{"gene":"FER1L5","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2010,"finding":"FER1L5 protein binds directly to endocytic recycling proteins EHD1 and EHD2, and the second C2 domain (C2B) of FER1L5 mediates this interaction. EHD2 is required for normal translocation of FER1L5 to the plasma membrane.","method":"Direct binding assay (pulldown), domain mapping with C2 domain constructs, localization studies in myoblasts","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding demonstrated with domain mapping and localization experiment, single lab but two orthogonal methods","pmids":["21177873"],"is_preprint":false},{"year":2010,"finding":"FER1L5 is expressed in small myotubes containing only two to four nuclei (early fusion stage), consistent with a role in early myoblast fusion events. Reduction of EHD1 and/or EHD2 inhibits myoblast fusion and impairs Fer1L5 membrane localization.","method":"Western blot / immunofluorescence of differentiating myoblasts; EHD1/EHD2 knockdown with fusion assay readout","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function (EHD knockdown) with defined cellular phenotype and localization data, single lab","pmids":["21177873"],"is_preprint":false},{"year":2014,"finding":"In EHD1-null myoblasts, Fer1L5 is mislocalized, demonstrating that EHD1 is required for proper subcellular localization of Fer1L5 during muscle development.","method":"EHD1-null mouse model; immunofluorescence localization of Fer1L5 in null vs. wildtype myoblasts","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout with direct localization readout, replicates finding from prior study by same lab","pmids":["24440153"],"is_preprint":false},{"year":2020,"finding":"Fer1L5 localizes to vesicular structures (low-density, non-detergent-resistant membrane vesicles) in C2C12 myoblasts, accumulates at fusion sites of apposed myoblast membranes, and inhibitory antibodies against Fer1L5 cause defects in myoblast fusion and impaired membrane repair.","method":"Confocal immunolabeling, biochemical fractionation into low-density vesicles, multiphoton laser wounding membrane repair assay, inhibitory antibody treatment","journal":"Biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (fractionation, localization, functional antibody inhibition), single lab","pmids":["33182221"],"is_preprint":false},{"year":2023,"finding":"FER1L5 is required for the Ca2+-activated acrosome reaction in spermatozoa. Fer1l5 mutant mice are male-infertile; their spermatozoa reach eggs normally but fail to undergo the acrosome reaction. Even Ca2+ ionophore treatment cannot rescue the acrosome reaction in Fer1l5 mutant sperm, placing FER1L5 downstream of Ca2+ influx as an essential mediator of this exocytotic event.","method":"Fer1l5 knockout/mutant mouse generation; male fertility assay; sperm migration assay in female reproductive tract; acrosome reaction assay with Ca2+ ionophore","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with specific functional phenotype (acrosome reaction failure), ionophore rescue experiment places FER1L5 downstream of Ca2+ entry, rigorous in vivo and ex vivo validation","pmids":["36696506"],"is_preprint":false},{"year":2022,"finding":"In silico full-length structural modeling (RoseTTAFold/AlphaFold2) of FER1L5 defined objective C2 domain boundaries and identified a previously uncharacterized C2 domain (C2-FerA) present in all human ferlins including FER1L5. The domain architecture is highly conserved across ferlins despite sequence divergence.","method":"Computational full-length protein structure prediction (RoseTTAFold, AlphaFold2) with domain boundary analysis","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 4 / Weak — purely computational prediction without experimental validation of the new domain in FER1L5 specifically","pmids":["35901179"],"is_preprint":false},{"year":2016,"finding":"Fer1L5 (type-I ferlin) localizes predominantly to the plasma membrane and late endosomes (Rab7-positive), distinguishing it from type-II ferlins (Fer1L6, otoferlin) that localize to the trans-Golgi/recycling endosome compartment.","method":"3D-structured illumination microscopy; endosomal transit assays with Rab7/Rab11 markers; live-cell imaging","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — super-resolution microscopy with organelle markers, but FER1L5-specific data is part of a broader family characterization in a single study","pmids":["26707827"],"is_preprint":false}],"current_model":"FER1L5 is a Ca2+-sensing, multi-C2-domain ferlin protein that localizes to low-density vesicles and the plasma membrane via EHD1/EHD2-dependent endocytic recycling (interacting through its C2B domain); it plays an essential role in myoblast fusion during early myotube formation and, most critically, acts downstream of Ca2+ influx as the required mediator of the exocytotic acrosome reaction in spermatozoa, with its absence rendering sperm completely unable to undergo this reaction even upon pharmacological Ca2+ elevation."