Bacteria push themselves forward by
coiling long, threadlike appendages into corkscrew shapes that act as makeshift
propellers. But how exactly they do this has baffled scientists, because the
“propellers” are made of a single protein.
An international team led by UVA’s
Edward H. Egelman, PhD, 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 traditional light microscope could
see: the strange structure of these propellers at the level of individual
atoms.
“While models have existed for 50 years for 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 upon such miniature propellers.”
BLUEPRINTS FOR BACTERIA’S ‘SUPERCOILS’
Different bacteria have one or many
appendages known as a flagellum, or, in the plural, flagella. A flagellum is
made of thousands of subunits, but all these subunits are exactly the same. You
might think that such a tail would be straight, or at best a bit flexible, but
that would leave the bacteria unable to move. That’s because such shapes can’t
generate thrust. It takes a rotating, corkscrew-like propeller to push a
bacterium forward. Scientists call the formation of this shape “supercoiling,”
and now, after more than 50 years, 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 causes the corkscrew
shape to form.
It has been known that the propeller in
bacteria is quite different than similar propellers used by hearty one-celled
organisms called archaea. Archaea are found in some of the most extreme
environments on Earth, such as in nearly boiling pools of acid, the very bottom
of the ocean and in petroleum deposits deep in the ground.
Egelman and colleagues used cryo-EM to examine the
flagella of one form of archaea, Saccharolobus islandicus,
and found that the protein forming its flagellum exists in 10 different states.
While the details were quite different than 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 via very different means. This shows
that even though bacteria and archaea’s propellers are similar in form and
function, the organisms evolved those traits independently.
“As with birds, bats and bees, which have all independently evolved wings for flying, the evolution of bacteria and archaea has converged on a similar solution for swimming in both,” said Egelman, whose prior imaging work saw him inducted into the National Academy of Sciences, one of the highest honors a scientist can receive. “Since these biological structures emerged on Earth billions of years ago, the 50 years that it has taken to understand them may not seem that long.”
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