Multiple Redox and Non-Redox Interactions Define 2-Cys Peroxiredoxin as a Regulatory Hub in the Chloroplast

In plants, the highly abundant 2-cysteine peroxiredoxin (2-CysPrx) is associated with the chloroplast and involved in protecting photosynthesis. This work addresses the multiple interactions of the 2-CysPrx in the chloroplast, which depend on its redox state. Transcript co-regulation analysis showed a strong linkage to the peptidyl-prolyl-cis/trans isomerase Cyclophilin 20-3 (Cyp20-3) and other components of the photosynthetic apparatus. Co-expression in protoplasts and quantification of fluorescence resonance energy transfer (FRET) efficiency in vivo confirmed protein interactions of 2-CysPrx with Cyp20-3 as well as NADPH-dependent thioredoxin reductase C (NTRC), while thioredoxin x (Trx-x) did not form complexes that could enable FRET. Likewise, changes in FRET of fluorescently labeled 2-CysPrx in vitro and in vivo proved redox dependent dynamics of 2-CysPrx. Addition of Cyp20-3 to an in vitro peroxidase assay with 2-CysPrx had no significant effect on peroxide reduction. Also, in the presence of NTRC, addition of Cyp20-3 did not further enhance peroxide reduction. In addition, 2-CysPrx functioned as chaperone and inhibited aggregation of citrate synthase during heat treatment. This activity was partly inhibited by Cyp20-3. As a new interaction partner of decameric 2-CysPrx, photosystem II could be identified after chloroplast fractionation and in pull-down assays after reconstitution. In summary, the data indicate a dynamic function of plant 2-CysPrx as redox sensor, chaperone, and regulator in the chloroplast with diverse functions beyond its role as thiol peroxidase.