Abstract
The evolution of eukaryotic cells necessitated the advent of machinery to transport molecular building blocks for organelles to proliferate. Extant eukaryotes share several classes of highly conserved lipid transfer proteins (LTP) that associate with donor membranes, bind individual lipid molecules, and shuttle them to acceptor membranes. Because cells lacking organelles do not require extensive lipid transport networks, it is not known if this machinery pre-dated eukaryotic organelles or had to evolve alongside them. Here we describe a class of putative lipid transporters in the Asgard archaeota superphylum that share a common ancestry with eukaryotic LTPs in the START domain superfamily. We identified three classes of Asgard START proteins, StarAsg1-3, which are conserved across most Asgard phyla. Of these, StarAsg1 family proteins contain the predicted structural features necessary for lipid transfer: large, hydrophobic binding pockets lined with amphipathic motifs for membrane docking. In contrast, StarAsg2 and StarAsg3 family proteins contain smaller binding cavities and minimal predicted membrane interactions. We found that StarAsg1 from Lokiarcheota interacts with anionic membranes both in vitro and in yeast cells and can transfer sterols between liposomes. Phylogenetic analysis of START domains across the tree of life indicates that eukaryotic LTPs share a common ancestry with StarAsg1 homologs, while StarAsg2 and StarAsg3 form a monophyletic group with eukaryotic heat shock protein cochaperones. We propose that the emergence of proteins for inter-membrane lipid transporters in the ancestors of eukaryotic cells could have facilitated the subsequent development of intracellular organelles.