Mechanisms of Chloroplast Protein Import in Plants

Abstract

Most chloroplast proteins are encoded as preproteins by the nuclear genome. Their import into chloroplasts occurs post-translationally. An N-terminal pre-sequence, the transit peptide, contains the organellar targeting information. It is specifically recognized by receptor components at the chloroplast surface. These receptors are components of the TOC (translocon at the outer envelope membrane of chloroplasts) complex. Together with the TIC (translocon at the inner envelope membrane of chloroplasts) machinery, this mediates the import of proteins into chloroplasts. In addition to the receptors, these complexes incorporate channel, motor and regulatory functions. Many putative or actual components have been identified. Multiple isoforms of the TOC receptors (and possibly of some other components) constitute the molecular basis of separate import pathways with distinct client preferences. This perhaps reduces competition effects between highly abundant and less abundant preproteins. Client preferences of different import pathways might also facilitate the differentiation of various plastid types. In addition to the canonical TOC/TIC-mediated import routes, alternative, mechanistically distinct pathways of protein transport to chloroplasts have been identified; one of these passes through the endoplasmic reticulum and Golgi apparatus. Other work has revealed several protein targeting pathways leading to the envelope membranes.