Affinage

FBXL4

F-box/LRR-repeat protein 4 · UniProt Q9UKA2

Length
621 aa
Mass
70.1 kDa
Annotated
2026-06-09
43 papers in source corpus 14 papers cited in narrative 14 extracted findings
Cross-family judge vs UniProt: tie faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FBXL4 is a nuclear-encoded F-box/leucine-rich-repeat protein that resides at the mitochondrial outer membrane and nucleates an SCF/CRL1 (SKP1–CUL1–FBXL4) ubiquitin E3 ligase complex governing mitochondrial quality control (PMID:36896912, PMID:37161784, PMID:37568009). Its principal function is to constitutively ubiquitinate the mitophagy receptors BNIP3 and NIX/BNIP3L, targeting them for proteasomal degradation and thereby suppressing basal mitophagy and preserving mtDNA content (PMID:36896912, PMID:37161784, PMID:37102372, PMID:37568009). Loss of FBXL4 — whether by gene deletion or by pathogenic variants that bind substrate normally but fail to assemble the active SCF/CRL1 complex — causes BNIP3/NIX accumulation, hyperactive mitophagy, respiratory chain deficiency, and mtDNA depletion; genetic complementation restores mtDNA copy number and bioenergetics (PMID:23993194, PMID:23993193, PMID:37568009), and ablation of NIX (or NIX/BNIP3) rescues the phenotype in mice (PMID:36896912, PMID:37102372). These deficits define mitochondrial DNA depletion syndrome type 13 (MTDPS13), with substrate accumulation reproduced in knock-in mice and patient hiPSC-derived neurons (PMID:37568009). Substrate engagement and complex assembly depend on FBXL4's modular architecture, with the F-box domain required for substrate ubiquitination and the LRR domains mediating protein–protein interaction (PMID:38359748, PMID:32559514), and the outer-membrane phosphatase PPTC7 acts as an activity-independent cofactor promoting SCF-FBXL4-mediated BNIP3/NIX turnover (PMID:40025034). Beyond mitophagy, FBXL4 ubiquitinates additional substrates in distinct cellular contexts: the histone demethylase KDM4A/JMJD2A to restrain S-phase progression (PMID:21757720), the fission GTPase Drp1 to limit mitochondrial fission (PMID:38359748), and profilin-1 (PFN1, K48-linked at K70) to preserve sarcomeric integrity in cardiomyocytes (PMID:41589689); in Drosophila, CLOCK-driven rhythmic Fbxl4 degrades GABA-A receptors in pacemaker neurons to control excitability and sleep (PMID:29174887).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2011 Medium

    Established FBXL4 as a substrate-recognition subunit of an SCF E3 ligase, showing it can drive proteasomal turnover of a defined substrate and thereby influence cell-cycle timing.

    Evidence Reciprocal Co-IP of SCF/CUL1 with JMJD2A plus ubiquitin-rescue and BrdU S-phase readouts in cultured cells

    PMID:21757720

    Open questions at the time
    • Did not localize this activity to mitochondria
    • Ubiquitination not reconstituted in vitro
    • Physiological relevance to organismal phenotype not tested
  2. 2013 High

    Identified FBXL4 as a mitochondrial protein whose loss causes mtDNA depletion and respiratory deficiency, and proved causality by rescue — defining the disease-relevant cellular phenotype.

    Evidence Subcellular fractionation, native gel (400 kDa complex), bioenergetics, mtDNA quantification, and wild-type cDNA complementation in patient fibroblasts

    PMID:23993193 PMID:23993194

    Open questions at the time
    • Molecular substrate driving mtDNA loss not identified at this stage
    • Composition of the 400 kDa complex unresolved
    • Mechanism linking FBXL4 loss to mtDNA depletion unknown
  3. 2017 Medium

    Showed FBXL4 ubiquitin-ligase activity can be transcriptionally clocked to provide rhythmic control of receptor abundance, linking it to neuronal excitability and behavior.

