By looking through a high-tech microscope at flash-frozen proteins, researchers have just solved a 50-year-old mystery about how bacteria and their archenemy, the archaea, actually swim.
We’ve long known that they use a small coiled “tail” called a flagellum, but details of how their stringy appendage forms its curled shape to propel them forward have eluded us—until now.
In animal cells, flagella act like the tails we are more familiar with – beating back and forth to propel their bodies forward. But cells belonging to bacteria and the third domain of life, unicellular archaea, have corkscrew-shaped flagella that cannot generate thrust by simple side-to-side motion.
Instead, these small coils spin like a twisted spindle helix. Its coils appear to be able to stretch and contract somewhat, similar to a slant, allowing the microbes to create different waveforms with their motor-driven rotations. Spins can also change directions.
Both bacteria and archaeal flagella are all composed of the same repeating flagellin protein subunits. However, the type of flagellum found in the tail of archaea is more similar to that found in another type of cell protrusion found in bacteria called pili.
Biophysicist Mark Kreutzberger and his colleagues at the University of Virginia used cryo-electron tomography to examine the molecular structure of flagellar filaments at a near-atomic level in rod-shaped bacteria. Escherichia coli and ancient Saccharolobus islandicus.
They saw that in bacteria, protein filaments could exist in 11 different states and 10 different states in archaea. The combination of these states causes the structure as a whole to fold into its coiled shape in both microbes, despite differences in protein structure.
The resulting supercoiled structure is so stable that it can withstand torsional stresses, maintaining its convex shape during rotation – that is, until the whip changes direction of spin.
In E.coli, Straight swimming involves counterclockwise rotation. But when the bacteria change the direction of their tail rotation, the forces exerted on the flagellum change its structure, twisting one or more of its filaments from their tight bundle and relaxing the supercoils into a semi-coiled or curly shape.
This changes the microbe’s straight swimming pattern to a flip with the tail now spinning clockwise.
These direction-induced changes were not seen in the archaea, although changing their environmental conditions by adding salt or acid changed the structure of their flagella.
Despite their differences in structure and the fact that they evolved independently, nature has shaped both bacteria and ancient flagella to have essentially the same form and function – a clear example of convergent evolution.
“As with birds, bats and bees, which all have independently evolved wings to fly, the evolution of bacteria and archaea has converged on a similar solution for swimming in both,” explains the biochemist at the University of Virginia Edward Eggelman.
“Our new understanding will help pave the way for technologies that could rely on such tiny propellers.”
This research was published in Cell.