The phototroph-specific β-hairpin structure of the γ subunit of FF-ATP synthase is important for efficient ATP synthesis of cyanobacteria.

The FF synthase produces ATP from ADP and inorganic phosphate. The γ subunit of FF ATP synthase in photosynthetic organisms, which is the rotor subunit of this enzyme, contains a characteristic β-hairpin structure. This structure is formed from an insertion sequence that has been conserved only in phototrophs. Using recombinant subcomplexes, we previously demonstrated that this region plays an essential role in the regulation of ATP hydrolysis activity, thereby functioning in controlling intracellular ATP levels in response to changes in the light environment. However, the role of this region in ATP synthesis has long remained an open question because its analysis requires the preparation of the whole FF complex and a transmembrane proton-motive force. In this study, we successfully prepared proteoliposomes containing the entire FF ATP synthase from a cyanobacterium, Synechocystis sp. PCC 6803, and measured ATP synthesis/hydrolysis and proton-translocating activities. The relatively simple genetic manipulation of Synechocystis enabled the biochemical investigation of the role of the β-hairpin structure of FF ATP synthase and its activities. We further performed physiological analyses of Synechocystis mutant strains lacking the β-hairpin structure, which provided novel insights into the regulatory mechanisms of FF ATP synthase in cyanobacteria via the phototroph-specific region of the γ subunit. Our results indicated that this structure critically contributes to ATP synthesis and suppresses ATP hydrolysis.