4au5
Structure of the NhaA dimer, crystallised at low pH
Structural highlights
FunctionNHAA_ECOLI Na(+)/H(+) antiporter that extrudes sodium in exchange for external protons. Catalyzes the exchange of 2 H(+) per Na(+). Can mediate sodium uptake when a transmembrane pH gradient is applied. Active at alkaline pH. Activity is strongly down-regulated below pH 6.5.[1] Publication Abstract from PubMedSodium-proton antiporters rapidly exchange protons and sodium ions across the membrane to regulate intracellular pH, cell volume, and sodium concentration. How ion binding and release is coupled to the conformational changes associated with transport is not clear. Here, we report a crystal form of the prototypical sodium-proton antiporter NhaA from Escherichia coli in which the protein is seen as a dimer. In this new structure, we observe a salt bridge between an essential aspartic acid (Asp163) and a conserved lysine (Lys300). An equivalent salt bridge is present in the homologous transporter NapA, but not in the only other known crystal structure of NhaA, which provides the foundation of most existing structural models of electrogenic sodium-proton antiport. Molecular dynamics simulations show that the stability of the salt bridge is weakened by sodium ions binding to Asp164 and the neighboring Asp163. This suggests that the transport mechanism involves Asp163 switching between forming a salt bridge with Lys300 and interacting with the sodium ion. pKa calculations suggest that Asp163 is highly unlikely to be protonated when involved in the salt bridge. As it has been previously suggested that Asp163 is one of the two residues through which proton transport occurs, these results have clear implications to the current mechanistic models of sodium-proton antiport in NhaA. Crystal structure of the sodium-proton antiporter NhaA dimer and new mechanistic insights.,Lee C, Yashiro S, Dotson DL, Uzdavinys P, Iwata S, Sansom MS, von Ballmoos C, Beckstein O, Drew D, Cameron AD J Gen Physiol. 2014 Dec;144(6):529-44. doi: 10.1085/jgp.201411219. PMID:25422503[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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