    Evidence GABA-A receptor ubiquitination assays, CLOCK-dependent rhythmic expression, fbxl4 loss-of-function genetics, and sleep/electrophysiology in Drosophila

    PMID:29174887

    Open questions at the time
    • Conservation of GABA-A receptor targeting in mammals not addressed
    • Relationship to mitochondrial function not examined
    • Direct ubiquitin linkage chemistry not characterized
  4. 2019 Medium

    Connected pathogenic FBXL4 variants to altered mitochondrial dynamics, indicating FBXL4 promotes fusion (or restrains fragmentation).

    Evidence Photo-activatable GFP fusion assay comparing wild-type vs. p.Cys584Arg in patient fibroblasts

    PMID:31442532

    Open questions at the time
    • Single-method, single-lab observation
    • Did not identify a molecular effector of fusion
    • Causal link to mtDNA depletion not established
  5. 2023 High

    Resolved the long-standing mechanistic gap by defining the SCF-FBXL4 substrates as the mitophagy receptors BNIP3 and NIX, showing FBXL4 suppresses basal mitophagy and that substrate removal rescues loss-of-function — establishing the molecular basis of MTDPS13.

    Evidence Mitochondria-targeted/CRISPR screens, Co-IP and ubiquitination/degradation assays, Fbxl4 knockout and knock-in mice, Bnip3/Nix epistasis, and patient hiPSC-derived neurons across multiple concurrent studies

    PMID:36896912 PMID:37102372 PMID:37161784 PMID:37568009

    Open questions at the time
    • How pathogenic variants selectively block complex assembly while retaining substrate binding not structurally resolved
    • Relative contribution of NIX vs. BNIP3 across tissues incompletely defined
    • Trigger for switching off FBXL4-mediated suppression during physiological mitophagy unknown
  6. 2024 Medium

    Extended the substrate repertoire to mitochondrial fission and PINK1 mitophagy machinery in cardiac cells, showing F-box-dependent targeting of Drp1 and site-specific PINK1 ubiquitination.

    Evidence Mass spectrometry, Co-IP, ΔF-box and lysine-site mutagenesis, ubiquitination assays, and epistasis in HFpEF/CIH cardiomyocyte models

    PMID:38333696 PMID:38359748

    Open questions at the time
    • Whether Drp1/PINK1 targeting operates outside cardiac stress contexts unknown
    • Hierarchy among multiple substrates not defined
    • circRNA-dependence may limit generality of PINK1 finding
  7. 2025 Medium

    Identified PPTC7 as an activity-independent cofactor required for efficient SCF-FBXL4-mediated BNIP3/NIX degradation, adding a regulatory input to the core mitophagy-suppression module.

    Evidence Co-IP, phosphatase-dead PPTC7 mutant stability assays, and Pptc7 knockout mouse phenocopying FBXL4 loss

    PMID:40025034

    Open questions at the time
    • Structural role of PPTC7 in complex assembly unresolved
    • Whether PPTC7 regulates non-mitophagy substrates untested
    • Single-lab observation
  8. 2026 Medium

    Demonstrated a cardiac structural role via K48-linked ubiquitination of profilin-1, with transcriptional repression of FBXL4 by SP1 during hypertrophy linking substrate control to sarcomere maintenance.

    Evidence Co-IP, K70 site-specific K48 ubiquitination assay, inducible cardiomyocyte-specific KO with AAV9 rescue, PFN1 epistasis, and hiPSC-derived cardiomyocytes

    PMID:41589689

    Open questions at the time
    • Relationship between PFN1 targeting and mitochondrial substrate functions not integrated
    • Generality beyond cardiomyocytes untested
    • Single-lab study

