3.E.1.1.9
Rhodopsin, RxR, of 239 aas and 7 TMSs. This rhodopsin is from the thermophilic eubacterium Rubrobacter xylanophilus DSM 9941(T) and was isolated from thermally polluted water. Although R. xylanophilus rhodopsin (RxR) is from an Actinobacterium, it is located between eukaryotic and archaeal rhodopsins in the phylogenetic tree (Kanehara et al. 2017). E. coli cells expressing RxR showed a light-induced decrease in environmental pH and inhibition by a protonophore, indicating that it works as a light-driven outward proton pump. Purified RxR has an absorption maximum at 541 nm and binds all-trans retinal. The pKa values for the protonated retinal Schiff base and its counterion were 10.7 and 1.3, respectively. Of note, RxR showed an extremely high thermal stability in comparison with other proton pumping rhodopsins such as thermophilic rhodopsin TR (by 16-times) and bacteriorhodopsin from Halobacterium salinarum (HsBR, by 4-times) (Kanehara et al. 2017). Hayashi et al. 2020 showed how RxR realizes its exceptionally high stability while retaining its original proton pumping function.  Biochemical synthesis of membrane-spanning lipids is one adaptation that organisms such as thermophilic archaea have evolved to prevent membrane leakiness (Kim et al. 2019).