The ion-translocating c ring of the Na(+) F1 Fo 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.
Keywords: ATPase; bioenergetics; c subunit; hybrid rotor; liposome; membrane enzyme; sodium transport.
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