Open questions

Synthesis pass · forward-looking unresolved questions
  • How a single outer-membrane E3 ligase coordinates its diverse substrates (BNIP3/NIX, Drp1, PINK1, KDM4A, PFN1) across tissues and physiological states, and the structural basis by which disease variants disrupt complex assembly, remains unresolved.
  • No structure of the assembled SCF-FBXL4 complex
  • Tissue-specific substrate selection mechanism unknown
  • Conditions that physiologically relieve FBXL4-mediated mitophagy suppression undefined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 6 GO:0016874 ligase activity 3 GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005739 mitochondrion 3
Pathway
R-HSA-392499 Metabolism of proteins 4 R-HSA-9612973 Autophagy 4 R-HSA-1852241 Organelle biogenesis and maintenance 3
Partners
BNIP3BNIP3LCUL1DNM1LPFN1PINK1PPTC7SKP1
Complex memberships
SCF-FBXL4 (SKP1–CUL1–FBXL4) / CRL1-FBXL4 ubiquitin E3 ligase

Evidence

Reading pass · 14 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2013 FBXL4 is targeted to mitochondria and localizes in the intermembrane space, where it participates in an approximately 400 kDa protein complex. Loss of FBXL4 results in mitochondrial respiratory chain deficiency, hyperfragmentation of the mitochondrial network, and substantially decreased mtDNA content in muscle and fibroblasts. Subcellular fractionation, mitochondrial targeting experiments, native gel electrophoresis (400 kDa complex), oxygen consumption measurements, mtDNA copy number quantification in patient muscle/fibroblasts American journal of human genetics High 23993193 23993194
2013 Expression of wild-type FBXL4 transcript in patient cell lines fully rescued mtDNA copy number levels and corrected the mitochondrial biochemical deficit, confirming that loss of FBXL4 function is the direct cause of mtDNA depletion. Genetic complementation assay — transfection of wild-type FBXL4 cDNA into patient-derived fibroblasts with measurement of mtDNA copy number and respiratory chain activity American journal of human genetics High 23993193
2011 The SCF ubiquitin E3 ligase complex containing CUL1 and FBXL4 (FbxL4) directly interacts with the histone demethylase KDM4A/JMJD2A and regulates its proteasomal degradation. Ubiquitin overexpression restored JMJD2A turnover and blocked JMJD2A-dependent faster S-phase progression in a CUL1-dependent manner. Co-immunoprecipitation (SCF complex with JMJD2A), ubiquitin overexpression rescue assay, S-phase progression measurement (BrdU incorporation), chromatin occupancy analysis The Journal of biological chemistry Medium 21757720
2017 Drosophila Fbxl4 promotes ubiquitination and degradation of GABA-A receptors in arousal-promoting large ventral lateral neurons (lLNvs). The transcription of fbxl4 in lLNvs is CLOCK-dependent, producing rhythmic Fbxl4 levels that reduce GABA sensitivity to increase pacemaker neuron excitability and promote wakefulness. GABA-A receptor ubiquitination assay, rhythmic protein expression analysis, genetic loss-of-function (fbxl4 mutants), electrophysiology/behavioral sleep assays, CLOCK-dependent transcription analysis Current biology : CB Medium 29174887
2019 FBXL4 promotes mitochondrial fusion; cells harboring a pathogenic FBXL4 variant (p.Cys584Arg) exhibit reduced mitochondrial fusion rates by photo-activatable GFP assay, while overexpression of wild-type FBXL4 (but not the variant) promotes mitochondrial hyperfusion. Photo-activatable GFP mitochondrial fusion assay, FBXL4 overexpression vs. pathogenic variant comparison in patient fibroblasts Biochimica et biophysica acta. Molecular basis of disease Medium 31442532
2023 FBXL4 functions as an integral outer mitochondrial membrane protein that forms an SCF-FBXL4 ubiquitin E3 ligase complex (with SKP1 and CUL1). This complex ubiquitinates the mitophagy receptors BNIP3 and NIX (BNIP3L), targeting them for proteasomal degradation to suppress basal mitophagy. Pathogenic FBXL4 mutations disrupt SCF-FBXL4 assembly and impair substrate degradation. Fbxl4-/- mice show elevated BNIP3/NIX, hyperactive mitophagy, and perinatal lethality; knockout of either Bnip3 or Nix rescues metabolic derangements and viability. Mitochondria-targeted genetic screen, co-immunoprecipitation (SCF complex components), ubiquitination assay, Fbxl4 knockout mice, Bnip3/Nix double-knockout epistasis, metabolic phenotyping The EMBO journal High 36896912
