The ATP-powered gymnastics of TRiC/CCT: an asymmetric protein folding machine with a symmetric origin story
Graphical abstract
Introduction
All kingdoms of life contain a large double ring chaperonin complex that uses ATP binding and hydrolysis to bind and fold non-native polypeptides within its central chamber. Chaperonins have diverged into two evolutionarily distinct groups, differing substantially in their structure and mechanism. Prokaryotes and organelles of endosymbiotic origin contain Group I chaperonins, including the well characterized Escherichia coli chaperonin GroEL/GroES (reviewed in Ref. [1]). GroEL consists of two stacked homo-heptameric rings. In response to ATP-binding, the GroEL chamber is capped by the homo-heptameric GroES complex, which acts as a detachable lid required for substrate encapsulation and subsequent folding. Eukaryotes and archaea contain Group II chaperonins, which differ significantly from their prokaryotic counterparts in their architecture and mechanisms. First, the two rings of Group II chaperonins are generally eight membered and are stacked in a different inter-ring geometry than bacterial chaperonins. In addition, Group II chaperonins lack a GroES-like cofactor, using instead a flexible protrusion at the top of each subunit to create a built-in-lid that opens and closes in an ATP-dependent manner. Because of their distinct architecture, Group II chaperonins also have diverged in their conformational cycle, allosteric regulation, and in their substrate-recognition and folding mechanisms. In addition, while most prokaryotic chaperonins are homo-oligomeric group II chaperonins have undergone a progressive diversification of subunit composition. In archaea, chaperonins typically consist of 1–3 paralog subunits, while the eukaryotic chaperonin TRiC (TCP1-ring complex, also called CCT for cytosolic chaperonin containing TCP1) has eight different subunits per ring. The underlying causes and consequences of subunit diversification, which have long been enigmatic, are now beginning to emerge, as discussed in this review.
Section snippets
TRiC architecture and overall structure
TRiC/CCT is a 1 MDa complex composed of eight paralogous subunits that assemble into a double ring hexadecamer. Each subunit has a characteristic three domains architecture; an apical domain at the top of the ring which contains the substrate recognition site and the lid-forming loop, an equatorial domain that contains the ATP-binding site, and an intermediate domain which contains a conserved Aspartic acid required for ATP hydrolysis and communicates ATP cycling at the equatorial domain to
ATP regulation of the TRiC/CCT conformational cycle
Group II chaperonins use ATP binding and hydrolysis to alternate between an open state, where the lid forming segments are unstructured and the substrate binding sites in each subunit are accessible and a closed state, where the lid segments form a beta-stranded iris (schematic in Figure 2a). The conformational cycle, allosteric regulation, and ATP-driven structural changes are largely conserved between TRiC and its simpler archaeal homologues. Many of the early insights into the ATP cycle came
The cellular functions and substrates of TRiC/CCT
TRiC is essential for viability as many essential proteins, such as actin, tubulin, and cell cycle regulators, exhibit an obligate TRiC requirement to achieve the native state. TRiC binds co-translationally and post-translationally to ∼10% of the proteome [26,27]. Why TRiC selects these particular set of proteins as substrates and how it facilitates their folding are key unanswered questions for future research. TRiC substrates tend to be topologically complex with aggregation prone β -sheet
TRiC substrate interactions: how subunit diversity impacts polypeptide binding and folding
Substrate folding is coordinated with the ATPase cycle. In the open state TRiC binds unfolded polypeptides through the apical domains; ATP-induced lid formation releases the substrate to the closed chamber where folding is thought to occur. While a detailed understanding of how TRiC assists substrate folding has remained elusive, recent studies revealed a key role of subunit diversity in both substrate binding and folding.
The important question of substrate recognition, and how TRiC can
Perspectives
The wealth of new structural and mechanistic insight emerging in recent years has completely changed our view of TRiC/CCT. Initially thought to be a variation on the theme of GroEL-ES, this chaperonin has emerged as a highly asymmetrical and complex machine, where subunit diversification has created unique substrate binding and folding capabilities and a unique ATP-driven cycle unlike that of any other chaperone. This must clearly be linked to TRiC’s unique ability to fold eukaryotic proteins.
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Work in the Frydman lab is supported by N.I.H. (R01GM074074 to JF; F32GM103124 to DG). We thank members of the Frydman lab for discussions.
References (46)
- et al.
The GroEL-GroES chaperonin machine: a nano-cage for protein folding
Trends Biochem Sci
(2016) - et al.
Crystal structure of the thermosome, the archaeal chaperonin and homolog of CCT
Cell
(1998) - et al.
Crystal structures of the group II chaperonin from Thermococcus strain KS-1: steric hindrance by the substituted amino acid, and inter-subunit rearrangement between two crystal forms
J Mol Biol
(2004) - et al.
Gene duplication and the evolution of group II chaperonins: implications for structure and function
J Struct Biol
(2001) - et al.
The molecular architecture of the eukaryotic chaperonin TRiC/CCT
Structure
(2012) - et al.
Mechanism of nucleotide sensing in group II chaperonins
EMBO J
(2012) - et al.
The asymmetric ATPase cycle of the thermosome: elucidation of the binding, hydrolysis and product-release steps
J Mol Biol
(2006) - et al.
An information theoretic framework reveals a tunable allosteric network in group II chaperonins
Nat Struct Mol Biol
(2017) - et al.
Role of the chaperonin CCT/TRiC complex in G protein betagamma-dimer assembly
J Biol Chem
(2006) - et al.
Interaction of p53 with the CCT complex promotes protein folding and wild-type p53 activity
Mol Cell
(2013)
Modulation of STAT3 folding and function by TRiC/CCT chaperonin
PLoS Biol
Exogenous delivery of chaperonin subunit fragment ApiCCT1 modulates mutant Huntingtin cellular phenotypes
Proc Natl Acad Sci U S A
T-complex protein 1-ring complex enhances retrograde axonal transport by modulating tau phosphorylation
Traffic
The chaperonin CCT inhibits assembly of alpha-synuclein amyloid fibrils by a specific, conformation-dependent interaction
Sci Rep
The structural basis of substrate recognition by the eukaryotic chaperonin TRiC/CCT
Cell
Structural features of the GroEL-GroES nano-cage required for rapid folding of encapsulated protein
Cell
The crystal structure of yeast CCT reveals intrinsic asymmetry of eukaryotic cytosolic chaperonins
EMBO J
Crystal structure of the open conformation of the mammalian chaperonin CCT in complex with tubulin
Nat Struct Mol Biol
4.0-A resolution cryo-EM structure of the mammalian chaperonin TRiC/CCT reveals its unique subunit arrangement
Proc Natl Acad Sci U S A
The inter-ring arrangement of the cytosolic chaperonin CCT
EMBO Rep
Subunit order of eukaryotic TRiC/CCT chaperonin by cross-linking, mass spectrometry, and combinatorial homology modeling
Proc Natl Acad Sci U S A
Staggered ATP binding mechanism of eukaryotic chaperonin TRiC (CCT) revealed through high-resolution cryo-EM
Nat Struct Mol Biol
Pathway of actin folding directed by the eukaryotic chaperonin TRiC
Cell
Cited by (56)
The critical role of co-translational folding: An evolutionary and biophysical perspective
2024, Current Opinion in Systems BiologyThe structural basis of eukaryotic chaperonin TRiC/CCT: Action and folding
2024, Molecules and CellsThe Essential Functions of Molecular Chaperones and Folding Enzymes in Maintaining Endoplasmic Reticulum Homeostasis
2024, Journal of Molecular Biology
- 1
These authors contributed equally and are listed alphabetically.