Photosynthesis and the biosynthesis of many important metabolites occur in chloroplasts. In these semi-autonomous organelles, the chloroplast genome encodes approximately 100 proteins. The remaining chloroplast proteins, close to 3000, are encoded by nuclear genes whose products are translated in the cytosol and imported into chloroplasts. However, there is still no consensus on the composition of the protein import machinery including its motor proteins and on how newly imported chloroplast proteins are refolded. In this study, we have examined the function of orf2971, the largest chloroplast gene of Chlamydomonas reinhardtii. Depletion of Orf2971 causes accumulation of protein precursors, partial proteolysis and aggregation of proteins, increased expression of chaperones and proteases, and autophagy. Orf2971 interacts with the TIC complex, catalyzes ATP hydrolysis and associates with chaperones and chaperonins. We propose that Orf2971 is intimately connected to the protein import machinery and plays an important role in chloroplast protein quality control.
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