cct-8 encodes the theta (CCT-theta / TCP-1-theta) subunit of the eukaryotic cytosolic chaperonin CCT (chaperonin-containing TCP-1), also known as TRiC. CCT/TRiC is an ATP-dependent, hetero-oligomeric double-ring chaperonin whose two rings are each built from eight distinct but paralogous subunits (CCT1-CCT8 / alpha-theta); cct-8 is the theta paralog. The assembled complex, not any individual subunit, is the folding machine: it uses cycles of ATP binding and hydrolysis by its subunits to fold roughly a tenth of the cytosolic proteome, most notably the obligate cytoskeletal substrates actin and tubulin, as well as WD40 beta-propeller proteins. The theta subunit contributes an equatorial nucleotide-binding module and an apical substrate-binding surface; it is one of the low-ATPase subunits and participates in substrate contacts and in allosteric coordination of the folding cycle. In C. elegans, cct-8 acts in the cytoplasm, is required for correct subcellular localization of the germ-granule protein PGL-1, and is a limiting determinant of TRiC/CCT assembly: raising CCT8 levels increases assembled chaperonin, suppresses aggregation of polyglutamine and mutant Huntingtin proteins, and extends lifespan, so cct-8 supports cytosolic protein homeostasis during aging and in the germline.
Definition: A molecular function of a single subunit of a group II chaperonin (CCT/TRiC) that presents an apical-domain substrate-binding surface and contributes subunit-specific contacts to the ATP-dependent folding of client proteins by the assembled chaperonin, without itself folding a substrate independently of the complex.
Justification: Individual CCT subunits have no adequate GO molecular-function term and can only be annotated with the complex-level foldase activity or a generic catalytic term, obscuring subunit-specific substrate selectivity.
Parent term: molecular_function
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006457 protein folding | IBA GO_REF:0000033 | ACCEPT | Summary: cct-8 is the theta subunit of the cytosolic chaperonin CCT/TRiC, which catalyzes ATP-dependent folding of cytoskeletal and other cytosolic substrates. Protein folding is the core biological process for this subunit. Reason: The phylogenetic (IBA) inference is fully consistent with the conserved chaperonin family assignment (PANTHER PTHR11353; InterPro theta-subunit signatures) and with direct biochemical demonstration that purified CCT catalyzes actin folding. In C. elegans, raising CCT8 levels increases assembled TRiC/CCT and suppresses protein aggregation, confirming the folding role in vivo. Supporting Evidence: PMID:16762366 The eukaryotic cytosolic chaperonin CCT is an essential ATP-dependent protein folding machine whose action is required for folding the cytoskeletal proteins actin and tubulin PMID:27892468 increased TRiC/CCT complex is required to avoid aggregation of mutant Huntingtin protein file:interpro/panther/PTHR11353/PTHR11353-metadata.yaml CHAPERONIN |
| GO:0005832 chaperonin-containing T-complex | IBA GO_REF:0000033 | ACCEPT | Summary: cct-8 is an integral subunit of the chaperonin-containing T-complex (CCT/TRiC), which is built from eight paralogous subunits per ring. Reason: The CCT complex is assembled from a stoichiometric array of subunits Cct1p-Cct8p; cct-8 is the theta subunit. In C. elegans, ectopic CCT8 is rate-limiting for assembly of the complex. Complex membership is the most important and best-supported annotation for this gene. Supporting Evidence: PMID:15704212 Eukaryotic chaperonins, the Cct complexes, are assembled into two rings, each of which is composed of a stoichiometric array of eight different subunits, which are denoted Cct1p-Cct8p. PMID:27892468 ectopic expression of a single subunit (CCT8) is sufficient to increase TRiC/CCT assembly |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: Each CCT subunit binds ATP/ADP through the conserved equatorial nucleotide-binding site of the group II chaperonin fold; nucleotide binding is integral to the folding cycle. Reason: ATP binding is a conserved, well-established property of CCT/TRiC subunits, consistent with the InterPro chaperonin domains and the ATP-dependent mechanism of the complex. The theta subunit is a low-ATPase subunit that binds and retains nucleotide, so ATP binding is the most accurate concrete molecular-function annotation for it. Supporting Evidence: PMID:16762366 The eukaryotic cytosolic chaperonin CCT is an essential ATP-dependent protein folding machine |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: CCT/TRiC is a cytosolic chaperonin; cct-8 acts in the cytoplasm on cytosolic substrates. Reason: Cytoplasmic localization is consistent with the known biology of the cytosolic chaperonin and with the UniProt subcellular location. The more specific term cytosol (GO:0005829) would be preferable, but the broader cytoplasm annotation is not incorrect and is retained. Supporting Evidence: PMID:16762366 The eukaryotic cytosolic chaperonin CCT |
| GO:0006457 protein folding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro domain-based annotation of protein folding, duplicating the IBA protein-folding annotation. Reason: Consistent electronic (InterPro) support for the core protein-folding process, in agreement with the IBA annotation and the conserved chaperonin mechanism. Supporting Evidence: PMID:16762366 Yeast CCT catalyses the folding of yeast ACT1p and human beta-actin with nearly identical rate constants and yields. |
