Affinage

TBCB

Tubulin-folding cofactor B · UniProt Q99426

Length
244 aa
Mass
27.3 kDa
Annotated
2026-06-10
11 papers in source corpus 9 papers cited in narrative 9 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/7 claims corpus-supported (86%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TBCB is a tubulin-folding cofactor that controls the cellular pool of αβ-tubulin heterodimers and thereby microtubule dynamics (PMID:17184771, PMID:33968934). It stably engages α-tubulin folding intermediates emerging from the CCT chaperonin as an obligate step in de novo heterodimer formation; a pachygyria-causing TUBA1A R264C mutation disrupts this interaction and reduces assembly efficiency (PMID:18199681). Acting together with TBCE, TBCB forms a binary complex that greatly enhances dissociation of αβ-tubulin heterodimers, after which TBCE, TBCB, and α-tubulin form a ternary complex while free β-tubulin is recovered by TBCA (PMID:17184771). Structural work on the human TBCE–TBCB–α-tubulin complex established that dissociation is energy-independent, driven by steric interaction between β-tubulin and the TBCE CAP-Gly and LRR domains, with protruding UBL domains positioned to direct α-tubulin to the proteasome (PMID:25908846); this TBCE/TBCB–TBCA system recycles pre-existing rather than newly synthesized heterodimers and is inhibited by colchicine (PMID:33968934). TBCB abundance is tightly regulated: HILI (PIWIL2) promotes HSP90 binding and suppresses Gigaxonin-mediated ubiquitination and PAK1-mediated phosphorylation to stabilize TBCB and destabilize microtubules (PMID:28393858), while miR-1825 controls TBCB translation, and excess TBCB depolymerizes and degrades the α-tubulin isoform TUBA4A, causing motor axon defects (PMID:30030593). The Salmonella effector SseK1 binds and Arg-GlcNAcylates TBCB to stabilize the host microtubule cytoskeleton (PMID:32366039). A homozygous loss-of-function TBCB variant (p.Tyr197Asn) reduces TBCB protein levels and causes a neurodevelopmental disorder with spastic paraparesis, with corroborating microtubule-destabilization and neuronal phenotypes in yeast and Drosophila models (PMID:40856104).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2006 High

    Established that TBCB does not act alone but partners with TBCE to actively dissociate tubulin heterodimers, defining its place in the tubulin-cofactor cycle.

    Evidence Overexpression, in vitro tubulin dissociation assays, and complex characterization showing TBCB enhances TBCE dissociation activity and forms a TBCE–TBCB–α-tubulin ternary complex with β-tubulin recovered by TBCA

    PMID:17184771

    Open questions at the time
    • Did not resolve the atomic mechanism of dissociation
    • Fate of the ternary complex (e.g., degradation routing) not established
  2. 2008 High

    Showed TBCB acts upstream in biogenesis by capturing CCT-generated α-tubulin intermediates, making it an obligate step in de novo heterodimer formation rather than only a recycling factor.

    Evidence In vitro folding/assembly reconstitution, co-IP of TBCB with CCT intermediates, and functional comparison of wild-type vs. R264C TUBA1A

    PMID:18199681

    Open questions at the time
    • Did not define how TBCB hands off intermediates downstream
    • Generalizability across α-tubulin isoforms not tested
  3. 2015 High

    Defined the structural and energetic basis of heterodimer dissociation, revealing it is steric/energy-independent and positions α-tubulin for proteasomal degradation.

    Evidence Cryo-EM 3D reconstruction of the TBCE–TBCB–α-tubulin complex, X-ray crystallography of the TBCE UBL domain, and biochemical dissociation assays with domain docking

    PMID:25908846

    Open questions at the time
    • Direct proteasome engagement via UBL domains inferred from geometry, not demonstrated
    • TBCB-specific contributions versus TBCE within the complex not fully separated
  4. 2017 Medium

    Identified post-translational control of TBCB stability, linking HILI/HSP90, Gigaxonin ubiquitination, and PAK1 phosphorylation to microtubule destabilization.

    Evidence Reciprocal co-IP of HILI–TBCB, HILI–HSP90–TBCB, and Gigaxonin–TBCB, plus ubiquitination, phosphorylation, and microtubule polymerization assays in a single lab

    PMID:28393858

    Open questions at the time
    • No independent replication
    • Phosphorylation sites and ubiquitination acceptor residues on TBCB not mapped
  5. 2018 Medium

    Connected TBCB dosage to isoform-specific tubulin degradation and an in vivo neuronal phenotype, showing miR-1825 sets TBCB levels that govern TUBA4A stability.

