Affinage

CABP4

Calcium-binding protein 4 · UniProt P57796

Length
275 aa
Mass
30.4 kDa
Annotated
2026-04-28
17 papers in source corpus 7 papers cited in narrative 7 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CaBP4 is a calmodulin-like neuronal calcium-binding protein that functions as a critical modulator of presynaptic calcium channel gating at photoreceptor ribbon synapses. CaBP4 binds the IQ motif of the Cav1.4 L-type calcium channel via its Ca²⁺-sensing C-lobe, displacing the channel's autoinhibitory ICDI domain to dramatically increase channel availability and sustain calcium influx required for tonic neurotransmitter release (PMID:22936811, PMID:25258313). Its modulatory activity is dynamically regulated by PKCζ-mediated phosphorylation at Ser37, which is enhanced in light-adapted retinas, and by PP2A-mediated dephosphorylation, creating a light-dependent tuning mechanism for presynaptic calcium signals (PMID:18003854, PMID:23341017). Loss of CaBP4 disrupts photoreceptor synapse structure, severely attenuates synaptic transmission to second-order neurons, and causes a congenital stationary night blindness-like (CSNB2-like) phenotype in mice and humans (PMID:16249514, PMID:16960802).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2005 High

    Whether CaBP4 is required for photoreceptor synaptic function was unknown; Cabp4-knockout mice demonstrated that CaBP4 is essential for both rod and cone synapse maintenance and for signal transmission to bipolar cells, establishing its non-redundant role at the outer plexiform layer.

    Evidence Double-knockout mouse (Cabp4−/−; Gnat1−/−), ERG, electron microscopy, immunocytochemistry

    PMID:16249514

    Open questions at the time
    • Molecular target through which CaBP4 exerts its synaptic effect was not identified
    • Whether CaBP4 acts pre- or postsynaptically was not resolved
  2. 2006 High

    The molecular target of CaBP4 at the synapse was identified as the Cav1.4 L-type calcium channel; direct association with the Cav1.4 C-terminal domain and phenotypic similarity between CaBP4 and Cav1.4 loss explained the synaptic transmission defect and linked CABP4 mutations to a CSNB2-like human disease.

    Evidence Candidate gene approach, protein interaction assays (CaBP4–Cav1.4 C-terminal domain binding), mouse knockout phenotype comparison, human patient analysis

    PMID:16960802

    Open questions at the time
    • Precise binding site on Cav1.4 and mechanism of channel modulation were undefined
    • Whether CaBP4 modulates other voltage-gated calcium channels was untested
  3. 2007 High

    How CaBP4 activity is regulated was unknown; identification of PKCζ phosphorylation at Ser37 and demonstration that this phosphorylation is light-dependent and required for CaBP4's ability to prolong Cav1.3 calcium currents revealed a post-translational switch linking ambient light conditions to presynaptic calcium dynamics.

    Evidence In vitro kinase assay, light/dark retinal phosphorylation assay, electrophysiology of S37A and EF-hand mutants in transfected cells

    PMID:18003854

    Open questions at the time
    • Identity of the phosphatase reversing Ser37 phosphorylation was unknown
    • Whether phosphorylation affects CaBP4–Cav1.4 interaction in native photoreceptors was not shown
  4. 2008 High

    Beyond Cav1.4, CaBP4's interactome at the synapse was uncharacterized; identification of Unc119 as a CaBP4-binding partner and its reduced synaptic localization in CaBP4-knockout mice suggested CaBP4 also stabilizes additional synaptic proteins at the photoreceptor terminal.

    Evidence Affinity chromatography, yeast two-hybrid, co-immunoprecipitation, immunohistochemistry and Western blot of CaBP4-KO retinas

    PMID:18296658

    Open questions at the time
    • Functional consequence of CaBP4–Unc119 interaction for synaptic transmission was not determined
    • Whether Unc119 loss contributes to the CaBP4-knockout phenotype was untested
  5. 2012 High

    The precise mechanism by which CaBP4 increases Cav1.4 channel availability was resolved: CaBP4 binds the IQ motif and counteracts the autoinhibitory ICDI domain, shifting voltage-dependent gating to open the channel at more negative potentials; disease-associated CaBP4 mutants partially lose this ability, explaining their pathogenicity.

    Evidence Patch-clamp electrophysiology of reconstituted Cav1.4 ± CaBP4 in heterologous cells, FRET, ICDI-deletion mutant, disease-associated CaBP4 mutant analysis

    PMID:22936811

    Open questions at the time
    • Atomic-resolution structure of the CaBP4–Cav1.4 complex was lacking
    • In vivo confirmation that ICDI relief is the primary CaBP4 mechanism at the synapse was absent
  6. 2013 High

    The phosphatase opposing PKCζ was identified as PP2A; PP2A associates with CaBP4 and dephosphorylates it, and inhibiting PP2A increases CaBP4 phosphorylation and potentiates its modulation of Cav1.3, establishing a bidirectional phosphorylation cycle that tunes presynaptic calcium signals.

