Archaeal lineages related to eukaryotes encode functional diterpenoid cyclases

The first eukaryotic cell originated through the union of an archaeon (Asgardarchaeota) and a bacterium (Alphaproteobacteria)1-4. Little is known about the molecular basis of eukaryogenesis, but it is likely that lipids played a key role in this process. Modern eukaryotic membranes contain polycyclic triterpenoids (mainly sterols) that are essential for a variety of cellular functions, but these lipids have not been identified in archaea. The lipid composition of Asgardarchaeota including the newly described class Hodarchaeales, which share a common ancestor with eukaryotes, is unknown. Here, we investigated the potential for Asgards to produce cyclic terpenoids. Phylogenomics coupled with structural prediction revealed that Asgards lack the capacity to make sterols, yet organisms from the clades Hodarchaeales and Kariarcheales encode for divergent homologs of sterol cyclases, predicted to produce diterpenoids. We tested the functionality of these enzymes in vitro, showing that they cyclize geranylgeranyl pyrophosphate to form bicyclic halimadienyl pyrophosphate. Halimadienyl lipids have previously been shown to mediate intracellular persistence of Mycobacterium tuberculosis in host endosomes5-7, and may function similarly Asgardarchaeota. This is the first evidence of experimentally validated diterpenoid cyclases in archaea, providing new insights into the biochemistry of these microbes pivotal in the evolution of complex cellular life.