{"gene":"FBXL4","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2013,"finding":"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.","method":"Subcellular fractionation, mitochondrial targeting experiments, native gel electrophoresis (400 kDa complex), oxygen consumption measurements, mtDNA copy number quantification in patient muscle/fibroblasts","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent labs published simultaneously with overlapping orthogonal methods (fractionation, functional rescue, bioenergetics), replicated across multiple patient cell lines","pmids":["23993194","23993193"],"is_preprint":false},{"year":2013,"finding":"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.","method":"Genetic complementation assay — transfection of wild-type FBXL4 cDNA into patient-derived fibroblasts with measurement of mtDNA copy number and respiratory chain activity","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct rescue experiment (complementation) in two patient cell lines, single lab but strong internal controls","pmids":["23993193"],"is_preprint":false},{"year":2011,"finding":"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.","method":"Co-immunoprecipitation (SCF complex with JMJD2A), ubiquitin overexpression rescue assay, S-phase progression measurement (BrdU incorporation), chromatin occupancy analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus functional rescue with multiple orthogonal readouts (turnover, S-phase, chromatin occupancy), single lab","pmids":["21757720"],"is_preprint":false},{"year":2017,"finding":"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.","method":"GABA-A receptor ubiquitination assay, rhythmic protein expression analysis, genetic loss-of-function (fbxl4 mutants), electrophysiology/behavioral sleep assays, CLOCK-dependent transcription analysis","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ubiquitination assay plus genetic epistasis and behavioral readout in Drosophila, single lab with multiple orthogonal methods","pmids":["29174887"],"is_preprint":false},{"year":2019,"finding":"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.","method":"Photo-activatable GFP mitochondrial fusion assay, FBXL4 overexpression vs. pathogenic variant comparison in patient fibroblasts","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct live-imaging fusion assay with wild-type vs. mutant comparison, single lab, single method","pmids":["31442532"],"is_preprint":false},{"year":2023,"finding":"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.","method":"Mitochondria-targeted genetic screen, co-immunoprecipitation (SCF complex components), ubiquitination assay, Fbxl4 knockout mice, Bnip3/Nix double-knockout epistasis, metabolic phenotyping","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro ubiquitination assay, genetic epistasis in mice (double KO rescue), multiple orthogonal methods across two simultaneous independent publications","pmids":["36896912"],"is_preprint":false},{"year":2023,"finding":"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.","method":"CRISPR/Cas9 screen for mitophagy regulators, co-immunoprecipitation (FBXL4 with NIX/BNIP3), protein stability/degradation assays, patient variant functional analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — CRISPR screen plus direct interaction and degradation assays, independently replicated by three concurrent publications","pmids":["37161784"],"is_preprint":false},{"year":2023,"finding":"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.","method":"CRISPR/Cas9 E3 ligase screen, direct Co-IP (FBXL4–NIX/BNIP3), selective NIX vs. BNIP3 knockdown epistasis, mitophagy flux assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR screen, reciprocal interaction, genetic epistasis, replicated across multiple concurrent studies","pmids":["37102372"],"is_preprint":false},{"year":2023,"finding":"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.","method":"Co-immunoprecipitation, ubiquitin-proteasome pathway assays, CRL1 complex assembly assays, knock-in mouse model (patient mutation), hiPSC-derived cortical neurons from MTDPS13 patients","journal":"Cell death and differentiation","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct binding assay, ubiquitination reconstitution, patient knock-in mice, hiPSC model; orthogonal methods across multiple systems","pmids":["37568009"],"is_preprint":false},{"year":2024,"finding":"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.","method":"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","journal":"Redox biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis, and genetic epistasis, single lab with multiple orthogonal methods","pmids":["38359748"],"is_preprint":false},{"year":2025,"finding":"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.","method":"Co-immunoprecipitation (PPTC7–FBXL4–BNIP3/3L), protein stability assay with phosphatase-dead PPTC7 mutant, Pptc7 knockout mouse model, BNIP3/3L-dependent mitophagy rescue experiments","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis (phosphatase-dead), KO mouse, single lab with multiple orthogonal methods","pmids":["40025034"],"is_preprint":false},{"year":2026,"finding":"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.","method":"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","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — direct ubiquitination with lysine mapping, genetic KO and AAV rescue, epistasis experiments; single lab but multiple orthogonal methods","pmids":["41589689"],"is_preprint":false},{"year":2024,"finding":"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.","method":"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","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct Co-IP and ubiquitination site mapping by mutagenesis, single lab; note this study focuses on a circRNA context but directly establishes FBXL4–PINK1 ubiquitination","pmids":["38333696"],"is_preprint":false},{"year":2020,"finding":"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.","method":"3D protein structural analysis, mtDNA/nuclear DNA ratio quantification (ND1/GAPDH) in patient lymphocytes and urine","journal":"Journal of the neurological sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — structural modeling plus indirect functional evidence (mtDNA ratio), no direct biochemical reconstitution","pmids":["32559514"],"is_preprint":false}],"current_model":"FBXL4 is a nuclear-encoded F-box/LRR protein that localizes to the mitochondrial outer membrane (and intermembrane space) as part of an SCF (SKP1-CUL1-FBXL4) ubiquitin E3 ligase complex; this complex constitutively ubiquitinates and targets the mitophagy receptors BNIP3 and NIX/BNIP3L for proteasomal degradation, thereby suppressing basal mitophagy and maintaining mtDNA copy number—loss-of-function mutations disrupt SCF assembly, cause BNIP3/NIX accumulation and hyperactive mitophagy, and underlie mitochondrial DNA depletion syndrome type 13 (MTDPS13). FBXL4 also ubiquitinates additional substrates including the histone demethylase KDM4A/JMJD2A (regulating cell-cycle S-phase), Drp1 (restraining mitochondrial fission), PFN1/profilin-1 (preserving sarcomere integrity), and PINK1 (modulating mitophagy in cardiomyocytes), and in Drosophila acts as a CLOCK-dependent E3 ligase that rhythmically degrades GABA-A receptors to control pacemaker neuron excitability and sleep timing."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":2011,"claim":"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","pmids":["21757720"],"confidence":"Medium","gaps":["Did not localize this activity to mitochondria","Ubiquitination not reconstituted in vitro","Physiological relevance to organismal phenotype not tested"]},{"year":2013,"claim":"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","pmids":["23993194","23993193"],"confidence":"High","gaps":["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"]},{"year":2017,"claim":"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","pmids":["29174887"],"confidence":"Medium","gaps":["Conservation of GABA-A receptor targeting in mammals not addressed","Relationship to mitochondrial function not examined","Direct ubiquitin linkage chemistry not characterized"]},{"year":2019,"claim":"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","pmids":["31442532"],"confidence":"Medium","gaps":["Single-method, single-lab observation","Did not identify a molecular effector of fusion","Causal link to mtDNA depletion not established"]},{"year":2023,"claim":"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","pmids":["36896912","37161784","37102372","37568009"],"confidence":"High","gaps":["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"]},{"year":2024,"claim":"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","pmids":["38359748","38333696"],"confidence":"Medium","gaps":["Whether Drp1/PINK1 targeting operates outside cardiac stress contexts unknown","Hierarchy among multiple substrates not defined","circRNA-dependence may limit generality of PINK1 finding"]},{"year":2025,"claim":"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","pmids":["40025034"],"confidence":"Medium","gaps":["Structural role of PPTC7 in complex assembly unresolved","Whether PPTC7 regulates non-mitophagy substrates untested","Single-lab observation"]},{"year":2026,"claim":"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","pmids":["41589689"],"confidence":"Medium","gaps":["Relationship between PFN1 targeting and mitochondrial substrate functions not integrated","Generality beyond cardiomyocytes untested","Single-lab