3.E.1.6.15
Bellilinea Na⁺-pumping rhodopsin, BeNaR, of 269 aas and 7 TMSs. Kurihara et al. 2023 identified and characterized a rhodopsin from a thermophilic bacterium, Bellilinea sp. Recombinant Escherichia coli cells expressing this rhodopsin showed light-induced alkalization of the medium only in the presence of Na⁺, and the alkalization signal was enhanced by addition of a protonophore, indicating an outward Na⁺ pump function across the cellular membrane. Its Na⁺-pumping activity is greater than that of the known Na⁺-pumping rhodopsin, KR2. Its photochemical properties included: (i) Visible spectroscopy and HPLC revealed that BeNaR had an absorption maximum at 524 nm with predominantly (>96%) the all-trans retinal conformer. (ii) Time-dependent thermal denaturation experiments revealed that BeNaR showed high thermal stability. (iii) The time-resolved flash-photolysis in the nanosecond to millisecond time domains revealed the presence of four kinetically distinctive photointermediates, K, L, M and O. (iv) Mutational analysis revealed that Asp101, which acts as a counterion, and Asp230 around the retinal were essential for the Na⁺-pumping activity. Kurihara et al. 2023 proposed a model for the outward Na⁺-pumping mechanism of BeNaR.