Molecular switch of F0F1-ATP synthase, G-protein, and other ATP-driven enzymes

The alpha/beta interfaces of alpha(1)beta(1), alpha(3)beta(3), and F1: domain motions and elastic energy stored during gamma rotation

ATP synthase (F(o)F(1)) consists of F(1) (ATP-driven motor) and F(o) (H(+)-driven motor). F(1) is a complex of alpha(3)beta(3)gammadeltaepsilon subunits, and gamma is the rotating cam in alpha(3)beta(3). Thermophilic F(1) (TF(1)) is exceptional in that it can be crystallized as a beta monomer and an alpha(3)beta(3) oligomer, and it is sufficiently stable to allow alphabeta refolding and reassembly of hybrid complexes containing 1, 2, and 3 modified alpha or beta. The nucleotide-dependent open-close conversion of conformation is an inherent property of an isolated beta and energy and signals are transferred through alpha/beta interfaces. The catalytic and noncatalytic interfaces of both mitochondrial F(1) (MF(1)) and TF(1) were analyzed by an atom search within the limits of 0.40 nm across the alphabeta interfaces. Seven (plus thermophilic loop in TF(1)) contact areas are located at both the catalytic and noncatalytic interfaces on the open beta form. The number of contact areas on closed beta increased to 11 and 9, respectively, in the catalytic and noncatalytic interfaces. The interfaces in the barrel domain are immobile. The torsional elastic strain applied through the mobile areas is concentrated in hinge residues and the P-loop in beta. The notion of elastic energy in F(o)F(1) has been revised. X-ray crystallography of F(1) is a static snap shot of one state and the elastic hypotheses are still inconsistent with the structure, dyamics, and kinetics of F(o)F(1). The domain motion and elastic energy in F(o)F(1) will be elucidated by time-resolved crystallography.

Linear and rotary molecular motors

A single molecule of F1-ATPase has been shown to be the smallest rotary motor ever found, with a central rotor of radius approximately 1 nm turning in a stator barrel of radius approximately 5 nm. Continuous rotation of the central gamma subunit was revealed under an optical microscope by attaching to gamma a huge marker, an actin filament. In a separate study, rotation of a sliding actin filament around its axis has been revealed by attaching a small probe, a single fluorescent dye molecule, to the actin filament and detecting the orientation of the fluorophore, and thus of the actin filament, through polarization imaging. The axial rotation was slow compared to the linear sliding, indicating that myosin does not 'walk' along the helical array of actin protomers but 'runs,' skipping many protomers. The two motors above, one rotary and the other linear, represent two extreme cases of the mode of motor operation: in the F1-ATPase the two partners, the rotor and stator, never detach from each other whereas myosin touches actin only occasionally. In considering the mechanisms of these and other molecular motors, distinction between bending and binding is important. The use of huge and small probes as described above should be useful in studies of protein machines in general, as a means of detecting conformational changes in a single protein molecule during function.

Direct observation of the rotation of F1-ATPase

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 5nm formed by three alpha- and three beta-subunits. 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. It has been suggested that the gamma-subunit of F1-ATPase rotates within the alphabeta-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 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(-1) under high load.