{"gene":"FER1L5","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":2010,"finding":"FER1L5 protein binds directly to the endocytic recycling proteins EHD1 and EHD2, with the second C2 domain of FER1L5 mediating this interaction. EHD2 is required for normal translocation of FER1L5 to the plasma membrane, and reduction of EHD1 and/or EHD2 inhibits myoblast fusion.","method":"Direct binding assay (pulldown), domain mapping (C2 domain deletion constructs), co-immunoprecipitation, knockdown experiments with myoblast fusion readout","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding assay with domain mapping plus functional KD phenotype, moderate evidence","pmids":["21177873"],"is_preprint":false},{"year":2014,"finding":"Loss of EHD1 causes mislocalization of FER1L5 in myoblasts, consistent with EHD1-dependent vesicle recycling being required for correct FER1L5 trafficking to the plasma membrane during muscle development.","method":"EHD1-null mouse model, immunofluorescence/confocal microscopy, protein localization analysis in primary myoblasts","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with direct localization readout, single study","pmids":["24440153"],"is_preprint":false},{"year":2016,"finding":"FER1L5 shows tissue-specific expression and subcellular localization consistent with the plasma membrane/late endosomal ferlin subgroup (type I ferlins), cycling between the plasma membrane and late endosomal compartments, as distinguished from trans-Golgi/recycling ferlins.","method":"3D-structured illumination microscopy, endosomal transit assays, colocalization with Rab7-positive late endosomes","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 — direct imaging with subcellular compartment markers, single study","pmids":["26707827"],"is_preprint":false},{"year":2020,"finding":"FER1L5 localizes to vesicular structures (low-density, non-detergent-resistant vesicles) in C2C12 myoblasts, is enriched at fusion sites of apposed myoblast membranes with peak expression at day 4 of differentiation, and is required for both myoblast fusion and membrane repair, as demonstrated by inhibitory antibody experiments causing fusion defects and impaired membrane repair.","method":"Confocal microscopy/immunolabeling, biochemical fractionation, multiphoton laser wounding assay, inhibitory antibody treatment","journal":"Biology","confidence":"Medium","confidence_rationale":"Tier 2–3 — multiple orthogonal methods (imaging, fractionation, functional antibody inhibition) in a single study","pmids":["33182221"],"is_preprint":false},{"year":2023,"finding":"FER1L5 is specifically required for the Ca2+-activated acrosome reaction in spermatozoa. Fer1l5 mutant mice are male-infertile; mutant spermatozoa can migrate to eggs but fail to undergo the acrosome reaction. Crucially, even exogenous Ca2+ ionophore cannot rescue the acrosome reaction in Fer1l5 mutants, identifying FER1L5 as an essential molecular link between Ca2+ signaling and exocytotic acrosome membrane fusion.","method":"CRISPR/gene-targeted mutant mouse generation, fertility assays, acrosome reaction assays with Ca2+ ionophore, sperm migration assays in female reproductive tract","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 — clean genetic KO with specific mechanistic readout (acrosome reaction) and ionophore rescue failure, strong evidence from multiple functional assays","pmids":["36696506"],"is_preprint":false},{"year":2022,"finding":"In silico full-length structural modeling of FER1L5 (using RoseTTAFold and AlphaFold2) defined objective domain boundaries and revealed a previously unrecognized C2 domain (C2-FerA) present in all six human ferlins including FER1L5, providing a revised domain architecture for the protein.","method":"RoseTTAFold and AlphaFold2 in silico structural modeling with cross-validation of domain boundaries","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 4 — computational prediction only, no experimental validation of domain structure","pmids":["35901179"],"is_preprint":false}],"current_model":"FER1L5 is a multi-C2-domain ferlin protein that localizes to low-density vesicles and the plasma membrane/late endosomal compartment; in myoblasts it interacts directly (via its C2B domain) with EHD1 and EHD2 endocytic recycling proteins to facilitate vesicle trafficking, myoblast fusion, and membrane repair, while in spermatozoa it serves as the essential molecular link between Ca2+ signaling and exocytotic acrosome membrane fusion required for male fertility."