Affinage

TBCB

Tubulin-folding cofactor B · UniProt Q99426

Length
244 aa
Mass
27.3 kDa
Annotated
2026-04-28
11 papers in source corpus 9 papers cited in narrative 10 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TBCB is a tubulin-folding cofactor that captures α-tubulin intermediates released from the CCT chaperonin and cooperates with TBCE to dissociate αβ-tubulin heterodimers through an energy-independent steric-displacement mechanism, forming a TBCE–TBCB–α-tubulin ternary complex that routes α-tubulin toward proteasomal degradation while freed β-tubulin is recycled by TBCA (PMID:18199681, PMID:25908846, PMID:33968934). This TBCE/TBCB+TBCA system sets the critical concentration of free tubulin heterodimers and thereby regulates microtubule dynamics; elevated TBCB destabilizes microtubules and promotes α-tubulin degradation, causing motor axon defects in vivo (PMID:17184771, PMID:30030593). TBCB protein stability is controlled by Gigaxonin-mediated ubiquitination, HSP90 binding, and PAK1-dependent phosphorylation (PMID:28393858). A homozygous loss-of-function TBCB variant causes a neurodevelopmental disorder with impaired motor function, establishing TBCB as essential for normal CNS development and axonal integrity (PMID:40856104).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 1996 Low

    Initial cloning of TBCB (then CKAP1) identified a CAP-Gly domain, placing it among cytoskeleton-associated proteins but leaving its biochemical function unknown.

    Evidence cDNA cloning from human fetal brain, Northern blot, FISH mapping

    PMID:8978778

    Open questions at the time
    • No functional or binding assay performed
    • CAP-Gly domain role in tubulin interaction not tested
    • Expression pattern limited to Northern blot
  2. 2006 High

    Demonstrating that TBCB forms a binary complex with TBCE and greatly enhances heterodimer dissociation established the functional partnership central to tubulin homeostasis, and showed that the products are a TBCE–TBCB–α-tubulin ternary complex plus free β-tubulin captured by TBCA.

    Evidence Overexpression, in vitro dissociation assays, co-immunoprecipitation, and immunofluorescence showing microtubule depolymerization upon TBCB overexpression

    PMID:17184771

    Open questions at the time
    • Structural basis of TBCB–TBCE cooperation unknown
    • Mechanism of energy-independent dissociation unclear
    • Physiological stoichiometry of TBCE/TBCB not determined
  3. 2008 High

    Showing that TBCB directly binds CCT-generated α-tubulin folding intermediates — and that a disease-causing α-tubulin mutation disrupts this interaction — established TBCB as the immediate downstream acceptor of nascent α-tubulin from the chaperonin.

    Evidence In vitro folding assays with CCT, pulldown assays with TBCB, TUBA1A R264C mutagenesis

    PMID:18199681

    Open questions at the time
    • Binding interface between TBCB and α-tubulin not structurally resolved
    • Whether TBCB functions as monomer or oligomer when receiving α-tubulin unknown
  4. 2015 High

    The cryo-EM structure of the TBCE–TBCB–α-tubulin ternary complex revealed that heterodimer dissociation is energy-independent, driven by steric clash between TBCE domains and β-tubulin, and that protruding UBL domains suggest a direct route to proteasomal degradation of α-tubulin.

    Evidence Cryo-EM 3D reconstruction, X-ray crystallography of TBCE UBL domain, atomic docking, biochemical dissociation assays

    PMID:25908846

    Open questions at the time
    • Direct proteasome recruitment by UBL domains not experimentally demonstrated
    • High-resolution atomic model of full complex not achieved
    • Contribution of TBCB CAP-Gly domain to steric displacement not individually dissected
  5. 2017 Medium

    Identification of TBCB post-translational regulation — Gigaxonin-mediated ubiquitination, HSP90-dependent stabilization, and PAK1 phosphorylation — revealed how TBCB protein levels are tuned upstream of its tubulin-destabilizing activity.