    Evidence In vitro phosphatase assay with okadaic acid/fostriecin, pull-down from retinal lysates, PP2A subunit overexpression, electrophysiology in HEK293T cells

    PMID:23341017

    Open questions at the time
    • Whether the PKCζ/PP2A phosphorylation cycle operates on CaBP4–Cav1.4 (versus Cav1.3) in native photoreceptors was not demonstrated
    • Temporal dynamics of phosphorylation during light adaptation in vivo remain uncharacterized
  7. 2014 High

    Solution structures of CaBP4 revealed how Ca²⁺ sensing is converted into channel modulation: Ca²⁺ triggers a closed-to-open transition in the C-lobe that exposes a hydrophobic surface contacting the Cav1.4 IQ motif, and disruption of this interface (Cav1.4 Y1595E) impairs binding, providing a structural basis for the ICDI-displacement mechanism.

    Evidence NMR structure determination of Mg²⁺- and Ca²⁺-bound CaBP4, structural modeling of CaBP4–IQ interaction, binding assay with Cav1.4 Y1595E mutant

    PMID:25258313

    Open questions at the time
    • Co-crystal or co-NMR structure of the CaBP4–Cav1.4 IQ–ICDI ternary complex has not been solved
    • Whether the N-lobe contributes to Cav1.4 modulation beyond Mg²⁺/Ca²⁺ sensing is unresolved
    • Role of the disordered N-terminal region (residues 1–99) in synaptic function is unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • A complete structural model of CaBP4 bound to the Cav1.4 channel in the context of the ICDI domain, and in vivo validation that the phosphorylation-dependent tuning mechanism operates at Cav1.4 (not only Cav1.3) in native photoreceptor terminals, remain unresolved.
  • No co-structure of CaBP4 with Cav1.4 IQ+ICDI domains exists
  • In vivo phospho-dynamics at the Cav1.4 synapse have not been measured
  • Functional significance of the CaBP4–Unc119 interaction for synaptic transmission is untested

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 4 GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005829 cytosol 2
Pathway
GO:0005829 cytosol 1
Partners
CACNA1DCACNA1FPPP2CAPRKCZUNC119

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 CABP4 is directly associated with the C-terminal domain of the Cav1.4 alpha subunit (L-type Ca2+ channel) in photoreceptor synaptic terminals, and loss of CABP4 causes a CSNB2-like phenotype similar to loss of Cav1.4 alpha. Candidate gene approach, genetic epistasis (mouse knockout phenotype comparison), protein interaction (direct association with Cav1.4 C-terminal domain) American journal of human genetics High 16960802
2005 CaBP4 is essential for the development and maintenance of both rod and cone photoreceptor synapses; Cabp4(-/-) mice show reduced outer plexiform layer thickness and severely attenuated ERG b-wave amplitude and sensitivity, establishing CaBP4 as critical for signal transmission from photoreceptors to second-order neurons. Double knockout mouse (Cabp4(-/-)Gnat1(-/-)), immunocytochemistry, electron microscopy, ERG Investigative ophthalmology & visual science High 16249514
2008 CaBP4 physically interacts with Unc119 (MRG4) at the photoreceptor synapse; CaBP4 knockout mice show reduced Unc119 levels specifically at photoreceptor terminals, suggesting CaBP4 stabilizes Unc119 at the synapse. Affinity chromatography, yeast two-hybrid, co-immunoprecipitation, gel overlay assay, immunohistochemistry, Western blot of synaptic fractions from CaBP4-KO retinas Investigative ophthalmology & visual science High 18296658
2007 CaBP4 is phosphorylated by protein kinase C zeta (PKCzeta) at serine 37 both in vitro and in the retina; phosphorylation is greater in light-adapted retinas; mutation S37A abolishes CaBP4's ability to prolong Ca2+ current through Cav1.3, while inactivating Ca2+-binding site mutations strengthen Cav1.3 modulation, demonstrating that phosphorylation and Ca2+ binding regulate CaBP4's modulation of presynaptic Ca2+ channels. In vitro kinase assay, retinal phosphorylation assay (light vs. dark), electrophysiology of transfected cells with CaBP4 mutants (S37A, Ca2+-binding site mutations), colocalization by immunohistochemistry The Journal of neuroscience High 18003854
2012 CaBP4 dramatically increases Cav1.4 channel availability by interacting with the IQ motif of Cav1.4 and relieving inhibitory effects of the ICDI (inhibitor of Ca2+-dependent inactivation) domain on voltage-dependent gating; this effect is absent in a Cav1.4 mutant lacking the ICDI domain. Disease-associated CaBP4 mutants interact with Cav1.4 but only partially preserve its functional effects, reducing channel availability. Heterologous expression of Cav1.4, patch-clamp electrophysiology, FRET experiments, ICDI-deletion mutant Cav1.4, disease-associated CaBP4 mutant analysis The Journal of biological chemistry High 22936811
2014 NMR structures of CaBP4 in Mg2+-bound and Ca2+-bound states reveal an unstructured N-terminal region (residues 1-99) and four EF-hands in two lobes: the N-lobe (EF1/EF2) adopts a closed conformation with Mg2+ or Ca2+ at EF1, while the C-lobe (EF3/EF4) undergoes a Ca2+-induced closed-to-open transition. Ca2+-bound CaBP4 exposes residues (Phe137, Glu168, Leu207, Phe214, Met251, Phe264, Leu268) that contact the IQ motif of Cav1.4; Cav1.4 mutant Y1595E strongly impairs CaBP4 binding. CaBP4 is proposed to collapse around the IQ motif, disrupting IQ-ICDI interaction to promote channel activation. NMR structure determination, structural modeling of CaBP4-Cav1.4 IQ motif interaction, binding assay with Cav1.4 Y1595E mutant The Journal of biological chemistry High 25258313
2013 CaBP4 is dephosphorylated by protein phosphatase 2A (PP2A) in the retina; PP2A inhibitors (okadaic acid, fostriecin) block CaBP4 dephosphorylation; PP2A associates with CaBP4 (pull-down from retinal and HEK293 lysates); PP2A overexpression enhances dephosphorylation rate; inhibition of phosphatase activity increases CaBP4 phosphorylation and potentiates CaBP4 modulation of Cav1.3 Ca2+ channels, indicating PP2A fine-tunes presynaptic Ca2+ signals. In vitro phosphatase assay with PP2A inhibitors, pull-down from retinal lysates, overexpression of PP2A subunits in HEK293 cells, electrophysiology of Cav1.3 + CaBP4 in HEK293T cells Investigative ophthalmology & visual science High 23341017