study"]},{"year":null,"claim":"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.","evidence":"","pmids":[],"confidence":"Medium","gaps":["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":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[2,5,8,9,11,12]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[5,6,8]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[13]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0,5,6]}],"pathway":[{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[5,6,7,8]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,5,8,11]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,4,9]}],"complexes":["SCF-FBXL4 (SKP1–CUL1–FBXL4) / CRL1-FBXL4 ubiquitin E3 ligase"],"partners":["SKP1","CUL1","BNIP3","BNIP3L","DNM1L","PINK1","PFN1","PPTC7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9UKA2","full_name":"F-box/LRR-repeat protein 4","aliases":["F-box and leucine-rich repeat protein 4","F-box protein FBL4/FBL5"],"length_aa":621,"mass_kda":70.1,"function":"Substrate-recognition component of the mitochondria-localized SCF-FBXL4 ubiquitin E3 ligase complex that plays a role in the restriction of mitophagy by controlling the degradation of BNIP3 and NIX mitophagy receptors (PubMed:36896912, PubMed:38992176). Rescues also mitochondrial injury through reverting hyperactivation of DRP1-mediated mitochondrial fission (By similarity)","subcellular_location":"Cytoplasm; Nucleus; Mitochondrion outer membrane","url":"https://www.uniprot.org/uniprotkb/Q9UKA2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FBXL4","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FBXL4","total_profiled":1310},"omim":[{"mim_id":"617847","title":"SPLICING FACTOR 3B, SUBUNIT 5; SF3B5","url":"https://www.omim.org/entry/617847"},{"mim_id":"615471","title":"MITOCHONDRIAL DNA DEPLETION SYNDROME 13 (ENCEPHALOMYOPATHIC TYPE); MTDPS13","url":"https://www.omim.org/entry/615471"},{"mim_id":"605654","title":"F-BOX AND LEUCINE-RICH REPEAT PROTEIN 4; FBXL4","url":"https://www.omim.org/entry/605654"},{"mim_id":"603041","title":"MITOCHONDRIAL DNA DEPLETION SYNDROME 1 (MNGIE TYPE); MTDPS1","url":"https://www.omim.org/entry/603041"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nuclear speckles","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/FBXL4"},"hgnc":{"alias_symbol":["FBL4","FBL5"],"prev_symbol":[]},"alphafold":{"accession":"Q9UKA2","domains":[{"cath_id":"2.60.120.260","chopping":"50-229","consensus_level":"high","plddt":93.735,"start":50,"end":229},{"cath_id":"3.80.10.10","chopping":"467-621","consensus_level":"medium","plddt":96.4539,"start":467,"end":621}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKA2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKA2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKA2-F1-predicted_aligned_error_v6.png","plddt_mean":86.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FBXL4","jax_strain_url":"https://www.jax.org/strain/search?query=FBXL4"},"sequence":{"accession":"Q9UKA2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UKA2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UKA2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKA2"}},"corpus_meta":[{"pmid":"23993194","id":"PMC_23993194","title":"Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy.","date":"2013","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23993194","citation_count":136,"is_preprint":false},{"pmid":"23993193","id":"PMC_23993193","title":"Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance.","date":"2013","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23993193","citation_count":136,"is_preprint":false},{"pmid":"36896912","id":"PMC_36896912","title":"A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease.","date":"2023","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/36896912","citation_count":98,"is_preprint":false},{"pmid":"37161784","id":"PMC_37161784","title":"FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors.","date":"2023","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/37161784","citation_count":68,"is_preprint":false},{"pmid":"21757720","id":"PMC_21757720","title":"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.