Representation of the M. villous archaellum highlighting the two alternating subunits in blue and orange (foreground) and swimming M. villosus cells (background)

Exeter biologists investigate smallest propeller on earth

University of Exeter scientists have discovered new information about the tiny propellers used by single-cell organisms called archaea.

Like bacteria, archaea are found in a vast range of habitats – including inside human bodies – but unlike bacteria they are not known to cause disease.

Some archaea propel themselves to incredible speeds by rotating a spiral-shaped filament called an archaellum.

Using a powerful cryo-electron microscope, the new study examined this closer than ever before.

The research team – which included the University of Regensburg – focussed on Methanocaldococcus villosus, a species found near underwater volcanoes off Iceland, where water temperatures can reach about 80°C.

"M. villosus swims at a speed of about 500 body lengths per second," said Dr Lavinia Gambelli, of Exeter's Living Systems Institute (LSI).

"Considering that the tiny cell is only about one micrometre in size, this means half a millimetre in one second.

"At first glance, this does not seem much.

"But in comparison, a cheetah achieves only 20 body lengths per second – so if an M. villosus cell had the size of a cheetah, it would swim at approximately 3,000 kilometres per hour.

"The incredible speed that M. villosus can achieve makes it one of the fastest organisms on the planet."

Using the cryo-electron microscope, researchers can see objects whose width is as small as only a few hydrogen atoms.

"At this resolution, we can see the very fabric of life and study fundamental biological processes at atomic detail," said Dr Bertram Daum, also of the LSI.

"In this study, we were able to look closely at the smallest propeller in the world, to find out more about its shape and how it works.

"As well as teaching us more about these fascinating organisms, this could have implications for human health and technology.

"Archaea make up a considerable percentage of the microorganisms found in the human body. None has so far been found to cause disease, but it remains a possibility.

"In the future, it might even be possible to develop microscopic robotic devices for drug delivery based on the tiny propellers used by archaea."

The study discovered that the filament used by M. villosus is made up of thousands of copies of two alternating proteins, whereas previously investigated filaments showed only one protein.

This suggests that the architecture and assembly of an archaellum is more complex than previously thought.

The researchers also identified two major structural elements that enable the archaellum filament to move, propelling the cell at high speed.

The study was funded by the European Research Council.

The paper, published in the journal Nature Communications, is entitled: "An archaellum filament composed of two alternating subunits."

Date: 7 February 2022