Affinage

FBXL4

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

Length
621 aa
Mass
70.1 kDa
Annotated
2026-04-28
43 papers in source corpus 13 papers cited in narrative 12 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FBXL4 is a mitochondria-associated F-box protein that functions as the substrate-recognition subunit of an SCF (SKP1-CUL1-FBXL4) E3 ubiquitin ligase complex, serving as a master regulator of mitochondrial homeostasis by controlling basal mitophagy, mitochondrial dynamics, and mtDNA maintenance. At the outer mitochondrial membrane, SCF-FBXL4 constitutively ubiquitinates the mitophagy receptors BNIP3 and NIX/BNIP3L, targeting them for proteasomal degradation and thereby suppressing basal mitophagy; the mitochondrial phosphatase PPTC7 acts as an essential cofactor of this complex in a phosphatase-activity-independent manner (PMID:36896912, PMID:37102372, PMID:37568009, PMID:40025034). FBXL4 additionally promotes mitochondrial fusion and ubiquitinates the fission GTPase Drp1 to restrain mitochondrial fragmentation, while in the nucleus the same SCF-FBXL4 complex targets the histone demethylase KDM4A/JMJD2A for degradation to regulate DNA replication timing (PMID:21757720, PMID:31442532, PMID:38359748). Biallelic loss-of-function mutations in FBXL4 cause mitochondrial DNA depletion syndrome 13 (MTDPS13), characterized by mtDNA depletion, combined OXPHOS deficiency, and mitochondrial hyperfragmentation; genetic ablation of BNIP3 or NIX rescues the lethal phenotype of Fbxl4-knockout mice, establishing hyperactive mitophagy as the proximal disease mechanism (PMID:23993194, PMID:23993193, PMID:36896912).

Mechanistic history

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

    Establishing that FBXL4 operates as a canonical SCF E3 ligase subunit with a defined nuclear substrate resolved the fundamental biochemical activity of the protein: SCF-FBXL4 ubiquitinates the histone demethylase KDM4A/JMJD2A to control replication timing and S-phase progression.

    Evidence Reciprocal Co-IP, ubiquitination assays, BrdU incorporation, and ChIP in human cell lines

    PMID:21757720

    Open questions at the time
    • Whether KDM4A degradation is relevant in mitochondrial contexts was unknown
    • No structural basis for substrate recognition by the leucine-rich repeat domain
    • Physiological consequences of FBXL4-KDM4A axis in vivo not tested
  2. 2013 High

    Identification of biallelic FBXL4 mutations in patients with mtDNA depletion syndrome established FBXL4 as essential for mitochondrial DNA maintenance and revealed its mitochondrial localization, shifting the functional focus from the nucleus to the mitochondrion.

    Evidence Patient exome sequencing, subcellular fractionation showing intermembrane space localization, native gel electrophoresis (~400 kDa complex), mtDNA quantification, complementation rescue in two independent cohorts

    PMID:23993193 PMID:23993194

    Open questions at the time
    • Identity of the mitochondrial substrates of FBXL4 was unknown
    • How an SCF complex operates at mitochondria was not explained
    • Mechanism linking FBXL4 loss to mtDNA depletion remained unclear
  3. 2017 Medium

    Demonstration in Drosophila that Fbxl4 ubiquitinates the GABA-A receptor in pacemaker neurons under CLOCK-dependent transcriptional control expanded the known substrate repertoire and linked FBXL4 to circadian-regulated proteolysis outside mitochondria.

    Evidence Genetic loss-of-function, GABA-A receptor ubiquitination assay, circadian behavior analysis, and receptor immunostaining in Drosophila lLNv neurons

    PMID:29174887

    Open questions at the time
    • Relevance of circadian GABA-A regulation to mammalian FBXL4 function not established
    • Whether this reflects a conserved substrate or lineage-specific adaptation is unclear
    • No direct biochemical reconstitution of Fbxl4-GABA-A ubiquitination
  4. 2019 Medium

    Direct measurement of mitochondrial fusion rates showed that FBXL4 actively promotes fusion, providing a mechanistic explanation for the hyperfragmented mitochondrial network observed in patient cells.

