Eukaryotic cells contain semi-autonomous organelles that have lost most of their coding capacity for essential proteins to the nucleus.Therefore, their functioning depends on the cytosolic translation products of nuclear-encoded genes that must be imported by a specialized protein import machinery.Chloroplasts import between 2500 and 3500 proteins for their functioning,demonstrating that the bulk of the chloroplast proteome is shuttied through the cytosol as precursor proteins.Precursor proteins usually contain an N-terminal transit peptide that is cleaved off by a signa/processing peptidase (SPP) and degraded by the chloroplast protease network upon import.In the cytosol, precursors can be co-and post-translationally modified, interact with a set of chaperones, co-chaperones, 14-3-3 proteins, and/or E3 ligases before they are either imported or degraded by the cytosolic proteasome complex.It is now clear that precursors are not only a transient transport form of functional proteins but that their cytosolic transition is of regulatory importance.