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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2012-11, Vol.109 (45), p.18395-18400
• Main Authors: Lü, Wei, Schwarzer, Nikola J, Du, Juan, Gerbig-Smentek, Elke, Andrade, Susana L. A, Einsle, Oliver
• Format: Article
• Language: English
• Subjects: Amino Acids - metabolism; Anion Transport Proteins - chemistry; Anion Transport Proteins - metabolism; Anions; aquaporins; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Biological Sciences; Biological Transport; Crystal structure; Crystallography, X-Ray; Cytoplasm; cytotoxins; Electric Conductivity; Electric current; electrophysiology; Escherichia coli; Formates; Formates - metabolism; Hydrogen-Ion Concentration; innate immunity; intestinal microorganisms; Ion Channel Gating; Ion Channels - chemistry; lipid bilayers; macrophages; Metabolism; Models, Molecular; Nitrates; nitric oxide; Nitrites; Nitrites - metabolism; Nitrogen; nitrogen metabolism; Oxides; pathogenicity; Protein subunits; Protein Subunits - chemistry; Protein Subunits - metabolism; Proteins; Protons; Salmonella; Salmonella Typhimurium; Salmonella typhimurium - metabolism; Structural Homology, Protein; Structure-Activity Relationship; Substrate Specificity; X-ray diffraction
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1210793109

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.