    Evidence Photo-activatable GFP mitochondrial fusion assay comparing WT and mutant FBXL4 in patient fibroblasts

    PMID:31442532

    Open questions at the time
    • The fusion substrate(s) of FBXL4 were not identified
    • Whether fusion promotion is direct or secondary to mitophagy suppression was unresolved
    • Mechanism distinguishing FBXL4's role in fusion versus fission not addressed
  5. 2023 High

    Four independent groups converged to identify BNIP3 and NIX/BNIP3L as the critical mitochondrial substrates of SCF-FBXL4, establishing that constitutive degradation of these mitophagy receptors is the primary mechanism by which FBXL4 maintains mitochondrial mass and mtDNA — and that pathogenic mutations impair SCF complex assembly rather than substrate binding.

    Evidence Genome-wide CRISPR E3 ligase screen, mitochondria-targeted genetic screen, Co-IP and ubiquitination assays, proteasome inhibitor treatment, Fbxl4-knockout mouse rescued by Bnip3 or Nix double KO, patient mutation analysis in CRL1 complex assembly, hiPSC-derived neuron validation

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

    Open questions at the time
    • Structural basis for FBXL4 engagement with BNIP3/NIX not determined
    • Whether additional mitochondrial substrates beyond BNIP3/NIX contribute to the disease phenotype
    • Tissue-specific regulation of SCF-FBXL4 activity is unexplored
  6. 2024 Medium

    Identification of Drp1 and PINK1 as additional FBXL4 substrates extended the model to show that FBXL4 simultaneously suppresses multiple pro-mitophagy and pro-fission pathways: ubiquitination of Drp1 restrains fission, and ubiquitination of PINK1 suppresses PINK1/Parkin-mediated mitophagy.

    Evidence Co-IP with F-box domain truncation mapping for Drp1; Co-IP with K319R/K433R site-directed mutagenesis for PINK1; cardiac functional readouts for Drp1; siRNA knockdown and colocalization for PINK1

    PMID:38333696 PMID:38359748

    Open questions at the time
    • Relative contribution of Drp1 vs. BNIP3/NIX degradation to the mtDNA depletion phenotype is unknown
    • PINK1 ubiquitination by FBXL4 demonstrated by single lab; independent confirmation needed
    • Whether Drp1 and PINK1 are relevant substrates in post-mitotic tissues (e.g., neurons) not tested
  7. 2025 Medium

    Discovery of PPTC7 as a scaffolding cofactor of SCF-FBXL4 revealed that the E3 ligase complex requires a mitochondrial phosphatase for efficient BNIP3/NIX degradation, operating independently of PPTC7's catalytic activity.

    Evidence Co-IP, Pptc7 knockout mouse, ubiquitination assay, phosphatase-dead mutant analysis

    PMID:40025034

    Open questions at the time
    • How PPTC7 structurally interfaces with SCF-FBXL4 is unresolved
    • Whether PPTC7 mutations phenocopy FBXL4 mutations in human disease not established
    • Single-lab finding; independent replication needed
  8. 2026 High

    Identification of profilin-1 (PFN1) as a cardiac FBXL4 substrate linked FBXL4 to sarcomere maintenance independent of mitophagy, demonstrating that FBXL4 loss causes heart failure through PFN1 accumulation and sarcomere disorganization.

    Evidence Co-IP, K48-linked ubiquitination assay with K70R mutant, cardiomyocyte-specific FBXL4 KO mouse, AAV9-PFN1 knockdown rescue, hiPSC-derived cardiomyocyte model

    PMID:41589689

    Open questions at the time
    • Whether PFN1 dysregulation contributes to MTDPS13 pathology or is confined to cardiac phenotypes
    • How FBXL4 discriminates among its multiple substrates in different cellular compartments is unknown
    • No structural model of FBXL4-PFN1 recognition

Open questions

Synthesis pass · forward-looking unresolved questions
  • A unified structural and regulatory model explaining how SCF-FBXL4 is directed to distinct substrates across mitochondrial, cytoplasmic, and nuclear compartments — and how its activity is regulated in different tissues — remains to be established.
  • No crystal or cryo-EM structure of SCF-FBXL4 with any substrate
  • Post-translational regulation of FBXL4 itself (e.g., phosphorylation, turnover) is largely uncharacterized
  • Relative pathogenic contribution of each substrate to the MTDPS13 phenotype across tissues remains unclear

