UC San Diego

Upon discovery of the first archaeal species in the 1970s, life has been subdivided into three domains: Eukarya, Archaea, and Bacteria. However, the organization of the three-domain tree of life has been challenged following the discovery of archaeal lineages such as the TACK Superphylum and, more recently, the Asgard Superphylum. The Asgard superphylum has emerged as the closest archaeal ancestor to eukaryotes and may allow us to improve our understanding of the evolution of life from relatively simple prokaryotes to complex eukaryotes. In this study, we characterize the transportomes and their substrates within four metagenomes of Asgard archaea (i.e., Loki-, Odin-, Thor-, and Heimdall-archaeota). Using the Transporter Classification Database (TCDB) as reference, candidate transporters were identified based on sequence similarity, alignment coverage, overlap of hydropathy profiles, TMS topologies and shared Pfam domain content. Identified transport systems are compared within the Asgard superphylum, and to other eukaryotic, archaeal and bacterial transport systems. We found that Asgard organisms rely mostly on secondary carriers for the transport of cofactors and vitamins. They seem to contain a diverse range of systems required for establishing the membrane potential and proton motive force for subsequent ATP production. To varying degrees, the results indicate that Asgard organisms depend mostly on the intake of organic molecules such as lipids, amino acids, and proteins. The transporters identified clearly resemble prokaryotic transporter more than eukaryotic specific transporters. Taken together, the results confirm the mixotrophic and diverse metabolic capabilities of the Asgard superphylum.