Internal ATP Is The Only Energy Requirement for the Translocation of Precursor Proteins Across Chloroplastic Membranes

The energy requirements for the import of nuclear-encoded proteins into isolated chloroplasts have been reinvestigated. We have shown that, in contrast to protein import into mitochondria, the translocation of the precursors to ferredoxin, plastocyanin (prPC) and the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (prSS) across all chloroplastic membranes is independent of a protonmotive force and requires only ATP. This extends previous works in which investigations were limited to prSS and demonstrates that our results are probably general to all chloroplastic protein precursors. Our results are particularly interesting for the import of prPC, since in addition to the two envelope membranes, this protein must traverse the energy-transducing thylakoid membranes en route to its proper location in the thylakoid lumen. This lack of involvement of a protonmotive force, specifically of a transmembrane electric potential, demonstrates that separate mechanisms operate during the import of proteins into chloroplasts and mitochondria. We also examined the question of whether ATP is utilized inside or outside of chloroplasts during protein import. Previous attempts to resolve this question have resulted in conflicting answers. We found, by two independent approaches, that ATP for protein import is utilized inside chloroplasts. The implications of these results on the possible mechanisms of protein import into chloroplasts are discussed.