(A) Summary of observed helix movements in response to Na+ binding. Schematic helix positions in absence of NaCl (left) or at pH 4 and pH 8 at >250 mM NaCl (right). Helix movements are indicated by arrows. Helix projections are shown as circles or ovals. The 6-helix bundle is color-coded as in Figure 2A. Helices at the dimer interface are grey. (B) Model drawing of changes in the positions of TMH 5, 6 and 12 that respond most strongly to Na+ binding. Residue D161 in TMH 6, thought to be directly involved in substrate binding, is shown in black. Na+ binding results in a transition from the apo or proton-bound state, where the putative ion-binding site is likely to be more accessible form the extracellular space (left), to a Na+-bound state, which we propose to be open to the cytoplasm (right).