Protein import into chloroplasts

Chloroplasts have evolved an elaborate system of membrane and soluble subco mpartments to organize and regulate photosynthesis and essential aspects of amino acid and lipid metabolism. The biogenesis and maintenance of organel lar architecture rely on protein subunits encoded by both nuclear and plast id genomes. Import of nuclear-encoded proteins is mediated by interactions between the intrinsic N-terminal transit sequence of the nuclear-encoded pr eprotein and a common import machinery at the chloroplast envelope. Recent investigations have shown that there are two unique membrane-bound transloc ation 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 o f these translocation systems and models for the way in which they might fu nction.