providing an opportunity to study the detailed aspects of the evolution of vertebrate visual signaling (Figure 1A)
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