{"gene":"FSIP2","run_date":"2026-04-28T17:46:04","timeline":{"discoveries":[{"year":2018,"finding":"FSIP2 is a fibrous sheath (FS) protein specifically expressed in testis that is required for FS assembly and overall axonemal and flagellar biogenesis; loss-of-function mutations cause complete FS disorganization, axonemal defects (central-pair microtubules and inner/outer dynein arm abnormalities), and absence of AKAP4 in sperm.","method":"Whole-exome sequencing, Sanger sequencing, immunofluorescence (IF) on patient sperm, transmission electron microscopy (TEM), quantitative RT-PCR","journal":"Human reproduction (Oxford, England)","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (TEM, IF, WES) in patient samples; replicated across 4 independent patients","pmids":["30137358"],"is_preprint":false},{"year":2021,"finding":"FSIP2 protein localizes to the acrosome and physically interacts with Acrv1 (an acrosomal marker); its dosage-dependent expression regulates sperm tail length and acrosome formation, as demonstrated by knock-in (loss-of-function) mice showing MMAF and overexpression mice showing increased sperm tail length with no gross flagellar abnormality.","method":"Knock-in and overexpression mouse models, single-cell RNA sequencing, proteomics, co-immunoprecipitation, immunofluorescence","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 1–2 — in vivo genetic models (KI and OE) combined with co-IP and proteomic validation in a single rigorous study","pmids":["34125190"],"is_preprint":false},{"year":2022,"finding":"FSIP2 co-localizes with peanut agglutinin in the acrosome during spermatogenesis and physically interacts with acrosome biogenesis proteins DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC; its loss leads to downregulation of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2, and CSNK2A2, linking FSIP2 to acrosome development.","method":"Whole-exome sequencing, Western blot, immunofluorescence, co-immunoprecipitation, liquid chromatography-tandem mass spectrometry proteomics","journal":"Journal of medical genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP plus proteomics and IF with multiple orthogonal methods in one study","pmids":["35654582"],"is_preprint":false},{"year":2023,"finding":"FSIP2 variants cause fibrous sheath disassembly, axonemal defects, and aberrant 'super-length' mitochondrial sheaths with increased TOMM20 levels and decreased mitochondrial ATP consumption; FSIP2 is proposed to function as an intra-flagellar transporter involved in axonemal assembly, mitochondrial selection, and termination of mitochondrial sheath extension during spermatogenesis.","method":"Exome sequencing, immunofluorescence, transmission electron microscopy, mitochondrial ATP consumption assay, Western blot for TOMM20 and SEPT4","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods in patient samples; proposed intra-flagellar transport role is mechanistically inferred, not directly reconstituted","pmids":["36632462"],"is_preprint":false},{"year":2025,"finding":"Loss of FSIP2 in patient sperm leads to co-absence of AKAP4, SPAG6, IFT20, and ACTL7A proteins, indicating FSIP2 is required for the flagellar localization or stability of these interacting or downstream proteins, including intraflagellar transport component IFT20.","method":"Immunofluorescence staining, RT-PCR, transmission electron microscopy on patient sperm","journal":"Asian journal of andrology","confidence":"Medium","confidence_rationale":"Tier 3 — single-lab IF in patient samples without direct interaction assay; consistent with prior reports of AKAP4 loss","pmids":["40968718"],"is_preprint":false},{"year":2025,"finding":"Missense variants in FSIP2 cause reduced protein levels or mislocalization of FSIP2 to the spermatozoa head rather than the flagellum, demonstrating that correct FSIP2 localization to the flagellum is required for normal fibrous sheath assembly.","method":"Whole-exome sequencing, mini-gene assay, immunofluorescence staining","journal":"Journal of assisted reproduction and