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⁺ F₁F_o ATP synthase of the anaerobic bacterium Acetobacterium woodii is the first heteromeric c ring found in nature that contains one V- (c₁) and two F-type-like c subunits (c₂/c₃), 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 c₁ was compensated by incorporation of two c₂/c₃ subunits but the enzyme was unstable and largely impaired in Na⁺ transport. Deletion of c₂ was compensated by incorporation of c₃ but also led to a reduction of Na⁺ transport. Deletion of c₃ had no effect. In contrast, deletion of both c₂ and c₃ led to a complete loss of ATPase activity at the cytoplasmic membrane. Mass spectrometric analysis of c₂+1 Ala and c₂+2 Ala variants revealed a copy number of 8 : 1 for c₂/c₃ which is consistent with the biochemical characteristics of the variants. These data indicate a role of c₁ in assembly and a function of c₂ as the predominant c ring constituent