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April 25th, 2025
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Osaka Metropolitan University
biophysics
biorxiv

Coral anthozoan-specific opsins employ a novel chloride counterion for spectral tuning

Sakai, Y.Open in Google Scholar•Sen, S.Open in Google Scholar•Sugihara, T.Open in Google Scholar•Kakeyama, Y.Open in Google Scholar•Iwasaki, M.Open in Google Scholar•Schertler, G.Open in Google Scholar•Deupi, X.Open in Google Scholar•Koyanagi, M.Open in Google Scholar•Terakita, A.Open in Google Scholar

Animal opsins are G protein coupled receptors that have evolved to sense light by covalently binding a retinal chromophore via a protonated (positively charged) Schiff base. A negatively charged amino acid in the opsin, acting as a counterion, stabilises the proton on the Schiff base, which is essential for sensitivity to visible light. In this study, we investigate the spectroscopic properties of a unique class of opsins from a reef-building coral belonging to the anthozoan-specific opsin II group (ASO-II opsins), which intriguingly lack a counterion residue at any of established sites. Our findings reveal that, unlike other known animal opsins, the protonated state of the Schiff base in visible light-sensitive ASO-II opsins is highly dependent on exogenously supplied chloride ions (Cl-). By using structural modelling and QM/MM calculations to interpret spectroscopy data, we conclude that, in the dark state, ASO-II opsins employ environmental Cl- as their native counterion, while a nearby polar residue, Glu292 in its protonated neutral form, facilitates Cl- binding. In contrast, Glu292 plays a crucial role in maintaining the protonation state of the Schiff base in the light-activated protein, serving as the counterion in the photoproduct. Furthermore, Glu292 is involved in G protein activation of the ASO-II opsin, suggesting that this novel counterion system coordinates multiple functional properties.

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