Mitochondrial Preprotein Translocase of Trypanosomatids Has a Bacterial Origin

SummaryMitochondria are found in all eukaryotic cells and derive from a bacterial endosymbiont [1 and 2]. The evolution of a protein import system was a prerequisite for the conversion of the endosymbiont into a true organelle. Tom40, the essential component of the protein translocase of the outer membrane, is conserved in mitochondria of almost all eukaryotes but lacks bacterial orthologs [3, 4, 5 and 6]. It serves as the gateway through which all mitochondrial proteins are imported. The parasitic protozoa Trypanosoma brucei and its relatives do not have a Tom40-like protein, which raises the question of how proteins are imported by their mitochondria [ 7 and 8]. Using a combination of bioinformatics and in vivo and in vitro studies, we have discovered that T. brucei likely employs a different import channel, termed ATOM (archaic translocase of the outer mitochondrial membrane). ATOM mediates the import of nuclear-encoded proteins into mitochondria and is essential for viability of trypanosomes. It is not related to Tom40 but is instead an ortholog of a subgroup of the Omp85 protein superfamily that is involved in membrane translocation and insertion of bacterial outer membrane proteins [ 9]. This suggests that the protein import channel in trypanosomes is a relic of an archaic protein transport system that was operational in the ancestor of all eukaryotes.

Graphical AbstractFigure optionsDownload high-quality image (312 K)Download as PowerPoint slideHighlights
► ATOM, the outer membrane translocase of trypanosomatids, is of bacterial type
► ATOM provides a missing link for the evolution of mitochondrial protein import
► ATOM suggests that trypanosomatids are early-diverging eukaryotes