TBCC (tubulin folding cofactor C) is a cytosolic tubulin-specific chaperone that acts at the final step of the post-chaperonin tubulin folding pathway. After the chaperonin CCT partially folds alpha- and beta-tubulin, cofactors A and D, and B and E, capture the subunits, and TBCD and TBCE (with the small GTPase ARL2) assemble them into a supercomplex. TBCC then joins and acts as a GTPase-activating protein: through the conserved arginine finger (R262) in its C-terminal CARP/beta-helix domain, homologous to the GAP domain of RP2, it stimulates GTP hydrolysis that releases the native alpha/beta-tubulin heterodimer. In vitro, TBCC with TBCD and TBCE also stimulates GTP hydrolysis by native beta-tubulin. In reconstituted yeast proteins it activates the ARL2 GTPase within the TBCD-TBCE-ARL2 cage. Its N-terminal spectrin-like domain binds tubulin. TBCC thus governs the supply of polymerization-competent tubulin; altering its levels changes the soluble/polymerized tubulin balance and microtubule dynamics. It is mostly cytoplasmic, concentrates at centrosomes and spindle poles, and its depletion causes multipolar spindles and mitotic failure. In retina it is enriched at the photoreceptor connecting cilium.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000226 microtubule cytoskeleton organization | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT-propagated microtubule cytoskeleton organization. TBCC supplies assembly-competent alpha/beta-tubulin heterodimers; altering TBCC levels changes the polymerizable tubulin pool and microtubule dynamics. Reason: True but indirect: TBCC acts on soluble tubulin upstream of microtubule assembly (the core process is GO:0007023). Loss or excess of TBCC alters microtubule mass and dynamics in human cells, and human TBCC complements the yeast cofactor C deletion microtubule phenotype, so the IBA node placement is sound; kept as non-core because it describes the downstream consequence rather than what TBCC does. Supporting Evidence: PMID:21216936 MC- cells displayed decreased content in nonpolymerizable tubulins and increased content of polymerizable/microtubule tubulins PMID:20384997 TBCC overexpression influenced tubulin fraction distribution, with higher content of nonpolymerizable tubulins and lower content of polymerizable dimers and microtubules PMID:11847227 both cofactor C and RP2 partially complement the microtubule phenotype resulting from deletion of the cofactor C homolog |
| GO:0003924 GTPase activity | IDA PMID:11847227 Functional overlap between retinitis pigmentosa 2 protein an... | MODIFY | Summary: GTPase activity (GTP + H2O = GDP + phosphate) assigned from Bartolini et al. 2002. The paper shows that cofactor C, with cofactors D and E, stimulates GTP hydrolysis by native beta-tubulin, and that the conserved arginine R262 acts as an arginine finger. The hydrolysis is catalysed by tubulin, not by TBCC. Reason: TBCC has no GTPase domain (no P-loop or tubulin/FtsZ fold); its C-terminal CARP beta-helix domain is the RP2-like GAP module that inserts an arginine finger into the GTPase active site. The cited abstract describes stimulation of tubulin GTPase activity, and the UniProt mutagenesis feature from the same paper reads "Inhibits stimulation of tubulin GTPase activity". GO:0003924 is defined as catalysis of GTP hydrolysis, whereas GO:0005096 GTPase activator activity ("Binds to and increases the activity of a GTPase") matches the experiment. The same paper is already annotated to GO:0005096 by MGI. The essence (TBCC drives tubulin GTP hydrolysis) is right; the term is wrong, so MODIFY rather than REMOVE. Flagged by rule FRR:0010. Proposed replacements: GTPase activator activity Supporting Evidence: PMID:11847227 Together with cofactors D and E, cofactor C stimulates the GTPase activity of native tubulin PMID:11847227 this residue acts as an "arginine finger" to trigger the tubulin GTPase activity file:human/TBCC/TBCC-uniprot.txt R->A: Inhibits stimulation of tubulin GTPase PMID:10446175 the cofactors behave as GTPase-activating proteins, stimulating the GTP-binding protein beta-tubulin to hydrolyze its GTP |
