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

Abstract

AbstractThe chloroplast chaperonin system is indispensable for the biogenesis of Rubisco, the key enzyme in photosynthesis. Using Chlamydomonas reinhardtii as the model system, we revealed that chloroplast chaperonin is consisted of CPN60α, CPN60β1, and CPN60β2, and co-chaperonin is composed of three subunits CPN20, CPN11 and CPN23 in vivo. CPN20 homo-oligomers and all possible other chloroplast co-chaperonin hetero-oligomers are functional, but only CPN11/20/23-CPN60αβ1β2 pair can fully replace GroES/GroEL in E. coli at stringent growth condition. 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 Å and 3.82Å, respectively. In both hetero-oligomeric complexes the chaperonin subunits within each ring are highly symmetric. The chloroplast co-chaperonin CPN11/20/23 formed seven GroES-like domains through hetero-oligomerization which symmetrically interact with CPN60αβ1β2. Our structures also reveal an uneven distribution of roof-like structures in the dome-shaped CPN11/20/23 and potentially diversified surface properties in the folding cavity of CPN60αβ1β2 that might enable the chloroplast chaperonin system to assist in the folding of specific substrates.