},"narrative":{"teleology":[{"year":2010,"claim":"Identification of FER1L5 as a direct binding partner of the endocytic recycling proteins EHD1 and EHD2 established the first mechanistic context for this ferlin, linking it to vesicle trafficking and revealing that its C2B domain mediates this interaction and that EHD-dependent recycling is required for myoblast fusion.","evidence":"Pulldown/co-immunoprecipitation with domain-deletion mapping and EHD knockdown in differentiating myoblasts","pmids":["21177873"],"confidence":"High","gaps":["Whether FER1L5 itself is required for fusion (as opposed to EHD proteins alone) was not shown by genetic loss-of-function at this point","No structural detail for the FER1L5–EHD interaction beyond domain identification"]},{"year":2014,"claim":"Genetic ablation of EHD1 confirmed that EHD1-dependent vesicle recycling is required for correct FER1L5 trafficking to the plasma membrane in primary myoblasts, validating the earlier biochemical interaction in vivo.","evidence":"EHD1-null mouse model with immunofluorescence localization of FER1L5 in primary myoblasts","pmids":["24440153"],"confidence":"Medium","gaps":["FER1L5 mislocalization was inferred from imaging in a single study; quantitative trafficking assays were not performed","Functional consequence of FER1L5 mislocalization on fusion or membrane repair was not directly tested in EHD1-null cells"]},{"year":2016,"claim":"Classification of FER1L5 as a type I (plasma membrane/late endosomal) ferlin, distinct from trans-Golgi ferlins, defined its intracellular itinerary and provided a subcellular framework for understanding its fusion role.","evidence":"3D-structured illumination microscopy with Rab7-positive late endosome colocalization and endosomal transit assays","pmids":["26707827"],"confidence":"Medium","gaps":["Cycling dynamics (rates, regulatory triggers) between the plasma membrane and late endosomes are uncharacterized","Whether late endosomal localization is functionally required or represents a degradation intermediate was not resolved"]},{"year":2020,"claim":"Direct functional evidence that FER1L5 is required for both myoblast fusion and membrane repair was obtained, showing it localizes to low-density vesicles at fusion sites and that antibody-mediated inhibition blocks both processes.","evidence":"Confocal imaging, biochemical fractionation, multiphoton laser wounding assay, and inhibitory antibody treatment in C2C12 myoblasts","pmids":["33182221"],"confidence":"Medium","gaps":["Antibody inhibition is indirect; a genetic knockout in myoblasts would strengthen causality","Identity of FER1L5-containing vesicle cargo is unknown","Whether membrane repair and fusion roles depend on the same or distinct FER1L5 molecular interactions was not dissected"]},{"year":2023,"claim":"CRISPR knockout of Fer1l5 in mice demonstrated that FER1L5 is indispensable for Ca²⁺-triggered acrosome exocytosis in spermatozoa, establishing it as the molecular effector downstream of Ca²⁺ signaling required for male fertility.","evidence":"Fer1l5 CRISPR-knockout mice, fertility assays, acrosome reaction assays with Ca²⁺ ionophore rescue attempts, sperm migration tracking","pmids":["36696506"],"confidence":"High","gaps":["Direct Ca²⁺-binding properties and which C2 domains sense Ca²⁺ during the acrosome reaction are undefined","Whether FER1L5 interacts with EHD proteins or other machinery in spermatozoa is untested","Mechanism by which FER1L5 drives outer acrosomal–plasma membrane fusion at the molecular level remains unknown"]},{"year":null,"claim":"Key unresolved questions include the structural basis for FER1L5 Ca²⁺-dependent membrane fusion, whether its roles in myoblast fusion and acrosome exocytosis share a common mechanism, and identification of additional protein partners beyond EHD1/EHD2.","evidence":"","pmids":[],"confidence":"Low","gaps":["No experimentally determined structure for FER1L5 or any of its C2 domains","No lipidomic or reconstitution data defining the membrane fusion mechanism","Genetic loss-of-function in skeletal muscle has not been performed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[2,3,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,3]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[2]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,1,2,3]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[4]}],"complexes":[],"partners":["EHD1","EHD2"],"other_free_text":[]},"mechanistic_narrative":"FER1L5 is a multi-C2-domain ferlin family protein that cycles between the plasma membrane and late endosomal compartments and functions as a Ca²⁺-dependent mediator of membrane fusion events in both skeletal muscle and spermatozoa [PMID:26707827, PMID:36696506]. In myoblasts, FER1L5 localizes to low-density vesicles enriched at fusion sites, where it interacts directly with the endocytic recycling proteins EHD1 and EHD2 via its C2B domain to support myoblast fusion and plasma membrane repair [PMID:21177873, PMID:33182221]. In spermatozoa, FER1L5 is the essential molecular link between Ca²⁺ signaling and exocytotic acrosome membrane fusion; Fer1l5-knockout mice are male-infertile because spermatozoa fail to undergo the acrosome reaction even when stimulated with exogenous Ca²⁺ ionophore [PMID:36696506]."