Discovered just a decade ago, Asgard archaea are a group of microbes that bridge the gap between bacteria, simple archaea, and complex life forms like plants and animals. Martin Pilhofer, NOMIS Professor of Cryo-Electron Microscopy at ETH Zurich, and fellow researchers recently studied one species, Lokiarchaeum ossiferum, and found that it contains cytoskeletal proteins strikingly similar to those in complex organisms. This discovery suggests that Asgard archaea are the ancient ancestors of humans, animals and plants. Their findings were published in Cell.
Who were our earliest ancestors? The answer could lie in a special group of single-celled organisms with a cytoskeleton similar to that of complex organisms, such as animals and plants. ETH researchers made these findings in a new study.
Ten years ago, nobody knew that Asgard archaea even existed. In 2015, however, researchers examining deep-sea sediments discovered gene fragments that indicated a new and previously undiscovered form of microbes.
With computer assistance, the researchers assembled these fragments like puzzle pieces to compile the entire genome. It was only then that they realised they were dealing with a previously unknown group of archaea.
Like bacteria, archaea are single-celled organisms. Genetically, however, there are significant differences between the two domains, especially regarding their cell envelopes and metabolic processes.
After a further search, microbiologists identified the corresponding organisms, described them and classified them as a separate archaeal sub-group: Asgard archaea. Their name, taken from the heavenly realm in Norse mythology, references their initial discovery close to Loki’s Castle – a black smoker on the mid-Atlantic ridge between Norway and Svalbard.
In fact, Asgard archaea appeared almost heaven-sent for research: they turned out to be a missing link between archaea and eukaryotes – that is, between archaea and organisms whose cells contain a nucleus, such as plants and animals.
Tree of life with one branch fewer
In recent years, researchers have found growing indications of close links between Asgard archaea and eukaryotes, and that the latter may have evolved from the former. The division of all living organisms into the three domains of bacteria, archaea and eukaryotes did not hold up to this surprising discovery.
Some researchers have since proposed regarding eukaryotes as a group within Asgard archaea. This would reduce the number of domains of life from three to two: archaea, including eukaryotes, and bacteria.
At ETH Zurich, Professor Martin Pilhofer and his team are fascinated by Asgard archaea and have examined the mysterious microbes for several years.
In an article published in Nature two years ago, the ETH researchers explored details of the cellular structure and architecture of Lokiarchaeum ossiferum. Originating in the sediments of a brackish water channel in Slovenia, this Asgard archaeon was isolated by researchers in Christa Schleper’s laboratory at the University of Vienna.
In that study, Pilhofer and his postdoctoral researchers Jingwei Xu and Florian Wollweber demonstrated that Lokiarchaeum ossiferum possesses certain structures also typical of eukaryotes. “We found an actin protein in that species that appears very similar to the protein found in eukaryotes – and occurs in almost all Asgard archaea discovered to date,” says Pilhofer.
In the first study, the researchers combined different microscopy techniques to demonstrate that this protein – called Lokiactin – forms filamentous structures, especially in the microbes’ numerous tentacle-like protrusions. “They appear to form the skeleton for the complex cell architecture of Asgard archaea,” adds Florian Wollweber.
In addition to actin filaments, eukaryotes also possess microtubules. These tube-shaped structures are the second key component of the cytoskeleton and are comprised of numerous tubulin proteins. These tiny tubes are important for transport processes within a cell and the segregation of chromosomes during cell division
The origin of these microtubules has been unclear – until now. In a newly published article in Cell , the ETH researchers discovered related structures in Asgard archaea and describe their structure. These experiments show that Asgard tubulins form very similar microtubules, albeit smaller than those in their eukaryotic relatives.
However, only a few Lokiarchaeum cells form these microtubules. And, unlike actin, these tubulin proteins only appear in very few species of Asgard archaea.
Continue reading this ETH story
Read the Cell publication: Microtubules in Asgard archaea
Feature image: An artist’s depiction of an Asgard archaeon, based on cryo-electron tomography data: the cell body and appendages feature thread-like skeletal structures, similar to those found in complex cells with nuclei. (Graphic: Margot Riggi, Max Planck Institute of Biochemistry)
