Affinage

CABP4

Calcium-binding protein 4 · UniProt P57796

Length
275 aa
Mass
30.4 kDa
Annotated
2026-06-09
17 papers in source corpus 8 papers cited in narrative 8 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CABP4 encodes a calmodulin-like neuronal Ca2+-binding protein localized to photoreceptor synaptic terminals, where it is essential for the development and maintenance of rod and cone synapses and for transmission of light signals to second-order neurons (PMID:16960802, PMID:16249514). CaBP4 controls presynaptic Ca2+ influx by binding the IQ motif of the Cav1.4 L-type Ca2+ channel and interfering with the channel's intrinsic C-terminal ICDI (inhibitor of Ca2+-dependent inactivation) domain, thereby relieving ICDI-mediated inhibition and dramatically increasing channel availability; disease-associated CaBP4 mutants retain Cav1.4 binding but only partially preserve this functional effect (PMID:22936811). NMR structures show that Ca2+ binding to the EF-hand C-lobe drives a closed-to-open transition that exposes hydrophobic residues engaging the Cav1.4 IQ motif, and mutation of the channel residue Y1595 abolishes this interaction (PMID:25258313). CaBP4 activity is bidirectionally regulated by light: PKCzeta phosphorylates CaBP4 at Ser37, an event required for its prolongation of Cav1.3 current, while PP2A reverses this phosphorylation, with the two activities shifting the channel-modulating set point between light- and dark-adapted states (PMID:18003854, PMID:23341017). CaBP4 also physically interacts with the synaptic protein Unc119 and is required to maintain Unc119 levels at photoreceptor terminals (PMID:18296658). Loss-of-function and protein-destabilizing CABP4 mutations cause a congenital stationary night blindness (CSNB2-like) phenotype (PMID:16960802, PMID:35378956).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2005 High

    Whether a dedicated synaptic factor is required to build and sustain photoreceptor ribbon synapses was unresolved; this work established CaBP4 as critical for photoreceptor synapse integrity and cone-to-bipolar signal transmission.

    Evidence Cabp4-/-Gnat1-/- knockout mice analyzed by immunocytochemistry, electron microscopy, and electroretinography

    PMID:16249514

    Open questions at the time
    • Did not define the molecular target through which CaBP4 acts
    • Mechanism linking synapse maintenance to channel function not addressed
  2. 2006 Medium

    The question of which channel CaBP4 operates on and its disease relevance was answered by placing CaBP4 at the photoreceptor terminal in association with the Cav1.4 C-terminus and linking its loss to a CSNB2-like defect.

    Evidence Candidate gene sequencing, immunolocalization, and qRT-PCR in patients and mice

    PMID:16960802

    Open questions at the time
    • Direct Cav1.4 binding cited from prior work, not re-demonstrated here
    • Functional consequence of binding for channel gating not shown
  3. 2007 High

    How CaBP4 activity is coupled to lighting conditions was unknown; this work showed light-regulated PKCzeta phosphorylation at Ser37 and Ca2+ binding together tune CaBP4 modulation of presynaptic Ca2+ channels.

    Evidence In vitro kinase assay, S37A and Ca2+-site mutants, electrophysiology of Cav1.3 co-transfections, and light/dark retinal phosphorylation measurements

    PMID:18003854

    Open questions at the time
    • Phosphorylation effect demonstrated on Cav1.3 rather than the native Cav1.4 partner
    • Structural basis of how Ser37 phosphorylation alters channel modulation not defined
  4. 2008 High

    Beyond the channel, whether CaBP4 has scaffolding partners at the synapse was open; this established a physical CaBP4–Unc119 interaction and a requirement for CaBP4 in maintaining synaptic Unc119.

