An Asgard archaeon with internal membrane compartments

Abstract

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 controversy1-3, close archaeal relatives of eukaryotes have recently been identified that possess homologues of eukaryotic trafficking machinery4,5 and a complex cell architecture6,7. 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 Y. umbracryptum in enriched mixed microbial communities. Possessing a small genome encoding few homologues of eukaryotic membrane remodelling machinery8, 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 an endomembrane system, whose appearance changes with cell state.