Mycoplasma mobile, a fish pathogen, exhibits gliding motility using ATP hydrolysis on solid surfaces, including animal cells. The gliding machinery can be divided into surface and internal structures. The internal structure of the motor is composed of 28 so-called "chains" that are each composed of 17 repeating protein units called "particles." These proteins include homologs of the catalytic α and β subunits of F1-ATPase. In this study, we isolated the particles and determined their structures using negative-staining electron microscopy and high-speed atomic force microscopy. The isolated particles were composed of five proteins, MMOB1660 (α-subunit homolog), -1670 (β-subunit homolog), -1630, -1620, and -4530, and showed ATP hydrolyzing activity. The two-dimensional (2D) structure, with dimensions of 35 and 26 nm, showed a dimer of hexameric ring approximately 12 nm in diameter, resembling F1-ATPase catalytic (αβ)3. We isolated the F1-like ATPase unit, which is composed of MMOB1660, -1670, and -1630. Furthermore, we isolated the chain and analyzed the three-dimensional (3D) structure, showing that dimers of mushroom-like structures resembling F1-ATPase were connected and aligned along the dimer axis at 31-nm intervals. An atomic model of F1-ATPase catalytic (αβ)3 from Bacillus PS3 was successfully fitted to each hexameric ring of the mushroom-like structure. These results suggest that the motor for M. mobile gliding shares an evolutionary origin with F1-ATPase. Based on the obtained structure, we propose possible force transmission processes in the gliding mechanism. IMPORTANCE F1Fo-ATPase, a rotary ATPase, is widespread in the membranes of mitochondria, chloroplasts, and bacteria and converts ATP energy with a proton motive force across the membrane by its physical rotation. Homologous protein complexes play roles in ion and protein transport. Mycoplasma mobile, a pathogenic bacterium, was recently suggested to have a special motility system evolutionarily derived from F1-ATPase. The present study isolated the protein complex from Mycoplasma cells and supported this conclusion by clarifying the detailed structures containing common and novel features as F1-ATPase relatives.
Keywords: F1-ATPase; atomic force microscopy; bacterial motility; electron microscopy; parasitic bacteria; rotary motor.