Tiny Tubes in archaea Reveal Clues to the Evolution of Complex Life

Amazing stuff!

"... Now, some of the first reports about the biology of certain Asgards are revealing new, provocative details about the interior lives of these cells. The latest paper ... describes how a portion of their cytoskeleton — the set of cellular structures that give a cell its shape — is surprisingly similar to what can be found in more complex organisms such as ourselves. ..."

"Asgard archaea were discovered in 2015 by Thijs Ettema and colleagues through the analysis of metagenomic data from sediment samples from Loki’s castle– a formation of hydrothermal deep-sea vents.

Phylogenetic analyses uncovered that Asgards were the closest known relatives of modern eukaryotes, supported by the fact that their genomes also harbor Eukaryotic Signature Proteins. Ancestors of Asgard archaea are therefore considered as a likely "host" that acquired a mitochondrion as a result of a symbiosis with other microbes. ..."

From the highlights and abstract:

"Highlights

• Asgard archaea express tubulins related to eukaryotic α/β-tubulin and bacterial BtubA/B

• Asgard tubulins (AtubA/B/B2) assemble into canonical and non-canonical heterodimers

• Asgard tubulin heterodimers polymerize into 5 or 7 protofilament microtubules

• AtubA/B form cytoskeletal structures in Ca. Lokiarchaeum ossiferum

Summary

Microtubules are a hallmark of eukaryotes. Archaeal and bacterial homologs of tubulins typically form homopolymers and non-tubular superstructures.

The origin of heterodimeric tubulins assembling into microtubules remains unclear.

Here, we report the discovery of microtubule-forming tubulins in Asgard archaea, the closest known relatives of eukaryotes. These Asgard tubulins (AtubA/B) are closely related to eukaryotic α/β-tubulins and the enigmatic bacterial tubulins BtubA/B. 

Proteomics of Candidatus Lokiarchaeum ossiferum showed that AtubA/B were highly expressed.

Cryoelectron microscopy structures demonstrate that AtubA/B form eukaryote-like heterodimers, which assembled into 5-protofilament bona fide microtubules in vitro. 

The additional paralog AtubB2 lacks a nucleotide-binding site and competitively displaced AtubB. These AtubA/B2 heterodimers polymerized into 7-protofilament non-canonical microtubules. In a sub-population of Ca. Lokiarchaeum ossiferum cells, cryo-tomography revealed tubular structures, while expansion microscopy identified AtubA/B cytoskeletal assemblies.

Our findings suggest a pre-eukaryotic origin of microtubules and provide a framework for understanding the fundamental principles of microtubule assembly."

Tiny Tubes Reveal Clues to the Evolution of Complex Life | Quanta Magazine "Scientists have identified tubulin structures in primitive Asgard archaea that may have been the precursor of our own cellular skeletons."

Asgard Archaea (overview article by one of the senior authors of this study)

Microtubules in Asgard archaea (open access)

Graphical abstract

Eukaryotes are thought having evolved from an Asgard archael ancestor that acquired a mitochondrion through symbiosis

Cell architecture and cytoskeleton of the Asgard archaeon Ca. Lokiarchaeum ossiferum | actin, orange; tubulin, blue | Illustration based on cryo-tomographic data