15 Apr 2021 by Navish

This post first appeared on the website of MCB department at Harvard.

The bacterial flagellar engine has a bidirectional gearshift

Bacteria are a nanotechnological marvel. Consider the bacterium Escherichia coli as an example. Barely a millionth the size of a typical human (E. coli is a rod-shaped cell about 2 µm long and 1 μm wide), each cell is chock-full of molecular machinery that enables it to carry out life’s myriad functions. One of these, the flagellum, is the miniature propeller that allows E. coli to swim at astonishing speeds, covering up to 20 body lengths every second. A typical cell has four or five flagella, each capable of rotating in either direction – clockwise (CW) or counterclockwise (CCW).

The engine that drives rotation of the flagella is the flagellar motor, a highly complex nanomachine about 50 nm in diameter. Made up of tens of different proteins, the flagellar motor embeds in the E. coli cell envelope and turns the flagellum from its base. Like a car engine, the flagellar motor converts chemical energy into mechanical work. Protein complexes called stator units attach to the cell wall (thus remaining stationary) and apply a force on the rotor (which is free to rotate) to drive the rotation.

It turns out that this miniature engine of E. coli has some neat tricks up its sleeve. Just like modern cars can automatically change gears to adapt to changing terrains, the flagellar motor can also adapt to changes in the mechanical load. But unlike cars, which adjust the transmission from the engine to the wheels, E. coli modifies the engine itself. When load increases, the flagellar motor adds additional stator units to increase its torque output, and when load decreases, the motor releases stator units, decreasing torque output. So, E. coli constantly dismantles and rebuilds its engines as the mechanical demands of its external environment change.

We studied this process using electrorotation, a technique in which we apply a high frequency rotating electric field to E. coli cells attached to a surface by the flagellum. The electric field applies a large external torque on the cell. This torque instantaneously and reversibly changes the mechanical load on the motor, allowing us to carefully measure its adaptation response. We can also control the amount of external torque by adjusting the strength of the electric field. Our electrorotation rig is thus a feature-rich platform for probing the function of bacterial flagellar engines.

Using electrorotation, we previously measured the mechano-adaptation response in CCW rotating flagellar motors. We found that the extent and the speed of the motor’s response depends on the torque produced by the stator units. When we decrease the mechanical load, torque output decreases, and stator units leave. When load increases, torque also increases, and the stator units stay. Torque somehow tunes the stator units’ binding to the cell wall, enabling the adaptation response.

When we sought to test these ideas, we had to look no further than the CW rotating flagellar motor. For reasons not fully understood, the flagellar motor produces much less torque when spinning CW than it does when spinning CCW. So, to test our model of torque-dependent response, all we had to do was to use electrorotation to measure mechano-adaptation in CW rotating motors and compare the results with data from CCW rotating motors. That is exactly what we did, reporting our findings in a new paper just published in PNAS.

To our great satisfaction, we found that remodeling rates from CW and CCW rotating motors fall on the same curve if plotted against torque. So, mechanosensitive remodeling in the bacterial flagellar motor is independent of its direction of rotation and depends only on torque. This result not only confirms our model, but also provides strong constraints on the molecular mechanisms of mechano-adaptation in the flagellar motor. We are currently developing mathematical theories and new experimental methods to probe these mechanisms.

We remain enamored by the bacterial flagellar motor, which turns out to be one of nature’s most complex yet elegant nanomachines, making most human-made machines look primitive in comparison. Going forward, we plan to continue probing it with novel experimental approaches, in the hope that this will allow us to reveal more of its secrets.