    Evidence Affinity chromatography, yeast two-hybrid, co-immunoprecipitation, gel overlay, and Western blot of Cabp4-/- retinal sections

    PMID:18296658

    Open questions at the time
    • Functional role of the CaBP4–Unc119 interaction in transmission not resolved
    • Whether Unc119 loss contributes to the synaptic phenotype unclear
  5. 2012 High

    The mechanism by which CaBP4 enhances Cav1.4 was defined: it binds the IQ motif and displaces the inhibitory ICDI domain to relieve suppression of voltage-dependent gating and increase channel availability.

    Evidence Heterologous Cav1.4 expression, FRET binding assays, ICDI-deletion electrophysiology, and disease-mutant functional analysis

    PMID:22936811

    Open questions at the time
    • Atomic interface of the IQ interaction not yet resolved at this stage
    • Disease mutants bind but partially fail—structural basis not defined here
  6. 2013 High

    How CaBP4 phosphorylation is reversed was unknown; PP2A was identified as the phosphatase that dephosphorylates CaBP4 and counteracts PKCzeta to set channel modulation in a light-dependent manner.

    Evidence In vitro phosphatase assays with selective inhibitors, pull-downs of PP2A subunits, and Cav1.3 electrophysiology in HEK293T cells

    PMID:23341017

    Open questions at the time
    • PP2A targeting subunit specificity not fully defined
    • In vivo demonstration of the phosphorylation cycle at native synapses limited
  7. 2014 High

    The structural logic of CaBP4 Ca2+ sensing and channel engagement was resolved: NMR structures revealed a Ca2+-induced C-lobe opening that exposes hydrophobic residues contacting the Cav1.4 IQ motif.

    Evidence NMR structures in Mg2+- and Ca2+-bound states with Cav1.4 IQ mutant (Y1595E) validation

    PMID:25258313

    Open questions at the time
    • No structure of the full CaBP4–Cav1.4 complex
    • How ICDI displacement occurs structurally not directly visualized
  8. 2022 Medium

    The molecular consequence of a specific patient mutation was clarified: p.G155D destabilizes CaBP4 protein without reducing transcription, linking loss of stable protein to disease.

    Evidence RT-PCR, Western blot, and recombinant expression in human neuron cells

    PMID:35378956

    Open questions at the time
    • Protein stability mechanism not structurally validated
    • Functional consequence for Cav1.4 modulation not measured

Open questions

Synthesis pass · forward-looking unresolved questions
  • How the phosphorylation cycle, Ca2+ sensing, ICDI displacement, and Unc119 scaffolding are integrated at the native photoreceptor synapse to control transmitter release in real time remains unresolved.
  • No structure of the intact CaBP4–Cav1.4 complex
  • Functional role of the CaBP4–Unc119 interaction undefined
  • In vivo dynamics of light-driven phosphorylation switching not directly observed

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 2
Localization
GO:0005886 plasma membrane 2
Pathway
R-HSA-112316 Neuronal System 2 R-HSA-9709957 Sensory Perception 2
Partners
CACNA1DCACNA1FPPP2CAPRKCZUNC119

