title: Structural and functional characterization of the nitrite channel NirC from Salmonella typhimurium.
abstract: Nitrite (NO(2)(-)) is a central intermediate in the nitrogen metabolism of microorganisms and plants, and is used as a cytotoxin by macrophages as part of the innate immune response. The bacterial membrane protein NirC acts as a specific channel to facilitate the transport of nitrite anions across lipid bilayers for cytoplasmic detoxification. Despite NirC's importance in nitrogen metabolism and in the pathogenicity of enteric bacteria, available biochemical data are scarce. Here we present a functional and structural characterization of NirC from Salmonella typhimurium by lipid bilayer electrophysiology and X-ray crystallography. NirC is a pentameric member of the formate/nitrite transporter family of membrane proteins that operates as a channel with high conductance. Single-channel measurements reveal fast and slow gating events but, in contrast to the related FocA formate channel, no pH-dependent gating. A 2.4Å crystal structure of NirC at pH 5 shows similarity to FocA and aquaporins, but lacks the structural asymmetry observed in the formate channel at similarly low pH. Resolved water molecules in the protomers suggest a transport mechanism that also permits a facultative NO(2)(-)/H(+) symport.
authors.affiliation: Lehrstuhl für Biochemie, Institut für organische Chemie und Biochemie and BIOSS Centre for Biological Signalling Studies, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: Wei
authors.initials: W
authors.lastname: Lü
authors.forename: Nikola J
authors.initials: NJ
authors.lastname: Schwarzer
authors.forename: Juan
authors.initials: J
authors.lastname: Du
authors.forename: Elke
authors.initials: E
authors.lastname: Gerbig-Smentek
authors.forename: Susana L A
authors.initials: SL
authors.lastname: Andrade
authors.forename: Oliver
authors.initials: O
authors.lastname: Einsle
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D027321:Anion Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D000838:Anions; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001692:Biological Transport; D018360:Crystallography, X-Ray; D004553:Electric Conductivity; D005561:Formates / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating; D007473:Ion Channels / Q000737:chemistry; D008958:Models, Molecular; D009573:Nitrites / Q000378:metabolism*; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D011522:Protons; D012486:Salmonella typhimurium / Q000378:metabolism*; D040681:Structural Homology, Protein; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D000596:Amino Acids; D027321:Anion Transport Proteins; D000838:Anions; D001426:Bacterial Proteins; D005561:Formates; D007473:Ion Channels; C088104:NirC protein, Bacteria; D009573:Nitrites; D021122:Protein Subunits; D011522:Protons