{"gene":"FCHO1","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":2016,"finding":"Arrayed DPF motifs within Eps15/R are differentially decoded by Fcho1/2 versus AP-2; the structure of an Eps15/R·Fcho1 μ-homology domain complex reveals a spacing-dependent DPF triad bound in a mechanistically distinct way from single DPF binding to AP-2. Fcho1/2, Eps15/R, and AP-2 form transient ternary nanoclusters, and the Fcho1/2 interdomain linker facilitates conformational activation of AP-2 to promote cargo engagement.","method":"Crystal structure of Eps15/R·Fcho1 μ-HD complex; cell-based loss-of-function (FCHO1/2 knockout + Eps15 sequestration); Co-IP; mutagenesis","journal":"Developmental Cell","confidence":"High","confidence_rationale":"Tier 1 — crystal structure combined with mutagenesis and epistatic cell-based rescue experiments in a single rigorous study","pmids":["27237791"],"is_preprint":false},{"year":2012,"finding":"The μ-homology domain of FCHO1/2 acts as an endocytic interaction hub; FCHO1 μ-HD directly interacts with the BMP receptor Alk8, and fcho1 knockdown in zebrafish impairs BMP signal transmission, demonstrating a positive modulatory role in BMP receptor endocytosis distinct from the essential role of AP-2.","method":"Translational silencing (morpholino) in zebrafish; co-immunoprecipitation of μ-HD with Alk8; genetic epistasis comparing fcho1 and AP-2 morphant phenotypes","journal":"Nature Cell Biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus in vivo epistasis in zebrafish with defined phenotypic readout, replicated across methods","pmids":["22484487"],"is_preprint":false},{"year":2020,"finding":"Disease-causing FCHO1 mutations either cause mislocalization of the protein or prevent its interaction with binding partners, leading to impaired clathrin-coated pit formation; FCHO1-deficient Jurkat T cells show severely perturbed TCR internalization that is rescued by wild-type FCHO1 re-expression.","method":"Live-cell imaging of mutant FCHO1 variants; shRNA/CRISPR loss-of-function in Jurkat cells; TCR internalization rescue assay","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 2 — clean loss-of-function with defined cellular phenotype (TCR internalization) and specific rescue experiment in human patient cells and Jurkat cells","pmids":["32098969"],"is_preprint":false},{"year":2007,"finding":"GFP-FCHO1 displays periodic fluctuations in intensity at perinuclear regions (~every 100 s) that are temporally correlated with clathrin periodicity, linking FCHO1 dynamics to clathrin-coated vesicle formation.","method":"Live-cell fluorescence imaging of GFP-FCHO1","journal":"Bioscience, Biotechnology, and Biochemistry","confidence":"Low","confidence_rationale":"Tier 3 — single localization/dynamics observation without functional perturbation","pmids":["17617719"],"is_preprint":false},{"year":2020,"finding":"PKB (AKT) phosphorylates FCHO1 at a substrate motif (560PPRRLRSRKVSC571), and a synthetic peptide derived from this motif inhibits cell proliferation via PKB/ERK/SMAD4 pathways, indicating FCHO1 is a PKB substrate involved in cell division regulation.","method":"In vitro kinase assay with synthetic PKB substrate motif peptide; cell proliferation and xenograft tumor assays","journal":"Biochemical and Biophysical Research Communications","confidence":"Low","confidence_rationale":"Tier 3 — peptide-based assay without direct site-specific phosphorylation mapping or mutagenesis of endogenous FCHO1","pmids":["32507602"],"is_preprint":false},{"year":2024,"finding":"Using super-resolution imaging and pixel-based correlation analysis (PC-coloring), Eps15, FCHo1/2, and intersectin-1 were shown to form a complex with Grb2 along the interior of Grb2-dominant regions within clathrin-coated structures upon EGF stimulation, with Eps15-FCHo1/2 complexes lining the interior bordered by Eps15-intersectin-1 complexes, revealing laminar complex architecture at cargo recruitment sites.","method":"IRIS super-resolution microscopy with Fab probes; pixel-based principal component analysis