Essential arginine residue of the F_o-a subunit in F_oF₁-ATP synthase has a role to prevent the proton shortcut without c-ring rotation in the F_o proton channel

In F_oF₁ (F_oF₁-ATP synthase), proton translocation through F_o drives rotation of the oligomer ring of F_o-c subunits (c-ring) relative to F_o-a. Previous reports have indicated that a conserved arginine residue in F_o-a plays a critical role in the proton transfer at the F_o-a/c-ring interface. Indeed, we show in the present study that thermophilic F_oF₁s with substitution of this arginine (aR169) to other residues cannot catalyse proton-coupled reactions. However, mutants with substitution of this arginine residue by a small (glycine, alanine, valine) or acidic (glutamate) residue mediate the passive proton translocation. This translocation requires an essential carboxy group of F_o-c (cE56) since the second mutation (cE56Q) blocks the translocation. Rotation of the c-ring is not necessary because the same arginine mutants of the ‘rotation-impossible’ (c₁₀-a)F_oF₁, in which the c-ring and F_o-a are fused to a single polypeptide, also exhibits the passive proton translocation. The mutant (aR169G/Q217R), in which the arginine residue is transferred to putatively the same topological position in the F_o-a structure, can block the passive proton translocation. Thus the conserved arginine residue in F_o-a ensures proton-coupled c-ring rotation by preventing a futile proton shortcut.