    Evidence Co-immunoprecipitation, ubiquitination assays, RNAi, overexpression in cells; HILI/PIWIL2 shown to modulate all three regulatory axes

    PMID:28393858

    Open questions at the time
    • Phosphorylation site(s) on TBCB not mapped
    • In vivo relevance of HILI–TBCB axis outside cancer cell lines not tested
    • Whether Gigaxonin disease mutations dysregulate TBCB levels in patients not shown
  6. 2018 Medium

    Demonstrating that elevated TBCB destabilizes TUBA4A and causes motor axon defects in vivo linked TBCB dosage directly to neuronal cytoskeletal integrity through a miR-1825/TBCB/TUBA4A regulatory axis.

    Evidence Proteomics, transcriptomics, gain/loss-of-function in cell lines, in vivo motor axon phenotype, patient tissue validation

    PMID:30030593

    Open questions at the time
    • Whether TBCB excess acts solely through TUBA4A or also other α-tubulin isoforms not resolved
    • miR-1825 regulation of TBCB not confirmed independently
    • Specificity of motor neuron vulnerability unclear
  7. 2020 Medium

    Discovery that the Salmonella effector SseK1 glycosylates TBCB on arginine residues to stabilize microtubules revealed TBCB as a pathogen target, showing that post-translational modification can switch TBCB from a microtubule-destabilizing to a stabilizing state.

    Evidence Yeast two-hybrid, glycosyltransferase assay, DxD motif mutagenesis, immunofluorescence in HEK293T cells

    PMID:32366039

    Open questions at the time
    • Specific arginine residues modified not identified
    • Mechanism by which glycosylation reverses TBCB's destabilizing activity unclear
    • Relevance during actual Salmonella infection not demonstrated
  8. 2021 High

    Showing that colchicine blocks TBCE/TBCB-mediated heterodimer dissociation and that TBCA levels independently control free heterodimer concentration established that the TBCE/TBCB+TBCA system functions as a homeostatic rheostat for soluble tubulin.

    Evidence In vitro dissociation assays with colchicine, TBCA RNAi and overexpression, western blot, colchicine/nocodazole/cycloheximide treatment in cells

    PMID:33968934

    Open questions at the time
    • Whether the system responds to microtubule polymerization state in a feedback loop not shown
    • Quantitative kinetic parameters of the dissociation reaction not determined
  9. 2025 Medium

    Identification of a homozygous TBCB loss-of-function variant causing neurodevelopmental disease — validated in patient fibroblasts, yeast, and Drosophila — established TBCB as essential for CNS development and axonal function in humans.

    Evidence Exome sequencing, patient fibroblast western blot/IF, yeast ALF1 benomyl sensitivity, CRISPR Drosophila climbing/survival assays

    PMID:40856104

    Open questions at the time
    • Single family reported; additional kindreds needed
    • Whether disease mechanism is tubulin destabilization or impaired folding not distinguished
    • CNS-specific role versus general cytoskeletal requirement not dissected

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unknown how the TBCE–TBCB–α-tubulin complex is delivered to the proteasome, whether the UBL domains directly engage proteasomal receptors, how PAK1 phosphorylation sites modulate TBCB activity, and whether TBCB loss-of-function disease operates through failed tubulin folding, impaired heterodimer turnover, or both.
  • Proteasome engagement by UBL domains not experimentally validated
  • Phosphorylation sites on TBCB unmapped
  • Pathomechanism of TBCB deficiency (folding vs. turnover) unresolved

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008092 cytoskeletal protein binding 4 GO:0044183 protein folding chaperone 3
Localization
GO:0005829 cytosol 3
Pathway
GO:0005856 cytoskeleton 3 R-HSA-392499 Metabolism of proteins 3
Partners
GANHSP90PAK1PIWIL2TBCATBCETUBA1ATUBA4A
Complex memberships
TBCE-TBCB-α-tubulin ternary complex