Evidence

Reading pass · 8 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 CABP4 protein is localized to photoreceptor synaptic terminals and directly associates with the C-terminal domain of the Cav1.4 alpha subunit (CACNA1F); loss-of-function mutations in CABP4 cause a CSNB2-like signaling defect, placing CABP4 in the photoreceptor synaptic transmission pathway upstream of second-order neurons. Candidate gene sequencing, immunolocalization, transcript quantification (qRT-PCR); prior functional association with Cav1.4 C-terminal domain cited from earlier work American journal of human genetics Medium 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, fewer photoreceptor terminals, and severely attenuated cone b-wave amplitude and sensitivity, establishing CaBP4 as critical for signal transmission from cones to second-order neurons. Double knockout mouse model (Cabp4−/−Gnat1−/−), immunocytochemistry, electron microscopy, single-flash and flicker electroretinography Investigative ophthalmology & visual science High 16249514
2012 CaBP4 dramatically increases Cav1.4 channel availability by interacting with the IQ motif of Cav1.4 and interfering with binding of the C-terminal ICDI (inhibitor of Ca2+-dependent inactivation) domain, thereby relieving ICDI-mediated inhibition of voltage-dependent channel gating; disease-associated CaBP4 mutants bind Cav1.4 but only partially preserve this functional effect, reducing channel availability. Heterologous expression of Cav1.4 in cells, FRET experiments to demonstrate CaBP4–IQ motif interaction, electrophysiology of Cav1.4 mutant lacking ICDI, functional characterization of disease-associated CaBP4 mutants The Journal of biological chemistry High 22936811
2008 CaBP4 physically interacts with Unc119 (MRG4) at the photoreceptor synapse; Unc119 levels are specifically reduced in photoreceptor terminals of Cabp4−/− mice, suggesting CaBP4 is required for maintaining Unc119 at the synapse. Affinity chromatography, yeast two-hybrid, co-immunoprecipitation, gel overlay assay, immunohistochemistry, Western blot of synaptic tangential sections from Cabp4−/− retinas Investigative ophthalmology & visual science High 18296658
2007 CaBP4 is phosphorylated by protein kinase C zeta (PKCζ) at serine 37 both in vitro and in the retina; phosphorylation is greater in light-adapted than dark-adapted retinas; mutation of serine 37 to alanine abolishes CaBP4's ability to prolong Ca2+ current through Cav1.3, while Ca2+-binding site mutations strengthen Cav1.3 modulation, demonstrating that light-regulated PKCζ phosphorylation and Ca2+ binding control presynaptic Ca2+ signaling. In vitro kinase assay, retinal phosphorylation measurements under light/dark conditions, electrophysiology of cells co-transfected with Cav1.3 and CaBP4 mutants (S37A and Ca2+-binding site mutants), co-localization in photoreceptors The Journal of neuroscience High 18003854
2013 CaBP4 is dephosphorylated by protein phosphatase 2A (PP2A) in the retina; PP2A inhibitors (okadaic acid, fostriecin) block CaBP4 dephosphorylation; PP2A subunits associate with CaBP4 in pull-down experiments; inhibition of PP2A increases CaBP4 phosphorylation and potentiates CaBP4 modulation of Cav1.3 channels; phosphatase activity is increased under light-adapted conditions, reversing PKCζ-mediated phosphorylation. In vitro phosphatase assays with selective inhibitors, pull-down experiments with retinal and HEK293 lysates, overexpression of PP2A subunits, electrophysiology of Cav1.3 in HEK293T cells Investigative ophthalmology & visual science High 23341017
2014 NMR structures of CaBP4 in Mg2+-bound and Ca2+-bound states show: Mg2+ binds EF1 and EF3; Ca2+ binds EF1, EF3, and EF4; the N-lobe (EF1/EF2) adopts a closed conformation regardless of ion, while the C-lobe (EF3/EF4) undergoes a Ca2+-induced closed-to-open transition; Ca2+-bound CaBP4 contacts the IQ motif of Cav1.4 through exposed hydrophobic residues (Phe137, Glu168, Leu207, Phe214, Met251, Phe264, Leu268); Cav1.4 mutant Y1595E strongly impairs CaBP4 binding, suggesting CaBP4 promotes channel activation by disrupting IQ–ICDI interaction. NMR structure determination, ion binding characterization, mutagenesis of Cav1.4 IQ motif (Y1595E) to validate binding interface The Journal of biological chemistry High 25258313
2022 The disease-associated CaBP4 missense mutation p.G155D (c.464G>A) reduces CaBP4 protein expression but increases CaBP4 mRNA, indicating the mutation destabilizes the CaBP4 protein without affecting transcription. RT-PCR for mRNA quantification, Western blot for protein levels, recombinant protein expression in human neuron cells Translational pediatrics Medium 35378956

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 148 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 45 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

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