Rocking Motion of a Protein-Folding Nano-Machine Revealed By Single-Particle Cryo-Em

The protein folding machine Methanococcus maripaludis chaperonin (Mm-cpn) is a type II archael chaperonin that has a built-in lid. It is a 16-subunit homo-oligomer of ∼1 MDa arranged in a two back-to-back rings that is structurally very similar to the mammalian chaperonin such as TRiC. The substrate folding is accompanied by a conformational change triggered by nucleotide binding and hydrolysis.Using single particle cryo-EM and image reconstruction, we solve both the wild type and lidless mutant Mm-cpn in open and closed states respectively at resolutions between 10 and 4.3 A. The open state is a nucleotide-free state while the closed state corresponds to the transition state of ATP hydrolysis. Cα backbone models of these four 3-D reconstructions have been hand traced or flexibly fitted depending on their resolutions. The models show clearly the subunits' equatorial domain rotation between the open and closed states, which is unique and different from the well-studied type I chaperonin (GroE) found in E.Coli.