},"prefetch_data":{"uniprot":{"accession":"A0AVI2","full_name":"Fer-1-like protein 5","aliases":[],"length_aa":2057,"mass_kda":237.9,"function":"Plays a crucial role in male fertility, primarily through its involvement in the acrosome reaction of spermatozoa. Required for the Ca(2+)-activated fusion of the acrosomal membrane with the plasma membrane during the acrosome. May play a role in skeletal muscle cell development by ensuring effective myoblast fusion and facilitating membrane repair","subcellular_location":"Cell membrane; Cytoplasmic vesicle membrane","url":"https://www.uniprot.org/uniprotkb/A0AVI2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FER1L5","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":74,"dependency_fraction":0.05405405405405406},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FER1L5","total_profiled":1310},"omim":[{"mim_id":"620884","title":"FER1-LIKE FAMILY, MEMBER 6; FER1L6","url":"https://www.omim.org/entry/620884"},{"mim_id":"620883","title":"FER1-LIKE FAMILY, MEMBER 5; FER1L5","url":"https://www.omim.org/entry/620883"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"testis","ntpm":4.0}],"url":"https://www.proteinatlas.org/search/FER1L5"},"hgnc":{"alias_symbol":["DKFZp434I0121"],"prev_symbol":[]},"alphafold":{"accession":"A0AVI2","domains":[{"cath_id":"2.60.40.150","chopping":"3-148","consensus_level":"high","plddt":77.5879,"start":3,"end":148},{"cath_id":"2.60.40.150","chopping":"166-323","consensus_level":"high","plddt":69.2684,"start":166,"end":323},{"cath_id":"2.60.40.150","chopping":"324-447_458-501","consensus_level":"medium","plddt":77.9158,"start":324,"end":501},{"cath_id":"2.60.40.150","chopping":"505-581_696-845_1003-1026_1062-1069","consensus_level":"medium","plddt":82.8694,"start":505,"end":1069},{"cath_id":"-","chopping":"857-862_874-918_938-949","consensus_level":"medium","plddt":83.6878,"start":857,"end":949},{"cath_id":"2.60.40.150","chopping":"1076-1239","consensus_level":"high","plddt":78.9275,"start":1076,"end":1239},{"cath_id":"2.60.40.150","chopping":"1243-1345_1427-1460","consensus_level":"high","plddt":81.462,"start":1243,"end":1460},{"cath_id":"2.60.40.150","chopping":"1482-1503_1523-1588_1672-1713","consensus_level":"medium","plddt":84.0125,"start":1482,"end":1713},{"cath_id":"2.60.40.150","chopping":"1720-1914","consensus_level":"high","plddt":78.9501,"start":1720,"end":1914}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/A0AVI2","model_url":"https://alphafold.ebi.ac.uk/files/AF-A0AVI2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-A0AVI2-F1-predicted_aligned_error_v6.png","plddt_mean":74.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FER1L5","jax_strain_url":"https://www.jax.org/strain/search?query=FER1L5"},"sequence":{"accession":"A0AVI2","fasta_url":"https://rest.uniprot.org/uniprotkb/A0AVI2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/A0AVI2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/A0AVI2"}},"corpus_meta":[{"pmid":"21177873","id":"PMC_21177873","title":"Endocytic 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EHD2 is required for normal translocation of FER1L5 to the plasma membrane, and reduction of EHD1 and/or EHD2 inhibits myoblast fusion.\",\n      \"method\": \"Direct binding assay (pulldown), domain mapping with C2 domain constructs, knockdown of EHD1/EHD2 with fusion phenotype readout, subcellular localization assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding with domain mapping, KD phenotype, and localization with functional consequence; moderate evidence from single lab with multiple orthogonal methods\",\n      \"pmids\": [\"21177873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"EHD1 loss causes mislocalization of Fer1L5 in myoblasts, indicating EHD1 is required for proper intracellular trafficking of Fer1L5 to the plasma membrane during muscle development.