2023 The SCF-FBXL4 ubiquitin ligase complex localizes to the mitochondrial outer membrane in unstressed cells and constitutively ubiquitinates and degrades NIX (BNIP3L) and BNIP3 to suppress basal mitophagy. Pathogenic FBXL4 variants causing MTDPS13 do not efficiently interact with core SCF machinery, leading to NIX/BNIP3 accumulation and excessive basal mitophagy. CRISPR/Cas9 screen for mitophagy regulators, co-immunoprecipitation (FBXL4 with NIX/BNIP3), protein stability/degradation assays, patient variant functional analysis The EMBO journal High 37161784
2023 FBXL4 restricts NIX and BNIP3 levels via direct interaction and protein destabilization; depletion of NIX (but not BNIP3 alone) is sufficient to restore elevated mitophagy caused by FBXL4 loss. VHL acts through a distinct mechanism (HIF1α-mediated transcription) on the same substrates. CRISPR/Cas9 E3 ligase screen, direct Co-IP (FBXL4–NIX/BNIP3), selective NIX vs. BNIP3 knockdown epistasis, mitophagy flux assays The EMBO journal High 37102372
2023 FBXL4 directly interacts with BNIP3 and BNIP3L (NIX) and promotes their degradation through the ubiquitin-proteasome pathway via assembly of an active CRL1-FBXL4 complex. Patient-derived FBXL4 mutations do not affect direct binding to BNIP3/3L but specifically impair assembly of the active CRL1 complex, resulting in BNIP3/3L accumulation and excessive mitophagy observed in knock-in mice and patient hiPSC-derived cortical neurons. Co-immunoprecipitation, ubiquitin-proteasome pathway assays, CRL1 complex assembly assays, knock-in mouse model (patient mutation), hiPSC-derived cortical neurons from MTDPS13 patients Cell death and differentiation High 37568009
2024 FBXL4 interacts with the mitochondrial fission protein Drp1, promotes its ubiquitination and proteasomal degradation, and thereby restrains Drp1-mediated mitochondrial fission. The F-box domain of FBXL4 is required for this interaction; a ΔF-box truncation mutant fails to interact with Drp1. Downstream, FBXL4-mediated Drp1 degradation preserves SERCA2a-dependent calcium handling. Mass spectrometry, co-immunoprecipitation (FBXL4–Drp1), truncation mutagenesis (ΔF-box), ubiquitination assay, Drp1 knock-in/knock-out epistasis, SERCA2a/calcium signaling measurement in HFpEF mouse model Redox biology Medium 38359748
2025 PPTC7, a phosphatase partly located on the outer mitochondrial membrane, interacts with FBXL4 and facilitates SCF-FBXL4-mediated ubiquitin-proteasomal degradation of BNIP3 and BNIP3L. This interaction is phosphatase-activity-independent. PPTC7 knockout activates high levels of BNIP3/3L-dependent basal mitophagy, phenocopying FBXL4 loss. Co-immunoprecipitation (PPTC7–FBXL4–BNIP3/3L), protein stability assay with phosphatase-dead PPTC7 mutant, Pptc7 knockout mouse model, BNIP3/3L-dependent mitophagy rescue experiments Cell death & disease Medium 40025034
2026 FBXL4 interacts with profilin-1 (PFN1) and promotes its K48-linked ubiquitination at lysine 70, leading to proteasomal degradation of PFN1 and preservation of sarcomeric integrity. Cardiomyocyte-specific FBXL4 ablation causes heart failure with myofibrillar disorganization; AAV9-mediated restoration rescues this. SP1 transcription factor represses FBXL4 expression during hypertrophy. Co-immunoprecipitation (FBXL4–PFN1), K48-linked ubiquitination assay with site-specific mutagenesis (K70), inducible cardiomyocyte-specific KO mice, AAV9 rescue, PFN1 knockdown/pharmacological inhibition epistasis, hiPSC-derived cardiomyocytes Advanced science Medium 41589689
2024 FBXL4 mediates ubiquitination and degradation of PINK1 at K319 and K433 in cardiomyocytes. A circRNA (circ-CIMIRC) promotes FBXL4-mediated PINK1 ubiquitination; silencing of circ-CIMIRC reduces PINK1 ubiquitination and enhances PINK1/Parkin-mediated mitophagy. Co-immunoprecipitation (FBXL4–PINK1), ubiquitination assay with site mutagenesis (K319/K433), FBXL4 siRNA knockdown, circ-CIMIRC overexpression/silencing in H9c2 cells and rat CIH model iScience Medium 38333696
2020 The LRR (leucine-rich repeat) domains of FBXL4 are required for its protein-protein interaction function; a frameshift mutation causing loss of the C-terminal LRR domains results in mitochondrial dysfunction and mtDNA depletion, as confirmed by 3D structural analysis and mtDNA quantification. 3D protein structural analysis, mtDNA/nuclear DNA ratio quantification (ND1/GAPDH) in patient lymphocytes and urine Journal of the neurological sciences Low 32559514

