ANCESTRAL MITOCHONDRIAL PROTEIN SECRETION MACHINERY

Modern mitochondria have preserved few traits of the original bacterial endosymbiont. Unexpectedly, we find that certain eukaryotes possess homologues of the type 2 secretion system (T2SS) so far restricted to eubacteria. We show that these components are localized to the mitochondrion, and their behaviour in functional assays is consistent with the formation of a mitochondrial T2SS-derived protein secretion system. We additionally identified 23 protein families exactly co-occurring in eukaryotes with the T2SS. Seven of these proteins could be directly linked to the core T2SS by functional data and/or sequence features, whereas others may represent different parts of a broader functional pathway, possibly linking the mitochondrion with the peroxisome. Its distribution in eukaryotes and phylogenetic evidence indicate that the whole mitochondrial T2SS-centred pathway is an ancestral eukaryotic trait. Our findings thus have direct implications for the functional properties of the early mitochondrion.