Rhodopsins possess retinal chromophore surrounded by seven transmembrane ␣-helices, are widespread in prokaryotes and in eukaryotes, and can be utilized as optogenetic tools. Although rhodopsins work as distinctly different photoreceptors in various organisms, they can be roughly divided according to their two basic functions, light-energy conversion and light-signal transduction. In microbes, light-driven proton transporters functioning as light-energy converters have been modified by evolution to produce sensory receptors that relay signals to transducer proteins to control motility. In this study, we cloned and characterized two newly identified microbial rhodopsins from Haloquadratum walsbyi. One of them has photochemical properties and a proton pumping activity similar to the well known proton pump bacteriorhodopsin (BR). The other, named middle rhodopsin (MR), is evolutionarily transitional between BR and the phototactic sensory rhodopsin II (SRII), having an SRII-like absorption maximum, a BR-like photocycle, and a unique retinal composition. The wild-type MR does not have a light-induced proton pumping activity. On the other hand, a mutant MR with two key hydrogen-bonding residues located at the interaction surface with the transducer protein HtrII shows robust phototaxis responses similar to SRII, indicating that MR is potentially capable of the signaling. These results demonstrate that color tuning and insertion of the critical threonine residue occurred early in the evolution of sensory rhodopsins. MR may be a missing link in the evolution from type 1 rhodopsins (microorganisms) to type 2 rhodopsins (animals), because it is the first microbial rhodopsin known to have 11-cis-retinal similar to type 2 rhodopsins.Rhodopsin molecules are photochemically active membrane-embedded proteins having seven transmembrane ␣-helices and retinal chromophore (vitamin A aldehyde) (1, 2). Rhodopsins are classified into two groups, microbial (type 1) and animal (type 2) rhodopsins (3). Type 1 rhodopsins are widespread in the microbial world in prokaryotes (bacteria and archaea) and in eukaryotes (fungi and algae) (1, 4 -7), and type 2 rhodopsins, such as visual pigments, are G-proteincoupled receptors, widespread in vertebrates and in invertebrates (3,8). An interesting feature of this photoactive protein family is a wide range of seemingly dissimilar functions performed by its members. Some rhodopsins are light-driven transporters, such as proton pumps bacteriorhodopsin (BR) 2 in haloarchaea, xanthorhodopsin in halophilic bacteria, proteorhodopsin in marine bacteria, and Leptosphaeria rhodopsin in fungi (1, 9 -13). Those proteins generate an electrochemical membrane potential upon light activation, which is utilized by ATP synthase to produce ATP (14). Other rhodopsins are light sensors, such as the phototaxis receptors sensory rhodopsins I (SRI) and II (SRII) in haloarchaea (1, 5) and mammalian rod rhodopsin and color visual pigments both in vertebrates and in invertebrates (3,8). These sensory receptors relay signals...

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The bacterial flagellar motor generates a rotational force by the flow of ions through the membrane. The rotational force is generated by the interaction between the cytoplasmic regions of the rotor and the stator. FliG is directly involved in the torque generation of the rotor protein by its interaction. FliG is composed of three domains: the N-terminal, Middle and C-terminal domains, based on its structure. The C-terminal domain of FliG is assumed to be important for the interaction with the stator that generates torque. In this study, using CD spectra, gel filtration chromatography and DSC (differential scanning calorimetry), we characterized the physical properties of the C-terminal domain (G214-Stop) of wild-type (WT) FliG and its non-motile phenotype mutant derivatives (L259Q, L270R and L271P), which were derived from the sodium-driven motor of Vibrio. The CD spectra and gel filtration chromatography revealed a slight difference between the WT and the mutant FliG proteins, but the DSC results suggested a large difference in their stabilities. That structural difference was confirmed by differences in protease sensitivity. Based on these results, we conclude that mutations which confer the non-motile phenotype destabilize the C-terminal domain of FliG.

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