The Asgard Archaeal-Unique Contribution to Protein Families of the Eukaryotic Common Ancestor Was 0.3

Deviating from the norm: Nuclear organisation in trypanosomes

At first glance the nucleus is a highly conserved organelle. Overall nuclear morphology, the octagonal nuclear pore complex, the presence of peripheral heterochromatin and the nuclear envelope appear near constant features right down to the ultrastructural level. New work is revealing significant compositional divergence within these nuclear structures and their associated functions, likely reflecting adaptations and distinct mechanisms between eukaryotic lineages and especially the trypanosomatids. While many examples of mechanistic divergence currently lack obvious functional interpretations, these studies underscore the malleability of nuclear architecture. I will discuss some recent findings highlighting these facets within trypanosomes, together with the underlying evolutionary framework and make a call for the exploration of nuclear function in non-canonical experimental organisms.

Prokaryotic membrane coat - like proteins: An update

Membrane coat proteins are essential players in the eukaryotic endomembrane traffic system. Previous work identified proteins with the membrane-coat architecture in prokaryotes, specifically in the Planctomycetes, Verrucomicrobia and Chlamydiae (PVC) superphylum, bacteria that display the most developed prokaryotic endomembrane system. Hence, the membrane coat-like (MCL) proteins are predicted to play a central role in this system but their actual function is still unknown. In this work we strengthened previous structure predictions for these prokaryotic MCL proteins. We also detected new putative MCL proteins in the Planctomycete Gemmata obscuriglobus. Structural analysis of these revealed the presence of additional domains apart from the β-propeller and α-solenoid combination, characteristic of the membrane-coat architecture. Functions associated with these domains include some related to carbohydrate or membrane/lipid binding. Using homology-based methods, we found MCL proteins in other bacterial phyla, but the most abundant hits are still restricted to Planctomycetes and Verrucomicrobia. Detailed inspection of neighbouring genes of MCL in G. obscuriglobus supports the idea that the function of these proteins is related to membrane manipulation. No significant hits were found in Archaea, including Asgard archaea. More than 10 years after their original detection, PVC bacteria are still uniquely linked to eukaryotes through the structure of the MCL proteins sustaining their endomembrane system.

Endosymbiotic selective pressure at the origin of eukaryotic cell biology

The dichotomy that separates prokaryotic from eukaryotic cells runs deep. The transition from pro- to eukaryote evolution is poorly understood due to a lack of reliable intermediate forms and definitions regarding the nature of the first host that could no longer be considered a prokaryote, the first eukaryotic common ancestor, FECA. The last eukaryotic common ancestor, LECA, was a complex cell that united all traits characterising eukaryotic biology including a mitochondrion. The role of the endosymbiotic organelle in this radical transition towards complex life forms is, however, sometimes questioned. In particular the discovery of the asgard archaea has stimulated discussions regarding the pre-endosymbiotic complexity of FECA. Here we review differences and similarities among models that view eukaryotic traits as isolated coincidental events in asgard archaeal evolution or, on the contrary, as a result of and in response to endosymbiosis. Inspecting eukaryotic traits from the perspective of the endosymbiont uncovers that eukaryotic cell biology can be explained as having evolved as a solution to housing a semi-autonomous organelle and why the addition of another endosymbiont, the plastid, added no extra compartments. Mitochondria provided the selective pressures for the origin (and continued maintenance) of eukaryotic cell complexity. Moreover, they also provided the energetic benefit throughout eukaryogenesis for evolving thousands of gene families unique to eukaryotes. Hence, a synthesis of the current data lets us conclude that traits such as the Golgi apparatus, the nucleus, autophagosomes, and meiosis and sex evolved as a response to the selective pressures an endosymbiont imposes.

The expanding Asgard archaea and their elusive relationships with Eukarya

The discovery of Asgard archaea and the exploration of their diversity over the last 6 years have deeply impacted the scientific community working on eukaryogenesis, rejuvenating an intense debate on the topology of the universal tree of life (uTol). Here, we discuss how this debate is impacted by two recent publications that expand the number of Asgard lineages and eukaryotic signature proteins (ESPs). We discuss some of the main difficulties that can impair the phylogenetic reconstructions of the uTol and suggest that the debate about its topology is not settled. We notably hypothesize the existence of horizontal gene transfers between ancestral Asgards and proto-eukaryotes that could result in the observed abnormal behaviors of some Asgard ESPs and universal marker proteins. This hypothesis is relevant regardless of the scenario considered regarding eukaryogenesis. It implies that the Asgards were already diversified before the last eukaryotic common ancestor and shared the same biotopes with proto-eukaryotes. We suggest that some Asgards might be still living in symbiosis today with modern Eukarya.