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+ F1Fo 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 ATPase 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.