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21757720","citation_count":56,"is_preprint":false},{"pmid":"37102372","id":"PMC_37102372","title":"FBXL4 ubiquitin ligase deficiency promotes mitophagy by elevating NIX levels.","date":"2023","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/37102372","citation_count":44,"is_preprint":false},{"pmid":"25868664","id":"PMC_25868664","title":"Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations.","date":"2015","source":"Journal of inherited metabolic disease","url":"https://pubmed.ncbi.nlm.nih.gov/25868664","citation_count":44,"is_preprint":false},{"pmid":"28940506","id":"PMC_28940506","title":"Molecular and clinical spectra of FBXL4 deficiency.","date":"2017","source":"Human mutation","url":"https://pubmed.ncbi.nlm.nih.gov/28940506","citation_count":38,"is_preprint":false},{"pmid":"37568009","id":"PMC_37568009","title":"FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome.","date":"2023","source":"Cell death and differentiation","url":"https://pubmed.ncbi.nlm.nih.gov/37568009","citation_count":36,"is_preprint":false},{"pmid":"29174887","id":"PMC_29174887","title":"Fbxl4 Serves as a Clock Output Molecule that Regulates Sleep through Promotion of Rhythmic Degradation of the GABAA Receptor.","date":"2017","source":"Current biology : 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Loss of FBXL4 results in mitochondrial respiratory chain deficiency, hyperfragmentation of the mitochondrial network, and substantially decreased mtDNA content in muscle and fibroblasts.\",\n      \"method\": \"Subcellular fractionation, mitochondrial targeting experiments, native gel electrophoresis (400 kDa complex), oxygen consumption measurements, mtDNA copy number quantification in patient muscle/fibroblasts\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent labs published simultaneously with overlapping orthogonal methods (fractionation, functional rescue, bioenergetics), replicated across multiple patient cell lines\",\n      \"pmids\": [\"23993194\", \"23993193\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"Genetic complementation assay — transfection of wild-type FBXL4 cDNA into patient-derived fibroblasts with measurement of mtDNA copy number and respiratory chain activity\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct rescue experiment (complementation) in two patient cell lines, single lab but strong internal controls\",\n      \"pmids\": [\"23993193\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"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.\",\n      \"method\": \"Co-immunoprecipitation (SCF complex with JMJD2A), ubiquitin overexpression rescue assay, S-phase progression measurement (BrdU incorporation), chromatin occupancy analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus functional rescue with multiple orthogonal readouts (turnover, S-phase, chromatin occupancy), single lab\",\n      \"pmids\": [\"21757720\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"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.\",\n      \"method\": \"GABA-A receptor ubiquitination assay, rhythmic protein expression analysis, genetic loss-of-function (fbxl4 mutants), electrophysiology/behavioral sleep assays, CLOCK-dependent transcription analysis\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ubiquitination assay plus genetic epistasis and behavioral readout in Drosophila, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"29174887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"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.\",\n      \"method\": \"Photo-activatable GFP mitochondrial fusion assay, FBXL4 overexpression vs. pathogenic variant comparison in patient fibroblasts\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct live-imaging fusion assay with wild-type vs. mutant comparison, single lab, single method\",\n      \"pmids\": [\"31442532\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"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.\",\n      \"method\": \"Mitochondria-targeted genetic screen, co-immunoprecipitation (SCF complex components), ubiquitination assay, Fbxl4 knockout mice, Bnip3/Nix double-knockout epistasis, metabolic phenotyping\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro ubiquitination assay, genetic epistasis in mice (double KO rescue), multiple orthogonal methods across two simultaneous independent publications\",\n      \"pmids\": [\"36896912\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"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.\",\n      \"method\": \"CRISPR/Cas9 screen for mitophagy regulators, co-immunoprecipitation (FBXL4 with NIX/BNIP3), protein stability/degradation assays, patient variant functional analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — CRISPR screen plus direct interaction and degradation assays, independently replicated by three concurrent publications\",\n      \"pmids\": [\"37161784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"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.\",\n      \"method\": \"CRISPR/Cas9 E3 ligase screen, direct Co-IP (FBXL4–NIX/BNIP3), selective NIX vs. BNIP3 knockdown epistasis, mitophagy flux assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR screen, reciprocal interaction, genetic epistasis, replicated across multiple concurrent studies\",\n      \"pmids\": [\"37102372\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"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.