InterPro domain: IPR012718

Members of this eukaryotic family are part of the group II chaperonin complex called CCT (chaperonin containing TCP-1 or Tailless Complex Polypeptide 1) or TRiC [ 1 , 2 ]. Chaperonins are involved in productive folding of proteins [ 3 ]. They share a common general morphology, a double toroid of 2 stacked rings. The archaeal equivalent group II chaperonin is often called the thermosome [ 4 ]. Both the thermosome and the TCP-1 family of proteins are weakly, but significantly [ 5 ], related to the cpn60/groEL chaperonin family (see IPR001844 ).

The TCP-1 protein was first identified in mice where it is especially abundant in testis but present in all cell types. It has since been found and characterised in many other animal species, as well as in yeast, plants and protists. The TCP1 complex has a double-ring structure with central cavities where protein folding takes place [ 6 ]. TCP-1 is a highly conserved protein of about 60kDa (556 to 560 residues) which participates in a hetero-oligomeric 900kDa double-torus shaped particle [ 7 ] with 6 to 8 other different, but homologous, subunits [ 8 ]. These subunits, the chaperonin containing TCP-1 (CCT) subunit beta, gamma, delta, epsilon, zeta and eta are evolutionary related to TCP-1 itself [ 9 , 10 ]. Non-native proteins are sequestered inside the central cavity and folding is promoted by using energy derived from ATP hydrolysis [ 11 , 12 , 13 ]. The CCT is known to act as a molecular chaperone for tubulin, actin and probably some other proteins [ 14 , 15 ].

This family consists exclusively of the CCT epsilon chain (part of a paralogous family) from animals, plants, fungi, and other eukaryotes.

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2. TCP1 - molecular chaperonin of the cytoplasm? Curr. Biol. 2, 487-9
3. Chaperonin-mediated protein folding. Annu Rev Biophys Biomol Struct 30, 245-69
4. Review: nucleotide binding to the thermoplasma thermosome: implications for the functional cycle of group II chaperonins. J. Struct. Biol. 135, 147-56
5. What is a chaperonin? Nature 357, 650
6. 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. 107, 4967-72
7. T-complex polypeptide-1 is a subunit of a heteromeric particle in the eukaryotic cytosol. Nature 358, 249-52
8. The chaperonin containing t-complex polypeptide 1 (TCP-1). Multisubunit machinery assisting in protein folding and assembly in the eukaryotic cytosol. Eur. J. Biochem. 230, 3-16
9. Identification of six Tcp-1-related genes encoding divergent subunits of the TCP-1-containing chaperonin. Curr. Biol. 4, 89-99
10. Cystosolic chaperonin subunits have a conserved ATPase domain but diverged polypeptide-binding domains. Trends Biochem. Sci. 19, 543-8
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13. Structure and function of a protein folding machine: the eukaryotic cytosolic chaperonin CCT. FEBS Lett. 529, 11-6
14. Activities of the chaperonin containing TCP-1 (CCT): implications for cell cycle progression and cytoskeletal organisation. Cell Stress Chaperones 14, 23-31
15. The substrate recognition mechanisms in chaperonins. J. Mol. Recognit. 17, 85-94