Mechanism of protein import across the chloroplast envelope

The development and maintenance of chloroplasts relies on the contribution of protein subunits from both plastid and nuclear genomes. Most chloroplast proteins are encoded by nuclear genes and are post-translationally importe d into the organelle across the double membrane of the chloroprast envelope . Protein import into the chloroprast consists of two essential elements: t he specific recognition of the targeting signals (transit sequences) of cyt oplasmic preproteins by receptors at the outer envelope membrane and the su bsequent translocation of preproteins simultaneously across the double memb rane of the envelope. These processes are mediated via the co-ordinate acti on of protein translocon complexes in the outer (Toc apparatus) and inner ( Tic apparatus) envelope membranes.