In FoF1 (FoF1-ATP synthase), proton translocation through Fo drives rotation of the oligomer ring of Fo-c subunits (c-ring) relative to Fo-a. Previous reports have indicated that a conserved arginine residue in Fo-a plays a critical role in the proton transfer at the Fo-a/c-ring interface. Indeed, we show in the present study that thermophilic FoF1s 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 Fo-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)FoF1, in which the c-ring and Fo-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 Fo-a structure, can block the passive proton translocation. Thus the conserved arginine residue in Fo-a ensures proton-coupled c-ring rotation by preventing a futile proton shortcut.

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