= Solution
\b[The proton current supplies energy, while a conformational switch changes how that current is coupled to rotor motion.] Reversing torque therefore does not require reversing the electrochemical gradient. In the proton-driven <bacterial flagellar motor> of <Escherichia coli>, the cell maintains a <proton-motive force> that favours proton entry. <MotA> and <MotB> form stator units anchored to the cell envelope. Proton-dependent conformational cycles of those units exert force on the rotor, including interactions with <FliG>.
The rotor's switch complex includes <FliG>, <FliM> and <FliN>. Cooperative changes in its organization alter the relative geometry of rotor and stator contacts and hence the direction in which the energy-driven cycle produces angular displacement. Structural work on the torque ring supported a mechanism in which reorientation of interacting regions changes the sign of torque.<Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching by Lawrence K. Lee and others>[Lee and colleagues, 2010, abstract and structural model of rotational switching]{ref} Such structural models constrain the explanation, but should not be treated as a complete direct movie of every proton-transfer and mechanical step. The central distinction is robust: \b[the sign of mechanical rotation changes through the motor's coupling state, while the thermodynamically favourable direction of proton flow remains inward].
This is consistent with energy conservation. With a fixed electrochemical difference, inward proton movement has a negative free-energy change. In either rotational state, the magnitude of that input can supply positive work against a resisting load. Both torque and angular displacement reverse when the motor reverses, leaving their product, the work delivered during motion against that load, positive. No principle requires a given sign of proton flow to correspond to one immutable sign of angular displacement when the <protein> coupling apparatus can change conformation.
Chemotaxis controls the switch bias. Receptor complexes regulate <CheA>, which transfers phosphoryl groups to <CheY>. Phosphorylated <CheY> binds the switch, particularly FliM-associated sites, increasing clockwise bias in the familiar E. coli swimming system. Counterclockwise rotation promotes a coherent flagellar bundle and a run; clockwise episodes disrupt the bundle and permit reorientation. An attractant response commonly reduces <CheA> activity and <CheY> <phosphorylation>, extending runs, while adaptation restores sensitivity. <CheY> <phosphorylation> is a regulatory signal; it does not provide the motor's main mechanical energy or reverse the proton gradient. Other bacteria can use different motor arrangements or sodium-driven stators, so this account should not be universalized to all species.
Experimentally, a tethered-cell or attached-bead assay resolves individual motor direction changes while external pH and membrane potential are controlled. Reversal without a measured reversal of the gradient supports switching of coupling. A protonophore should suppress energy supply and rotation, rather than reliably command the opposite direction. <CheY> perturbations and switch mutations can alter directional bias while leaving ion-driven torque generation intact. Simultaneous gradient measurements, motor speed and direction distinguish an energy deficit from a switch defect; a population swimming assay alone cannot do so. \b[Directional control and energetic drive are separable functions of the same molecular machine.]
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