F-subunit reinforces torque generation in V-ATPase

Vacuolar-type H⁺-pumping ATPases (V-ATPases) perform remarkably diverse functions in eukaryotic organisms. They are present in the membranes of many organelles and regulate the pH of several intracellular compartments. A family of V-ATPases is also present in the plasma membranes of some bacteria. Such V-ATPases function as ATP-synthases. Each V-ATPase is composed of a water-soluble domain (V₁) and a membrane-embedded domain (V _o). The ATP-driven rotary unit, V ₁, is composed of A, B, D, and F subunits. The rotary shaft (the DF subcomplex) rotates in the central cavity of the A₃B₃-ring (the catalytic hexamer ring). The D-subunit, which has a coiled-coil domain, penetrates into the ring, while the F-subunit is a globular-shaped domain protruding from the ring. The minimal ATP-driven rotary unit of V₁ is comprised of the A₃B ₃D subunits, and we therefore investigated how the absence of the globular-shaped F-subunit affects the rotary torque generation of V₁. Using a single-molecule technique, we observed the motion of the rotary motors. To obtain the torque values, we then analyzed the measured motion trajectories based on the fluctuation theorem, which states that the law of entropy production in non-equilibrium conditions and has been suggested as a novel and effective method for measuring torque. The measured torque of A ₃B₃D was half that of the wild-type V₁, and full torque was recovered in the mutant V₁, in which the F-subunit was genetically fused with the D-subunit, indicating that the globular-shaped F-subunit reinforces torque generation in V₁.