Ciliary opsins were classically thought to function only in vertebrates for vision, but they have been recently identified also in invertebrates for non-visual photoreception. Larvae of the annelid Platynereis dumerilii are used as a zooplankton model, and this zooplankton species possesses a "vertebrate-type" ciliary opsin (named c-opsin) in the brain. Platynereis c -opsin is suggested to relay light signals to melatonin production and circadian behaviors. Thus, the spectral and biochemical characteristics of this c-opsin would be directly related to non-visual photoreception in this zooplankton model. Here, we demonstrate that the c-opsin can sense UV to activate intracellular signaling cascades, and tha... More
Ciliary opsins were classically thought to function only in vertebrates for vision, but they have been recently identified also in invertebrates for non-visual photoreception. Larvae of the annelid Platynereis dumerilii are used as a zooplankton model, and this zooplankton species possesses a "vertebrate-type" ciliary opsin (named c-opsin) in the brain. Platynereis c -opsin is suggested to relay light signals to melatonin production and circadian behaviors. Thus, the spectral and biochemical characteristics of this c-opsin would be directly related to non-visual photoreception in this zooplankton model. Here, we demonstrate that the c-opsin can sense UV to activate intracellular signaling cascades, and that it can directly bind exogenous all-trans-retinal. These results suggest that this c-opsin regulates circadian signaling in a UV-dependent manner and that it does not require supply of 11 -cis-retinal for photoreception. Avoidance of damaging UV irradiation is a major cause of a large-scale daily zooplankton movement, and the observed capability of the c-opsin to transmit UV signals and bind all -trans-retinal is ideally suited for sensing UV radiation in the brain, which presumably lacks enzymes producing 11-cis-retinal. Mutagenesis analyses indicated that a unique amino acid residue (Lys-94) is responsible for c-opsin-mediated UV sensing in the Platynereis brain. We therefore propose that acquisition of the lysine residue in the c-opsin would be a critical event in the evolution of Platynereis to enable detection of ambient UV. In summary, our findings indicate that the c-opsin possesses spectral and biochemical properties suitable for UV sensing by the zooplankton model.
