Hetero‐oligomeric CPN60 resembles highly symmetric group‐I chaperonin structure revealed by Cryo‐EM

The chloroplast chaperonin system is indispensable for the biogenesis of Rubisco, the key enzyme in photosynthesis. Using Chlamydomonas reinhardtii as a model system, we found that in vivo the chloroplast chaperonin consists of CPN60α, CPN60β1 and CPN60β2 and the co‐chaperonin of the three subunits CPN20, CPN11 and CPN23. In Escherichia coli, CPN20 homo‐oligomers and all possible other chloroplast co‐chaperonin hetero‐oligomers are functional, but only that consisting of CPN11/20/23‐CPN60αβ1β2 can fully replace GroES/GroEL under stringent stress conditions. Endogenous CPN60 was purified and its stoichiometry was determined to be 6:2:6 for CPN60α:CPN60β1:CPN60β2. The cryo‐EM structures of endogenous CPN60αβ1β2/ADP and CPN60αβ1β2/co‐chaperonin/ADP were solved at resolutions of 4.06 and 3.82 A, respectively. In both hetero‐oligomeric complexes the chaperonin subunits within each ring are highly symmetric. Through hetero‐oligomerization, the chloroplast co‐chaperonin CPN11/20/23 forms seven GroES‐like domains, which symmetrically interact with CPN60αβ1β2. Our structure also reveals an uneven distribution of roof‐forming domains in the dome‐shaped CPN11/20/23 co‐chaperonin and potentially diversified surface properties in the folding cavity of the CPN60αβ1β2 chaperonin that might enable the chloroplast chaperonin system to assist in the folding of specific substrates.