Source papers

Stage 0 corpus · 17 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Mutations in CABP4, the gene encoding the Ca2+-binding protein 4, cause autosomal recessive night blindness. American journal of human genetics 146 16960802
2008 A novel homozygous nonsense mutation in CABP4 causes congenital cone-rod synaptic disorder. Investigative ophthalmology & visual science 68 19074807
2012 Complex regulation of voltage-dependent activation and inactivation properties of retinal voltage-gated Cav1.4 L-type Ca2+ channels by Ca2+-binding protein 4 (CaBP4). The Journal of biological chemistry 44 22936811
2005 A critical role of CaBP4 in the cone synapse. Investigative ophthalmology & visual science 44 16249514
2010 A null mutation in CABP4 causes Leber's congenital amaurosis-like phenotype. Molecular vision 40 20157620
2008 Interaction and colocalization of CaBP4 and Unc119 (MRG4) in photoreceptors. Investigative ophthalmology & visual science 38 18296658
2012 Clinical characterisation of the CABP4-related retinal phenotype. The British journal of ophthalmology 35 23099293
2007 Phosphorylation of the Ca2+-binding protein CaBP4 by protein kinase C zeta in photoreceptors. The Journal of neuroscience : the official journal of the Society for Neuroscience 20 18003854
2017 Exome sequencing identified a novel missense mutation c.464G>A (p.G155D) in Ca2+-binding protein 4 (CABP4) in a Chinese pedigree with autosomal dominant nocturnal frontal lobe epilepsy. Oncotarget 17 29108277
2014 Structural insights into activation of the retinal L-type Ca²⁺ channel (Cav1.4) by Ca²⁺-binding protein 4 (CaBP4). The Journal of biological chemistry 15 25258313
2017 Multimodal imaging in CABP4-related retinopathy. Ophthalmic genetics 13 28635425
2013 Protein phosphatase 2A dephosphorylates CaBP4 and regulates CaBP4 function. Investigative ophthalmology & visual science 10 23341017
2018 Retinal findings in a patient of French ancestry with CABP4-related retinal disease. Documenta ophthalmologica. Advances in ophthalmology 8 29525873
2022 A novel missense creatine mutant of CaBP4, c.464G>A (p.G155D), associated with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), reduces the expression of CaBP4. Translational pediatrics 5 35378956
2016 Mutation screening of the LRIT3, CABP4, and GPR179 genes in Chinese patients with Schubert-Bornschein congenital stationary night blindness. Ophthalmic genetics 4 27428514
2024 A novel homozygous nonsense variant in CABP4 causing stationary cone/rod synaptic dysfunction. Ophthalmic genetics 1 39148310
2025 Identification of a novel CABP4 frameshift variant and a secondary USH2A missense variant in congenital cone-rod synaptic disorder. Ophthalmic genetics 0 41126388