| GO:0016887 ATP hydrolysis activity | IEA GO_REF:0000002 | ACCEPT | Summary: The CCT subunits form ATPases; ATP hydrolysis around the ring powers the substrate-folding cycle of the chaperonin. Reason: ATP hydrolysis is a conserved catalytic activity of the CCT/TRiC family, supported by the InterPro chaperonin domains. This is a family-level electronic annotation; the theta subunit specifically is one of the low-ATPase subunits that retains bound ADP, but it preserves the catalytic P-loop and aspartate, so the term is retained rather than removed. Supporting Evidence: PMID:16762366 The eukaryotic cytosolic chaperonin CCT is an essential ATP-dependent protein folding machine file:worm/cct-8/cct-8-deep-research-falcon.md CCT-8 occupies a defined position in the low-ATPase hemisphere and plays a specialized role in substrate binding, allosteric regulation, and complex assembly |
| GO:0140662 ATP-dependent protein folding chaperone | IEA GO_REF:0000002 | ACCEPT | Summary: ATP-dependent protein folding chaperone is the most informative molecular-function term for CCT/TRiC; cct-8 contributes to this activity as a subunit of the complex. Reason: This term precisely captures the activity of the CCT chaperonin. Strictly it is a complex-level activity to which the theta subunit contributes rather than one it enables alone, but the annotation is appropriate and is the best available molecular-function descriptor for this gene. Supporting Evidence: PMID:16762366 The eukaryotic cytosolic chaperonin CCT is an essential ATP-dependent protein folding machine |
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Download this section (compressed HTML)Q: Which native C. elegans clients depend specifically on the theta (cct-8) subunit contacts, as opposed to the CCT/TRiC complex as a whole?
Q: Is PGL-1 a direct CCT/TRiC substrate, or is its P-granule mislocalization on cct-8 depletion an indirect consequence of impaired actin/tubulin folding?
Q: Is the low-ATPase, substrate-binding character of the theta subunit seen in mammalian/yeast CCT structures conserved and functionally important in C. elegans?
Experiment: Perform affinity purification/mass spectrometry of tagged cct-8 (or the assembled CCT/TRiC) from C. elegans to identify native clients, and test whether PGL-1 co-purifies with the chaperonin.
Hypothesis: cct-8 engages a defined set of native C. elegans clients that includes cytoskeletal proteins and may include germ-granule components.
Experiment: Map theta-subunit apical-domain substrate contacts by crosslinking mass spectrometry or cryo-EM of CCT/TRiC caught with folding substrates, to define the subunit-specific substrate-binding contribution.
Hypothesis: The theta apical domain makes distinct substrate contacts that differ from the other seven CCT subunits.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The native C. elegans client repertoire that specifically depends on the theta (CCT8) subunit is undefined, and there is no GO molecular-function term that expresses "substrate-binding subunit of the CCT chaperonin". The theta subunit's own activity can therefore only be annotated as the complex-level foldase (GO:0140662) or as generic ATP binding/hydrolysis, leaving cct-8 molecular-function-dark at the subunit level.
OPEN ONTOLOGYBIOLOGY MF_DARK
What is known: It is firmly established that cct-8 is the theta subunit of the eight-membered CCT/TRiC ring, that the assembled complex folds actin, tubulin, WD40 proteins, and about 10% of the cytosolic proteome, and that the eight subunits are functionally non-equivalent. Structural work in CCT/TRiC further shows theta is a low-ATPase subunit that makes substrate contacts (actin and tubulin contact CCT8-containing apical domains) and contributes to allostery and assembly.
Significance: CCT subunits are individually essential and non-redundant, so subunit-specific substrate engagement underlies the complex's client selectivity. Because no ontology term names a chaperonin substrate-binding subunit activity, the theta subunit's characterized structural/regulatory role cannot be curated, and its native worm-specific clients remain unmapped.
What would resolve it: Affinity-proteomics of the C. elegans CCT/TRiC to define native theta clients, plus crosslinking or cryo-EM mapping of theta apical-domain contacts, would define the subunit contribution; a new GO molecular-function term for a chaperonin substrate-binding subunit activity would let the knowledge be expressed.
Provenance (the field's own admissions):
Proposed term (ontology gap):
Gap: It is unknown whether the germ-granule protein PGL-1 is a direct CCT/TRiC client of cct-8 or whether the pgl-1 mislocalization seen on cct-8 depletion is an indirect consequence of impaired folding of another substrate (for example actin, which scaffolds germ granules).
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: cct-8 depletion causes low, diffuse PGL-1 localization in C. elegans embryos rather than confinement to granules, and cct-8 knockdown promotes neuronal polyglutamine aggregation, identifying cct-8 as required for correct protein solubility/localization in vivo. Whether PGL-1 physically engages the chaperonin has not been tested.
Significance: Distinguishing a direct chaperonin-client relationship from an indirect cytoskeletal effect would clarify how the general folding machinery supports germline biomolecular-condensate (P-granule) assembly.
What would resolve it: Test whether PGL-1 physically engages CCT/TRiC (co-purification, in vitro folding assays) and whether actin/tubulin folding defects alone reproduce the P-granule phenotype.
Provenance (the field's own admissions):
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