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitin-proteasome pathway assays, CRL1 complex assembly assays, knock-in mouse model (patient mutation), hiPSC-derived cortical neurons from MTDPS13 patients\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct binding assay, ubiquitination reconstitution, patient knock-in mice, hiPSC model; orthogonal methods across multiple systems\",\n      \"pmids\": [\"37568009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"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.\",\n      \"method\": \"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\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis, and genetic epistasis, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"38359748\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"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.\",\n      \"method\": \"Co-immunoprecipitation (PPTC7–FBXL4–BNIP3/3L), protein stability assay with phosphatase-dead PPTC7 mutant, Pptc7 knockout mouse model, BNIP3/3L-dependent mitophagy rescue experiments\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis (phosphatase-dead), KO mouse, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"40025034\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"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.\",\n      \"method\": \"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\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct ubiquitination with lysine mapping, genetic KO and AAV rescue, epistasis experiments; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"41589689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"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.\",\n      \"method\": \"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\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct Co-IP and ubiquitination site mapping by mutagenesis, single lab; note this study focuses on a circRNA context but directly establishes FBXL4–PINK1 ubiquitination\",\n      \"pmids\": [\"38333696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"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.\",\n      \"method\": \"3D protein structural analysis, mtDNA/nuclear DNA ratio quantification (ND1/GAPDH) in patient lymphocytes and urine\",\n      \"journal\": \"Journal of the neurological sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — structural modeling plus indirect functional evidence (mtDNA ratio), no direct biochemical reconstitution\",\n      \"pmids\": [\"32559514\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FBXL4 is a nuclear-encoded F-box/LRR protein that localizes to the mitochondrial outer membrane (and intermembrane space) as part of an SCF (SKP1-CUL1-FBXL4) ubiquitin E3 ligase complex; this complex constitutively ubiquitinates and targets the mitophagy receptors BNIP3 and NIX/BNIP3L for proteasomal degradation, thereby suppressing basal mitophagy and maintaining mtDNA copy number—loss-of-function mutations disrupt SCF assembly, cause BNIP3/NIX accumulation and hyperactive mitophagy, and underlie mitochondrial DNA depletion syndrome type 13 (MTDPS13). FBXL4 also ubiquitinates additional substrates including the histone demethylase KDM4A/JMJD2A (regulating cell-cycle S-phase), Drp1 (restraining mitochondrial fission), PFN1/profilin-1 (preserving sarcomere integrity), and PINK1 (modulating mitophagy in cardiomyocytes), and in Drosophila acts as a CLOCK-dependent E3 ligase that rhythmically degrades GABA-A receptors to control pacemaker neuron excitability and sleep timing.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"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 [#5, #6, #8]. 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 [#5, #6, #7, #8]. 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 [#0, #1, #8], and ablation of NIX (or NIX/BNIP3) rescues the phenotype in mice [#5, #7]. These deficits define mitochondrial DNA depletion syndrome type 13 (MTDPS13), with substrate accumulation reproduced in knock-in mice and patient hiPSC-derived neurons [#8]. 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 [#9, #13], and the outer-membrane phosphatase PPTC7 acts as an activity-independent cofactor promoting SCF-FBXL4-mediated BNIP3/NIX turnover [#10]. Beyond mitophagy, FBXL4 ubiquitinates additional substrates in distinct cellular contexts: the histone demethylase KDM4A/JMJD2A to restrain S-phase progression [#2], the fission GTPase Drp1 to limit mitochondrial fission [#9], and profilin-1 (PFN1, K48-linked at K70) to preserve sarcomeric integrity in cardiomyocytes [#11]; in Drosophila, CLOCK-driven rhythmic Fbxl4 degrades GABA-A receptors in pacemaker neurons to control excitability and sleep [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2011,\n      \"claim\": \"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.