Scientists end a 50-year mystery and reveal how bacteria can move

Researchers at the University of Virginia School of Medicine and their collaborators have solved a decades-old mystery E. coli and other bacteria can move.

Bacteria propel themselves by rolling long, threadlike appendages into corkscrew shapes that act as makeshift propellers. But exactly how they do this has amazed scientists, because the “propellers” consist of a single protein.

An international team led by Edward H. Egelman, PhD of UVA, a leader in the field of high-tech cryo-electron microscopy (cryo-EM), has cracked the case. The researchers used cryo-EM and advanced computer modeling to reveal what no conventional light microscope could see: the strange structure of these propellers at the level of individual atoms.

“While there have been models for 50 years of how these filaments might form such regular coiled shapes, we have now determined the structure of these filaments in atomic detail,” said Egelman of UVA’s Department of Biochemistry and Molecular Genetics. “We can show that these models were wrong, and our new understanding will help pave the way for technologies that could be based on such miniature propellers.”

Blueprints for the “supercoils” of bacteria

Various bacteria have one or more appendages known as the flagellum, or in the plural flagella. A flagellum is made up of thousands of subunits, but all of these subunits are exactly the same. One might think that such a tail would be straight, or at best somewhat flexible, but that would immobilize the bacteria. This is because such shapes cannot generate thrust. It takes a spinning, corkscrew-like propeller to propel a bacterium forward. Scientists call the formation of this shape “supercoiling,” and now, more than 50 years later, they understand how bacteria do it.

Using cryo-EM, Egelman and his team found that the protein that makes up the flagellum can exist in 11 different states. It is the precise mixture of these states that leads to the formation of the corkscrew shape.

The propeller in bacteria is known to be quite different from similar propellers used by powerful single-celled organisms called archaea. Archaea are found in some of the most extreme environments on Earth, such as near-boiling acid pools, at the bottom of the ocean, and in deep-soil petroleum deposits.

Egelman and colleagues used cryo-EM to study the flagella of one form of archaea, Saccharolobus islandicus, and found that the protein that makes up its flagellum exists in 10 different states. While the details were very different from what the researchers saw in bacteria, the result was the same, with the filaments forming regular corkscrews. They conclude that this is an example of “convergent evolution” – when nature arrives at similar solutions in very different ways. This shows that although the propellers of bacteria and archaea are similar in form and function, the organisms evolved these traits independently.

“As with birds, bats, and bees, all of which independently evolved wings for flight, the evolution of bacteria and archaea evolved towards a similar solution for swimming in both,” said Egelman, whose previous imaging work led to his finding in was admitted to the National Academy of Sciences, one of the highest honors a scientist can receive. “Because these biological structures formed on Earth billions of years ago, the 50 years it took to understand them doesn’t seem that long.”

results published

The researchers published their results in the journal cell. The team consisted of Mark AB Kreutzberger, Ravi R. Sonani, Junfeng Liu, Sharanya Chatterjee, Fengbin Wang, Amanda L. Sebastian, Priyanka Biswas, Cheryl Ewing, Weili Zheng, Frédéric Poly, Gad Frankel, BF Luisi, Chris Calladine, Mart Krupovic , Birgit E. Scharf and Egelmann.

Work was supported by National Institutes of Health grants GM122150 and T32 GM080186; US Navy Work Unit Program 6000.RAD1.DA3.A0308; and through a Robert R. Wagner grant. The researchers’ paper does not represent the official policy or position of the Department of Navy, Department of Defense, or the U.S. government.

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