},"narrative":{"mechanistic_narrative":"FER1L5 is a multi-C2-domain ferlin protein that functions as a Ca2+-sensing mediator of regulated membrane fusion and repair [PMID:33182221, PMID:36696506]. In myoblasts it resides on low-density vesicles and concentrates at the apposed membranes of fusing cells, where it is required for myoblast fusion and plasma-membrane repair [PMID:33182221]. Its delivery to the plasma membrane depends on the endocytic recycling machinery: FER1L5 binds the ATPases EHD1 and EHD2 directly through its second C2 domain (C2B), and loss of EHD1 or EHD2 mislocalizes the protein and blocks fusion [PMID:21177873, PMID:24440153]. FER1L5 distributes between the plasma membrane and Rab7-positive late endosomes, distinguishing it from trans-Golgi/recycling-endosome-localized ferlins [PMID:26707827]. Its most sharply defined physiological role is in spermatozoa, where FER1L5 is essential for the Ca2+-activated acrosome reaction: Fer1l5 mutant males are infertile and their sperm fail to undergo the acrosome reaction even when intracellular Ca2+ is forced upward by ionophore, placing FER1L5 genetically downstream of Ca2+ entry as a required effector of this exocytotic event [PMID:36696506].","teleology":[{"year":2010,"claim":"Established how FER1L5 is delivered to the plasma membrane by identifying a direct molecular link between the protein and the endocytic recycling machinery and mapping the responsible domain.","evidence":"Pulldown binding assay with C2-domain constructs and localization in myoblasts, plus EHD1/EHD2 knockdown with fusion readout","pmids":["21177873"],"confidence":"Medium","gaps":["Single lab; reciprocal validation of the EHD interaction not established","Whether C2B-EHD binding is Ca2+-regulated is unaddressed","Structural basis of the C2B-EHD interface unknown"]},{"year":2014,"claim":"Confirmed in vivo that EHD1 governs FER1L5 subcellular targeting, moving the recycling-dependent localization model from knockdown into a genetic-null context.","evidence":"EHD1-null mouse myoblasts with immunofluorescence localization of Fer1L5","pmids":["24440153"],"confidence":"Medium","gaps":["Functional consequence of mislocalization for muscle in the animal not quantified","Does not separate EHD1 direct vs. indirect effects"]},{"year":2016,"claim":"Defined FER1L5's steady-state compartments, classifying it as a type-I ferlin enriched at the plasma membrane and Rab7+ late endosomes rather than the trans-Golgi/recycling-endosome pool of other ferlins.","evidence":"3D-SIM super-resolution microscopy and Rab7/Rab11 endosomal transit assays","pmids":["26707827"],"confidence":"Medium","gaps":["FER1L5 data embedded in broader family survey","Functional role of the late-endosomal pool not tested"]},{"year":2020,"claim":"Linked FER1L5 localization to function by showing it accumulates at myoblast fusion sites on low-density vesicles and is required for both fusion and membrane repair.","evidence":"Biochemical fractionation, confocal immunolabeling, laser-wounding repair assay, and inhibitory antibody treatment in C2C12 cells","pmids":["33182221"],"confidence":"Medium","gaps":["Antibody inhibition not corroborated by genetic loss-of-function in muscle","Ca2+-dependence of the vesicle accumulation not directly tested"]},{"year":2023,"claim":"Identified the essential physiological role of FER1L5 as a required effector of the Ca2+-triggered acrosome reaction acting downstream of Ca2+ influx, explaining male infertility in its absence.","evidence":"Fer1l5 mutant mouse with fertility, sperm migration, and Ca2+-ionophore acrosome-reaction assays","pmids":["36696506"],"confidence":"High","gaps":["Molecular partners of FER1L5 in the acrosomal fusion machinery not identified","Whether C2 domains directly sense the Ca2+ signal during the reaction untested","Connection between sperm role and muscle fusion role mechanistically unresolved"]},{"year":null,"claim":"How FER1L5's C2 domains transduce Ca2+ signals into membrane fusion, and which SNARE or fusion-machinery partners it engages during the acrosome reaction and myoblast fusion, remains unresolved.","evidence":"No direct experimental evidence in the available corpus","pmids":[],"confidence":"Low","gaps":["No identified fusion-machinery partners","No experimental validation of Ca2+ binding by specific C2 domains","No structural model of the membrane-engaged