title: Spectroscopic study of a light-driven chloride ion pump from marine bacteria.
abstract: Thousands of light-driven proton-pumping rhodopsins have been found in marine microbes, and a light-driven sodium-ion pumping rhodopsin was recently discovered, which utilizes sunlight for the energy source of the cell. Similarly, a light-driven chloride-ion pump has been found from marine bacteria, and three eubacterial light-driven pumps possess the DTE (proton pump), NDQ (sodium-ion pump), and NTQ (chloride-ion pump) motifs corresponding to the D85, T89, and D96 positions in bacteriorhodopsin (BR). The corresponding motif of the known haloarchaeal chloride-ion pump, halorhodopsin (HR), is TSA, which is entirely different from the NTQ motif of a eubacterial chloride-ion pump. It is thus intriguing to compare the molecular mechanism of these two chloride-ion pumps. Here we report the spectroscopic study of Fulvimarina rhodopsin (FR), a eubacterial light-driven chloride-ion pump from marine bacterium. FR binds a chloride-ion near the retinal chromophore and chloride-ion binding causes a spectral blue-shift. FR predominantly possesses an all-trans retinal, which is responsible for the light-driven chloride-ion pump. Upon light absorption, the red-shifted K intermediate is formed, followed by the appearance of the L and O intermediates. When the M intermediate does not form, this indicates that the Schiff base remains in the protonated state during the photocycle. These molecular mechanisms are common in HR, and a common mechanism for chloride-ion pumping by evolutionarily distant proteins suggests the importance of the electric quadrupole in the Schiff base region and their changes through hydrogen-bonding alterations. One noticeable difference between FR and HR is the uptake of chloride-ion from the extracellular surface. While the uptake occurs upon decay of the O intermediate in HR, chloride-ion uptake accompanies the rise of the O intermediate in FR. This suggests the presence of a second chloride-ion binding site near the extracellular surface of FR, which is unique to the NTQ rhodopsin.
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology , Showa-ku, Nagoya 466-8555, Japan.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Inoue
authors.forename: Faisal Hammad Mekky
authors.initials: FH
authors.lastname: Koua
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Kato
authors.forename: Rei
authors.initials: R
authors.lastname: Abe-Yoshizumi
authors.forename: Hideki
authors.initials: H
authors.lastname: Kandori
mesh_terms: D018407:Bacterial Physiological Phenomena; D002712:Chlorides / Q000378:metabolism*; D007473:Ion Channels / Q000737:chemistry / Q000378:metabolism*; D008027:Light
chemical_list: D002712:Chlorides; D007473:Ion Channels