    Evidence Transcriptomic/proteomic analysis, miRNA manipulation, in vivo motor axon phenotype assay, and western blot validation in patient tissue

    PMID:30030593

    Open questions at the time
    • Mechanism of TUBA4A selectivity over other α-tubulins unresolved
    • Single lab, no independent replication
  6. 2020 Medium

    Revealed TBCB as a target of bacterial subversion, with the Salmonella effector SseK1 modifying TBCB to stabilize host microtubules.

    Evidence Yeast two-hybrid screen, in cellulo glycosyltransferase assay, DxD active-site mutagenesis, and microtubule stability assay in HEK293T cells

    PMID:32366039

    Open questions at the time
    • Arg-GlcNAcylated residues on TBCB not mapped
    • How modification mechanistically alters TBCB function in the cofactor cycle unknown
  7. 2021 Medium

    Clarified that the TBCE/TBCB + TBCA system recycles pre-existing heterodimers to control free tubulin pools, and that colchicine blocks this dissociation step.

    Evidence In vitro dissociation assays with colchicine, TBCA RNAi and overexpression, and western blot of TBCA/β-tubulin complexes in human cells with control comparisons

    PMID:33968934

    Open questions at the time
    • Direct colchicine binding site within the dissociation machinery not defined
    • Single lab
  8. 2025 Medium

    Established TBCB loss-of-function as a cause of human neurodevelopmental disease, tying its tubulin-folding role to axonal/neuronal function across species.

    Evidence Exome sequencing, patient fibroblast western blot/immunofluorescence, yeast ALF1 mutant benomyl-sensitivity assay, and CRISPR-Cas9 Drosophila climbing/survival phenotype

    PMID:40856104

    Open questions at the time
    • Single index variant/family; allelic spectrum unknown
    • Cellular mechanism linking reduced TBCB to spastic paraparesis not defined

Open questions

Synthesis pass · forward-looking unresolved questions
  • How TBCB dosage, post-translational regulation, and isoform-selective α-tubulin handling integrate to produce tissue-specific neuronal vulnerability remains open.
  • No structural account of how TBCB discriminates α-tubulin isoforms such as TUBA4A
  • Regulatory inputs (Gigaxonin, PAK1, HILI, miR-1825) not integrated into a quantitative model of TBCB abundance in neurons

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008092 cytoskeletal protein binding 4 GO:0098772 molecular function regulator activity 2
Localization
GO:0005829 cytosol 2 GO:0005856 cytoskeleton 2
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 3 R-HSA-392499 Metabolism of proteins 2
Partners
CCTGANHSP90PAK1PIWIL2SSEK1TBCATBCE
Complex memberships
TBCE–TBCB–α-tubulin ternary complex

