Structural Characterization of a Thorarchaeota Profilin Indicates Eukaryotic‐Like Features but with an Extended N‐Terminus

Abstract

The emergence of the first eukaryotic cell is preceded by evolutionary events, which are still highly debatable. Clues of the exact sequence of events are beginning to emerge. Recent metagenomics analyses has uncovered the Asgard super‐phylum as the closest yet known archaea host of eukaryotes. Some of these have been tested and confirmed experimentally. However, the bulk of eukaryotic signature proteins predicted to be encoded by the Asgard super‐phylum have not been studied, and their true functions, at least in the context of a eukaryotic cell, are still elusive. For example, there are several different variants of the profilin within each Asgardian Achaea, and there are some conflicting results of their actual roles. Here, the 3D structure of profilin from Thorarchaeota is determined by nuclear magnetic resonance spectroscopy and shows that this profilin has a eukaryotic‐like profilin with a rigid core and an extended N‐terminus previously implicated in polyproline binding. In addition, it is also shown that Thorarchaeota Profilin co‐localizes with eukaryotic actin in cultured HeLa cells. This finding reaffirms the notion that Asgardian encoded proteins possess eukaryotic‐like characteristics and strengthen the likely existence of a complex cytoskeleton already in a last eukaryotic common ancestor.

Actin cytoskeleton and complex cell architecture in an Asgard archaeon

“…While F-actin-like assemblies have been identified in other archaea 22 , 23 , Asgard archaea also possess actin-related proteins (ARPs), as well as actin-binding proteins. Notably, Asgard profilins and gelsolins were found to be able to modulate the dynamics of eukaryotic actin 5 , 19 , 24 – 26 , indicating the existence of an elaborate and dynamic cytoskeleton 4 . However, the in situ structures and functions of archaeal actins remain unclear.…”

Archaeal actins and the origin of a multi-functional cytoskeleton

“…Unfortunately, recombinant lokiactin has proved challenging to express and purify in sufficient quantities to investigate its assembly properties. Akıl and Robinson, however, bypassed this roadblock by employing eukaryotic actin to study the regulatory potential of Asgard profilins and gelsolins ( 53 – 55 ). In vitro , purified Asgard profilins bind monomers of rabbit skeletal muscle actin and alter their assembly kinetics ( 53 ).…”

“…In vitro , purified Asgard profilins bind monomers of rabbit skeletal muscle actin and alter their assembly kinetics ( 53 ). When expressed in human tissue culture cells, a GFP fused to a Thorarchaeal profilin interacts with cytoplasmic actin ( 55 ). Like their eukaryotic cousins, purified Asgard profilins also bind phosphatidylinositol-4,5-bisphosphate (PIP2) lipids.…”

“…Similar to their eukaryotic cousins, these various activities of Thorarchaeal gelsolins are altered—possibly regulated—by calcium. When expressed in human tissue culture cells, Asgard gelsolins fused to GFP co-localize with endogenous actin structures in a calcium-sensitive manner ( 55 ). Recently, Survery et al reported purifying small amounts of a recombinant and polymerization-competent lokiactin from Heimdallarchaeota ( 56 ), paving the way for more in vitro biochemical studies.…”