Chloroplasts are the defining plant organelles with responsibility for photosynthesis and other vital functions. To deliver these functions, they possess a complex
proteome comprising thousands of largely nucleus-encoded
proteins. Composition of the
proteome is controlled by diverse processes affecting protein translocation and degradation-our focus here. Most
chloroplast proteins are imported from the cytosol via multiprotein translocons in the outer and inner envelope membranes (the TOC and
TIC complexes, respectively), or via one of several noncanonical pathways, and then sorted by different systems to organellar subcompartments. Chloroplast proteolysis is equally complex, involving the concerted action of internal
proteases of prokaryotic origin and the nucleocytosolic
ubiquitin-
proteasome system (UPS). The UPS degrades unimported
proteins in the cytosol and chloroplast-resident
proteins via chloroplast-associated protein degradation (CHLORAD). The latter targets the TOC apparatus to regulate
protein import, as well as numerous internal
proteins directly, to reconfigure chloroplast functions in response to developmental and environmental signals.