Protein stability and degradation in plastids

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.