How Bacterial Motors Shift Into High Gear

Using high-speed atomic force microscopy, scientists in Japan have uncovered the mechanism by which bacterial motors are assembled and activated.

AsianScientist (Nov. 24, 2017) – A team of researchers in Japan have gained a better understanding of how bacterial motors, known as flagella, are assembled and activated in response to ions. They published their findings in Science Advances.

Bacteria swim in many different ways, and the motors that drive their swimming are widely varied, implying an adaptive response to an environment. One of the most commonly identified of such motors is flagella. The flagellar motor consists of a rotor and a dozen stator units and is driven by the energy created by ions when they migrate across the cell membrane.

Flagellar motors are powered by protons (hydrogen ions, H+) or sodium ions (Na+). In Bacillus subtilis (B. subtilis), a bacterium commonly found in the soil, the flagellar motor has two distinct types of stator units consisting of the proteins MotAB, which runs on H+, and MotPS which runs on Na+. Although the components of the motor have been identified, the mechanism that powers the flagellar motor remains unknown.

In this study, a team of scientists led by Professor Tohru Minamino at Osaka University, Japan, used high-speed atomic force microscopy (HS-AFM) combined with mutational analysis to glean insights into the torque generation mechanism of the flagellar motor.

“We made real-time observations of the Na+-induced structural changes to the domain of MotS (a subunit of MotPS) that binds peptidoglycan, a polymer consisting of sugars and amino acids in bacteria,” said Assistant Professor Naoya Terahara of Osaka Universiy who is the first author of the study.

To understand how the MotPS protein complex responds to changes in the environment, the researchers first characterized the torque-speed relationship of the B. subtilis motor over a wide range of Na+ concentrations. The maximum speed of the motor was decreased from about 200 to 80 Hz when the external Na+ concentration was increased from 0 to 200 mM, although the stall torque was not changed at all.

Notably, when the domain of MotB that binds to peptidoglycan was replaced by that of MotS, the chimeric motor maintained the same torque-speed curve as the MotAB motor in the presence of Na+ ions. This means that Na+ is the main driver of the assembly and activation of flagellar motor, regardless of its composition.

“In the absence of sodium ions, MotPS exists as an inactive form in the cytoplasmic membrane because MotSc, the C-terminal periplasmic domain of MotS, adopts an unfolded conformation. When the concentration of sodium ions is increased, the ions bind to MotSc, changing its structure and causing its dimerization in a highly cooperative manner,” said Minamino.

These experiments have demonstrated the feasibility of using HS-AFM combined with mutational analysis to study intricate bacterial structures, which can provide valuable insights for industrial production of commercially viable products, such as proteases and therapeutics.

The article can be found at: Terahara et al. (2017) Na+-induced Structural Transition of MotPS for Stator Assembly of the Bacillus Flagellar Motor.


Source: Osaka University.
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