Protein stability and degradation in plastids

Abstract

Steady-state levels of chloroplast proteins rely on the balance between synthesis and degradation rates. Thus, the importance of protein-degradation processes in shaping the chloroplast proteome, and hence proper organellar functioning, cannot be overestimated. Chloroplast proteases and peptidases participate in chloroplast biogenesis through maturation or activation of pre-proteins, adaptation to changing environmental conditions through degradation of certain proteins, and maintenance of protein quality through degradation of unassembled or damaged proteins. These activities are mediated by ATP-dependent and -independent proteases, many of which are encoded by multigene families. Newly imported proteins are processed by stroma-and thylakoid-localized peptidases that remove signal sequences, which are then further degraded. The multisubunit ATP-dependent Clp and FtsH complexes degrade housekeeping and oxidatively damaged proteins in the stroma and thylakoid membranes, respectively. A number of other chloroplast proteases have been identified, but their function and substrates are still unknown, as are the nature of degradation signals and determinants of protein instability. Future research is expected to focus on these questions.

Genome-Wide Analysis of Plastid Gene Expression in Potato Leaf Chloroplasts and Tuber Amyloplasts: Transcriptional and Posttranscriptional Control

“…This indicates that, in amyloplasts, the association of mRNAs with polysomes may represent an additional regulatory step in gene expression. It is important to note that, in addition to polysome loading, other translational and posttranslational regulatory steps can also influence protein accumulation (Adam, 2007;Peled-Zehavi and Danon, 2007) and adjust subunit stoichiometries in multiprotein complexes in plastids.…”

Plastid production of protein antibiotics against pneumonia via a new strategy for high-level expression of antimicrobial proteins

“…The enormous lysin expression levels obtained here and in a previous study (3) are caused by high protein stability inside the chloroplast, which is consistent with the idea that phage lysins have evolved considerable resistance to the proteases of their host bacteria. Because plastids possess a prokaryotic-type proteolytic machinery (22), it seems conceivable that lysins are poor substrates also for chloroplast proteases. Thus, chloroplasts seem to provide an ideal expression platform for lysin-type protein antibiotics.…”

Expression of Plastid Genes: Organelle-Specific Elaborations on a Prokaryotic Scaffold

“…In fact, the stoichiometric accumulation of subunits within each photosynthetic enzyme complex is mediated, in part, by proteolysis of unassembled subunits (Adam, 2007). The relative contributions of coordinated protein synthesis versus posttranslational proteolysis has been most thoroughly explored in Chlamydomonas, where a set of mechanisms known as control by epistasy of synthesis (CES) coordinate the synthesis of subunits of each photosynthetic enzyme complex via negative feedback loops that are triggered by specific unassembled subunits (Choquet and Wollman, 2009).…”