pH- and sodium-induced changes in a sodium/proton antiporter.
We examined substrate-induced conformational changes in MjNhaP1, an archaeal electroneutral Na(+)/H(+)-antiporter resembling the human antiporter NHE1, by electron crystallography of 2D crystals in a range of physiological pH and Na(+) conditions. In the absence of sodium, changes in pH had no major effect. By contrast, changes in Na(+) concentration caused a marked conformational change that was largely pH-independent. Crystallographically determined, apparent dissociation constants indicated ∼10-fold stronger Na(+) binding at pH 8 than at pH 4, consistent with substrate competition for a common ion-binding site. Projection difference maps indicated helix movements by about 2 Å in the 6-helix bundle region of MjNhaP1 that is thought to contain the ion translocation site. We propose that these movements convert the antiporter from the proton-bound, outward-open state to the Na(+)-bound, inward-open state. Oscillation between the two states would result in rapid Na(+)/H(+) antiport. DOI: http://dx.doi.org/10.7554/eLife.01412.001.
Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.
Cristina
C
Paulino
Werner
W
Kühlbrandt
D001105:Archaea / Q000201:enzymology*; D003461:Crystallography; D006863:Hydrogen-Ion Concentration; D008854:Microscopy, Electron; D011487:Protein Conformation / Q000187:drug effects; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000737:chemistry* / Q000378:metabolism*
D017923:Sodium-Hydrogen Exchangers; D012964:Sodium
Na+/H+ antiporter; conformational changes; electron cryo-microscopy; electron crystallography; membrane protein structure; transport mechanism