Conformations of NhaA, the Na+/H+ exchanger from Escherichia coli, in the pH-activated and ion-translocating states.
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. The 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-A 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.
Appel
Matthias
M
Department of Structural Biology, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, Frankfurt am Main, Germany.
Hizlan
Dilem
D
Vinothkumar
Kutti R
KR
Ziegler
Christine
C
Kühlbrandt
Werner
W
D000494:Allosteric Regulation; D020285:Cryoelectron Microscopy; D003460:Crystallization; D004926:Escherichia coli / Q000737:chemistry*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000648:ultrastructure; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D011487:Protein Conformation; D017923:Sodium-Hydrogen Exchangers / Q000737:chemistry* / Q000648:ultrastructure
D029968:Escherichia coli Proteins; C102839:NhaA protein, E coli; D017923:Sodium-Hydrogen Exchangers