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016874 ligase activity 7 GO:0140096 catalytic activity, acting on a protein 7
Localization
GO:0005739 mitochondrion 5
Pathway
R-HSA-392499 Metabolism of proteins 6 R-HSA-1852241 Organelle biogenesis and maintenance 4 R-HSA-9612973 Autophagy 2 GO:0005739 mitochondrion 1 R-HSA-69306 DNA Replication 1
Partners
BNIP3BNIP3LCUL1DRP1KDM4APFN1PPTC7SKP1
Complex memberships
SCF-FBXL4 (SKP1-CUL1-FBXL4)

Evidence

Reading pass · 12 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 causes mtDNA depletion, hyperfragmentation of the mitochondrial network, and combined OXPHOS deficiency. Subcellular fractionation, native gel electrophoresis, cell-based respirometry, mtDNA quantification in patient muscle and fibroblasts American journal of human genetics High 23993193 23993194
2013 Expression of wild-type FBXL4 in patient cell lines fully rescues mtDNA copy number and corrects mitochondrial biochemical deficits, demonstrating a direct role for FBXL4 in mtDNA maintenance. Complementation assay — re-expression of WT FBXL4 in patient fibroblasts with measurement of mtDNA copy number and respiratory chain activity American journal of human genetics High 23993193 23993194
2011 FBXL4 forms an SCF (SKP1-CUL1-FBXL4) ubiquitin E3 ligase complex that interacts with and promotes proteasomal degradation of the histone demethylase KDM4A/JMJD2A, thereby regulating DNA replication timing and S-phase progression. Co-immunoprecipitation, ubiquitin overexpression rescue assay, BrdU incorporation, chromatin immunoprecipitation The Journal of biological chemistry High 21757720
2019 FBXL4 promotes mitochondrial fusion; patient fibroblasts carrying a pathogenic FBXL4 variant show reduced mitochondrial fusion rates by photo-activatable GFP assay, while overexpression of WT FBXL4 induces mitochondrial hyperfusion. Photo-activatable GFP mitochondrial fusion assay, overexpression of WT vs. mutant FBXL4 in patient fibroblasts Biochimica et biophysica acta. Molecular basis of disease Medium 31442532
2023 FBXL4 functions as an integral outer mitochondrial membrane protein and forms an SCF-FBXL4 ubiquitin E3 ligase complex that ubiquitinates mitophagy receptors BNIP3 and NIX/BNIP3L, targeting them for proteasomal degradation to suppress basal mitophagy; pathogenic FBXL4 mutations disrupt SCF complex assembly and impair substrate degradation, causing hyperactive mitophagy and mitochondrial DNA depletion. Mitochondria-targeted genetic screen, CRISPR-KO, Co-IP, ubiquitination assay, proteasome inhibitor treatment, Fbxl4-/- mouse model with BNIP3/NIX rescue by double KO The EMBO journal High 36896912 37102372 37161784 37568009
2023 FBXL4 directly interacts with BNIP3 and BNIP3L; patient-derived FBXL4 mutations do not disrupt interaction with BNIP3/BNIP3L but instead impair assembly of an active CRL1-FBXL4 complex, preventing substrate ubiquitination. Co-immunoprecipitation, ubiquitination assay, patient mutation analysis in CRL1 complex assembly Cell death and differentiation High 37568009
2023 VHL and FBXL4 are the two most profound negative regulators of basal mitophagy among E3 ubiquitin ligases; FBXL4 restricts NIX and BNIP3 protein levels via direct interaction and protein destabilization, and depletion of NIX (but not BNIP3 alone) is sufficient to restore mitophagy levels in FBXL4-deficient cells. Genome-wide CRISPR/Cas9 E3 ligase screen for mitophagy regulators, Co-IP, siRNA depletion, flow cytometry-based mitophagy assays The EMBO journal High 37102372
2024 FBXL4 interacts with and promotes ubiquitination and degradation of the mitochondrial fission protein Drp1; the F-box domain of FBXL4 is required for this interaction, and FBXL4-mediated suppression of Drp1-dependent fission protects against HFpEF-associated mitochondrial dysfunction. Mass spectrometry, Co-IP, truncation mutant analysis (Delta-Fbox), overexpression/knockdown with cardiac functional readouts Redox biology Medium 38359748
2024 FBXL4 interacts with PINK1 and promotes its ubiquitination at K319 and K433, leading to PINK1 degradation; circ-CIMIRC promotes this FBXL4-mediated ubiquitination of PINK1, thereby suppressing PINK1/Parkin-mediated mitophagy. Co-IP, ubiquitination assay with site-directed mutagenesis (K319R, K433R), siRNA knockdown of FBXL4, colocalization assay iScience Medium 38333696
2025 PPTC7, a mitochondrial phosphatase, localizes to the outer mitochondrial membrane where it interacts with FBXL4 and BNIP3/BNIP3L, and acts as an essential co-factor of the SCF-FBXL4 complex to facilitate ubiquitin-proteasomal degradation of BNIP3 and BNIP3L in a phosphatase-activity-independent manner. Co-IP, Pptc7 knockout mouse model, in vitro cell culture ubiquitination assay, phosphatase-dead mutant analysis Cell death & disease Medium 40025034
2017 In Drosophila, Fbxl4 acts as a clock output E3 ligase that promotes ubiquitination and rhythmic degradation of the GABA-A receptor in arousal-promoting lLNv neurons; transcription of fbxl4 in these neurons is CLOCK-dependent, and Fbxl4 regulates sleep timing by rhythmically reducing GABA sensitivity to modulate pacemaker neuron excitability. Genetic loss-of-function, GABA-A receptor ubiquitination assay, circadian behavior analysis, immunostaining of receptor levels in lLNvs Current biology : CB Medium 29174887
2026 FBXL4 interacts with profilin-1 (PFN1) and promotes its K48-linked polyubiquitination at lysine 70, leading to proteasomal degradation of PFN1 to preserve sarcomere integrity; cardiomyocyte-specific FBXL4 ablation causes heart failure with sarcomere disorganization that is rescued by AAV9-PFN1 knockdown or pharmacological PFN1 inhibition. Co-IP, ubiquitination assay with K70R mutant, FBXL4 inducible cardiac KO mouse, cardiac-specific overexpression, AAV9 rescue, hiPSC-derived cardiomyocyte model Advanced science High 41589689