| GO:0005096 GTPase activator activity | IDA PMID:11847227 Functional overlap between retinitis pigmentosa 2 protein an... | ACCEPT | Summary: GTPase activator activity: TBCC, with TBCD and TBCE, stimulates GTP hydrolysis by beta-tubulin; the conserved R262 arginine finger is required. Reason: This is the core molecular function of TBCC. The cofactors were shown to act as GAPs for beta-tubulin, and GTP hydrolysis by beta-tubulin in the cofactor supercomplex is the switch that releases the native heterodimer. Reconstitution with yeast proteins suggests TBCC may also (or instead) activate GTP hydrolysis by ARL2 in the TBCD-TBCE-ARL2 complex, and fission yeast Tbc1 is a GAP for Arl2; either target is captured by this term. Supporting Evidence: PMID:11847227 Together with cofactors D and E, cofactor C stimulates the GTPase activity of native tubulin PMID:11847227 this residue acts as an "arginine finger" to trigger the tubulin GTPase activity PMID:10446175 the cofactors behave as GTPase-activating proteins, stimulating the GTP-binding protein beta-tubulin to hydrolyze its GTP PMID:10831612 stimulating it to hydrolyze GTP and thus acting together as a beta-tubulin GTPase activating protein (GAP) PMID:26208336 further supporting the idea that TBCC is a true GAP |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | REMOVE | Summary: Yeast two-hybrid interaction with UniProtKB:P06396 from a neurodegenerative disease interactome screen. Reason: Generic protein binding is uninformative. This interaction comes from a large yeast two-hybrid neurodegenerative-disease interactome screen and has no follow-up linking it to TBCC function; removal does not assert that the interaction is false. Supporting Evidence: PMID:32814053 generated by systematic yeast two-hybrid interaction screening of βΌ500 ND-related proteins |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | REMOVE | Summary: Yeast two-hybrid interaction with UniProtKB:P22607 from a neurodegenerative disease interactome screen. Reason: Generic protein binding is uninformative. This interaction comes from a large yeast two-hybrid neurodegenerative-disease interactome screen and has no follow-up linking it to TBCC function; removal does not assert that the interaction is false. Supporting Evidence: PMID:32814053 generated by systematic yeast two-hybrid interaction screening of βΌ500 ND-related proteins |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | REMOVE | Summary: Yeast two-hybrid interaction with UniProtKB:P28329-3 from a neurodegenerative disease interactome screen. Reason: Generic protein binding is uninformative. This interaction comes from a large yeast two-hybrid neurodegenerative-disease interactome screen and has no follow-up linking it to TBCC function; removal does not assert that the interaction is false. Supporting Evidence: PMID:32814053 generated by systematic yeast two-hybrid interaction screening of βΌ500 ND-related proteins |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | MODIFY | Summary: Cytoplasm, propagated by PAINT. TBCC is a soluble cytoplasmic chaperone cofactor. Reason: Correct but less specific than the evidence supports: TBCC acts on soluble tubulin and is detected in the cytosol (HPA IDA), with a concentration at centrosomes. Refine to cytosol, which the gene already carries. Propagation Review Root cause: NO FAILURE CORE Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000388691 SUPPORTS TRANSFER TBCC-family node; cytoplasmic localization of the soluble tubulin cofactor is conserved and correctly transferred; only the granularity (cytosol) is refined here. Proposed replacements: cytosol Supporting Evidence: PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells PMID:12417528 Cofactor C was cytoplasmic in distribution |