and clustering (PC-coloring); colocalization correlation analysis","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 2 — novel super-resolution approach with quantitative colocalization, but preprint not yet peer-reviewed","pmids":["bio_10.1101_2024.07.02.601646"],"is_preprint":true},{"year":2026,"finding":"FCHO1 regulates synaptic vesicle endocytosis at central synapses in an activity-dependent and domain-specific manner: the F-BAR domain is sufficient under low stimulation conditions, whereas the μ-homology domain becomes essential at higher stimulation intensities; shRNA-mediated FCHO1 depletion markedly slows endocytic kinetics and is rescued by shRNA-resistant FCHO1.","method":"shRNA knockdown; pHluorin-based live imaging of synaptic vesicle recycling; domain-specific rescue constructs (F-BAR only vs. full-length vs. μ-HD mutants)","journal":"Molecular Brain","confidence":"High","confidence_rationale":"Tier 2 — clean KD with specific rescue and domain dissection providing mechanistic resolution of functional contributions","pmids":["41943152"],"is_preprint":false},{"year":2024,"finding":"In budding yeast, the FCHO1/2 ortholog Syp1 cooperates with CALM/AP180 orthologs (Yap1801/Yap1802) and anionic phospholipids/cargo to trigger Ede1 (Eps15)-centric CME initiation complex assembly at the plasma membrane, preferentially in regions of higher cargo/acidic phospholipid density.","method":"Live-cell imaging; yeast genetics (double mutant analysis); quantitative comparison of CME protein dynamics in mother vs. daughter cells","journal":"PLoS Biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis and live imaging in yeast ortholog system with quantitative analysis","pmids":["39316607"],"is_preprint":false}],"current_model":"FCHO1 is an early endocytic pioneer whose F-BAR domain binds and curves membranes, whose μ-homology domain serves as an interaction hub (binding DPF motifs in Eps15/R, the BMP receptor Alk8, and components of the clathrin coat), and whose interdomain linker facilitates conformational activation of AP-2; together with Eps15/R it forms transient ternary nanoclusters that prime AP-2 for cargo binding, while at synapses FCHO1 operates as a demand-sensitive scaffold in which the F-BAR domain supports basal vesicle retrieval and the μ-HD becomes essential under high stimulation load, and FCHO1 is itself phosphorylated by PKB/AKT to regulate cell division."},"narrative":{"teleology":[{"year":2007,"claim":"Initial live-cell imaging linked FCHO1 dynamics to clathrin-coated vesicle formation by showing that GFP-FCHO1 intensity oscillates with periodicity matching clathrin at perinuclear regions, establishing FCHO1 as temporally coordinated with the clathrin coat cycle.","evidence":"Live-cell fluorescence imaging of GFP-FCHO1 in mammalian cells","pmids":["17617719"],"confidence":"Low","gaps":["No functional perturbation performed; correlation does not demonstrate causality","Overexpression of GFP-fusion may not reflect endogenous behavior","No binding partners or domain requirements identified"]},{"year":2012,"claim":"The question of whether FCHO1 has functions beyond coat assembly was answered by showing that the μ-HD directly binds the BMP receptor Alk8 and that fcho1 loss in zebrafish impairs BMP signaling, establishing FCHO1 as a cargo-selective endocytic modulator for receptor signaling.","evidence":"Co-immunoprecipitation of μ-HD with Alk8; morpholino knockdown in zebrafish with genetic epistasis versus AP-2","pmids":["22484487"],"confidence":"High","gaps":["Structural basis of μ-HD–Alk8 interaction not resolved","Whether mammalian FCHO1 similarly regulates BMP signaling not tested"]},{"year":2016,"claim":"The molecular mechanism by which FCHO1 activates AP-2 was elucidated: crystal structures showed that the μ-HD decodes arrayed DPF motifs in Eps15/R in a spacing-dependent manner distinct from AP-2, and the interdomain linker enables conformational opening of AP-2 for cargo binding, establishing the FCHO1–Eps15/R–AP-2 ternary nanocluster as the functional unit