Source papers

Stage 0 corpus · 43 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2013 Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy. American journal of human genetics 136 23993194
2013 Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance. American journal of human genetics 134 23993193
2023 A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease. The EMBO journal 91 36896912
2023 FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors. The EMBO journal 62 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
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
2023 FBXL4 ubiquitin ligase deficiency promotes mitophagy by elevating NIX levels. The EMBO journal 42 37102372
2017 Molecular and clinical spectra of FBXL4 deficiency. Human mutation 38 28940506
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
2023 FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome. Cell death and differentiation 32 37568009
2024 FBXL4 protects against HFpEF through Drp1-Mediated regulation of mitochondrial dynamics and the downstream SERCA2a. Redox biology 24 38359748
2017 FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome. Clinical genetics 23 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
2017 Identification of FBXL4 as a Metastasis Associated Gene in Prostate Cancer. Scientific reports 18 28698647
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 17 31442532
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
2020 Novel homozygous mutation in the FBXL4 gene is associated with mitochondria DNA depletion syndrome-13. Journal of the neurological sciences 12 32559514
2022 Dichloroacetate improves mitochondrial function, physiology, and morphology in FBXL4 disease models. JCI insight 11 35881484
2024 Excessive BNIP3- and BNIP3L-dependent mitophagy underlies the pathogenesis of FBXL4-mutated mitochondrial DNA depletion syndrome. Autophagy 9 37876279
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
2024 FBXL4: safeguarding against mitochondrial depletion through suppression of mitophagy. Autophagy 8 38423516
2023 FBXL4 mutation-caused mitochondrial DNA depletion syndrome is driven by BNIP3/BNIP3L-dependent excessive mitophagy. Trends in molecular medicine 7 38123379
2022 Prenatal phenotype of FBXL4-associated encephalomyopathic mitochondrial DNA depletion syndrome-13. Prenatal diagnosis 6 36411461
2020 Different clinical presentation in a patient with two novel pathogenic variants of the FBXL4 gene. The Turkish journal of pediatrics 6 32779419
2024 Circ-CIMIRC inhibition alleviates CIH-induced myocardial damage via FbxL4-mediated ubiquitination of PINK1. iScience 5 38333696
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
2025 FBXL4-Related Mitochondrial Depletion Syndrome Underscores the role of Mitophagy in Stem Cell Differentiation during Embryogenesis. Stem cell reviews and reports 2 39937392
2021 A Mild Phenotype of Mitochondrial DNA Depletion Syndrome Type 13 with a Novel FBXL4 Variant. Molecular syndromology 2 34602956
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