title: Crystal structures of the L1, L2, N, and O states of pharaonis halorhodopsin.
abstract: Halorhodopsin from Natronomonas pharaonis (pHR) functions as a light-driven halide ion pump. In the presence of halide ions, the photochemical reaction of pHR is described by the scheme: K→ L1 → L2 → N → O → pHR' → pHR. Here, we report light-induced structural changes of the pHR-bromide complex observed in the C2 crystal. In the L1-to-L2 transition, the bromide ion that initially exists in the extracellular vicinity of retinal moves across the retinal Schiff base. Upon the formation of the N state with a bromide ion bound to the cytoplasmic vicinity of the retinal Schiff base, the cytoplasmic half of helix F moves outward to create a water channel in the cytoplasmic interhelical space, whereas the extracellular half of helix C moves inward. During the transition from N to an N-like reaction state with retinal assuming the 13-cis/15-syn configuration, the translocated bromide ion is released into the cytoplasmic medium. Subsequently, helix F relaxes into its original conformation, generating the O state. Anion uptake from the extracellular side occurs when helix C relaxes into its original conformation. These structural data provide insight into the structural basis of unidirectional anion transport.
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Japan; RIKEN Harima Branch, 1-1-1, Kouto, Sayo, Hyogo, Japan. Electronic address: kouyama@bio.phys.nagoya-u.ac.jp.
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Kouyama
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Japan.
authors.forename: Haruki
authors.initials: H
authors.lastname: Kawaguchi
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Japan.
authors.forename: Taichi
authors.initials: T
authors.lastname: Nakanishi
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Kubo
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, Japan.
authors.forename: Midori
authors.initials: M
authors.lastname: Murakami
mesh_terms: D018360:Crystallography, X-Ray; D006216:Halobacteriaceae; D025602:Halorhodopsins / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008027:Light; D008958:Models, Molecular; D055503:Protein Multimerization / Q000528:radiation effects; D020836:Protein Structure, Quaternary; D012172:Retinaldehyde / Q000378:metabolism; D013696:Temperature
chemical_list: D025602:Halorhodopsins; D012172:Retinaldehyde