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.