| GO:0005737 cytoplasm | IDA PMID:12417528 Localization in the human retina of the X-linked retinitis p... | MODIFY | Summary: Cytoplasm from immunolocalization in human retina and cell lines (Grayson et al. 2002). Reason: Correct; the abstract describes cofactor C as cytoplasmic in distribution. TBCC is a soluble protein acting on soluble tubulin, and HPA detects it in the cytosol, so cytosol (already on the gene) is the more specific supported location. Proposed replacements: cytosol Supporting Evidence: PMID:12417528 Cofactor C was cytoplasmic in distribution PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | MODIFY | Summary: Cytoplasm, mapped from the UniProt subcellular location (Cytoplasm, PMID:12417528). Reason: Correct but general; refined to cytosol, consistent with the other cytoplasm rows. Proposed replacements: cytosol Supporting Evidence: file:human/TBCC/TBCC-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: Cytosol from Human Protein Atlas immunofluorescence. Reason: Consistent with TBCC being a soluble cofactor acting on unpolymerized tubulin in the cytosol, and with independent immunostaining showing a mostly cytoplasmic distribution. Supporting Evidence: PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. |
| GO:0005829 cytosol | IEA GO_REF:0000117 | ACCEPT | Summary: Cytosol predicted by an ARBA rule. Reason: Agrees with the HPA cytosol IDA and with the biochemistry of the soluble tubulin-folding pathway. Supporting Evidence: PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells |
| GO:0005856 cytoskeleton | TAS PMID:8706133 Pathway leading to correctly folded beta-tubulin. | MODIFY | Summary: Cytoskeleton, traceable author statement from Tian et al. 1996, which describes the tubulin folding pathway and does not localize TBCC to cytoskeletal structures. Reason: TBCC is not a component of microtubule polymers, but it does concentrate at the centrosome (part of the microtubule cytoskeleton) in HeLa cells and at spindle poles in mitosis, and its depletion causes multipolar spindles. The general term is therefore replaced with the specific supported structure, centrosome. Proposed replacements: centrosome Supporting Evidence: PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells PMID:22028797 TBCC is found at the centrosome and is implicated in bipolar spindle formation |
| GO:0005874 microtubule | TAS PMID:8706133 Pathway leading to correctly folded beta-tubulin. | REMOVE | Summary: Microtubule, traceable author statement from Tian et al. 1996. Reason: TBCC acts on soluble tubulin folding intermediates and native alpha/beta-tubulin heterodimers in the cytosol, not on microtubule polymers. The cited paper describes a folding pathway of soluble tubulin and does not report TBCC on microtubules; no later study localizes TBCC to microtubules (it is cytoplasmic with centrosomal enrichment). This non-experimental statement is not supported. Supporting Evidence: PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. PMID:22028797 TBCC is mostly a cytoplasmic protein but concentrates at the centrosomes of HeLa cells |
| GO:0006457 protein folding | TAS PMID:8706133 Pathway leading to correctly folded beta-tubulin. | MODIFY | Summary: Protein folding, from the paper describing the beta-tubulin folding pathway. Reason: TBCC does not fold polypeptides in the general chaperone sense; partial folding is done by the chaperonin, and TBCC acts in the final step of the cofactor pathway that assembles and releases the native heterodimer. GO models this as GO:0007023 post-chaperonin tubulin folding pathway (is_a tubulin complex assembly), which the gene already carries and which is the precise term. Proposed replacements: post-chaperonin tubulin folding pathway Supporting Evidence: PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. |
| GO:0007023 post-chaperonin tubulin folding pathway | IBA GO_REF:0000033 | ACCEPT | Summary: Post-chaperonin tubulin folding pathway, propagated by PAINT; TBCC itself is among the experimental sources. Reason: Core process. TBCC acts at the last step of the cofactor pathway, triggering release of the native alpha/beta-tubulin heterodimer from the cofactor supercomplex; the function is conserved in yeast and plants. Supporting Evidence: PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. PMID:10446175 hydrolysis of GTP by beta-tubulin in the supercomplex acts as a switch for the release of native tubulin heterodimer |
