Mechanism and Function of the Eukaryotic Chaperonin CCT

Abstract

Chaperonins are a family of molecular chaperones that utilise the cavity formed by its two rings for the folding of substrates. The eukaryotic cytosolic chaperonin CCT or TRiC is thought to be involved in the folding of over 10% of newly synthesised proteins, although its major substrates are the cytoskeletal proteins such as actin and tubulin. CCT is the most complex of all chaperonins, with eight different subunits able to recognise specific sequences within substrate proteins, and it interacts with a variety of chaperones, cochaperones and cofactors to perform its activity. Substrate folding is mediated by an ATP‐driven cycle that induces conformational changes resulting in the opening or closing of the cavity. The different subunits are spatially arranged into an ATP‐binding and a substrate‐binding cluster, showing an asymmetric behaviour of the chaperonin. The increasing number of CCT substrates involved in cell signalling, cancer and neurodegenerative diseases suggest a central role of the chaperonin in the cellular proteostasis.

Key Concepts

  • CCT is a chaperonin composed of two rings of eight subunits each that form a cavity for substrate folding.
  • Each CCT subunit is divided into apical, intermediate and equatorial domains with specialised functions.
  • Actin and tubulin are the two main substrates of CCT and have driven the evolution of the chaperonin.
  • ATP hydrolysis induces the closing of the cavity required for substrate folding.
  • WD40 proteins have been described to be an important family of CCT substrates.
  • The folding process often requires collaboration with other chaperones, cochaperones or cofactors that are the key for CCT specificity.
  • CCT subunits display different affinities for ATP and substrates, leading to a spatial and functional clustering.

Keywords: molecular chaperones; chaperonins; CCT/TRiC; cochaperones; ATP hydrolysis; protein folding; substrates; cytoskeletal proteins; WD40 proteins; functional clustering

Figure 1. CCT architecture. (a) The ATP‐driven conformational cycle of CCT. Side (left) and top (right) views of CCT showing the different conformations in the folding process. From top to bottom: ATP‐free state or Apo‐CCT (PDB:5GW4; EMBD:9540), ATP‐bound state (PDB:5GW5; EMBD:9541) and ADP‐bound state after ATP hydrolysis (PDB:4V8R). Subunit identity is indicated in the top view: α/1 (orange), β/2 (green), γ/3 (purple), δ/4 (pink), ϵ/5 (red), ζ/6 (yellow), η/7 (gold) and θ/8 (blue). (b) Schematic inter‐ring arrangement of the CCT subunits, showing that CCT2 and CCT5 are the only ones that contact their homologue subunit in the opposite ring (c) domain topology of CCT subunits. Equatorial (purple), intermediate (yellow) and apical (light blue) domains of the CCT7 subunit (PDB:5GW4). The proximal loop (PL) (red) and the helix 11 (H11) (dark blue) provide the substrate‐binding surface, while the helical protrusion (circled in orange) constitutes a built‐in lid of the cavity. N‐ and C‐termini are shown in green.
Figure 2. Functional clustering of the CCT oligomer showing the ATP and substrate affinities in a blue and orange gradient, respectively. Subunits CCT5/2/4 display high ATP affinity and provide a negatively charged surface, whereas low‐affinity subunits CCT3/6/8 show opposite electrostatic properties. A generic substrate (green ellipse) is shown interacting with the latter subunits.
Figure 3. CCT interacts with a defined set of substrates involved in different cellular processes. A noncomprehensive classification of the major CCT substrates: cytoskeletal proteins actin (PDB:1J6Z) and tubulin (PDB:1TUB); WD40 repeat‐containing proteins CDC20 (PDB:4N14), Gβ (PDB:2TRC) and mLST8 (PDB:4JSN); oncogenic proteins VHL (PDB:1VCB) and p53 (PDB:2XWR); viral proteins gp41 from HIV (PDB:1J5X); neurodegenerative disease‐related proteins huntingtin (PDB:6EZ8) and α‐synuclein (PDB:2N0A).
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Further Reading

Gómez‐Puertas P, Martín‐Benito J, Carrascosa JL, et al. (2004) The substrate recognition mechanisms in chaperonins. Journal of Molecular Recognition 17 (2): 85–94.

Gruber R and Horovitz A (2016) Allosteric mechanisms in chaperonin machines. Chemical Reviews 116 (11): 6588–6606.

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Spiess C, Meyer AS, Reissmann S, et al. (2004) Mechanism of the eukaryotic chaperonin: protein folding in the chamber of secrets. Trends in Cell Biology 14 (11): 598–604.

Willardson BM and Tracy CM (2012) Chaperone‐mediated assembly of G protein complexes. Subcellular Biochemistry 63: 131–153.

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Bueno‐Carrasco, María T, and Cuéllar, Jorge(Nov 2018) Mechanism and Function of the Eukaryotic Chaperonin CCT. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028208]