Common ground for protein translocation: access control for mitochondria and chloroplasts

Mitochondria and chloroplasts import the vast majority of their proteins across two membranes, and use translocases of the outer membrane as an entry gate. These translocases interact with the incoming precursor protein and guiding chaperone factors. Within the translocon, precursor-protein receptors dock to a central component that mediates both transfer through a cation-selective channel and initial sorting towards internal subcompartments. Despite these similarities, the mode of translocation differs between the two organelles: in chloroplasts, GTP-binding and hydrolysis by the receptors is required for transport, whereas in mitochondria passage of the preprotein is driven by its increasing affinity for the translocase subunits.