Protein import into chloroplasts.

Chloroplasts have evolved an elaborate system of membrane and soluble subcompartments to organize and regulate photosynthesis and essential aspects of amino acid and lipid metabolism. The biogenesis and maintenance of organellar architecture rely on protein subunits encoded by both nuclear and plastid genomes. Import of nuclear-encoded proteins is mediated by interactions between the intrinsic N-terminal transit sequence of the nuclear-encoded preprotein and a common import machinery at the chloroplast envelope. Recent investigations have shown that there are two unique membrane-bound translocation systems, in the outer and inner envelope membranes, which physically associate during import to transport preproteins from the cytoplasm to the internal stromal compartment. This review discusses current understanding of these translocation systems and models for the way in which they might function.