Organelle import of proteins with dual targeting properties into mitochondria and chloroplasts takes place by the general import pathways.

As a consequence of the endosymbiotic gene transfer, most mitochondrial and chloroplastic proteins are nuclear encoded and synthesized in the cytosol as precursor proteins with transit peptides mediating transport to their subcellular destination. It is often assumed that these transit peptides are strictly monospecific for a single organelle. But in recent years more and more proteins have been identified which carry transit peptides that are capable of mediating transport into both mitochondria and chloroplasts. In a recent study we showed with a combination of in silico, in organello, and in vivo approaches that the frequency of such proteins is apparently much higher than usually anticipated.(1) Here we demonstrate with in organello competition experiments that the import of 2 of these dually targeted proteins (GrpE and EF-Tu) takes place by the same import pathways that are used by organelle proteins with "typical" monospecific targeting properties.