An antimicrobial origin of targeting peptides to endosymbiotic organelles

Mitochondria and chloroplasts emerged from primary endosymbiosis. Most endosymbiont proteins have subsequently been expressed in the nucleo-cytosol of the host and organelle-targeted via the acquisition of N-terminal presequences, whose evolutionary origin remains enigmatic. Using a quantitative assessment of their physico-chemical properties, we show that organelle targeting peptides, which are distinct from signaling peptides targeting other subcellular compartments, group with a subset of antimicrobial peptides. We demonstrate that extant antimicrobial peptides target a fluorescent reporter to either the mitochondria or the chloroplast in the green alga Chlamydomonas reinhardtii and, conversely, that extant targeting peptides display antimicrobial activity. Thus, we provide conclusive computational and functional evidence for an evolutionary link between organelle-targeting and antimicrobial peptides. Our results support the view that resistance of bacterial progenitors of organelles to the attack of host antimicrobial peptides, has been instrumental in eukaryogenesis and in the subsequent emergence of photosynthetic eukaryotes.