state"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,6]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[4]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1,3]}],"complexes":[],"partners":["EHD1","EHD2"],"other_free_text":[]}},"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 recycling proteins EHD1 and EHD2 interact with fer-1-like-5 (Fer1L5) and mediate myoblast fusion.","date":"2010","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21177873","citation_count":51,"is_preprint":false},{"pmid":"24440153","id":"PMC_24440153","title":"EHD1 mediates vesicle trafficking required for normal muscle growth and transverse tubule development.","date":"2014","source":"Developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/24440153","citation_count":44,"is_preprint":false},{"pmid":"26707827","id":"PMC_26707827","title":"Ferlins Show Tissue-Specific Expression and Segregate as Plasma Membrane/Late Endosomal or Trans-Golgi/Recycling Ferlins.","date":"2016","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/26707827","citation_count":42,"is_preprint":false},{"pmid":"30109710","id":"PMC_30109710","title":"Genetic dissection of bull fertility in US Jersey dairy cattle.","date":"2018","source":"Animal genetics","url":"https://pubmed.ncbi.nlm.nih.gov/30109710","citation_count":41,"is_preprint":false},{"pmid":"32106631","id":"PMC_32106631","title":"Functions of Vertebrate Ferlins.","date":"2020","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/32106631","citation_count":39,"is_preprint":false},{"pmid":"36696506","id":"PMC_36696506","title":"Testis-enriched ferlin, FER1L5, is required for Ca2+-activated acrosome reaction and male fertility.","date":"2023","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/36696506","citation_count":24,"is_preprint":false},{"pmid":"35901179","id":"PMC_35901179","title":"Redefining the architecture of ferlin proteins: Insights into multi-domain protein structure and function.","date":"2022","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/35901179","citation_count":20,"is_preprint":false},{"pmid":"37538852","id":"PMC_37538852","title":"Otoferlin as a multirole Ca2+ signaling protein: from inner ear synapses to cancer pathways.","date":"2023","source":"Frontiers in cellular neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/37538852","citation_count":15,"is_preprint":false},{"pmid":"37227814","id":"PMC_37227814","title":"Identification and validation of diagnostic signature genes in non-obstructive azoospermia by machine learning.","date":"2023","source":"Aging","url":"https://pubmed.ncbi.nlm.nih.gov/37227814","citation_count":8,"is_preprint":false},{"pmid":"33182221","id":"PMC_33182221","title":"Fer1L5, a Dysferlin Homologue Present in Vesicles and Involved in C2C12 Myoblast Fusion and Membrane Repair.","date":"2020","source":"Biology","url":"https://pubmed.ncbi.nlm.nih.gov/33182221","citation_count":4,"is_preprint":false},{"pmid":"35330493","id":"PMC_35330493","title":"Neoexpression of JUNO in Oral Tumors Is Accompanied with the Complete Suppression of Four Other Genes and Suggests the Application of New Biomarker Tools.","date":"2022","source":"Journal of personalized medicine","url":"https://pubmed.ncbi.nlm.nih.gov/35330493","citation_count":2,"is_preprint":false},{"pmid":"41846291","id":"PMC_41846291","title":"Identification of biomarkers for non-obstructive azoospermia based on microRNA and bioinformatics screening.","date":"2026","source":"Yi chuan = Hereditas","url":"https://pubmed.ncbi.nlm.nih.gov/41846291","citation_count":0,"is_preprint":false},{"pmid":"41274580","id":"PMC_41274580","title":"Genomic regions affecting perinatal and early life survival in dairy calves.","date":"2025","source":"Journal of dairy science","url":"https://pubmed.ncbi.nlm.nih.gov/41274580","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8732,"output_tokens":1722,"usd":0.026013,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8516,"output_tokens":2143,"usd":0.048078,"stage2_stop_reason":"end_turn"},"total_usd":0.074091,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2010,\n      \"finding\": \"FER1L5 protein binds directly to endocytic recycling proteins EHD1 and EHD2, and the second C2 domain (C2B) of FER1L5 mediates this interaction. EHD2 is required for normal translocation of FER1L5 to the plasma membrane.\",\n      \"method\": \"Direct binding assay (pulldown), domain mapping with C2 domain constructs, localization studies in myoblasts\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding demonstrated with domain mapping and localization experiment, single lab but two orthogonal methods\",\n      \"pmids\": [\"21177873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FER1L5 is expressed in small myotubes containing only two to four nuclei (early fusion stage), consistent with a role in early myoblast fusion events. Reduction of EHD1 and/or EHD2 inhibits myoblast fusion and impairs Fer1L5 membrane localization.