Heimdallarchaea encodes profilin with eukaryotic-like actin regulation and polyproline binding
The evolutionary events which led to the first eukaryotic cell are still controversial1-4. The Asgard genome encodes a variety of eukaryotic signature proteins previously unseen in prokaryotes. Functional and structural characterization of these proteins is beginning to shed light on the complexity and pedigree of the ancestral eukaryotic cell5,6. In eukaryotes, the key cytoskeletal protein actin is important for diverse cellular processes such as membrane remodeling and cell motility7. Dynamic polymerization of actin provides both structure and generates the force which drives motility and membrane remodeling. These processes demand rapid filament assembly and disassembly on microsecond timescales. In eukaryotes, a variety of highly adapted proteins including gelsolin, profilin, VASP, ARP2/3 and signaling molecules (Phosphatidylinositol-4,5-bisphosphate (PIP2)) are crucial for organizing cellular cytoskeleton dynamics. Amongst others, the Asgard genomes encode predicted putative profilin homologues that regulate eukaryotic actin polymerization in vitro5,8. Interestingly, Asgard profilins appear to be regulated by PIP2, but not by polyproline rich motifs which are important for recruitment of actin:profilin complexes in eukaryotes5,9. These findings indicate that the Asgard archaea may have possessed analogous membrane organization to present-day eukaryotes, but that polyproline-mediated profilin regulation may have emerge later in the eukaryotic lineage5. Here, we show that Heimdallarchaeota, a candidate phylum within the Asgard superphylum, encodes a putative profilin (heimProfilin) that interacts with PIP2 and is regulated by polyproline motifs, implicative of an origin predating the rise of the eukaryotes. Additionally, we provide evidence for a novel regulatory mechanism whereby an extended N-terminal loop abolishes PIP2 and polyproline interactions. Lastly, we provide the first evidence for actin polymerization of an Asgard actin homologue. In context, these findings provide further evidence for the existence of a complex cytoskeleton already in Last eukaryotic common ancestor (LECA).