Symmetry-free cryo-EM structures of the chaperonin TRiC along its ATPase-driven conformational cycle

Abstract

Symmetry-free cryo-EM structures of the chaperonin TRiC along its ATPase-driven conformational cycleChaperonins are multisubunit entities that are composed of two stacked rings enclosing a central chamber for ATP-dependent protein folding. A series of cryo-EM structures of the eukaryotic group II chaperonin TRiC/CCT reveal the conformational changes during the ATPase cycle and provide insight into how the subunits cooperate to close the lid.

Asymmetric perturbations of signalling oligomers

“…Inherent in its nature, EM is a powerful experimental technique for distinguishing between structural symmetry and asymmetry in macromolecular complexes (Mi et al, 2011;Cerritelli et al, 2003;Hafenstein et al, 2007;Oshima et al, 2011;Cong et al, 2012;Matyskiela et al, 2013). It is important to note though that averaging and symmetry constraints used in EM often eliminate the chance of detecting oligomeric and small local asymmetries.…”

“…A recent comparative XRC and cryo-EM study has revealed that functional transition of GroEL into a relaxed state is associated with breaking of two salt bridges and transient asymmetry of the apical domains (Fei et al, 2013). Asymmetries can be also revealed in the octameric rings of TRiC, an eukaryotic group II chaperonin; in its ATP-binding hierarchy; and even in its transition states (Cong et al, 2012;Reissmann et al, 2012).…”

Folding of large multidomain proteins by partial encapsulation in the chaperonin TRiC/CCT

“…Interestingly, many of these proteins have complex domain topologies with a pronounced β-sheet propensity (18), a property that is also characteristic of the substrates of archaeal group II chaperonins (31). Similar to GroEL-GroES, TRiC alternates between open and closed conformations in an ATP-regulated manner (19)(20)(21)(32)(33)(34)(35)(36). In contrast to GroEL, however, the closed conformation is induced by formation of the ATP hydrolysis transition state, which can be artificially stabilized by addition of ATP·AlF x (33).…”

“…S1). However, the folding chamber of TRiC is equivalent in size to that of GroEL-GroES (19)(20)(21). How then does TRiC assist the folding of proteins that exceed the apparent size limit of its cavity?…”

“…Lid closure is mediated by a camera iris-like rearrangement of the apical subunit extensions (37), inducing formation of an eight-stranded β-barrel structure at the apical pore. The resulting cavity would be large enough to enclose proteins of up to ∼70 kDa (20,21) but not the numerous larger substrates. TRiC might thus encapsulate only those domains of modular proteins that critically require it for folding.…”