Stoichiometry and deletion analyses of subunits in the heterotrimeric F‐ ATP synthase c ring from the acetogenic bacterium Acetobacterium woodii

The ion‐translocating c ring of the Na + F 1 F o ATP synthase of the anaerobic bacterium Acetobacterium woodii is the first heteromeric c ring found in nature that contains one V‐ ( c 1 ) and two F‐type‐like c subunits ( c 2 / c 3 ), the latter of identical amino acid sequence. To address whether they are of equal or different importance for function, they were deleted in combination or individually. Deletion of c 1 was compensated by incorporation of two c 2 / c 3 subunits but the enzyme was unstable and largely impaired in Na + transport. Deletion of c 2 was compensated by incorporation of c 3 but also led to a reduction of Na + transport. Deletion of c 3 had no effect. In contrast, deletion of both c 2 and c 3 led to a complete loss of ATP ase activity at the cytoplasmic membrane. Mass spectrometric analysis of c 2 +1 Ala and c 2 +2 Ala variants revealed a copy number of 8 : 1 for c 2 / c 3 which is consistent with the biochemical characteristics of the variants. These data indicate a role of c 1 in assembly and a function of c 2 as the predominant c ring constituent.