Characterization of the import process of a transit peptide into chloroplasts

In order to get insight into the functioning of transit sequences in chloroplast protein transport, the import of the full-length transit peptide of ferredoxin (trfd) was investigated. trfd rapidly associated with chloroplasts under import conditions and becomes protected against externally added proteases. Import of radiolabeled trfd is inhibited equally efficiently by nonlabeled trfd as well as by the intact precursor of ferredoxin. This strongly suggests that trfd enters the general import pathway of proteins into chloroplasts. trfd import was stimulated by ATP, which is the first demonstration that ATP is involved in membrane translocation of a targeting signal. Imported trfd was membrane-associated but was also partially degraded by internal proteases, most likely present in the stroma, indicating that the membrane-associated fraction of trfd is en route to its functional localization. The degradation products are exported out of the organelle. In contrast to the import of the precursor of ferredoxin, the import of trfd was independent of protease-sensitive components on the chloroplast surface, indicating that the initial binding of precursor proteins may be facilitated by transit sequence-lipid interactions.