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Figure 1 Nature’s tiniest rotary motor shown here as a highly diagrammatic version of an ATP synthase complex — a device for generating ATP from free energy stored as a transmembrane protonmotive force (a gradient in electrochemical potential of protons, consisting of a pH gradient and a gradient in electric potential). The ATP synthase complex of chloroplasts was referred to earlier as a ‘coupling factor’ (Figure 1.11). (a) In mitochondria, an asymmetric portion F0 spans the inner membrane and provides a conduit for proton movement across this inner membrane (from intermembrane space to matrix), movement that imparts a torque between two adjacent subunits ‘a’ and ‘c’.Torque so generated is transmitted to F1 via the shaft ‘γ’ plus the ‘ε’ subunit. (b) ‘a’, ‘b’ and ‘δ’ subunits are linked to the F1 hexainer (α3β3) and constitute the ‘stator’ of this rotary motor. The ‘rotor’ itself is represented here as a shaded portion of the overall complex, and comprises ‘c’, ‘γ’ and ‘ε’ subunits. The ‘γ’ subunit behaves as a rotating shaft that mediates an exchange of energy derived from proton flow through F0 for ATP synthesis via the cooperative activity of three catalytic sites within F1 (three ATP are generated for every 12 protons that pass through this rotary motor).

In a widely acclaimed technical achievement, Hiroyuki Noji and colleagues at Yokohama (Noji et. al. 1997) attached a fluorescent actin filament to the tip of a 'γ' subunit and recorded continuous rotation during synthesis of ATP, thus confirming a rotary motion that had been predicted on biophysical grounds. Unrestrained by a long actin filament, rotation rate in vivo would peak around 150 revs per second. Significantly, when provided with a source of ATR this self same device (an ATP synthase complex) can draw energy from ATP hydrolysis to pump protons against a gradient. Now working as an ATPase, these rotary motors sustain energy-dependent processes including nutrient ion uptake and salt exclusion by plant roots. (Based on Elston et al. 1998; reproduced with permission of Nature)