西坂 崇之

Mycoplasma mobile is one of the fastest gliding bacteria, gliding with a speed of 4.5 μm s-1. This gliding motility is driven by a concerted movement of 450 supramolecular motor units composed of three proteins, Gli123, Gli349, and Gli521, in the gliding motility machinery. With general experimental setups, it is difficult to obtain the information on how each motor unit works. This chapter describes strategies to decrease the number of active motor units to extract stepwise cell movements driven by a minimum number of motor units. We also describe an unforeseen motility mode in which the leg motions convert the gliding motion into rotary motion, which enables us to characterize the motor torque and energy-conversion efficiency by adding some more assumptions.

F1-ATPase is a world's smallest biological rotary motor driven by ATP hydrolysis at three catalytic β subunits. The 120° rotational step of the central shaft γ consists of 80° substep driven by ATP binding and a subsequent 40° substep. In order to correlate timing of ATP cleavage at a specific catalytic site with a rotary angle, we designed a new F1-ATPase from thermophilic Bacillus PS3 carrying β(E190D/F414E/F420E) mutations which cause extremely slow rates of both ATP cleavage and ATP binding. We produced an F1 molecule which consists of one mutant β and two wild type βs (hybrid F1). As a result, the new hybrid F1 showed two pausing angles which are separated by 200°. They are attributable to two slowed reaction steps in the mutated β, thus providing the direct evidence that ATP cleavage occurs at 200° rather than 80° subsequent to ATP binding at 0°. This scenario resolves the long-standing unclarified issue in the chemomechanical coupling scheme and give insights into the mechanism of driving unidirectional rotation.

Length control is a fundamental requirement for molecular architecture. Even small wall-less bacteria have specially developed macro-molecular structures to support their survival. <italic>Mycoplasma pneumoniae</italic>, a human pathogen, forms a polar extension called an attachment organelle, which mediates cell division, cytadherence, and cell movement at host cell surface. This characteristic ultrastructure has a constant size of 250–300 nm, but its design principle remains unclear. In this study, we constructed several mutants by genetic manipulation to increase or decrease coiled-coil regions of HMW2, a major component protein of 200 kDa aligned in parallel along the cell axis. HMW2-engineered mutants produced both long and short attachment organelles, which we quantified by transmission electron microscopy and fluorescent microscopy with nano-meter precision. This simple design of HMW2 acting as a molecular ruler for the attachment organelle should provide an insight into bacterial cellular organization and its function for their parasitic lifestyles.