New study on Asgard archaea suggests a possible clue to eukaryotic membrane complexity

Eukaryotes are the cells that build up most plants, animals, and human bodies. They differ from unicellular prokaryotes by having their genetic material enclosed in a nucleus, having a cytoskeleton, and containing membrane-bound organelles such as mitochondria or chloroplasts. These organelles subdivide the eukaryotic cell in multiple compartments for specific cellular functions. Cross-talk between these organelles is carried out via vesicles that pinch out from organelles and fuse to target compartments hence facilitating the exchange of materials. Specialized proteins called SNARE proteins play a crucial role in pulling membranes together. How eukaryotes achieved this level of complexity for intracellular traffic remains a mystery.

The endosymbiosis theory proposed by Lynn Margulis in the 1960s suggests that a larger prokaryotic cell engulfed a small one which eventually became the mitochondria. This proposal prompted years of research in cracking the code of the missing link between the two cell types. But the science of reconstruction of eukaryotic cell evolution remains a daunting task as no living intermediate has yet been found. The recent discovery of the Asgard archaea showed that some genomic components resembled eukaryotic ones suggesting them to be the closest relative to eukaryotes.

Researchers from the University of Lausanne have now shown that SNARE-like proteins are found in genomes of the Heimdallarchaeota clade of Asgard archaea. Results show them to be a prototypic version of the eukaryotic SNAREs capable of complex formation. Since no archaeal member of Heimdallarchaeota has been successfully cultured for in vivo study, whether these proteins are involved in vesicular fusion akin to eukaryotes or some other function involving membranes is yet to be determined. The authors in the paper concur,

It is apparent that prototypic trafficking factors were present in the archaeal genomes before they hosted mitochondria, and it is conceivable that the entry of an α-proteobacterium sparked the evolution of a true eukaryotic membrane trafficking system , which was then refined by duplications of a primordial set of vesicle formation and fusion proteins.

Moreover, the absence of these SNARE-like genes in any other Asgard clade warrants further studies regarding the presence of a minimalist endomembrane system that evolved in archaea. But we are one step closer to bridge the gap between these ancient organisms and eukaryotes.

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