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 TBCB forms a binary complex with TBCE that greatly enhances the efficiency of TBCE to dissociate tubulin heterodimers both in vivo and in vitro; TBCE, TBCB, and α-tubulin form a ternary complex after heterodimer dissociation, while free β-tubulin is recovered by TBCA. Overexpression, in vitro dissociation assays, co-immunoprecipitation, biochemical fractionation Experimental cell research High 17184771
2006 Overexpression of TBCB depolymerizes microtubules in vivo, demonstrating a direct role for TBCB in regulating microtubule dynamics. Overexpression in cells, immunofluorescence microscopy of microtubule network Experimental cell research Medium 17184771
2008 TBCB binds CCT-generated folding intermediates of α-tubulin; the pachygyria-causing R264C mutation in TUBA1A fails to stably interact with TBCB, contributing to deficient tubulin heterodimer formation. In vitro folding assays with CCT, pulldown/binding assays with TBCB, mutagenesis Molecular biology of the cell High 18199681
2015 Cryo-EM structure of the human TBCE–TBCB–α-tubulin (αEB) ternary complex reveals that heterodimer dissociation is energy-independent and is driven by steric clash between β-tubulin and the TBCE CAP-Gly and LRR domains; UBL domains of the chaperones protrude in a configuration suggesting direct interaction with the proteasome for α-tubulin degradation. Electron microscopy, image processing (3D reconstruction), X-ray crystallography of TBCE UBL domain, atomic docking, biochemical dissociation assays Journal of cell science High 25908846
2017 HILI (PIWIL2) interacts with TBCB and inhibits Gigaxonin-mediated ubiquitination and proteasomal degradation of TBCB by promoting HSP90–TBCB binding and blocking Gigaxonin–TBCB interaction; HILI also reduces PAK1-induced phosphorylation of TBCB, collectively increasing TBCB levels and destabilizing microtubules. Co-immunoprecipitation, ubiquitination assays, RNAi knockdown, overexpression, western blot, immunofluorescence Scientific reports Medium 28393858
2018 Elevated TBCB leads to depolymerization and degradation of α-tubulin TUBA4A; excess TBCB causes motor axon defects in an in vivo model, placing TBCB upstream of TUBA4A stability in a miR-1825/TBCB/TUBA4A pathway. Proteomic and transcriptomic analysis, loss-of-function/gain-of-function in cell lines, in vivo motor axon phenotype assay, patient tissue validation Cellular and molecular life sciences Medium 30030593
2020 The Salmonella effector SseK1 directly binds TBCB (identified by yeast two-hybrid) and catalyzes N-acetylglucosamine addition to arginine residues on TBCB; this modification promotes microtubule cytoskeleton stabilization in host cells, and requires the conserved DxD catalytic motif of SseK1. Yeast two-hybrid, glycosyltransferase activity assay, mutagenesis of DxD motif, immunofluorescence of microtubule network in HEK293T cells International journal of molecular sciences Medium 32366039
2021 Colchicine inhibits tubulin heterodimer dissociation by the TBCE/TBCB complex in vitro, likely by interfering with TBCE–tubulin dimer interactions; this leads to accumulation of free TBCA and reduced tubulin heterodimer recycling. Manipulation of TBCA levels by RNAi or overexpression decreases tubulin heterodimer levels, indicating the TBCE/TBCB+TBCA system controls the critical concentration of free tubulin heterodimers. In vitro dissociation assays with colchicine, RNAi knockdown and overexpression of TBCA, western blot, human cell treatment with colchicine/nocodazole/cycloheximide Frontiers in cell and developmental biology High 33968934
2025 A homozygous p.Tyr197Asn variant in TBCB reduces TBCB protein levels in patient fibroblasts and causes a loss-of-function phenotype (benomyl hypersensitivity in the yeast ortholog ALF1); the homologous Drosophila mutant shows reduced survival and impaired motor function, establishing that TBCB is required for normal axonal function and CNS development. Exome sequencing, western blot and immunofluorescence in patient fibroblasts, yeast ALF1 ortholog benomyl sensitivity assay, CRISPR-Cas9 Drosophila mutant climbing/survival assay Genetics in medicine Medium 40856104
1996 TBCB (CKAP1) was cloned from human fetal brain cDNA and found to encode a protein containing a CAP-Gly domain conserved among cytoskeleton-associated proteins; the gene was mapped to chromosome 19q13.11–q13.12. cDNA cloning, Northern blot, FISH chromosomal mapping Cytogenetics and cell genetics Low 8978778

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 40 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 36 25908846
2021 Colchicine Blocks Tubulin Heterodimer Recycling by Tubulin Cofactors TBCA, TBCB, and TBCE. Frontiers in cell and developmental biology 19 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