Translocation of Proteins into Primary Plastids

Most chloroplast proteins are encoded in the nucleus and must be imported into plastids. They are synthesized as preproteins in the cytoplasm and have N-terminal transit peptides that enable their translocation across the dual plastid membrane. This requires the translocons at the chloroplast’s outer (TOC) and inner membranes (TIC). The transit peptide first engages the TOC import receptors, then inserts across the outer membrane protein translocating channel followed by full translocation across the inner membrane with the help of ATP-driven motors. Once the preprotein reaches the chloroplast stroma, stromal processing peptidase cleaves the transit peptide, allowing protein maturation to progress. In Arabidopsis, a family of TOC receptors exists. In varying compositions, they may mediate the import of specific proteome subsets into different plastid types. The remodeling of TOC -complexes is mediated by the ubiquitin–proteasome system in a process termed CHLORAD. This chapter will discuss the latest advances in the field: