Structural and functional characterization of the nitrite channel NirC from Salmonella typhimurium

22.10.2012

Lü W, Schwarzer NJ, Du J, Gerbig-Smentek E, Andrade SL, Einsle O

Proc Natl Acad Sci U S A. 2012; 109(45):18395-400

Proc Natl Acad Sci U S A.         online article

Nitrite, NO2–, is a central intermediate in the nitrogen metabolism of microorganisms and plants, and it is also 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. In spite of its importance for nitrogen metabolism and for 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 FNT family of membrane proteins that operates as a channel with high conductance. Single-channel measurements show fast and slow gating events, but contrary to the related formate channel FocA, no pH-dependent gating was observed. A 2.4 Å 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 NO2–/H+ symport.