Mechanism of lid closure in the eukaryotic chaperonin TRiC/CCT

Abstract

Group II chaperonins, such as TriC/CCT, have a build-in lid that can cover the folding chamber and functions in an analogous way to the GroES-like proteins used by their Group I counterparts. Structural and modeling data suggest an allosteric mechanism of TriC lid closure that differs from GroES–GroEL systems. All chaperonins mediate ATP-dependent polypeptide folding by confining substrates within a central chamber. Intriguingly, the eukaryotic chaperonin TRiC (also called CCT) uses a built-in lid to close the chamber, whereas prokaryotic chaperonins use a detachable lid. Here we determine the mechanism of lid closure in TRiC using single-particle cryo-EM and comparative protein modeling. Comparison of TRiC in its open, nucleotide-free, and closed, nucleotide-induced states reveals that the interdomain motions leading to lid closure in TRiC are radically different from those of prokaryotic chaperonins, despite their overall structural similarity. We propose that domain movements in TRiC are coordinated through unique interdomain contacts within each subunit and, further, these contacts are absent in prokaryotic chaperonins. Our findings show how different mechanical switches can evolve from a common structural framework through modification of allosteric networks.