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Dissecting chloroplast protein quality control specificity for rational plant reprogramming

Periodic Reporting for period 1 - ChloroQuality (Dissecting chloroplast protein quality control specificity for rational plant reprogramming)

Reporting period: 2016-01-01 to 2017-12-31

Chloroplasts are specialized compartments (organelles) of the cell that are essential in plants and algae for photosynthesis. In the thylakoid membrane of the chloroplast, photosynthetic complexes harvest the light energy allowing the fixation of carbon dioxide and the subsequent production of nutrients for plants. Furthermore, the biosynthesis of important compounds including pigments and plant hormones is performed in this organelle. Therefore, chloroplasts play a primary role in plant development and fitness.

While the composition and structure of chloroplastic high molecular protein complexes such as photosystems or ribosomes is well characterized, less information is available about their regulation, specially under stress situations. Similarly to other compartments, chaperones and proteases are required to monitor protein homeostasis in the chloroplast, assisting the folding, assembly and degradation of proteins during their lifetime. Although several types of chaperones are known to assist in the correct functioning of the chloroplast, the molecular basis behind these requirements remain unknown. Thus, it is necessary to increase our understanding of how the levels and the activity of chloroplastic proteins are regulated. ChloroQuality address this challenge by studying the DNAJ protein family of chaperones in the plant model Arabidopsis thaliana.
The main objective of the ChloroQuality project is to unveil and characterize chloroplastic mechanisms involved in protein complex assembly and stability in plant chloroplasts. One of the main accomplishments of the project is the classification of the HSP70-independent DNAJ proteins in three distinct groups, leading to the identification of 20 new isoforms in plants. Classical DNAJ proteins work together with HSP70 chaperones in the folding of substrate proteins. However, during evolution several DNAJ proteins have lost the capability of interaction with HSP70, displaying HSP70-independent chaperone activity. In fact, we focused our attention on SNOWY COTYLEDON 2 (SCO2), a DNAJ-related protein involved in thylakoid membrane biogenesis. Briefly, we demonstrated that SCO2 is required for the development of cotyledons and true leaves, and plays a role in leaf variegation in both Arabidopsis and Lotus japonicus, a forage crop widely used as legume model. We demonstrated that SCO2 is a specific assembly factor of the light-harvesting chlorophyll-binding protein LHCB1, assisting the formation of the megacomplexes with the photosystems at the thylakoid membranes. Moreover, we demonstrated that in the absence of SCO2, the Clp protease is indispensable for chloroplast proteostasis. In addition to the assembly of photosynthetic machinery, a widespread function of chaperones is the folding of nascent proteins at ribosomes. Unexpectedly, during our screening for potential chaperones at chloroplastic ribosomes, we identified the Arabidopsis protein CHLOROPLAST RIBOSOME ASSOCIATED (CRASS), a new plant-specific factor unrelated to other assembly factors described previously. We demonstrated that CRASS interacts mainly with ribosomal proteins of the 30S ribosomal subunit. Interestingly, CRASS is critical under stressful conditions when ribosomal activity is compromised.
The knowledge gained during the ChloroQuality project will contribute to approaches aiming to engineer the protein synthesis and modify the assembly of the proteins complexes in the chloroplast. Improving the stability of the photosynthetic complexes is a fundamental achievement for future strategies in synthetic photosynthesis. Therefore, the outcomes arising from ChloroQuality could lead to increase the tolerance of plants to stress situations.