AbstractWe experimentally challenged the endosymbiotic hypothesis that organelle-targeting peptides derive from antimicrobial amphipathic peptides delivered by the host cell, to which organelle progenitors became resistant. To explore the molecular changes required to convert such antimicrobial peptides into bona fide organelle-targeting peptides, we expressed a set of 13 antimicrobial peptides of various origins in the green alga Chlamydomonas reinhardtii that serves as a model for both mitochondrial and chloroplast import. The peptides were modified to match distinctive features of mitochondrial and chloroplast targeting peptides, and we assessed their targeting potential by following the intracellular localization and maturation of a Venus fluorescent reporter used as cargo protein. We present a temporal evolutionary scenario that emphasizes the early contribution of exchanging Lysines with Arginines in the sequence of the antimicrobial peptide, the evolution of a processing site followed by the addition of unstructured sequence and protein interaction sites that allow the selective targeting to the chloroplast.
Retracing the evolution from antimicrobial to targeting peptides
