ATP synthase (F 0F 1) is made of two motors, a proton-driven motor (F 0) and an ATP-driven motor (F 1), connected by a common rotary shaft, and catalyzes proton flow-driven ATP synthesis and ATP-driven proton pumping. In F 1, the central γ subunit rotates inside the α 3β 3 ring. Here we report structural features of F 1 responsible for torque generation and the catalytic ability of the low-torque F 0F 1. (i) Deletion of one or two turns in the α-helix in the C-terminal domain of catalytic β subunit at the rotor/stator contact region generates mutant F 1s, termed F 1(1/2)s, that rotate with about half of the normal torque. This helix would support the helix-loop-helix structure acting as a solid "pushrod" to push the rotor γ subunit, but the short helix in F 1(1/2)s would fail to accomplish this task. (ii) Three different half-torque F 0F 1(1/2)s were purified and reconstituted into proteoliposomes. They carry out ATP-driven proton pumping and build up the same small transmembrane ΔpH, indicating that the final ΔpH is directly related to the amount of torque. (iii) The half-torque F 0F 1(1/2)s can catalyzeATPsynthesis, although slowly. The rate of synthesis varies widely among the three F 0F 1(1/2)s, which suggests that the rate reflects subtle conformational variations of individual mutants.
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