Functional production of the Na+ F1FO ATP synthase from Acetobacterium woodii in Escherichia coli requires the native AtpI

The Na+ F1FO ATP synthase of the anaerobic, acetogenic bacterium Acetobacterium woodii has a unique FOVO hybrid rotor that contains nine copies of a F-O-like c subunit and one copy of a V-O-like c (1) subunit with one ion binding site in four transmembrane helices whose cellular function is obscure. Since a genetic system to address the role of different c subunits is not available for this bacterium, we aimed at a heterologous expression system. Therefore, we cloned and expressed its Na+ F1FO ATP synthase operon in Escherichia coli. A Delta atp mutant of E. coli produced a functional, membrane-bound Na+ F1FO ATP synthase that was purified in a single step after inserting a His(6)-tag to its beta subunit. The purified enzyme was competent in Na+ transport and contained the FOVO hybrid rotor in the same stoichiometry as in A. woodii. Deletion of the atpI gene from the A. woodii operon resulted in a loss of the c ring and a mis-assembled Na+ F1FO ATP synthase. AtpI from E. coli could not substitute AtpI from A. woodii. These data demonstrate for the first time a functional production of a FOVO hybrid rotor in E. coli and revealed that the native AtpI is required for assembly of the hybrid rotor.