An Asgard archaeon with internal membrane compartments

The emergence of eukaryotes from a merger between an archaeon and a bacterial cell ∼two billion years ago involved a profound change in cellular organisation. While the order in which different features of the eukaryotic cell arose remains a matter of controversy ^1-3 , close archaeal relatives of eukaryotes have recently been identified that possess homologues of eukaryotic trafficking machinery ^4,5 and a complex cell architecture ^6,7 . The members of this phylum, the Asgard archaea ( syn . Prometheoarchaeota) described so far, however, lack internal membrane-bound compartments, and therefore have shed little light on origins of the hallmark eukaryotic endomembrane system. Here we report the cell biological analysis of a member of the Heimdallarchaeia, Candidatus ‘ Yibarchaeum umbracryptum’ in enriched mixed microbial communities. Possessing a small genome encoding few homologues of eukaryotic membrane remodelling machinery ⁸ , Ca . Y. umbracryptum cells in late-stage cultures resemble previously described Asgard archaea with extensive cellular protrusions. Surprisingly, during early stages of culture growth Ca . Y. umbracryptum cells have fewer protrusions but possess numerous intracellular vesicles, most of which have a luminal surface that morphologically resembles the outer coat of the plasma membrane. These data alter our view of eukaryogenesis by identifying a close archaeal relative of eukaryotes with a regulated endomembrane system.