\",\n      \"method\": \"Western blot / immunofluorescence of differentiating myoblasts; EHD1/EHD2 knockdown with fusion assay readout\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function (EHD knockdown) with defined cellular phenotype and localization data, single lab\",\n      \"pmids\": [\"21177873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In EHD1-null myoblasts, Fer1L5 is mislocalized, demonstrating that EHD1 is required for proper subcellular localization of Fer1L5 during muscle development.\",\n      \"method\": \"EHD1-null mouse model; immunofluorescence localization of Fer1L5 in null vs. wildtype myoblasts\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout with direct localization readout, replicates finding from prior study by same lab\",\n      \"pmids\": [\"24440153\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Fer1L5 localizes to vesicular structures (low-density, non-detergent-resistant membrane vesicles) in C2C12 myoblasts, accumulates at fusion sites of apposed myoblast membranes, and inhibitory antibodies against Fer1L5 cause defects in myoblast fusion and impaired membrane repair.\",\n      \"method\": \"Confocal immunolabeling, biochemical fractionation into low-density vesicles, multiphoton laser wounding membrane repair assay, inhibitory antibody treatment\",\n      \"journal\": \"Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (fractionation, localization, functional antibody inhibition), 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 mutant mice are male-infertile; their spermatozoa reach eggs normally but fail to undergo the acrosome reaction. Even Ca2+ ionophore treatment cannot rescue the acrosome reaction in Fer1l5 mutant sperm, placing FER1L5 downstream of Ca2+ influx as an essential mediator of this exocytotic event.\",\n      \"method\": \"Fer1l5 knockout/mutant mouse generation; male fertility assay; sperm migration assay in female reproductive tract; acrosome reaction assay with Ca2+ ionophore\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with specific functional phenotype (acrosome reaction failure), ionophore rescue experiment places FER1L5 downstream of Ca2+ entry, rigorous in vivo and ex vivo validation\",\n      \"pmids\": [\"36696506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In silico full-length structural modeling (RoseTTAFold/AlphaFold2) of FER1L5 defined objective C2 domain boundaries and identified a previously uncharacterized C2 domain (C2-FerA) present in all human ferlins including FER1L5. The domain architecture is highly conserved across ferlins despite sequence divergence.\",\n      \"method\": \"Computational full-length protein structure prediction (RoseTTAFold, AlphaFold2) with domain boundary analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — purely computational prediction without experimental validation of the new domain in FER1L5 specifically\",\n      \"pmids\": [\"35901179\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Fer1L5 (type-I ferlin) localizes predominantly to the plasma membrane and late endosomes (Rab7-positive), distinguishing it from type-II ferlins (Fer1L6, otoferlin) that localize to the trans-Golgi/recycling endosome compartment.\",\n      \"method\": \"3D-structured illumination microscopy; endosomal transit assays with Rab7/Rab11 markers; live-cell imaging\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — super-resolution microscopy with organelle markers, but FER1L5-specific data is part of a broader family characterization in a single study\",\n      \"pmids\": [\"26707827\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FER1L5 is a Ca2+-sensing, multi-C2-domain ferlin protein that localizes to low-density vesicles and the plasma membrane via EHD1/EHD2-dependent endocytic recycling (interacting through its C2B domain); it plays an essential role in myoblast fusion during early myotube formation and, most critically, acts downstream of Ca2+ influx as the required mediator of the exocytotic acrosome reaction in spermatozoa, with its absence rendering sperm completely unable to undergo this reaction even upon pharmacological Ca2+ elevation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FER1L5 is a multi-C2-domain ferlin protein that functions as a Ca2+-sensing mediator of regulated membrane fusion and repair [#3, #4]. In myoblasts it resides on low-density vesicles and concentrates at the apposed