Hybrid rotors in F1Fo ATP synthases: subunit composition, distribution, and physiological significance

The c ring of the Na+ F1Fo ATP synthase from the anaerobic acetogenic bacterium Acetobacterium woodii is encoded by three different genes: atpE(1), atpE(2) and atpE(3). Subunit c(1) is similar to typical V-type c subunits and has four transmembrane helices with one ion binding site. Subunit c(2) and c(3) are identical at the amino acid level and are typical F-type c subunits with one ion binding site in two transmembrane helices. All three constitute a hybrid FoVo c ring, the first found in nature. To analyze whether other species may have similar hybrid rotors, we searched every genome sequence publicly available as of 23 February 2015 for F1Fo ATPase operons that have more than one gene encoding the c subunit. This revealed no other species that has three different c subunit encoding genes but twelve species that encode one F-o- and one V-o-type c sub-unit in one operon. Their c subunits have the conserved binding motif for Na+. The organisms are all anaerobic. The advantage of hybrid c rings for the organisms in their environments is discussed.