Source papers

Stage 0 corpus · 43 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2013 Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance. American journal of human genetics 136 23993193
2013 Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy. American journal of human genetics 136 23993194
2023 A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease. The EMBO journal 98 36896912
2023 FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors. The EMBO journal 68 37161784
2011 The SKP1-Cul1-F-box and leucine-rich repeat protein 4 (SCF-FbxL4) ubiquitin ligase regulates lysine demethylase 4A (KDM4A)/Jumonji domain-containing 2A (JMJD2A) protein. The Journal of biological chemistry 56 21757720
2023 FBXL4 ubiquitin ligase deficiency promotes mitophagy by elevating NIX levels. The EMBO journal 44 37102372
2015 Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations. Journal of inherited metabolic disease 44 25868664
2017 Molecular and clinical spectra of FBXL4 deficiency. Human mutation 38 28940506
2023 FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome. Cell death and differentiation 36 37568009
2017 Fbxl4 Serves as a Clock Output Molecule that Regulates Sleep through Promotion of Rhythmic Degradation of the GABAA Receptor. Current biology : CB 35 29174887
2024 FBXL4 protects against HFpEF through Drp1-Mediated regulation of mitochondrial dynamics and the downstream SERCA2a. Redox biology 28 38359748
2017 FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome. Clinical genetics 24 27743463
2015 Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion. JIMD reports 22 26404457
2019 FBXL4-Related Mitochondrial DNA Depletion Syndrome 13 (MTDPS13): A Case Report With a Comprehensive Mutation Review. Frontiers in genetics 21 30804983
2016 Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene. Clinical case reports 20 27099744
2019 Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion. Biochimica et biophysica acta. Molecular basis of disease 19 31442532
2017 Identification of FBXL4 as a Metastasis Associated Gene in Prostate Cancer. Scientific reports 18 28698647
2016 A novel mutation in FBXL4 in a Norwegian child with encephalomyopathic mitochondrial DNA depletion syndrome 13. European journal of medical genetics 15 27182039
2019 Whole exome sequencing revealed mutations in FBXL4, UNC80, and ADK in Thai patients with severe intellectual disabilities. Gene 14 30771478
2022 Dichloroacetate improves mitochondrial function, physiology, and morphology in FBXL4 disease models. JCI insight 12 35881484
2020 Novel homozygous mutation in the FBXL4 gene is associated with mitochondria DNA depletion syndrome-13. Journal of the neurological sciences 12 32559514
2024 Excessive BNIP3- and BNIP3L-dependent mitophagy underlies the pathogenesis of FBXL4-mutated mitochondrial DNA depletion syndrome. Autophagy 9 37876279
2024 FBXL4: safeguarding against mitochondrial depletion through suppression of mitophagy. Autophagy 9 38423516
2020 Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome. Frontiers in genetics 9 31969900
