1. ^*Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226, Japan
2. ^†Department of Physics, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223, Japan

Abstract

Cells employ a variety of linear motors, such as myosin^1–3, kinesin⁴ and RNA polymerase⁵, which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum⁶ (a complex of about 100 protein molecules⁷). 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 1 nm, formed by its -subunit, turns in a stator barrel of radius 5nm formed by three - and three -subunits⁸. F₁ ATPase, together with the membrane-embedded proton-conducting unit F₀, forms the H⁺-ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthetic cells^9,10. It has been suggested that the -subunit of F₁-ATPase rotates within the -hexamer¹¹, a conjecture supported by structural⁸, biochemical^12,13 and spectroscopic¹⁴ studies. We attached a fluorescent actin filament to the -subunit as a marker, which enabled us to observe 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 ^-l under high load.