Structural basis of HSP90C, a highly active chloroplastic HSP90 chaperone from A. thaliana

Chloroplasts are the main energy organelles in plants, primary through photosynthesis. Thereby, they are responsible for CO2 fixation and dioxygen production, which are essential for living species on Earth. To ensure these processes, numerous proteins encoded from the nuclear DNA need to be imported inside the chloroplast, and eventually to the thylakoids. Whereas the translocation systems from both chloroplastic and thylakoids membranes have been studied in recent years, the stromal route between these two membranes is largely unknown. Notably, the chloroplastic HSP90 (HSP90C) is likely to play an important role in this process, but its structure and molecular mechanisms remain to be unveiled. In this study, we used a combination of structural and biophysical approaches to elucidate the features of Arabidopsis thalianas HSP90C. Principally, we found that HSP90C has a remarkably high ATPase activity among the HSP90 family proteins. Further investigation allowed us to pinpoint atypical mechanisms responsible for this high activity. First, the N-terminal cap is involved in a disulfide bond that accelerates the ATPase activity of HSP90C. Second, its C-terminal domain features an extension that is mandatory for its dimerization. Third, our crystal structures reveal a wide opening of the HSP90Cs dimer with reduced intermonomeric interfaces. Lastly, we identified a helical switch which is required for HSP90Cs high activity. Three of these four features are due to sequence signatures of HSP90C, which we found to be shared by most of green plants representatives. Our study provides first insights of HSP90Cs non-canonical mechanisms, which will help in the understanding of processes related to protein import in the chloroplast.