ATP synthase uses a unique rotary mechanism to couple ATP synthesis and hydrolysis to transmembrane proton translocation. As part of the synthesis mechanism, the torque of the rotor has to be converted into conformational rearrangements of the catalytic binding sites on the stator to allow synthesis and release of ATP. The ␥ subunit of the rotor, which plays a central role in the energy conversion, consists of two long helices inside the central cavity of the stator cylinder plus a globular portion outside the cylinder. Here, we show that the N-terminal helix alone is able to fulfill the function of full-length ␥ in ATP synthesis as long as it connects to the rest of the rotor. This connection can occur via the ⑀ subunit. No direct contact between ␥ and the c ring seems to be required. In addition, the results indicate that the ⑀ subunit of the rotor exists in two different conformations during ATP synthesis and ATP hydrolysis.

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