Cnidopsins Characterized as Bistable Opsins From a Reef-Building Coral, Acropora tenuis

Many cnidarian animals possess multiple opsins, including a type known as cnidopsin, which is found throughout the phylum Cnidaria and is divided into several subgroups. Previous studies have suggested that cnidopsins from jellyfish and coral can light-dependently elevate intracellular cAMP levels, likely via activation of Gs-type G protein in cultured cells. However, their spectroscopic properties remain largely unclear, with the exception of jellyfish opsins. We recently identified eight cnidopsins from the reef-building coral Acropora tenuis and showed that they phylogenetically belong to different cnidopsin subgroups. Here, we spectroscopically analyzed one A. tenuis cnidopsin from each subgroup as a representative. We successfully obtained the absorption spectrum of one purified cnidopsin and found that it was blue-sensitive, with an absorption maximum at ∼460 nm. Blue light irradiation (460 nm) resulted in a red-shifted absorption spectrum, while subsequent irradiation with orange light (> 560 nm) caused a blue-shift. The difference spectra of after versus before repeated irradiation with alternating blue and orange light produced mirror images, indicating a bistable nature. We also successfully observed the absorption spectra of one of two other cnidopsins belonging to the same subgroup as the blue-sensitive A. tenuis cnidopsin. The spectral changes suggested that it was also a bistable opsin. These results suggest that these two coral cnidopsins, which belong to the same cnidopsin subgroup, are bistable-unlike jellyfish opsins, which belong to a different cnidopsin subgroup.