Abstract
Disruption of protein homeostasis in chloroplasts impairs the correct functioning of essential metabolic pathways, including the methylerythritol 4-phosphate (MEP) pathway for the production of plastidial isoprenoids involved in photosynthesis and growth. We previously found that misfolded and aggregated forms of the first enzyme of the MEP pathway are degraded by the Clp protease with the involvement of Hsp70 and Hsp100/ClpC1 chaperones in Arabidopsis thaliana. By contrast, the combined unfolding and disaggregating actions of Hsp70 and Hsp100/ClpB3 chaperones allow solubilization and hence reactivation of the enzyme. The repair pathway is promoted when the levels of ClpB3 proteins increase upon reduction of Clp protease activity in mutants or wild-type plants treated with the chloroplast protein synthesis inhibitor lincomycin (LIN). Here we show that LIN treatment rapidly increases the levels of aggregated proteins in the chloroplast, unleashing a specific retrograde signaling pathway that up-regulates expression of ClpB3 and other nuclear genes encoding plastidial chaperones. As a consequence, folding capacity is increased to restore protein homeostasis. This sort of chloroplast unfolded protein response (cpUPR) mechanism appears to be mediated by the heat shock transcription factor HsfA2. Expression of HsfA2 and cpUPR-related target genes is independent of GUN1, a central integrator of retrograde signaling pathways. However, double mutants defective in both GUN1 and plastome gene expression (or Clp protease activity) are seedling lethal, confirming that the GUN1 protein is essential for protein homeostasis in chloroplasts.
Highlights
Endosymbiotic organelles such as mitochondria and chloroplasts play fundamental roles in eukaryotic organisms
Chloroplasts are central metabolic factories for plant cells. They are constantly challenged by stress episodes that alter protein homeostasis and disrupt normal chloroplast functions
In this paper we report that saturated or defective Clp protease activity triggers a chloroplast unfolded protein response that results in the up-regulation of nuclear genes encoding chloroplast chaperones
Summary
Endosymbiotic organelles such as mitochondria and chloroplasts play fundamental roles in eukaryotic organisms They both contain their own genome but most of their proteins are encoded by the nuclear genome. When misfolded proteins accumulate and aggregate in mitochondria, an adaptive transcriptional response known as unfolded protein response (UPR) is activated to communicate with the nucleus and induce the expression of nuclear genes encoding mitochondria-targeted chaperones and proteases [5,6,7]. The Clp protease is a key component of the UPR mechanism in mitochondria [6,18] While these observations suggest that a UPR conceptually similar to that observed in mitochondria might operate in chloroplasts, the physiological signal(s) triggering this putative cpUPR and the specific consequences for chloroplast function remain unexplored
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