Structures of the Na+-binding sites in the membrane rotor subunits of the Na+-translocating ATP synthase of F-type and V-type. Left, c subunit of the Na+-translocating F-type ATP synthase of Ilyobacter tartaricus (PDB entry 1YCE ); right, NtpK subunit of the Na+-translocating V-type ATP synthase of Enterococcus hirae (PDB entry 2BL2 ). Note that in I. tartaricus the Na+ ion (purple) crosslinks two identical subunits, A (green) and B (ice-blue), while in E. hirae the Na+ ion is bound by a four-helical bundle that results from a subunit duplication. In both structures, major coordinating bonds to the Na+ ion are provided by the principal ligand (Glu65A in I. tartaricus and Glu139 in E. hirae); other bonds come from a conserved glutamine (Gln32A in I. tartaricus and Gln110 in E. hirae), a hydroxy group of Ser66B in I. tartaricus and Thr64 in E. hirae (as initially predicted from sequence comparison  and a backbone carbonyl (Val63B in I. tartaricus and Leu61 in E. hirae). In E. hirae one more bond is provided by Gln65 (not shown, see the text). The remaining bonds are provided, most likely, by the unseen water molecules.