An archaebacterium that expresses actin protein — a trait thought to be eukaryotie-specific

From time to time, these GEITP pages consider topics on Evolution. In biology, there are six Kingdoms of Life: Archaebacteria, Eubacteria, Protists, Fungi, Plants and Animals. The first two are prokaryotes (microscopic single-celled organisms having unpaired chromosomes and neither a distinct nucleus with membrane nor other specialized organelles), while the latter four are eukaryotes (organisms consisting of a cell or cells, having paired chromosomes contained within a distinct nucleus with membrane, plus many different organelles). Eukaryotes are thought to have evolved from a merger between an archaebacterial host cell, and a bacterium — from which eukaryotic organelles (called mitochondria) emerged.

Some insights into the biological properties of the host have been learned from the closest known archaeal rela­tives of eukaryotes, the Asgard superphylum. Genomes of organisms in the Asgard superfamily encode a suite of proteins typi­cally involved in functions, or processes, thought to be eukaryote-specific. The functions of these ‘eukaryotic genes’ in Asgard archaea have been elusive. However, authors [see attached article] provide evidence that some Asgard archaea encode proteins that are structurally and functionally similar to their eukaryotic counterparts.

In addition to their nucleus and energy-producing mitochondria, eukaryotic cells are characterized by a complex internal system of membrane-bound compart­ments (the endomembrane system), and by a dynamic network of proteins such as actin, forming cytoskeleton. The cytoskeleton gives the cells their shape and structure, but is also involved in a variety of cellular processes spe­cific to eukaryotes; these features are thought to have been present in the last common ances­tor (LCA) of all eukaryotes, which lived ~1.8 bil­lion years ago. Nevertheless, no life forms have been found that represent an intermediate between eukaryotes and their bacterial and archaeal ancestors. The seemingly sudden emergence of cellular complexity in the eukaryotic line­age has remained a mystery for evolutionary biologists.

Authors [attached article] show that Asgard archaea encode functional profilins (actin-binding proteins involved in the dynamic turnover and restructuring of the actin cytoskeleton) — thereby establishing that this archaeal superphylum has a regulated actin cytoskeleton. Loki profilin-1, Loki profilin-2 and Odin profilin adopt the typical profilin fold and are able to interact with rabbit actin (i.e. an interaction that involves proteins from species that diverged more than 1.2 billion years ago). These data suggest that Asgard archaea possess a primordial polar profilin-regulated actin system, which might be localized to membranes (considering the sensitivity of Asgard profilins to phospholipids). Because Asgard archaea are also predicted to encode potential eukaryotic-like genes involved in membrane-trafficking and endocytosis (uptake of matter by a living cell –– by invagination of its membrane to form a vacuole), imaging will now be necessary to elucidate whether these organisms are capable of generating eukaryotic-like membrane dynamics that are regulated by actin — such as those observed in eukaryotic cell movement, podosome formation (dynamic actin-rich cellular protrusions that degrade the extracellular matrix through local protein degradation; podosomes are involved in the motile function of normal cells, e.g. osteoclasts and certain immune cells), and endocytosis.


Nature 18 Oct 2o18; 562: 439–443 [article] and 352–353 (editorial)

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