Travelling of proteins through membranes: translocation into chloroplasts.

Most proteins involved in plastid biogenesis are encoded by the nuclear genome. They are synthesised in the cytosol and have to be transported toward and subsequently translocated into the organelle. This targeting and import process is initiated by a specific chloroplast-targeting signal. The targeting signal of the preprotein is recognised and modified by cytosolic proteins which function in transport toward the chloroplast and in maintaining the import-competent state of the preprotein. The precursor is transferred onto a multi-component complex in the outer envelope of the chloroplasts, which is formed by receptor proteins and the translocation channel. Some proteins, not containing transit sequences, are directly sorted into the outer membrane whereas the majority, containing transit sequences, will be translocated into the stroma. This involves the joint action of a protein complex in the outer envelope, one complex in the inner envelope, and soluble proteins in the intermembrane space and the stroma. The origin of this translocation complex following the endosymbiotic events is an unsolved question. Recent identification of homologous proteins to some members of this machinery in the cyanobacterium Synechocystis PCC6803 gives an initial insight into the origin of the translocation complex.