Mechanism of Sodium/Proton Antiport in NhaA

Abstract

The transmembrane protein NhaA from Escherichia coli is a prototypical sodium/proton antiporter. It enables the bacterium to grow under high salt conditions while homologous proteins in eukaryotes are involved in pH and cell volume regulation. A number of acidic and basic residues have been shown to be essential for the transport of one sodium ion for two protons but the mechanistic details of their involvement have not been fully determined. Furthermore, the conformational changes involved in the transport mechanism were not known. We present an unpublished crystal structure of NhaA in the inward facing conformation and of the homolog NapA in the outward facing conformation [1]. Using modelling and computer simulations we show how NhaA can function according to the alternating access model, resulting in large relative domain motions that are incompatible with previous structural models for transport in NhaA. Our structure of NhaA contains a salt bridge between the two conserved residues Asp163 and Lys300. With the help of molecular dynamics simulations we critically examine competing models for the molecular mechanism of the stoichiometric transport of two protons for one sodium ion, including one in which Lys300 maintains an active role in proton transport.[1] Lee et al, Nature 501 (2013), 573.