membranes of fusing cells, where it is required for myoblast fusion and plasma-membrane repair [#3]. Its delivery to the plasma membrane depends on the endocytic recycling machinery: FER1L5 binds the ATPases EHD1 and EHD2 directly through its second C2 domain (C2B), and loss of EHD1 or EHD2 mislocalizes the protein and blocks fusion [#0, #1, #2]. FER1L5 distributes between the plasma membrane and Rab7-positive late endosomes, distinguishing it from trans-Golgi/recycling-endosome-localized ferlins [#6]. Its most sharply defined physiological role is in spermatozoa, where FER1L5 is essential for the Ca2+-activated acrosome reaction: Fer1l5 mutant males are infertile and their sperm fail to undergo the acrosome reaction even when intracellular Ca2+ is forced upward by ionophore, placing FER1L5 genetically downstream of Ca2+ entry as a required effector of this exocytotic event [#4].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Established how FER1L5 is delivered to the plasma membrane by identifying a direct molecular link between the protein and the endocytic recycling machinery and mapping the responsible domain.\",\n      \"evidence\": \"Pulldown binding assay with C2-domain constructs and localization in myoblasts, plus EHD1/EHD2 knockdown with fusion readout\",\n      \"pmids\": [\"21177873\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; reciprocal validation of the EHD interaction not established\", \"Whether C2B-EHD binding is Ca2+-regulated is unaddressed\", \"Structural basis of the C2B-EHD interface unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Confirmed in vivo that EHD1 governs FER1L5 subcellular targeting, moving the recycling-dependent localization model from knockdown into a genetic-null context.\",\n      \"evidence\": \"EHD1-null mouse myoblasts with immunofluorescence localization of Fer1L5\",\n      \"pmids\": [\"24440153\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of mislocalization for muscle in the animal not quantified\", \"Does not separate EHD1 direct vs. indirect effects\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined FER1L5's steady-state compartments, classifying it as a type-I ferlin enriched at the plasma membrane and Rab7+ late endosomes rather than the trans-Golgi/recycling-endosome pool of other ferlins.\",\n      \"evidence\": \"3D-SIM super-resolution microscopy and Rab7/Rab11 endosomal transit assays\",\n      \"pmids\": [\"26707827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"FER1L5 data embedded in broader family survey\", \"Functional role of the late-endosomal pool not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked FER1L5 localization to function by showing it accumulates at myoblast fusion sites on low-density vesicles and is required for both fusion and membrane repair.\",\n      \"evidence\": \"Biochemical fractionation, confocal immunolabeling, laser-wounding repair assay, and inhibitory antibody treatment in C2C12 cells\",\n      \"pmids\": [\"33182221\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Antibody inhibition not corroborated by genetic loss-of-function in muscle\", \"Ca2+-dependence of the vesicle accumulation not directly tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified the essential physiological role of FER1L5 as a required effector of the Ca2+-triggered acrosome reaction acting downstream of Ca2+ influx, explaining male infertility in its absence.\",\n      \"evidence\": \"Fer1l5 mutant mouse with fertility, sperm migration, and Ca2+-ionophore acrosome-reaction assays\",\n      \"pmids\": [\"36696506\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular partners of FER1L5 in the acrosomal fusion machinery not identified\", \"Whether C2 domains directly sense the Ca2+ signal during the reaction untested\", \"Connection between sperm role and muscle fusion role mechanistically unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How FER1L5's C2 domains transduce Ca2+ signals into membrane fusion, and which SNARE or fusion-machinery partners it engages during the acrosome reaction and myoblast fusion, remains unresolved.\",\n      \"evidence\": \"No direct experimental evidence in the available corpus\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No identified fusion-machinery partners\", \"No experimental validation of Ca2+ binding by specific C2 domains\", \"No structural model of the membrane-engaged state\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005544\", \"supporting_discovery_ids\": []}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 6]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"EHD1\", \"EHD2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}