Characterization of Anthozoan-Specific Opsins from a Reef-Building Coral, Acropora tenuis, as Gq-Coupled Opsins.

Most animals capture light through opsins, which are light-sensitive G protein-coupled receptors (GPCRs). Recent genome analyses of anthozoans, including corals and sea anemones, have identified novel opsins that are phylogenetically classified into two groups distinct from previously known opsin groups. Despite their significance in clarifying biological functions, the specific molecular properties of these opsins remain largely unknown. In this study, we investigated the G protein activations and biochemical responses light-dependently induced by two anthozoan opsins, Antho2a and Antho2d, obtained from the reef-building coral Acropora tenuis, in mammalian cultured cells. Using jumping spider Rh1 (SpiRh1), which belongs to a known Gq-type G protein (Gq)-coupled opsin group as a control, we observed that Antho2a and Antho2d elicited light-dependent increases in Ca²+ levels in cultured cells. This response was inhibited by a Gq inhibitor, indicating that these opsins activated Gq in a light-dependent manner. Interestingly, Antho2d also activated the Gi-type G protein (Gi), similar to SpiRh1, while Antho2a showed limited or negligible Gi activation. We also found that Gi activation additionally contributed to the Ca²⁺ elevation, suggesting it enhances Gq-dependent Ca²⁺ elevation in Antho2d- and SpiRh1-expressing cells. In contrast, Antho2a demonstrated a higher specificity for Gq activation compared to SpiRh1 and was nearly equivalent to hM3Dq, a GPCR known for its strong Gq specificity and widely used as a chemogenetic tool for manipulating Gq activation. Our results suggest that this new anthozoan opsin group consists of Gq-coupled opsins with varying levels of Gi activation, demonstrating their potential for optogenetic applications.