Participation of Proteases in the Degradation of Chloroplast Proteins

Degradation of chloroplast proteins within the organelle is supported by the observation that chloroplasts contain several proteases of the ClpP, FtsH, Deg, and Lon families. Clp proteases were among the first identified chloroplasts’ proteases and may play an important role during chloroplast biogenesis. Some members of the ClpP family (i.e., nclpP3 and nclpP5) are up-regulated during senescence, whereas the expression of other Clp proteases is constitutive, with no changes during leaf ontogeny. Interestingly, the mRNA levels of erd1, a Clp regulatory subunit are up-regulated during senescence of Arabidopsis thaliana leaves, but the levels of the corresponding ERD1 protein decline. Homologs of the bacterial FtsH protease are also found in plastids. At least 12 FtsH proteases have been identified in Arabidopsis thaliana, and some of them may play roles in thylakoid protein degradation. An FtsH protease is involved in the breakdown of the 23-kDa fragment of the D1 protein of the PSII reaction centre, which is formed upon photooxidative damage. Chloroplast DegP and FtsH proteases seem to cooperate in D1 degradation during photoinhibition, and it seems likely that they might also be responsible for D1 degradation during senescence. In vitro studies with thylakoids isolated from knock-out lines for FtsH6 have implicated the involvement of this protease in LHCII degradation during senescence. Other FtsH subunits may function in chloroplast biogenesis rather than senescence. In this article, we show which proteases are involved in the degradation of chloroplast proteins. We will focus on both: intrachloroplast and non-chloroplast proteases and their mechanism of the process.