\",\n      \"method\": \"EHD1-null mouse model, immunofluorescence localization of Fer1L5 in EHD1-null myoblasts\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with localization readout, single lab, corroborates prior interaction data\",\n      \"pmids\": [\"24440153\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Fer1L5 localizes to vesicular structures (low-density vesicles non-resistant to non-ionic detergent) and accumulates at fusion sites of apposed C2C12 myoblast membranes. Inhibition with Fer1L5 antibodies causes defects in myoblast fusion and impaired membrane repair.\",\n      \"method\": \"Confocal immunolabeling, biochemical fractionation, multiphoton laser wounding assay, inhibitory antibody treatment with fusion and membrane repair phenotype readout\",\n      \"journal\": \"Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple orthogonal methods (fractionation, imaging, functional inhibition) from single lab\",\n      \"pmids\": [\"33182221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FER1L5 is required for the Ca2+-activated acrosome reaction in spermatozoa. Fer1l5 knockout mice are male-infertile; mutant spermatozoa can reach eggs but cannot undergo the acrosome reaction even when stimulated with a Ca2+ ionophore, establishing FER1L5 as an essential link between Ca2+ signaling and acrosomal exocytosis.\",\n      \"method\": \"Fer1l5 knockout mouse generation, fertility assay, acrosome reaction assay with Ca2+ ionophore stimulation, comparison with Fer1l4 and Fer1l6 knockouts\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — rigorous KO with defined cellular phenotype (acrosome reaction failure), ionophore rescue experiment rules out upstream Ca2+ signaling defect, replicated across multiple mutant lines\",\n      \"pmids\": [\"36696506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In silico full-length structural modeling of FER1L5 using RoseTTAFold and AlphaFold2 identified a previously unrecognized C2 domain (C2-FerA) conserved across all human ferlins and defined objective domain boundaries consistent across all six ferlin family members.\",\n      \"method\": \"Computational protein structure prediction (RoseTTAFold, AlphaFold2) with cross-validation of domain boundaries\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational prediction only, no experimental validation of structural findings\",\n      \"pmids\": [\"35901179\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FER1L5 is a multi-C2-domain, Ca2+-sensing ferlin protein that localizes to vesicular compartments and is trafficked to the plasma membrane via interaction (through its second C2 domain) with endocytic recycling proteins EHD1 and EHD2; in muscle it facilitates myoblast fusion and membrane repair, while in spermatozoa it is an essential mediator of Ca2+-triggered acrosomal exocytosis, acting downstream of Ca2+ influx to execute the acrosome reaction required for male fertility.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2010,\n      \"finding\": \"FER1L5 protein binds directly to the endocytic recycling proteins EHD1 and EHD2, with the second C2 domain of FER1L5 mediating this interaction. EHD2 is required for normal translocation of FER1L5 to the plasma membrane, and reduction of EHD1 and/or EHD2 inhibits myoblast fusion.\",\n      \"method\": \"Direct binding assay (pulldown), domain mapping (C2 domain deletion constructs), co-immunoprecipitation, knockdown experiments with myoblast fusion readout\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding assay with domain mapping plus functional KD phenotype, moderate evidence\",\n      \"pmids\": [\"21177873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Loss of EHD1 causes mislocalization of FER1L5 in myoblasts, consistent with EHD1-dependent vesicle recycling being required for correct FER1L5 trafficking to the plasma membrane during muscle development.\",\n      \"method\": \"EHD1-null mouse model, immunofluorescence/confocal microscopy, protein localization analysis in primary myoblasts\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with direct localization readout, single study\",\n      \"pmids\": [\"24440153\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FER1L5 shows tissue-specific expression and subcellular localization consistent with the plasma membrane/late endosomal ferlin subgroup (type I ferlins), cycling between the plasma membrane and late endosomal compartments, as distinguished from trans-Golgi/recycling ferlins.