for endocytic initiation.","evidence":"Crystal structure of Eps15/R·Fcho1 μ-HD complex; FCHO1/2 knockout cells; mutagenesis and rescue","pmids":["27237791"],"confidence":"High","gaps":["How the ternary nanocluster transitions to a mature clathrin-coated pit is not defined","Relative contributions of FCHO1 versus FCHO2 in different cell types remain unclear"]},{"year":2020,"claim":"The pathophysiological relevance of FCHO1 was established when disease-causing mutations were shown to either mislocalize the protein or abolish partner interactions, impairing clathrin-coated pit formation and TCR internalization in T cells, directly linking FCHO1 loss-of-function to human immunodeficiency.","evidence":"Live-cell imaging of patient-derived FCHO1 mutants; CRISPR knockout and shRNA in Jurkat cells with wild-type rescue","pmids":["32098969"],"confidence":"High","gaps":["Which specific interaction is disrupted by each mutation not fully mapped","Whether FCHO1 deficiency affects endocytosis of receptors beyond TCR in T cells not systematically tested"]},{"year":2024,"claim":"Ortholog studies in yeast showed that Syp1 (FCHO1/2 ortholog) cooperates with CALM/AP180 family proteins and anionic lipids to trigger Ede1 (Eps15)-centric CME initiation preferentially in cargo- and lipid-rich membrane regions, establishing an evolutionarily conserved mechanism for site selection during endocytic initiation.","evidence":"Live-cell imaging and double-mutant genetics in budding yeast comparing CME dynamics in mother versus daughter cells","pmids":["39316607"],"confidence":"Medium","gaps":["Direct functional equivalence of Syp1 and mammalian FCHO1 not demonstrated by cross-species rescue","Whether lipid-dependent site selection also applies in mammalian cells not tested"]},{"year":2026,"claim":"The long-standing question of whether FCHO1 functions at synapses and how its domains contribute was resolved: the F-BAR domain is sufficient for basal synaptic vesicle endocytosis, but the μ-HD becomes essential under high-frequency stimulation, establishing FCHO1 as a demand-sensitive endocytic scaffold at central synapses.","evidence":"shRNA knockdown with pHluorin-based live imaging of synaptic vesicle recycling; domain-specific rescue constructs in neurons","pmids":["41943152"],"confidence":"High","gaps":["Identity of μ-HD binding partners at synapses that are required under high stimulation is unknown","Whether FCHO2 compensates at synapses is not addressed"]},{"year":null,"claim":"Key unresolved questions include the full repertoire of cargo receptors and signaling receptors engaged by the FCHO1 μ-HD, how FCHO1 versus FCHO2 are differentially deployed across tissues, whether FCHO1 phosphorylation by PKB regulates endocytic function in vivo, and how the transient nanocluster matures into a productive clathrin-coated pit.","evidence":"","pmids":[],"confidence":"Low","gaps":["No in vivo phospho-site mutagenesis of endogenous FCHO1","FCHO1/FCHO2 tissue-specific functional redundancy not systematically addressed","Structural basis for activity-dependent μ-HD requirement at synapses unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,1,6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,6,7]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[2,3]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,2,6,7]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1]}],"complexes":["FCHO1–Eps15/R–AP-2 endocytic initiation nanocluster"],"partners":["EPS15","EPS15L1","AP2A1","ACVRL1","ITSN1"],"other_free_text":[]},"mechanistic_narrative":"FCHO1 is an early-acting endocytic pioneer that nucleates clathrin-coated pit formation by coupling membrane curvature sensing to adaptor activation and cargo recruitment. Its N-terminal F-BAR domain binds and bends membranes, while its C-terminal μ-homology domain (μ-HD) serves as a versatile interaction hub that directly engages arrayed DPF motifs in Eps15/R, forming transient ternary nanoclusters with AP-2; the FCHO1 interdomain linker facilitates conformational activation of AP-2 to promote cargo engagement [PMID:27237791]. The μ-HD also directly binds the BMP receptor Alk8 to modulate receptor endocytosis and signaling in vivo [PMID:22484487], and disease-causing FCHO1 mutations that disrupt protein localization or partner interactions impair clathrin-coated pit formation and TCR internalization in T cells [PMID:32098969]. At central synapses, FCHO1 operates in a demand-sensitive manner: the F-BAR domain supports basal synaptic vesicle retrieval, whereas the μ-HD becomes essential under high-frequency stimulation [PMID:41943152]."