title: A new group of eubacterial light-driven retinal-binding proton pumps with an unusual cytoplasmic proton donor.
abstract: One of the main functions of microbial rhodopsins is outward-directed light-driven proton transport across the plasma membrane, which can provide sources of energy alternative to respiration and chlorophyll photosynthesis. Proton-pumping rhodopsins are found in Archaea (Halobacteria), multiple groups of Bacteria, numerous fungi, and some microscopic algae. An overwhelming majority of these proton pumps share the common transport mechanism, in which a proton from the retinal Schiff base is first transferred to the primary proton acceptor (normally an Asp) on the extracellular side of retinal. Next, reprotonation of the Schiff base from the cytoplasmic side is mediated by a carboxylic proton donor (Asp or Glu), which is located on helix C and is usually hydrogen-bonded to Thr or Ser on helix B. The only notable exception from this trend was recently found in Exiguobacterium, where the carboxylic proton donor is replaced by Lys. Here we describe a new group of efficient proteobacterial retinal-binding light-driven proton pumps which lack the carboxylic proton donor on helix C (most often replaced by Gly) but possess a unique His residue on helix B. We characterize the group spectroscopically and propose that this histidine forms a proton-donating complex compensating for the loss of the carboxylic proton donor.
authors.affiliation: Department of Physics, University of Guelph, ON, Canada.
authors.forename: Andrew
authors.initials: A
authors.lastname: Harris
authors.affiliation: Department of Physics, University of Guelph, ON, Canada.
authors.forename: Milena
authors.initials: M
authors.lastname: Ljumovic
authors.affiliation: Department of Physics, Freie University, Berlin, Germany.
authors.forename: Ana-Nicoleta
authors.initials: AN
authors.lastname: Bondar
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Nagoya, Japan.
authors.forename: Yohei
authors.initials: Y
authors.lastname: Shibata
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Nagoya, Japan.
authors.forename: Shota
authors.initials: S
authors.lastname: Ito
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Nagoya, Japan; PRESTO, Japan Science and Technology Agency, Japan.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Inoue
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Nagoya, Japan. Electronic address: kandori@nitech.ac.jp.
authors.forename: Hideki
authors.initials: H
authors.lastname: Kandori
authors.affiliation: Department of Physics, University of Guelph, ON, Canada. Electronic address: lebrown@uoguelph.ca.
authors.forename: Leonid S
authors.initials: LS
authors.lastname: Brown
mesh_terms: D003593:Cytoplasm / Q000737:chemistry*; D005051:Eubacterium / Q000737:chemistry* / Q000235:genetics; D005798:Genes, Bacterial; D016297:Mutagenesis, Site-Directed; D011522:Protons; D012172:Retinaldehyde / Q000737:chemistry*; D012243:Rhodopsin / Q000737:chemistry*; D013059:Spectrum Analysis, Raman
chemical_list: D011522:Protons; D012243:Rhodopsin; D012172:Retinaldehyde
keywords: Eubacterial rhodopsins; FTIR spectroscopy; Photochemical cycle; Proton pumping; Retinal-binding proteins