\",\n      \"evidence\": \"Reciprocal Co-IP of SCF/CUL1 with JMJD2A plus ubiquitin-rescue and BrdU S-phase readouts in cultured cells\",\n      \"pmids\": [\"21757720\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not localize this activity to mitochondria\", \"Ubiquitination not reconstituted in vitro\", \"Physiological relevance to organismal phenotype not tested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"Subcellular fractionation, native gel (400 kDa complex), bioenergetics, mtDNA quantification, and wild-type cDNA complementation in patient fibroblasts\",\n      \"pmids\": [\"23993194\", \"23993193\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"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\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed FBXL4 ubiquitin-ligase activity can be transcriptionally clocked to provide rhythmic control of receptor abundance, linking it to neuronal excitability and behavior.\",\n      \"evidence\": \"GABA-A receptor ubiquitination assays, CLOCK-dependent rhythmic expression, fbxl4 loss-of-function genetics, and sleep/electrophysiology in Drosophila\",\n      \"pmids\": [\"29174887\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Conservation of GABA-A receptor targeting in mammals not addressed\", \"Relationship to mitochondrial function not examined\", \"Direct ubiquitin linkage chemistry not characterized\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected pathogenic FBXL4 variants to altered mitochondrial dynamics, indicating FBXL4 promotes fusion (or restrains fragmentation).\",\n      \"evidence\": \"Photo-activatable GFP fusion assay comparing wild-type vs. p.Cys584Arg in patient fibroblasts\",\n      \"pmids\": [\"31442532\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single-method, single-lab observation\", \"Did not identify a molecular effector of fusion\", \"Causal link to mtDNA depletion not established\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"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.\",\n      \"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\",\n      \"pmids\": [\"36896912\", \"37161784\", \"37102372\", \"37568009\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"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\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"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.\",\n      \"evidence\": \"Mass spectrometry, Co-IP, ΔF-box and lysine-site mutagenesis, ubiquitination assays, and epistasis in HFpEF/CIH cardiomyocyte models\",\n      \"pmids\": [\"38359748\", \"38333696\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Whether Drp1/PINK1 targeting operates outside cardiac stress contexts unknown\", \"Hierarchy among multiple substrates not defined\", \"circRNA-dependence may limit generality of PINK1 finding\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"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.\",\n      \"evidence\": \"Co-IP, phosphatase-dead PPTC7 mutant stability assays, and Pptc7 knockout mouse phenocopying FBXL4 loss\",\n      \"pmids\": [\"40025034\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Structural role of PPTC7 in complex assembly unresolved\", \"Whether PPTC7 regulates non-mitophagy substrates untested\", \"Single-lab observation\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"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.\",\n      \"evidence\": \"Co-IP, K70 site-specific K48 ubiquitination assay, inducible cardiomyocyte-specific KO with AAV9 rescue, PFN1 epistasis, and hiPSC-derived cardiomyocytes\",\n      \"pmids\": [\"41589689\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Relationship between PFN1 targeting and mitochondrial substrate functions not integrated\", \"Generality beyond cardiomyocytes untested\", \"Single-lab study\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"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.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No structure of the assembled SCF-FBXL4 complex\", \"Tissue-specific substrate selection mechanism unknown\", \"Conditions that physiologically relieve FBXL4-mediated mitophagy suppression undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 5, 8, 9, 11, 12]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [5, 6, 8]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 5, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [5, 6, 7, 8]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 5, 8, 11]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 4, 9]}\n    ],\n    \"complexes\": [\"SCF-FBXL4 (SKP1–CUL1–FBXL4) / CRL1-FBXL4 ubiquitin E3 ligase\"],\n    \"partners\": [\"SKP1\", \"CUL1\", \"BNIP3\", \"BNIP3L\", \"DNM1L\", \"PINK1\", \"PFN1\", \"PPTC7\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":6,"faith_total":6,"faith_pct":100.0}}