},"prefetch_data":{"uniprot":{"accession":"O14526","full_name":"F-BAR domain only protein 1","aliases":[],"length_aa":889,"mass_kda":96.9,"function":"Functions in an early step of clathrin-mediated endocytosis (PubMed:30822429). Has both a membrane binding/bending activity and the ability to recruit proteins essential to the formation of functional clathrin-coated pits. May regulate Bmp signaling by regulating clathrin-mediated endocytosis of Bmp receptors. Involved in the regulation of T-cell poliferation and activation (PubMed:30822429, PubMed:32098969). Affects TCR clustering upon receptor triggering and modulates its internalisation, playing a role in TCR-dependent T-cell activation (PubMed:32098969)","subcellular_location":"Membrane, clathrin-coated pit","url":"https://www.uniprot.org/uniprotkb/O14526/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FCHO1","classification":"Not Classified","n_dependent_lines":27,"n_total_lines":1208,"dependency_fraction":0.022350993377483443},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FCHO1","total_profiled":1310},"omim":[{"mim_id":"619164","title":"IMMUNODEFICIENCY 76; IMD76","url":"https://www.omim.org/entry/619164"},{"mim_id":"613438","title":"FCH DOMAIN ONLY PROTEIN 2; FCHO2","url":"https://www.omim.org/entry/613438"},{"mim_id":"613437","title":"FCH DOMAIN ONLY PROTEIN 1; FCHO1","url":"https://www.omim.org/entry/613437"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":23.8},{"tissue":"lymphoid tissue","ntpm":14.9},{"tissue":"skin 1","ntpm":20.2}],"url":"https://www.proteinatlas.org/search/FCHO1"},"hgnc":{"alias_symbol":["KIAA0290"],"prev_symbol":[]},"alphafold":{"accession":"O14526","domains":[{"cath_id":"1.20.1270.60","chopping":"15-249","consensus_level":"medium","plddt":94.8126,"start":15,"end":249},{"cath_id":"-","chopping":"627-745","consensus_level":"medium","plddt":89.8929,"start":627,"end":745},{"cath_id":"2.60.40.1170","chopping":"754-852","consensus_level":"medium","plddt":87.6633,"start":754,"end":852}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O14526","model_url":"https://alphafold.ebi.ac.uk/files/AF-O14526-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O14526-F1-predicted_aligned_error_v6.png","plddt_mean":71.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FCHO1","jax_strain_url":"https://www.jax.org/strain/search?query=FCHO1"},"sequence":{"accession":"O14526","fasta_url":"https://rest.uniprot.org/uniprotkb/O14526.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O14526/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O14526"}},"corpus_meta":[{"pmid":"27237791","id":"PMC_27237791","title":"Transient Fcho1/2⋅Eps15/R⋅AP-2 Nanoclusters Prime the AP-2 Clathrin Adaptor for Cargo Binding.","date":"2016","source":"Developmental cell","url":"https://pubmed.ncbi.nlm.nih.gov/27237791","citation_count":83,"is_preprint":false},{"pmid":"22484487","id":"PMC_22484487","title":"Distinct and separable activities of the endocytic clathrin-coat components Fcho1/2 and AP-2 in developmental patterning.","date":"2012","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/22484487","citation_count":76,"is_preprint":false},{"pmid":"32098969","id":"PMC_32098969","title":"Human FCHO1 deficiency reveals role for clathrin-mediated endocytosis in development and function of T cells.","date":"2020","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/32098969","citation_count":24,"is_preprint":false},{"pmid":"30822429","id":"PMC_30822429","title":"F-BAR