The Ins and Outs of Chloroplast Protein Transport

Abstract

Much of the chloroplast proteome is encoded in the nuclear genome and needs to be imported post-translationally. Information for the organellar targeting of these imported proteins lies in an N-terminal leader sequence, the transit peptide, which is specifically bound by receptor components at the chloroplast surface. These receptor components are part of the TOC (translocon at the outer envelope membrane of chloroplasts) complex, which, together with the TIC (translocon at the inner envelope membrane of chloroplasts) machinery, mediates the translocation of precursor proteins into chloroplasts. Apart from the receptors, these complexes incorporate channel, motor and regulatory functions. Many components of this TOC/TIC apparatus have been identified. Multiple isoforms of the TOC receptors (and possibly of some other components) enable the operation of different import pathways with different substrate preferences, perhaps so that non-abundant proteins can be imported without serious competition from highly-abundant proteins of the photosynthetic apparatus. The different import pathways might also play a role in the differentiation of different plastid types. While much research has focused on these canonical TOC/TIC-mediated import routes, a number of studies have revealed alternative protein transport pathways to chloroplasts that employ different mechanisms; one of these passes through the endoplasmic reticulum and the Golgi apparatus. Other recent studies have revealed several protein targeting pathways leading to the envelope itself.