Researchers from the University of Virginia School of Medicine and their associates have answered a long-standing question about how E. coli and other bacteria can move.
By coiling their long, threadlike appendages into corkscrew shapes that serve as makeshift propellers, bacteria propel themselves forward. However, because the “propellers” are made of a single protein, scientists are baffled as to how exactly they accomplish this.
The case has been solved by an international team led by UVA’s Edward H. Egelman, PhD, a pioneer in the high-tech field of cryo-electron microscopy (cryo-EM). The strange structure of these propellers at the atomic level, which couldn’t be seen with a regular light microscope, was shown by cryo-EM and advanced computer modeling.
“We have now determined the structure of these filaments in atomic detail,” said Egelman, of UVA’s Department of Biochemistry and Molecular Genetics. “Models have existed for 50 years for how these filaments might form such regular coiled shapes.” We can demonstrate that these models were inaccurate, and our new knowledge will make it possible for technologies to be built around these tiny propellers.
Bacterial “Supercoil” blueprints
Different bacteria have one or more protrusions called flagellums, or flagella in the plural. The thousands of individual parts that make up a flagellum are all identical. You might assume that such a tail would be straight or, at most, slightly flexible, but the bacteria would be unable to move if that were the case. This is because they are unable to produce thrust. A propeller that rotates and resembles a corkscrew is needed to move a bacterium forward. After more than 50 years, scientists have finally figured out how bacteria produce this shape, which they refer to as “supercoiling.”
Using cryo-EM, Egelman and his team discovered the flagellum protein can exist in 11 different states. The exact combination of these states is what gives rise to the corkscrew shape.
It is well known that the bacteria’s propeller differs significantly from the robust one-celled organisms known as the archaea’s propeller. Archaea are organisms that can be found in some of the most hostile environments on Earth, including oil deposits buried deep in the ground and acid pools that are nearly boiling.
When Egelman and colleagues used cryo-EM to look at the flagella of Saccharolobus islandicus, a particular type of archaea, they discovered that the protein that makes up its flagellum exists in ten different states. The filaments formed regular corkscrews, though the specifics were quite different from what the researchers observed in bacteria. They come to the conclusion that this is an illustration of “convergent evolution,” which occurs when nature finds solutions through very dissimilar means. This shows that, even though bacteria and archaea have propellers that look and work the same, the two groups of organisms came up with the same features on their own.
Egelman, whose prior imaging work saw him inducted into the National Academy of Sciences, one of the highest honors a scientist can receive, said that just as birds, bats, and bees independently evolved wings for flying, the evolution of bacteria and archaea “converged on a similar solution for swimming in both.” “The 50 years it has taken to understand these biological structures may not seem like a long time given that they first appeared on Earth billions of years ago.”
