Conformations of NhaA the Na+/H+ Exchanger from Escherichia coli, in the pH-Activated and Ion-Translocating States (vol 386, pg 351, 2009)

NhaA, the main sodium-proton exchanger in the inner membrane of Escherichia coli, regulates the cytosolic concentrations of H+ and Na+. It is inactive at acidic pH, becomes active between pH 6 and pH 7, and reaches maximum activity at pH 8. By cryo-electron microscopy of two-dimensional crystals grown at pH 4 and incubated at higher pH, we identified two sequential conformational changes in the protein in response to pH or substrate ions. Pie first change is induced by a rise in pH from 6 to 7 and marks the transition from the inactive state to the pH-activated state. pH activation, which precedes the ion-induced conformational change, is accompanied by an overall expansion of the NhaA monomer and a local ordering of the N-terminus. The second conformational change is induced by the substrate ions Na+ and Li+ at pH above 7 and involves a 7-angstrom displacement of helix IVp. This movement would cause a charge imbalance at the ion-binding site that may trigger the release of the substrate ion and open a periplasmic exit channel. (C) 2009 Elsevier Ltd. All rights reserved.