Ancestral Mitochondrial Protein Secretion Machinery

Modern mitochondria have preserved few traits of the original bacterial endosymbiont. Unexpectedly, we find that certain representatives of heteroloboseans, jakobids and malawimonads possess homologues of four core components 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.