\",\n      \"method\": \"3D-structured illumination microscopy, endosomal transit assays, colocalization with Rab7-positive late endosomes\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct imaging with subcellular compartment markers, single study\",\n      \"pmids\": [\"26707827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FER1L5 localizes to vesicular structures (low-density, non-detergent-resistant vesicles) in C2C12 myoblasts, is enriched at fusion sites of apposed myoblast membranes with peak expression at day 4 of differentiation, and is required for both myoblast fusion and membrane repair, as demonstrated by inhibitory antibody experiments causing fusion defects and impaired membrane repair.\",\n      \"method\": \"Confocal microscopy/immunolabeling, biochemical fractionation, multiphoton laser wounding assay, inhibitory antibody treatment\",\n      \"journal\": \"Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple orthogonal methods (imaging, fractionation, functional antibody inhibition) in a single study\",\n      \"pmids\": [\"33182221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FER1L5 is specifically required for the Ca2+-activated acrosome reaction in spermatozoa. Fer1l5 mutant mice are male-infertile; mutant spermatozoa can migrate to eggs but fail to undergo the acrosome reaction. Crucially, even exogenous Ca2+ ionophore cannot rescue the acrosome reaction in Fer1l5 mutants, identifying FER1L5 as an essential molecular link between Ca2+ signaling and exocytotic acrosome membrane fusion.\",\n      \"method\": \"CRISPR/gene-targeted mutant mouse generation, fertility assays, acrosome reaction assays with Ca2+ ionophore, sperm migration assays in female reproductive tract\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic KO with specific mechanistic readout (acrosome reaction) and ionophore rescue failure, strong evidence from multiple functional assays\",\n      \"pmids\": [\"36696506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In silico full-length structural modeling of FER1L5 (using RoseTTAFold and AlphaFold2) defined objective domain boundaries and revealed a previously unrecognized C2 domain (C2-FerA) present in all six human ferlins including FER1L5, providing a revised domain architecture for the protein.\",\n      \"method\": \"RoseTTAFold and AlphaFold2 in silico structural modeling with cross-validation of domain boundaries\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational prediction only, no experimental validation of domain structure\",\n      \"pmids\": [\"35901179\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FER1L5 is a multi-C2-domain ferlin protein that localizes to low-density vesicles and the plasma membrane/late endosomal compartment; in myoblasts it interacts directly (via its C2B domain) with EHD1 and EHD2 endocytic recycling proteins to facilitate vesicle trafficking, myoblast fusion, and membrane repair, while in spermatozoa it serves as the essential molecular link between Ca2+ signaling and exocytotic acrosome membrane fusion required for male fertility.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FER1L5 is a multi-C2-domain ferlin family protein that functions as a Ca2+-sensing membrane fusion effector in both skeletal muscle and spermatozoa. It localizes to intracellular vesicular compartments and is trafficked to the plasma membrane through direct interaction of its second C2 domain with the endocytic recycling proteins EHD1 and EHD2, where it accumulates at myoblast fusion sites and participates in membrane repair [PMID:21177873, PMID:33182221]. In spermatozoa, FER1L5 is essential for Ca2+-triggered acrosomal exocytosis: Fer1l5 knockout mice are male-infertile, and their spermatozoa fail to undergo the acrosome reaction even when stimulated with a Ca2+ ionophore, placing FER1L5 downstream of Ca2+ influx as a direct executor of acrosomal membrane fusion [PMID:36696506].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Establishing how FER1L5 reaches the plasma membrane resolved a key trafficking question: FER1L5 directly binds EHD1 and EHD2 via its second C2 domain, and EHD2 is required for its translocation to the plasma membrane, linking ferlin trafficking to endocytic recycling machinery and myoblast fusion.\",\n      \"evidence\": \"Direct pulldown binding assays with C2 domain constructs, EHD1/EHD2 knockdown with fusion phenotype and localization readout in myoblasts\",\n      \"pmids\": [\"21177873\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether other C2 domains contribute to EHD binding or additional trafficking interactions\",\n        \"The Ca2+-dependence of the FER1L5–EHD interaction was not tested\",\n        \"No in vivo validation of the EHD-dependent trafficking mechanism\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"An EHD1-null mouse model confirmed the in vivo requirement: loss of EHD1 causes FER1L5 mislocalization in myoblasts, corroborating the trafficking dependence identified in vitro.