Abstract
The lifespan and activity of proteins depend on protein quality control systems formed by chaperones and proteases that ensure correct protein folding and prevent the formation of toxic aggregates. We previously found that the Arabidopsis thaliana J-protein J20 delivers inactive (misfolded) forms of the plastidial enzyme deoxyxylulose 5-phosphate synthase (DXS) to the Hsp70 chaperone for either proper folding or degradation. Here we show that the fate of Hsp70-bound DXS depends on pathways involving specific Hsp100 chaperones. Analysis of individual mutants for the four Hsp100 chaperones present in Arabidopsis chloroplasts showed increased levels of DXS proteins (but not transcripts) only in those defective in ClpC1 or ClpB3. However, the accumulated enzyme was active in the clpc1 mutant but inactive in clpb3 plants. Genetic evidence indicated that ClpC chaperones might be required for the unfolding of J20-delivered DXS protein coupled to degradation by the Clp protease. By contrast, biochemical and genetic approaches confirmed that Hsp70 and ClpB3 chaperones interact to collaborate in the refolding and activation of DXS. We conclude that specific J-proteins and Hsp100 chaperones act together with Hsp70 to recognize and deliver DXS to either reactivation (via ClpB3) or removal (via ClpC1) depending on the physiological status of the plastid.
Highlights
Organelles like mitochondria and plastids play fundamental roles in all eukaryotic organisms
In this paper we report a relatively simple mechanism by which plant chloroplasts deal with inactive forms of deoxyxylulose 5-phosphate synthase (DXS), the main rate-determining enzyme for the production of plastidial isoprenoids relevant for photosynthesis and development
We provide evidence supporting that particular members of the Hsp100 chaperone family contribute to either refold or degrade inactive DXS proteins recognized by the J-protein adaptor J20 and delivered to Hsp70 chaperones
Summary
Organelles like mitochondria and plastids play fundamental roles in all eukaryotic organisms. Plastids (like mitochondria) are intimately integrated into the metabolism of plant cells but they still remain as separate functional entities that regulate their own biochemistry by relatively independent mechanisms. An important part of this regulation relies on the effective control of plastidial enzyme activities. Specific proteases cleave the transit peptides and complex networks of plastidial chaperones ensure proper folding, assembly, or suborganellar targeting of the mature proteins. While plant plastids contain many groups of prokaryotic-like chaperones (such as Hsp and Hsp100) and proteases (including Clp, Lon, Deg, and FstH), their specific targets and PQCrelated roles remain little studied [1,2,3,4]
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