▎WuXi AppTec Content Team Editor
Since Leeuwenhoek observed fast-swimming bacteria under a microscope, scientists have been eager to uncover the secrets of bacterial movement. But it is not until today, more than 300 years later, that the answer has gradually emerged.
We’ve learned from textbooks that bacteria rely on flagella for movement. Indeed, flagella are the characteristic motor organs of most bacteria, and they are composed of three parts: motors on bacterial membranes, extracellular linker apparatus and flagellar filaments. The mechanism by which flagella makes bacteria move involves two key questions: how does the flagellar motor provide power, and how does the flagellar filament as a “propeller” transform into a shape suitable for movement?
Image source: 123RF
For the first question, an important advance was presented in a Cell paper last year. Professor Zhu Yongqun and Professor Zhang Xing of Zhejiang University cooperated to analyze the structure of bacterial flagellar motor at atomic resolution, and how to assemble and provide power for the efficient operation of flagellar filaments. (Read more:
Milestone! Explain in detail how one of the most sophisticated molecular motors in nature works, the Zhejiang University research team published an article in “Cell”
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As for the second question, it’s an unsolved case that scientists have debated for half a century. What is known is that by rolling the slender flagellar filaments into a spiral, bacteria can obtain a temporary “propeller”, like the propeller of a helicopter, by turning to generate power and propel itself to move quickly .
But how is this transformation process accomplished? In a recent study published in the journal Cell, Professor Edward H. Egelman of the University of Virginia led a team to unravel the mystery. Using cryo-electron microscopy and computer simulations, the research team deciphered the helical mechanism of flagellar filaments at near-atomic resolution, and revealed the convergent evolution of bacterial and archaeal flagellar filaments.
“Models began to describe how these flagellar filaments form such regular spirals as early as 50 years ago, and now we have revealed the details of the structure of the flagellar filaments,” said Professor Egelman. “We ‘s research shows that those models are wrong, and the new understanding we bring will facilitate the development of new technologies based on these tiny ‘thrusters’.”
Each flagellar filament of bacteria consists of thousands of identical subunits. We might think that the flagellar filaments should be nearly straight, or only a little elastic. But in fact, such a structure simply cannot generate enough thrust, so the bacteria will have difficulty moving. Only when it is coiled into a spiral can the bacteria move. Scientists call this process supercoil (supercoil).
Under a cryo-electron microscope, the research team observed the core domain of flagellar filaments. In the lowest energy state, the protofilaments that make up the flagellar filaments have 11 different conformations, as shown in the figure below, these protofilaments are arranged circularly along the longitudinal axis to form a cylinder. Due to their different conformations, they vary in length and the originally straight cylinders bend towards the side of the shorter protofilaments, thus curling up to form supercoils.
▲The process of the formation of supercoiled filaments of flagellar filaments of bacteria (top) and archaea (bottom) (Image source: Reference [1])
In addition, the study analyzed the structure of archaeal flagellar filaments. Compared with bacteria, people’s understanding of archaea is more limited.
Under the cryo-electron microscope, the protofilaments that make up the archaeal flagellar filaments have 10 different conformations. Although many details differ between archaea and bacteria (for example, the core domain of archaeal flagellar filaments is single-stranded, while bacteria are multi-stranded), the end result is quite the same: flagellar filaments are transformed into regular superstructures. Spiral.
The team concluded that, in this case, archaea and bacteria evolved convergently: nature found similar solutions in different ways. In other words, although the flagellar filaments of bacteria and archaea share similar composition and function, they evolved these characteristics independently.
▲Through convergent evolution, the flagellar filaments of bacteria and archaea have similar supercoiled structures (Image source: Reference [1]) strong>
“Given that these structures have existed on Earth for billions of years, 50 years doesn’t seem like that long to understand,” Professor Egelman concluded.
References:
[1] Mark A.B. Kreutzberger et al, Convergent evolution in the supercoiling of prokaryotic flagellar filaments, Cell (2022). DOI: 10.1016/j.cell.2022.08.009
[2] Ending a 50-year mystery, scientists reveal how bacteria can move. Retrieved September 27, 2022 from https:https://www.eurekalert.org/news-releases/965985