Evidence

Reading pass · 9 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 TBCB overexpression depolymerizes microtubules in vivo. TBCB forms a binary complex with TBCE that greatly enhances TBCE's efficiency in dissociating tubulin heterodimers both in vivo and in vitro. After heterodimer dissociation, TBCE, TBCB, and α-tubulin form a ternary complex, while free β-tubulin is recovered by TBCA. Overexpression assays, in vitro tubulin dissociation assays, co-immunoprecipitation/complex formation assays Experimental cell research High 17184771
2008 TBCB stably interacts with CCT-generated α-tubulin folding intermediates as an obligate step in de novo tubulin heterodimer formation. A pachygyria-causing TUBA1A mutation (R264C) disrupts this stable interaction with TBCB, thereby reducing the efficiency of tubulin heterodimer assembly. In vitro tubulin folding/assembly assays, co-immunoprecipitation of TBCB with CCT-generated intermediates, functional comparison of wild-type vs. R264C mutant α-tubulin Molecular biology of the cell High 18199681
2015 Cryo-EM and X-ray crystallography revealed the 3D structure of the human TBCE–TBCB–α-tubulin (αEB) ternary complex formed upon heterodimer dissociation. Heterodimer dissociation is an energy-independent process driven by a steric interaction between β-tubulin and the TBCE CAP-Gly and LRR domains. The protruding arrangement of UBL domains in the αEB complex suggests direct interaction with the proteasome, mediating α-tubulin degradation. Electron microscopy and image processing (3D reconstruction), X-ray crystallography of TBCE UBL domain, biochemical dissociation assays, atomic domain docking Journal of cell science High 25908846
2017 HILI (PIWIL2) interacts with TBCB, promotes binding of HSP90 to TBCB, and suppresses both Gigaxonin-mediated ubiquitination/degradation of TBCB and PAK1-induced phosphorylation of TBCB, thereby stabilizing TBCB protein levels and destabilizing microtubules. Co-immunoprecipitation of HILI–TBCB, HILI–HSP90–TBCB, and Gigaxonin–TBCB interactions; ubiquitination assays; phosphorylation assays; microtubule polymerization assays Scientific reports Medium 28393858
2018 Excess TBCB causes depolymerization and degradation of α-tubulin TUBA4A, establishing TBCB as a regulator of TUBA4A protein stability. Reduced miR-1825 leads to translational upregulation of TBCB, which in turn reduces TUBA4A levels and causes motor axon defects in vivo. Transcriptomic/proteomic analysis, miRNA manipulation, in vivo motor axon phenotype assay, western blot validation in patient tissue Cellular and molecular life sciences : CMLS Medium 30030593
2020 The Salmonella effector SseK1 binds TBCB (identified by yeast two-hybrid) and catalyzes N-acetylglucosamine addition to arginine residues on TBCB; this modification stabilizes the host microtubule cytoskeleton. The SseK1 DxD motif is required for both TBCB binding/modification and the cytoskeletal effect. Yeast two-hybrid screen, glycosyltransferase activity assay (in cellulo), DxD active-site mutagenesis, microtubule stability assay in HEK293T cells International journal of molecular sciences Medium 32366039
2021 TBCE/TBCB together dissociate tubulin heterodimers, and colchicine inhibits this dissociation, likely by interfering with TBCE–tubulin dimer interactions, leading to accumulation of free TBCA. The TBCE/TBCB + TBCA system recycles pre-existing tubulin heterodimers (rather than newly synthesized tubulins) to control the pool of free tubulin heterodimers and microtubule dynamics. Manipulation of TBCA levels by RNAi or overexpression decreased tubulin heterodimer levels, confirming functional coupling. In vitro tubulin dissociation assays with colchicine, RNAi knockdown and overexpression of TBCA, western blot of TBCA/β-tubulin complexes in colchicine-treated human cells, comparison with nocodazole/cold shock/cycloheximide controls Frontiers in cell and developmental biology Medium 33968934
1996 TBCB (CKAP1) encodes a protein containing a CAP-Gly domain conserved among cytoskeleton-associated proteins; the CAP-Gly domain is thought to be essential for microtubule association. The gene maps to chromosome 19q13.11→q13.12. cDNA cloning from human fetal-brain library, Northern blot, FISH chromosomal mapping Cytogenetics and cell genetics Low 8978778
2025 A homozygous loss-of-function variant in TBCB (p.Tyr197Asn) reduces TBCB protein levels in patient fibroblasts and causes a neurodevelopmental disorder with spastic paraparesis. The yeast ortholog ALF1 carrying the equivalent mutation shows increased benomyl sensitivity (a microtubule-destabilizing agent), consistent with impaired tubulin-folding function. A CRISPR-Cas9 homologous mutant in Drosophila displays reduced survival and impaired climbing, confirming an essential role of TBCB in neuronal/axonal function. Exome sequencing, western blot and immunofluorescence in patient fibroblasts, yeast ALF1 mutant benomyl sensitivity assay, CRISPR-Cas9 Drosophila model with climbing/survival phenotype Genetics in medicine : official journal of the American College of Medical Genetics Medium 40856104

Source papers

Stage 0 corpus · 11 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Role of cofactors B (TBCB) and E (TBCE) in tubulin heterodimer dissociation. Experimental cell research 63 17184771
2008 A pachygyria-causing alpha-tubulin mutation results in inefficient cycling with CCT and a deficient interaction with TBCB. Molecular biology of the cell 41 18199681
2018 Dysregulation of a novel miR-1825/TBCB/TUBA4A pathway in sporadic and familial ALS. Cellular and molecular life sciences : CMLS 38 30030593
2015 The structure of the complex between α-tubulin, TBCE and TBCB reveals a tubulin dimer dissociation mechanism. Journal of cell science 38 25908846
2021 Colchicine Blocks Tubulin Heterodimer Recycling by Tubulin Cofactors TBCA, TBCB, and TBCE. Frontiers in cell and developmental biology 20 33968934
2017 HILI destabilizes microtubules by suppressing phosphorylation and Gigaxonin-mediated degradation of TBCB. Scientific reports 15 28393858
1996 Cloning, expression, and mapping of CKAPI, which encodes a putative cytoskeleton-associated protein containing a CAP-GLY domain. Cytogenetics and cell genetics 12 8978778
2020 Tubulin Folding Cofactor TBCB is a Target of the Salmonella Effector Protein SseK1. International journal of molecular sciences 7 32366039
2023 High Expression of Microtubule-associated Protein TBCB Predicts Adverse Outcome and Immunosuppression in Acute Myeloid Leukemia. Journal of Cancer 3 37476188
2026 Multi-omics analysis identifies TBCB as a therapeutic target in sepsis-induced liver injury. International journal of surgery (London, England) 0 41570284
2025 A founder variant in TBCB is associated with global developmental delay, autism spectrum, and spastic paraparesis. Genetics in medicine : official journal of the American College of Medical Genetics 0 40856104

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