The integration of the cyclophilin 20-3 and 2-cysteine peroxiredoxin thiol switches in the redox regulatory network of the chloroplast

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The chloroplast contains a physically and functionally interacting regulatory protein module consisting of cyclophilin 20-3 (Cyp20-3), 2-cysteine peroxiredoxin (2-CysPrxA/B), serine acetyl transferase (SERAT2.1) and O-acetylserine thiolyase (OASTL-B), the COPS module. This module controls plastid cysteine synthesis, affects redox homeostasis, interferes with nuclear gene expression and, if compromised, impedes growth performance, particularly under stress. Both Cyp20-3 and 2-CysPrx function as established thiol switches and likely have redox-dependent functions beyond their specific role within the COPS module. This projects aims to understand the physiological integration and redox regulatory significance of the COPS module with emphasis on Cyp20-3. The redox control of the thiol switch protein Cyp20-3 will be studied in vitro to dissect the efficiency of reductants and identify possible oxidants. Structural requirements for the interaction between Cyp20-3, 2-CysPrx, SERAT2.1 and OASTLB will be explored in transfected protoplasts and in Arabidopsis thaliana lines. To this end knockout lines will be complemented with site-directed mutated variants of Cyp20-3 and 2-CysPrx. Emphasis is given to functions of the COPS module in cellular protein homeostasis as indicated by the profound deregulation of transcripts belonging to this particular gene ontology group. The role of Cyp20-3 and 2-CysPrx in oxylipin signaling and carbohydrate metabolism will be scrutinized and linked to novel interactors and involved retrograde signaling from the chloroplast to the nucleus. Furthermore, the project will address the COPS module and its components for their function in balancing chloroplast electron distribution. The overarching hypothesis assumes that the components of the COPS module contribute to the metabolic decision of the plants between investment in growth or defense, and in the distribution of reducing power provided by the photosynthetic electron transport chain to the different consuming pathways.