domain only protein 1 (FCHO1) deficiency is a novel cause of combined immune deficiency in human subjects.","date":"2019","source":"The Journal of allergy and clinical immunology","url":"https://pubmed.ncbi.nlm.nih.gov/30822429","citation_count":22,"is_preprint":false},{"pmid":"17617719","id":"PMC_17617719","title":"Dynamic behavior of FCHO1 revealed by live-cell imaging microscopy: its possible involvement in clathrin-coated vesicle formation.","date":"2007","source":"Bioscience, biotechnology, and biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/17617719","citation_count":11,"is_preprint":false},{"pmid":"32507602","id":"PMC_32507602","title":"FCHO1560-571 peptide, a PKB kinase motif, inhibits tumor progression.","date":"2020","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/32507602","citation_count":5,"is_preprint":false},{"pmid":"34413495","id":"PMC_34413495","title":"FCH domain only 1 (FCHo1), a potential new biomarker for lung cancer.","date":"2021","source":"Cancer gene therapy","url":"https://pubmed.ncbi.nlm.nih.gov/34413495","citation_count":3,"is_preprint":false},{"pmid":"39498505","id":"PMC_39498505","title":"Human FCHO1 deficiency: review of the literature and additional two cases.","date":"2025","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/39498505","citation_count":1,"is_preprint":false},{"pmid":"39166479","id":"PMC_39166479","title":"Comprehensive exploration of FCHO1 mutations: Clinical manifestations and implications across disorders.","date":"2024","source":"American journal of medical genetics. Part A","url":"https://pubmed.ncbi.nlm.nih.gov/39166479","citation_count":1,"is_preprint":false},{"pmid":"39316607","id":"PMC_39316607","title":"The conserved protein adaptors CALM/AP180 and FCHo1/2 cooperatively recruit Eps15 to promote the initiation of clathrin-mediated endocytosis in yeast.","date":"2024","source":"PLoS biology","url":"https://pubmed.ncbi.nlm.nih.gov/39316607","citation_count":1,"is_preprint":false},{"pmid":"41943152","id":"PMC_41943152","title":"FCHO1 fine-tunes synaptic vesicle endocytosis in an activity-dependent manner.","date":"2026","source":"Molecular brain","url":"https://pubmed.ncbi.nlm.nih.gov/41943152","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.07.02.601646","title":"Nanoscale cluster mapping of molecular complexes with pixel-based correlations","date":"2024-07-03","source":"bioRxiv","url":"https://doi.org/10.1101/2024.07.02.601646","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6159,"output_tokens":2125,"usd":0.025176},"stage2":{"model":"claude-opus-4-6","input_tokens":5422,"output_tokens":2105,"usd":0.119603},"total_usd":0.144779,"stage1_batch_id":"msgbatch_0118RScg3WGaGkaZRtWZbtce","stage2_batch_id":"msgbatch_01Cpfzvtbj42aqp5oYZ5Te2s","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2016,\n      \"finding\": \"Arrayed DPF motifs within Eps15/R are differentially decoded by Fcho1/2 versus AP-2; the structure of an Eps15/R·Fcho1 μ-homology domain complex reveals a spacing-dependent DPF triad bound in a mechanistically distinct way from single DPF binding to AP-2. Fcho1/2, Eps15/R, and AP-2 form transient ternary nanoclusters, and the Fcho1/2 interdomain linker facilitates conformational activation of AP-2 to promote cargo engagement.\",\n      \"method\": \"Crystal structure of Eps15/R·Fcho1 μ-HD complex; cell-based loss-of-function (FCHO1/2 knockout + Eps15 sequestration); Co-IP; mutagenesis\",\n      \"journal\": \"Developmental Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure combined with mutagenesis and epistatic cell-based rescue experiments in a single rigorous study\",\n      \"pmids\": [\"27237791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The μ-homology domain of FCHO1/2 acts as an endocytic interaction hub; FCHO1 μ-HD directly interacts with the BMP receptor Alk8, and fcho1 knockdown in zebrafish impairs BMP signal transmission, demonstrating a positive modulatory role in BMP receptor endocytosis distinct from the essential role of AP-2.