Torque Generation and Utilization in Motor Enzyme F0F1-ATP Synthase HALF-TORQUE F1 WITH SHORT-SIZED PUSHROD HELIX AND REDUCED ATP SYNTHESIS BY HALF-TORQUE F0F1

ATP synthase (F0F1) 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 gamma subunit rotates inside the alpha(3)beta(3) ring. Here we report structural features of F-1 responsible for torque generation and the catalytic ability of the low-torque F0F1. (i) Deletion of one or two turns in the alpha-helix in the C-terminal domain of catalytic beta subunit at the rotor/stator contact region generates mutant F(1)s, 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 gamma subunit, but the short helix in F-1(1/2) s would fail to accomplish this task. (ii) Three different half-torque F0F1(1/2)s were purified and reconstituted into proteoliposomes. They carry out ATP-driven proton pumping and build up the same small transmembrane Delta pH, indicating that the final Delta pH is directly related to the amount of torque. (iii) The half-torque F0F1(1/2)s can catalyze ATP synthesis, although slowly. The rate of synthesis varies widely among the three F0F1(1/2)s, which suggests that the rate reflects subtle conformational variations of individual mutants.