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.

A collective motion of self-driven particles has been a fascinating subject in physics and biology. Sophisticated macroscopic behavior emerges through a population in thousands or millions of bacterial cells, propelling itself by flagellar rotation and its chemotactic response. Here we found a series of collective motions accompanying successive phase-transitions in a non-flagellated rod-shaped soil bacterium, Flavobacterium johnsoniae, which was driven by a surface cell movement known as gliding motility. When we spot the cells on an agar plate with a low level of nutrients, the bacterial community exhibited vortex patterns that spontaneously appeared as lattice and integrated into a large-scale circular plate. All patterns exhibit with monolayer of bacteria, which enable to visualize an individual cell with a high resolution among a wide-range pattern two-dimensionally. The single cells moved at random orientation, but the cells connected with one another showed left-turn biased trajectories in starved environment. This feature is possibly due to the collision of cells inducing a nematic alignment of dense cells as self-propelled rods. Subsequently, each vortex oscillated independently, and then transformed to the rotating mode as an independent circular plate. Notably, the rotational direction of the circular plate was counterclockwise without exception. The plates developed accompanying rotation with constant angular velocity, suggesting that the mode is an efficient strategy for bacterial survival.ImportanceSelf-propelled bacteria propelled by flagella rotation often display highly organized dynamic patterns at high cell densities. Here we found a new mode of collective motion in non-flagellated bacteria: vortex patterns were spontaneously appeared as lattice and integrated into a large-scale circular plate comprising hundreds of thousands of cells, which exhibited unidirectional rotation in a counterclockwise manner and expanded in size on agar. A series of collective motions was driven by gliding motility of the rod-shaped soil bacterium Flavobacterium johnsoniae In a low nutrient environment, single cells moved at random orientation while cells at high density moved together as a unitary cluster. This might be an efficient strategy for cells of this species to find nutrients.