| GO:0007023 post-chaperonin tubulin folding pathway | IDA PMID:11847227 Functional overlap between retinitis pigmentosa 2 protein an... | ACCEPT | Summary: Post-chaperonin tubulin folding pathway; Bartolini et al. show tubulin-GAP activity of cofactor C and complementation of yeast cofactor C deletion. Reason: Core process, directly supported by the GAP assays and the in vivo complementation reported in the cited paper. Supporting Evidence: PMID:11847227 Together with cofactors D and E, cofactor C stimulates the GTPase activity of native tubulin PMID:11847227 both cofactor C and RP2 partially complement the microtubule phenotype resulting from deletion of the cofactor C homolog file:human/TBCC/TBCC-uniprot.txt Tubulin-folding protein; involved in the final step of the |
| GO:0007023 post-chaperonin tubulin folding pathway | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the TBCC family signature (IPR027684). Reason: Family membership is sound and the mapped process is the core process of TBCC. Supporting Evidence: file:human/TBCC/TBCC-uniprot.txt Belongs to the TBCC family PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. |
| GO:0007023 post-chaperonin tubulin folding pathway | TAS PMID:8706133 Pathway leading to correctly folded beta-tubulin. | ACCEPT | Summary: Post-chaperonin tubulin folding pathway, from the paper that defined the pathway. Reason: Tian et al. 1996 established that cofactor C acts in the final step releasing native beta-tubulin. Supporting Evidence: PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. |
| GO:0015631 tubulin binding | IEA GO_REF:0000002 | ACCEPT | Summary: Tubulin binding, mapped from the TBCC N-terminal domain signature (IPR031925). Reason: The TBCC N-terminal domain provides a tubulin-interaction platform, and TBCC binds tubulin within the cofactor supercomplex. Tubulin binding is the substrate-recognition activity that underlies its GAP function; the evidence does not resolve whether alpha- or beta-tubulin is the primary contact, so the general term is appropriate. Supporting Evidence: PMID:22028797 This unstructured region is involved in the interaction with tubulin. PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. |
| GO:0032391 photoreceptor connecting cilium | IDA PMID:12417528 Localization in the human retina of the X-linked retinitis p... | KEEP AS NON CORE | Summary: Photoreceptor connecting cilium; cofactor C immunostaining is enriched at the connecting cilium of rod and cone photoreceptors. Reason: Experimentally observed, tissue-specific localization; TBCC is broadly expressed and acts in the cytosol, so this is peripheral to its core function. Supporting Evidence: PMID:12417528 cofactor C and Arl3 localized predominantly to the photoreceptor connecting cilium in rod and cone photoreceptors |
| GO:0051087 protein-folding chaperone binding | TAS PMID:8706133 Pathway leading to correctly folded beta-tubulin. | KEEP AS NON CORE | Summary: Protein-folding chaperone binding: TBCC interacts with the TBCD/TBCE-tubulin complex (tubulin-specific chaperones) at the final step of the pathway. Reason: TBCC does engage the tubulin-specific chaperones TBCD and TBCE in the supercomplex, so the statement is true, but the binding is a means to its GAP activity rather than a function in itself. Supporting Evidence: PMID:8706133 Cofactor E binds to the cofactor D-beta-tubulin complex; interaction with cofactor C then causes the release of beta-tubulin polypeptides that are committed to the native state. PMID:26208336 forming a ternary complex in which Arl2 GTP hydrolysis is activated to alter Ξ±Ξ²-tubulin conformation |
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Download this section (compressed HTML)Q: Is the physiological GAP target of human TBCC beta-tubulin, ARL2, or both in sequence? The Cowan-lab assays measured tubulin GTP hydrolysis, whereas reconstitution with yeast proteins attributes the hydrolysis to ARL2 in the TBCD-TBCE-ARL2 cage.
Q: Is TBCC centrosomal enrichment linked to its tubulin-folding role, or does it reflect a separate spindle-pole function, given the multipolar spindle phenotype on depletion?
Experiment: Reconstitute the human TBCD-TBCE-ARL2 complex with alpha/beta-tubulin, add wild-type or R262A TBCC, and measure single-turnover GTP hydrolysis using ARL2 locked in the GTP state (Q70L) and GTP-locked tubulin to assign which GTPase hydrolyses.
Hypothesis: Human TBCC R262 is the arginine finger for ARL2 GTP hydrolysis within the human TBCD-TBCE-ARL2-tubulin complex.
Type: enzyme kinetics
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