Affinage

CABP2

Calcium-binding protein 2 · UniProt Q9NPB3

Length
220 aa
Mass
24.5 kDa
Annotated
2026-04-28
12 papers in source corpus 9 papers cited in narrative 9 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CABP2 is a calmodulin-related calcium-binding protein that functions as a critical modulator of CaV1.3 voltage-gated calcium channels in sensory neurons, sustaining presynaptic calcium influx required for indefatigable synaptic transmission. In inner hair cells, CABP2 inhibits steady-state inactivation of CaV1.3 channels, thereby maintaining channel availability for continuous synaptic sound encoding; loss of CABP2 causes enhanced CaV1.3 inactivation, impaired presynaptic calcium-dependent signaling, elevated auditory brainstem response thresholds, and autosomal recessive hearing loss DFNB93, a phenotype rescued by AAV-mediated gene delivery in knockout mice (PMID:22981119, PMID:34489639, PMID:29661613). CABP2 also modulates retinal ganglion cell light responses through analogous presynaptic calcium signaling without affecting synapse morphology (PMID:27822497).

Mechanistic history

Synthesis pass · year-by-year structured walk · 4 steps
  1. 2012 High

    Establishing that CABP2 directly regulates CaV1.3 calcium channel activity and that a human loss-of-function truncation (p.Phe164Serfs*4) disrupts both calcium binding and channel modulation, linking CABP2 to DFNB93 hearing loss.

    Evidence Isothermal titration calorimetry and functional CaV1.3 channel regulation assays comparing wild-type and truncated CABP2 protein

    PMID:22981119

    Open questions at the time
    • Mechanism of CABP2–CaV1.3 physical interaction and binding site not defined
    • In vivo consequences of CABP2 loss on hair cell physiology not yet demonstrated
  2. 2016 High

    Demonstrating that CABP2 functions beyond the cochlea, as Cabp2 knockout alters retinal ganglion cell light response amplitude and kinetics without morphological defects, establishing a broader sensory role through presynaptic calcium signaling.

    Evidence Whole-cell patch clamp of retinal ganglion cells in Cabp2 KO mice with immunohistochemistry and electron microscopy controls

    PMID:27822497

    Open questions at the time
    • Whether retinal phenotype is mediated specifically through CaV1.3 or other CaV channels not resolved
    • Contribution of individual retinal cell types to the signaling deficit unclear
  3. 2018 High

    Confirming that CABP2 is essential for normal peripheral auditory function in vivo, with knockout mice showing elevated ABR thresholds and reduced mid-frequency DPOAEs alongside impaired IHC presynaptic calcium signaling.

    Evidence ABR, DPOAE, and whole-cell patch clamp recordings in Cabp2 KO mice

    PMID:29661613

    Open questions at the time
    • Precise biophysical mechanism by which CABP2 prevents CaV1.3 steady-state inactivation not yet dissected
    • Whether outer hair cell function contributes to the mid-frequency DPOAE deficit not fully delineated
  4. 2021 High

    Defining the specific biophysical mechanism — CABP2 prevents enhanced steady-state inactivation of CaV1.3 channels in IHCs — and demonstrating that AAV-mediated Cabp2 gene delivery restores CaV1.3 function and improves hearing, establishing proof-of-concept gene therapy for DFNB93.

    Evidence Whole-cell patch clamp of IHCs in Cabp2 KO mice; AAV2/1 and AAV-PHP.eB delivery with functional and audiometric assessment

    PMID:34489639

    Open questions at the time
    • Structural basis of CABP2 inhibition of CaV1.3 inactivation gating not resolved
    • Long-term durability and cell-type specificity of AAV-mediated rescue in vivo not established
    • Whether CABP2 modulates other ion channels or signaling partners in IHCs unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • The structural interface between CABP2 and CaV1.3, the stoichiometry of the interaction, and whether CABP2 engages additional molecular partners at the IHC active zone remain unresolved.
  • No structural or cryo-EM model of the CABP2–CaV1.3 complex exists
  • Whether CABP2 participates in a larger presynaptic signaling complex is unknown
  • Functional redundancy with other CaBP family members (CaBP1, CaBP4) in IHCs not systematically tested

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 3
Localization
GO:0005829 cytosol 2
Pathway
R-HSA-112316 Neuronal System 4 R-HSA-9709957 Sensory Perception 2
Partners
CACNA1D