\",\n      \"method\": \"Translational silencing (morpholino) in zebrafish; co-immunoprecipitation of μ-HD with Alk8; genetic epistasis comparing fcho1 and AP-2 morphant phenotypes\",\n      \"journal\": \"Nature Cell Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus in vivo epistasis in zebrafish with defined phenotypic readout, replicated across methods\",\n      \"pmids\": [\"22484487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Disease-causing FCHO1 mutations either cause mislocalization of the protein or prevent its interaction with binding partners, leading to impaired clathrin-coated pit formation; FCHO1-deficient Jurkat T cells show severely perturbed TCR internalization that is rescued by wild-type FCHO1 re-expression.\",\n      \"method\": \"Live-cell imaging of mutant FCHO1 variants; shRNA/CRISPR loss-of-function in Jurkat cells; TCR internalization rescue assay\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean loss-of-function with defined cellular phenotype (TCR internalization) and specific rescue experiment in human patient cells and Jurkat cells\",\n      \"pmids\": [\"32098969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"GFP-FCHO1 displays periodic fluctuations in intensity at perinuclear regions (~every 100 s) that are temporally correlated with clathrin periodicity, linking FCHO1 dynamics to clathrin-coated vesicle formation.\",\n      \"method\": \"Live-cell fluorescence imaging of GFP-FCHO1\",\n      \"journal\": \"Bioscience, Biotechnology, and Biochemistry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single localization/dynamics observation without functional perturbation\",\n      \"pmids\": [\"17617719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PKB (AKT) phosphorylates FCHO1 at a substrate motif (560PPRRLRSRKVSC571), and a synthetic peptide derived from this motif inhibits cell proliferation via PKB/ERK/SMAD4 pathways, indicating FCHO1 is a PKB substrate involved in cell division regulation.\",\n      \"method\": \"In vitro kinase assay with synthetic PKB substrate motif peptide; cell proliferation and xenograft tumor assays\",\n      \"journal\": \"Biochemical and Biophysical Research Communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — peptide-based assay without direct site-specific phosphorylation mapping or mutagenesis of endogenous FCHO1\",\n      \"pmids\": [\"32507602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Using super-resolution imaging and pixel-based correlation analysis (PC-coloring), Eps15, FCHo1/2, and intersectin-1 were shown to form a complex with Grb2 along the interior of Grb2-dominant regions within clathrin-coated structures upon EGF stimulation, with Eps15-FCHo1/2 complexes lining the interior bordered by Eps15-intersectin-1 complexes, revealing laminar complex architecture at cargo recruitment sites.\",\n      \"method\": \"IRIS super-resolution microscopy with Fab probes; pixel-based principal component analysis and clustering (PC-coloring); colocalization correlation analysis\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — novel super-resolution approach with quantitative colocalization, but preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2024.07.02.601646\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"FCHO1 regulates synaptic vesicle endocytosis at central synapses in an activity-dependent and domain-specific manner: the F-BAR domain is sufficient under low stimulation conditions, whereas the μ-homology domain becomes essential at higher stimulation intensities; shRNA-mediated FCHO1 depletion markedly slows endocytic kinetics and is rescued by shRNA-resistant FCHO1.\",\n      \"method\": \"shRNA knockdown; pHluorin-based live imaging of synaptic vesicle recycling; domain-specific rescue constructs (F-BAR only vs. full-length vs. μ-HD mutants)\",\n      \"journal\": \"Molecular Brain\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KD with specific rescue and domain dissection providing mechanistic resolution of functional contributions\",\n      \"pmids\": [\"41943152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In budding yeast, the FCHO1/2 ortholog Syp1 cooperates with CALM/AP180 orthologs (Yap1801/Yap1802) and anionic phospholipids/cargo to trigger Ede1 (Eps15)-centric CME initiation complex assembly at the plasma membrane, preferentially in regions of higher cargo/acidic phospholipid density.