Structure and Mechanism of a Nitrate Transporter

• Published in: Cell reports (Cambridge) 2013-03, Vol.3 (3), p.716-723
• Main Authors: Yan, Hanchi, Huang, Weiyun, Yan, Chuangye, Gong, Xinqi, Jiang, Sirui, Zhao, Yu, Wang, Jiawei, Shi, Yigong
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Anion Transport Proteins - chemistry; Anion Transport Proteins - metabolism; Biological Transport, Active; Crystallography, X-Ray; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - metabolism; Molecular Sequence Data; Nitrates - metabolism; Protein Conformation
• ISSN: 2211-1247; 2211-1247
• DOI: 10.1016/j.celrep.2013.03.007

The nitrate/nitrite transporters NarK and NarU play an important role in nitrogen homeostasis in bacteria and belong to the nitrate/nitrite porter family (NNP) of the major facilitator superfamily (MFS) fold. The structure and functional mechanism of NarK and NarU remain unknown. Here, we report the crystal structure of NarU at a resolution of 3.1 Å and systematic biochemical characterization. The two molecules of NarU in an asymmetric unit exhibit two distinct conformational states: occluded and partially inward-open. The substrate molecule nitrate appears to be coordinated by four highly conserved, charged, or polar amino acids. Structural and biochemical analyses allowed the identification of key amino acids that are involved in substrate gating and transport. The observed conformational differences of NarU, together with unique sequence features of the NNP family transporters, suggest a transport mechanism that might deviate from the canonical rocker-switch model. [Display omitted] ► Structure of the nitrate transporter NarU in substrate-free and -bound states ► Distinct conformations of NarU in substrate-free and -bound states ► Identification of a substrate binding site and transport path ► Transport mechanism may deviate from the rocker-switch model Nitrate uptake by the transporters of the nitrate-nitrite porter (NNP) family plays an important physiological role in plants, animals, fungi, and bacteria. Shi and colleagues report the crystal structures of a bacterial nitrate transporter, NarU, and associated functional analyses. NarU specifically transports nitrate or nitrite, but not other examined anions. The substrate binding site and transport path are identified, and a new substrate transport mechanism for NNP family transporters is proposed on the basis of unique structural features in NarU.