\",\n      \"evidence\": \"EHD1 knockout mouse with immunofluorescence of Fer1L5 in primary myoblasts\",\n      \"pmids\": [\"24440153\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"EHD1 loss affects many cargo proteins; the specificity of the FER1L5 mislocalization phenotype was not dissected\",\n        \"No rescue experiment restoring EHD1 to confirm reversibility\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defining FER1L5's subcellular compartment and dual functional roles showed it resides on detergent-sensitive low-density vesicles, accumulates at myoblast fusion sites, and is required for both myoblast fusion and plasma membrane repair.\",\n      \"evidence\": \"Confocal imaging, biochemical fractionation, multiphoton laser wounding assay, and inhibitory antibody treatment in C2C12 myoblasts\",\n      \"pmids\": [\"33182221\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Antibody inhibition does not distinguish direct versus indirect effects on fusion and repair\",\n        \"No genetic loss-of-function (knockout/knockdown) was performed in the muscle repair context\",\n        \"Whether FER1L5 acts redundantly with dysferlin in membrane repair is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Computational structural modeling identified an additional C2 domain (C2-FerA) conserved across all ferlins and defined objective domain boundaries for FER1L5, refining the domain architecture of the ferlin family.\",\n      \"evidence\": \"RoseTTAFold and AlphaFold2 full-length structure prediction with cross-validation\",\n      \"pmids\": [\"35901179\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No experimental structural data (X-ray, cryo-EM) validates the predicted domain boundaries\",\n        \"Functional significance of the newly identified C2-FerA domain is untested\",\n        \"Lipid-binding or Ca2+-binding properties of individual C2 domains remain uncharacterized\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A definitive in vivo function was established: Fer1l5 knockout mice are male-infertile because their spermatozoa cannot execute the acrosome reaction, even when Ca2+ is artificially raised by ionophore, proving FER1L5 acts downstream of Ca2+ influx as a direct effector of acrosomal exocytosis.\",\n      \"evidence\": \"Fer1l5 knockout mouse, fertility assays, acrosome reaction assay with Ca2+ ionophore, comparison with Fer1l4 and Fer1l6 knockouts\",\n      \"pmids\": [\"36696506\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The molecular mechanism by which FER1L5 drives acrosomal membrane fusion is unknown\",\n        \"Whether FER1L5 acts alone or requires co-factors/SNARE partners for exocytosis\",\n        \"Redundancy or cooperation between FER1L5, FER1L4, and FER1L6 in the acrosome reaction is only partially addressed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of FER1L5-mediated membrane fusion, including which C2 domains sense Ca2+ and which engage target membranes, remains unknown, as does the relationship between its muscle and sperm functions at the molecular level.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No high-resolution structure of FER1L5 or any of its individual C2 domains\",\n        \"Ca2+-binding affinities of individual C2 domains are uncharacterized\",\n        \"Whether FER1L5 directly engages lipid bilayers to catalyze fusion or recruits downstream effectors is untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 2, 3]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"EHD1\",\n      \"EHD2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"FER1L5 is a multi-C2-domain ferlin family protein that cycles between the plasma membrane and late endosomal compartments and functions as a Ca²⁺-dependent mediator of membrane fusion events in both skeletal muscle and spermatozoa [PMID:26707827, PMID:36696506]. In myoblasts, FER1L5 localizes to low-density vesicles enriched at fusion sites, where it interacts directly with the endocytic recycling proteins EHD1 and EHD2 via its C2B domain to support myoblast fusion and plasma membrane repair [PMID:21177873, PMID:33182221]. In spermatozoa, FER1L5 is the essential molecular link between Ca²⁺ signaling and exocytotic acrosome membrane fusion; Fer1l5-knockout mice are male-infertile because spermatozoa fail to undergo the acrosome reaction even when stimulated with exogenous Ca²⁺ ionophore [PMID:36696506].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Identification of FER1L5 as a direct binding partner of the endocytic recycling proteins EHD1 and EHD2 established the first mechanistic context for this ferlin, linking it to vesicle trafficking and revealing that its C2B domain mediates this interaction and that EHD-dependent recycling is required for myoblast fusion.