\",\n      \"method\": \"Live-cell imaging; yeast genetics (double mutant analysis); quantitative comparison of CME protein dynamics in mother vs. daughter cells\",\n      \"journal\": \"PLoS Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis and live imaging in yeast ortholog system with quantitative analysis\",\n      \"pmids\": [\"39316607\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FCHO1 is an early endocytic pioneer whose F-BAR domain binds and curves membranes, whose μ-homology domain serves as an interaction hub (binding DPF motifs in Eps15/R, the BMP receptor Alk8, and components of the clathrin coat), and whose interdomain linker facilitates conformational activation of AP-2; together with Eps15/R it forms transient ternary nanoclusters that prime AP-2 for cargo binding, while at synapses FCHO1 operates as a demand-sensitive scaffold in which the F-BAR domain supports basal vesicle retrieval and the μ-HD becomes essential under high stimulation load, and FCHO1 is itself phosphorylated by PKB/AKT to regulate cell division.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FCHO1 is an early-acting endocytic pioneer that nucleates clathrin-coated pit formation by coupling membrane curvature sensing to adaptor activation and cargo recruitment. Its N-terminal F-BAR domain binds and bends membranes, while its C-terminal μ-homology domain (μ-HD) serves as a versatile interaction hub that directly engages arrayed DPF motifs in Eps15/R, forming transient ternary nanoclusters with AP-2; the FCHO1 interdomain linker facilitates conformational activation of AP-2 to promote cargo engagement [PMID:27237791]. The μ-HD also directly binds the BMP receptor Alk8 to modulate receptor endocytosis and signaling in vivo [PMID:22484487], and disease-causing FCHO1 mutations that disrupt protein localization or partner interactions impair clathrin-coated pit formation and TCR internalization in T cells [PMID:32098969]. At central synapses, FCHO1 operates in a demand-sensitive manner: the F-BAR domain supports basal synaptic vesicle retrieval, whereas the μ-HD becomes essential under high-frequency stimulation [PMID:41943152].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Initial live-cell imaging linked FCHO1 dynamics to clathrin-coated vesicle formation by showing that GFP-FCHO1 intensity oscillates with periodicity matching clathrin at perinuclear regions, establishing FCHO1 as temporally coordinated with the clathrin coat cycle.\",\n      \"evidence\": \"Live-cell fluorescence imaging of GFP-FCHO1 in mammalian cells\",\n      \"pmids\": [\"17617719\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No functional perturbation performed; correlation does not demonstrate causality\", \"Overexpression of GFP-fusion may not reflect endogenous behavior\", \"No binding partners or domain requirements identified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"The question of whether FCHO1 has functions beyond coat assembly was answered by showing that the μ-HD directly binds the BMP receptor Alk8 and that fcho1 loss in zebrafish impairs BMP signaling, establishing FCHO1 as a cargo-selective endocytic modulator for receptor signaling.\",\n      \"evidence\": \"Co-immunoprecipitation of μ-HD with Alk8; morpholino knockdown in zebrafish with genetic epistasis versus AP-2\",\n      \"pmids\": [\"22484487\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of μ-HD–Alk8 interaction not resolved\", \"Whether mammalian FCHO1 similarly regulates BMP signaling not tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The molecular mechanism by which FCHO1 activates AP-2 was elucidated: crystal structures showed that the μ-HD decodes arrayed DPF motifs in Eps15/R in a spacing-dependent manner distinct from AP-2, and the interdomain linker enables conformational opening of AP-2 for cargo binding, establishing the FCHO1–Eps15/R–AP-2 ternary nanocluster as the functional unit for endocytic initiation.