F₁-ATPase: a highly coupled reversible rotary motor

F₁ (F₁-ATPase) is a highly coupled rotary molecular motor and hydrolyses three ATP molecules per turn (3 ATP/turn). Recently, we have developed femtolitre reaction chamber arrays for highly sensitive measurement of biological reactions. By combining this technique with the rotating magnetic tweezers, the coupling ratio of the reverse reaction, ATP synthesis catalysed by single F₁ molecules, has been investigated. The low coupling ratio of 10% (0.3 ATP/turn), catalysed by the α₃β₃γ subcomplex of F₁, was significantly improved to 77% (2.3 ATP/turn) after reconstitution of the ϵ subunit. This result revealed the novel function of the ϵ subunit as a coupling factor of ATP synthesis catalysed by F₁. The possible mechanism for highly coupled ATP synthesis supported by the ϵ subunit is discussed.