2016 Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants. JIMD reports 9 27858371
2023 FBXL4 mutation-caused mitochondrial DNA depletion syndrome is driven by BNIP3/BNIP3L-dependent excessive mitophagy. Trends in molecular medicine 7 38123379
2020 Different clinical presentation in a patient with two novel pathogenic variants of the FBXL4 gene. The Turkish journal of pediatrics 7 32779419
2024 Circ-CIMIRC inhibition alleviates CIH-induced myocardial damage via FbxL4-mediated ubiquitination of PINK1. iScience 6 38333696
2022 Prenatal phenotype of FBXL4-associated encephalomyopathic mitochondrial DNA depletion syndrome-13. Prenatal diagnosis 6 36411461
2025 PPTC7 acts as an essential co-factor of the SCFFBXL4 ubiquitin ligase complex to restrict BNIP3/3L-dependent mitophagy. Cell death & disease 4 40025034
2010 Expression and SNP association analysis of porcine FBXL4 gene. Molecular biology reports 4 19768576
2025 FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome: A rare cause of hyperammonemia. Molecular genetics and metabolism reports 3 40161922
2021 The first case with FBXL4 mutation successfully treated with a parenteral ketogenic diet for lactic acidosis. JPEN. Journal of parenteral and enteral nutrition 3 33882172
2021 A Mild Phenotype of Mitochondrial DNA Depletion Syndrome Type 13 with a Novel FBXL4 Variant. Molecular syndromology 3 34602956
2025 FBXL4-Related Mitochondrial Depletion Syndrome Underscores the role of Mitophagy in Stem Cell Differentiation during Embryogenesis. Stem cell reviews and reports 2 39937392
2025 Prenatal FBXL4-Associated Mitochondrial DNA Depletion Syndrome-13: A New Case and Review of the Literature. Prenatal diagnosis 1 40252080
2023 Fbxl4 Regulates the Photic Entrainment of Circadian Locomotor Rhythms in the Cricket Gryllus bimaculatus. Zoological science 1 36744710
2026 F-Box and Leucine-Rich Repeat Protein 4 (FBXL4) Maintains Sarcomere Integrity and Cardiac Function by Enhancing K48-Linked Ubiquitinated Degradation of Profilin-1 (PFN1). Advanced science (Weinheim, Baden-Wurttemberg, Germany) 0 41589689
2025 A Patient with Organic Acidemia, Hyperammonemia, and a FBXL4 Variant Suggesting Mitochondrial DNA Depletion Syndrome. Molecular syndromology 0 40352449
2025 Prenatal diagnosis of a compound heterozygous variation in the FBXL4 gene by trio-WES and imaging monitoring: a case report. Frontiers in genetics 0 40352787
2025 Siblings of FBXL4-related mitochondrial DNA depletion syndrome, leading to fatal fulminant pneumonia. Molecular genetics and metabolism reports 0 41142849
2025 Cepharanthine-induced mitophagy through regulation of FBXL4-BNIP3 drives ferroptosis leading to robust anti-lung cancer efficacy. Free radical biology & medicine 0 41177235
2024 [A case of neonatal Mitochondrial DNA depletion syndrome type 13 caused by FBXL4 gene mutation]. Zhonghua yi xue yi chuan xue za zhi = Zhonghua yixue yichuanxue zazhi = Chinese journal of medical genetics 0 39653352

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