ATP synthase uses a unique rotary mechanism to couple ATP synthesis and hydrolysis to transmembrane proton translocation. The F1 subcomplex has three catalytic nucleotide binding sites, one on each β subunit, at the interface to the adjacent α subunit. In the x-ray structure of F 1 (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628), the three catalytic β/α interfaces differ in the extent of inter-subunit interactions between the C termini of the β and α subunits. At the closed βDP/α DP interface, a hydrogen-bonding network is formed between both subunits, which is absent at the more open βTP/α TP interface and at the wide open βE/α E interface. The hydrogen-bonding network reaches from βL328 (Escherichia coli numbering) and βQ441 via αQ399, βR398, and αE402 to βR394, and ends in a cation/π interaction between βR394 and αF406. Using mutational analysis in E. coli ATP synthase, the functional importance of the βDP/αDP hydrogen-bonding network is demonstrated. Its elimination results in a severely impaired enzyme but has no pronounced effect on the binding affinities of the catalytic sites. A possible role for the hydrogen-bonding network in coupling of ATP synthesis/hydrolysis and rotation will be discussed.