Evidence

Reading pass · 9 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2012 CaBP2 modulates CaV1.3 Ca2+ channel activity; a truncated CaBP2 (p.Phe164Serfs*4) caused by splice-site mutation showed altered Ca2+ binding by isothermal titration calorimetry and less potent regulation of CaV1.3 Ca2+ channels compared to wild-type CaBP2. Isothermal titration calorimetry (Ca2+ binding), functional channel regulation assay comparing wild-type vs. truncated CaBP2 American journal of human genetics High 22981119
2021 Loss of CaBP2 in inner hair cells causes enhanced steady-state inactivation of CaV1.3 Ca2+ channels, limiting their availability to trigger synaptic transmission; AAV-mediated delivery of Cabp2 restored IHC CaV1.3 function and improved hearing in Cabp2 knockout mice. Whole-cell patch clamp of IHCs in Cabp2 KO mice; AAV2/1 and AAV-PHP.eB gene delivery with in vitro and in vivo functional assessment Frontiers in molecular neuroscience High 34489639
2018 CaBP2 KO mice exhibit significant ABR threshold elevations and, at 9 weeks, reduced DPOAEs in the mid-frequency range, demonstrating that CaBP2 is required for normal peripheral auditory function; CaBP2 KO IHC presynaptic Ca2+-dependent signaling is affected. Auditory brainstem responses (ABRs), distortion product otoacoustic emissions (DPOAEs), and whole-cell patch clamp in CaBP2 KO mice Hearing research High 29661613
2016 CaBP2 KO mice show altered retinal ganglion cell light response amplitude and kinetics without gross retinal or synapse morphology changes, indicating CaBP2 is required for proper transmission of light responses through the retina, likely via modulation of presynaptic Ca2+-dependent signaling. Whole-cell patch clamp of retinal ganglion cells in Cabp2 KO mice; immunohistochemistry; transmission electron microscopy eNeuro High 27822497
1993 CaBP2 (rat homolog of ERp72) possesses protein disulfide isomerase activity, contains three thioredoxin-like motifs (-EFYAPNCGHCK-) and a C-terminal -KEEL ER retention sequence, and is a glycoprotein with O-linked glycans localized in the ER. cDNA sequencing, PDI activity assay, glycosylation analysis European journal of biochemistry Medium 8477750
1994 CaBP2 (ERp72/rat ER protein) catalyzes renaturation of denatured reduced proteins (Fab fragment, RNase AIII) in a GSH/GSSG-dependent manner, demonstrating disulfide bond formation and protein disulfide isomerase activity in the ER secretory pathway. In vitro renaturation assay with denatured reduced model proteins, variation of redox conditions The Journal of biological chemistry Medium 8300576
1995 CaBP2 (ERp72) is a substrate for thioredoxin reductase and catalyzes NADPH-dependent insulin disulfide reduction; its active-site disulfide/dithiol has a redox potential similar to PDI (~-235 mV), consistent with a role in forming protein disulfide bonds in the ER. Thioredoxin reductase substrate assay, NADPH oxidation stoichiometry, redox potential measurement using mutant Trx FEBS letters Medium 7835433
1997 CaBP2 (ERp72) is phosphorylated by protein kinase CK2 and by an endogenous CK2-type kinase in microsomes, as identified by 32P-labeling, purification, and amino acid sequencing of phosphopeptides. In vitro phosphorylation with CK2 and [32P]ATP, SDS-PAGE, purification, proteolytic cleavage, amino acid sequencing Journal of biochemistry Medium 9058200
2025 CaBP2 modulates presynaptic CaV1.3 Ca2+ channel function in inner hair cells and is required for indefatigable synaptic sound encoding; lack of CaBP2 impairs synaptic sound encoding via enhanced steady-state inactivation of CaV1.3 channels. Summary/review of DFNB93 mouse model electrophysiology and preclinical gene therapy studies MedComm Medium 40927552

Source papers

Stage 0 corpus · 12 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2012 A mutation in CABP2, expressed in cochlear hair cells, causes autosomal-recessive hearing impairment. American journal of human genetics 84 22981119
1994 Effects of CaBP2, the rat analog of ERp72, and of CaBP1 on the refolding of denatured reduced proteins. Comparison with protein disulfide isomerase. The Journal of biological chemistry 77 8300576
1995 Two resident ER-proteins, CaBP1 and CaBP2, with thioredoxin domains, are substrates for thioredoxin reductase: comparison with protein disulfide isomerase. FEBS letters 72 7835433
1993 CaBP2 is a rat homolog of ERp72 with proteindisulfide isomerase activity. European journal of biochemistry 57 8477750
2021 Cabp2-Gene Therapy Restores Inner Hair Cell Calcium Currents and Improves Hearing in a DFNB93 Mouse Model. Frontiers in molecular neuroscience 21 34489639
2018 Functions of CaBP1 and CaBP2 in the peripheral auditory system. Hearing research 21 29661613
2016 Lack of CaBP1/Caldendrin or CaBP2 Leads to Altered Ganglion Cell Responses. eNeuro 13 27822497
2019 A Novel Pathogenic Variant in the CABP2 Gene Causes Severe Nonsyndromic Hearing Loss in a Consanguineous Iranian Family. Audiology & neuro-otology 10 31661684
1997 Phosphorylation of CaBP1 and CaBP2 by protein kinase CK2. Journal of biochemistry 10 9058200
2018 Genetic Linkage Analysis of DFNB4, DFNB28, DFNB93 Loci in Autosomal Recessive Non-syndromic Hearing Loss: Evidence for Digenic Inheritance in GJB2 and GJB3 Mutations. Iranian journal of public health 8 29318123
2021 First reported CABP2-related non-syndromic hearing loss in Northern Europe. Molecular genetics & genomic medicine 6 33666369
2025 Is CABP2-Associated Hearing Loss (DFNB93) a Gene Therapy Target? Preclinical Progress and Patient Registry. MedComm 1 40927552