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Cells employ a variety of linear motors, such as myosin, Kinesin and RNA polymerase, which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum (a complex of about 100 protein molecules). We now show that a single molecule of F1-ATPase acts as a rotary motor, the smallest known, by direct observation of its motion. A central rotor of radius [approximately] 1 nm, formed by its gamma-subunit, turns in a stator barrel of radius [approximately] 5 nm formed by three alpha-and three beta-subunits. F1-ATPase, together with the membrane-embedded proton-conducting unit F0, forms the H sup -ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthesis cells. It has been suggested that the gamma-subunit of F1-ATPase rotates within the alpha beta-hexamer, a conjecture supported by structural, biochemical and spectroscopic studies. We attached a fluorescent actin filament to the gamma-subunit as a marker, which enabled us to observed this motion directly. In the presence of ATP, the filament rotated for more than 100 revolutions in an anticlockwise direction when viewed from the 'membrane' side. The rotary torque produced reached more than 40 pN nm sup -1 under high load.

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