\",\n      \"evidence\": \"Crystal structure of Eps15/R·Fcho1 μ-HD complex; FCHO1/2 knockout cells; mutagenesis and rescue\",\n      \"pmids\": [\"27237791\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the ternary nanocluster transitions to a mature clathrin-coated pit is not defined\", \"Relative contributions of FCHO1 versus FCHO2 in different cell types remain unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The pathophysiological relevance of FCHO1 was established when disease-causing mutations were shown to either mislocalize the protein or abolish partner interactions, impairing clathrin-coated pit formation and TCR internalization in T cells, directly linking FCHO1 loss-of-function to human immunodeficiency.\",\n      \"evidence\": \"Live-cell imaging of patient-derived FCHO1 mutants; CRISPR knockout and shRNA in Jurkat cells with wild-type rescue\",\n      \"pmids\": [\"32098969\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific interaction is disrupted by each mutation not fully mapped\", \"Whether FCHO1 deficiency affects endocytosis of receptors beyond TCR in T cells not systematically tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Ortholog studies in yeast showed that Syp1 (FCHO1/2 ortholog) cooperates with CALM/AP180 family proteins and anionic lipids to trigger Ede1 (Eps15)-centric CME initiation preferentially in cargo- and lipid-rich membrane regions, establishing an evolutionarily conserved mechanism for site selection during endocytic initiation.\",\n      \"evidence\": \"Live-cell imaging and double-mutant genetics in budding yeast comparing CME dynamics in mother versus daughter cells\",\n      \"pmids\": [\"39316607\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct functional equivalence of Syp1 and mammalian FCHO1 not demonstrated by cross-species rescue\", \"Whether lipid-dependent site selection also applies in mammalian cells not tested\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"The long-standing question of whether FCHO1 functions at synapses and how its domains contribute was resolved: the F-BAR domain is sufficient for basal synaptic vesicle endocytosis, but the μ-HD becomes essential under high-frequency stimulation, establishing FCHO1 as a demand-sensitive endocytic scaffold at central synapses.\",\n      \"evidence\": \"shRNA knockdown with pHluorin-based live imaging of synaptic vesicle recycling; domain-specific rescue constructs in neurons\",\n      \"pmids\": [\"41943152\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of μ-HD binding partners at synapses that are required under high stimulation is unknown\", \"Whether FCHO2 compensates at synapses is not addressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full repertoire of cargo receptors and signaling receptors engaged by the FCHO1 μ-HD, how FCHO1 versus FCHO2 are differentially deployed across tissues, whether FCHO1 phosphorylation by PKB regulates endocytic function in vivo, and how the transient nanocluster matures into a productive clathrin-coated pit.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No in vivo phospho-site mutagenesis of endogenous FCHO1\", \"FCHO1/FCHO2 tissue-specific functional redundancy not systematically addressed\", \"Structural basis for activity-dependent μ-HD requirement at synapses unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 1, 6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 6, 7]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 2, 6, 7]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"complexes\": [\n      \"FCHO1–Eps15/R–AP-2 endocytic initiation nanocluster\"\n    ],\n    \"partners\": [\n      \"EPS15\",\n      \"EPS15L1\",\n      \"AP2A1\",\n      \"ACVRL1\",\n      \"ITSN1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}