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Pineal organs of lower vertebrates contain several kinds of photosensitive molecules, opsins that are suggested to be involved in different light-regulated physiological functions. We previously reported that parapinopsin is an ultraviolet (UV)-sensitive opsin that underlies hyperpolarization of the pineal photoreceptor cells of lower vertebrates to achieve pineal wavelength discrimination. Although, parapinopsin is phylogenetically close to vertebrate visual opsins, it exhibits a property similar to invertebrate visual opsins and melanopsin: the photoproduct of parapinopsin is stable and reverts to the original dark states, demonstrating the nature of bistable pigments. Therefore, it is of evolutionary interest to identify a phototransduction cascade driven by parapinopsin and to compare it with that in vertebrate visual cells. Here, we showed that parapinopsin is coupled to vertebrate visual G protein transducin in the pufferfish, zebrafish, and lamprey pineal organs. Biochemical analyses demonstrated that parapinopsins activated transducin  in vitro  in a light-dependent manner, similar to vertebrate visual opsins. Interestingly, transducin activation by parapinopsin was provoked and terminated by UV- and subsequent orange-lights irradiations, respectively, due to the bistable nature of parapinopsin, which could contribute to a wavelength-dependent control of a second messenger level in the cell as a unique optogenetic tool. Immunohistochemical examination revealed that parapinopsin was colocalized with Gt2 in the teleost, which possesses rod and cone types of transducin, Gt1, and Gt2. On the other hand, in the lamprey, which does not possess the Gt2 gene,  in situ  hybridization suggested that parapinopsin-expressing photoreceptor cells contained Gt1 type transducin GtS, indicating that lamprey parapinopsin may use GtS in place of Gt2. Because it is widely accepted that vertebrate visual opsins having a bleaching nature have evolved from non-bleaching opsins similar to parapinopsin, these results implied that ancestral bistable opsins might acquire coupling to the transducin-mediated cascade and achieve light-dependent hyperpolarizing response of the photoreceptor cells.

Activation of Transducin by Bistable Pigment Parapinopsin in the Pineal Organ of Lower Vertebrates