\",\n      \"evidence\": \"Pulldown/co-immunoprecipitation with domain-deletion mapping and EHD knockdown in differentiating myoblasts\",\n      \"pmids\": [\"21177873\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether FER1L5 itself is required for fusion (as opposed to EHD proteins alone) was not shown by genetic loss-of-function at this point\",\n        \"No structural detail for the FER1L5–EHD interaction beyond domain identification\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Genetic ablation of EHD1 confirmed that EHD1-dependent vesicle recycling is required for correct FER1L5 trafficking to the plasma membrane in primary myoblasts, validating the earlier biochemical interaction in vivo.\",\n      \"evidence\": \"EHD1-null mouse model with immunofluorescence localization of FER1L5 in primary myoblasts\",\n      \"pmids\": [\"24440153\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"FER1L5 mislocalization was inferred from imaging in a single study; quantitative trafficking assays were not performed\",\n        \"Functional consequence of FER1L5 mislocalization on fusion or membrane repair was not directly tested in EHD1-null cells\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Classification of FER1L5 as a type I (plasma membrane/late endosomal) ferlin, distinct from trans-Golgi ferlins, defined its intracellular itinerary and provided a subcellular framework for understanding its fusion role.\",\n      \"evidence\": \"3D-structured illumination microscopy with Rab7-positive late endosome colocalization and endosomal transit assays\",\n      \"pmids\": [\"26707827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Cycling dynamics (rates, regulatory triggers) between the plasma membrane and late endosomes are uncharacterized\",\n        \"Whether late endosomal localization is functionally required or represents a degradation intermediate was not resolved\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Direct functional evidence that FER1L5 is required for both myoblast fusion and membrane repair was obtained, showing it localizes to low-density vesicles at fusion sites and that antibody-mediated inhibition blocks both processes.\",\n      \"evidence\": \"Confocal imaging, biochemical fractionation, multiphoton laser wounding assay, and inhibitory antibody treatment in C2C12 myoblasts\",\n      \"pmids\": [\"33182221\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Antibody inhibition is indirect; a genetic knockout in myoblasts would strengthen causality\",\n        \"Identity of FER1L5-containing vesicle cargo is unknown\",\n        \"Whether membrane repair and fusion roles depend on the same or distinct FER1L5 molecular interactions was not dissected\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"CRISPR knockout of Fer1l5 in mice demonstrated that FER1L5 is indispensable for Ca²⁺-triggered acrosome exocytosis in spermatozoa, establishing it as the molecular effector downstream of Ca²⁺ signaling required for male fertility.\",\n      \"evidence\": \"Fer1l5 CRISPR-knockout mice, fertility assays, acrosome reaction assays with Ca²⁺ ionophore rescue attempts, sperm migration tracking\",\n      \"pmids\": [\"36696506\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct Ca²⁺-binding properties and which C2 domains sense Ca²⁺ during the acrosome reaction are undefined\",\n        \"Whether FER1L5 interacts with EHD proteins or other machinery in spermatozoa is untested\",\n        \"Mechanism by which FER1L5 drives outer acrosomal–plasma membrane fusion at the molecular level remains unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis for FER1L5 Ca²⁺-dependent membrane fusion, whether its roles in myoblast fusion and acrosome exocytosis share a common mechanism, and identification of additional protein partners beyond EHD1/EHD2.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No experimentally determined structure for FER1L5 or any of its C2 domains\",\n        \"No lipidomic or reconstitution data defining the membrane fusion mechanism\",\n        \"Genetic loss-of-function in skeletal muscle has not been performed\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [2, 3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 3]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 1, 2, 3]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"EHD1\",\n      \"EHD2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}