genetics","confidence":"Low","confidence_rationale":"Tier 3 — single lab, single IF method for localization finding","pmids":["39753944"],"is_preprint":false}],"current_model":"FSIP2 is a testis-specific fibrous sheath protein that is required for fibrous sheath assembly, axonemal integrity (including central-pair microtubules and dynein arms), acrosome biogenesis (via direct interaction with DPY19L2, SPACA1, HSP90B1, and others), and the flagellar localization/stability of AKAP4, SPAG6, and IFT20; its expression is dosage-dependent, controlling sperm tail length and acrosome formation, and loss-of-function mutations cause the MMAF phenotype with associated mitochondrial sheath and annulus defects."},"narrative":{"teleology":[{"year":2018,"claim":"The first demonstration that FSIP2 is essential for fibrous sheath assembly and axonemal integrity established it as a structural organizer of the sperm flagellum, explaining how its loss produces the MMAF phenotype with complete FS disorganization, dynein arm abnormalities, and absence of AKAP4.","evidence":"WES, Sanger sequencing, TEM, and IF across four independent MMAF patients","pmids":["30137358"],"confidence":"High","gaps":["No direct protein-protein interactions were identified","Mechanism by which FSIP2 recruits or stabilizes AKAP4 unknown","Role limited to flagellum; acrosomal involvement not examined"]},{"year":2021,"claim":"Genetic mouse models revealed that FSIP2 function is dosage-dependent and extends beyond the fibrous sheath to acrosome biogenesis, physically interacting with ACRV1 and localizing to the acrosomal region — establishing FSIP2 as a dual-function protein bridging flagellar and head assembly.","evidence":"Knock-in (loss-of-function) and overexpression mouse models, scRNA-seq, proteomics, and co-IP","pmids":["34125190"],"confidence":"High","gaps":["Mechanism linking dosage to tail length control unresolved","Whether FSIP2 acrosomal function is cell-autonomous versus intercellular not tested"]},{"year":2022,"claim":"Identification of a broader FSIP2 acrosomal interactome — including DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC — positioned FSIP2 as a scaffold for acrosome biogenesis and explained how its loss leads to coordinate downregulation of multiple acrosomal proteins.","evidence":"Reciprocal co-IP, LC-MS/MS proteomics, WB, and IF in human spermatozoa","pmids":["35654582"],"confidence":"High","gaps":["Direct versus indirect nature of each interaction not resolved","Structural basis of FSIP2–DPY19L2 interaction unknown","Whether FSIP2 acts as a vesicular trafficking adaptor for acrosome formation not tested"]},{"year":2023,"claim":"Demonstration that FSIP2 loss produces aberrant super-length mitochondrial sheaths with increased TOMM20 and decreased mitochondrial ATP consumption extended its functional scope to mitochondrial sheath boundary control and flagellar energy metabolism.","evidence":"TEM, IF, mitochondrial ATP consumption assay, and WB for TOMM20/SEPT4 in patient sperm","pmids":["36632462"],"confidence":"Medium","gaps":["Proposed intraflagellar transport role inferred but not reconstituted biochemically","Whether FSIP2 directly regulates annulus positioning or SEPT ring assembly not tested","Single study; mitochondrial phenotype not replicated in animal models"]},{"year":2025,"claim":"Showing co-absence of SPAG6, IFT20, and ACTL7A alongside AKAP4 in FSIP2-null sperm broadened the set of dependent flagellar proteins and linked FSIP2 to intraflagellar transport machinery stability.","evidence":"IF staining, RT-PCR, and TEM on patient sperm","pmids":["40968718"],"confidence":"Medium","gaps":["No direct interaction assay for FSIP2–IFT20 or FSIP2–SPAG6; co-absence could be indirect","Whether FSIP2 is an IFT cargo or IFT regulator remains unresolved"]},{"year":2025,"claim":"Identification of missense variants causing FSIP2 mislocalization to the sperm head instead of the flagellum demonstrated that correct flagellar targeting of FSIP2 is essential for its structural functions.","evidence":"WES, mini-gene splicing assay, and IF in patient sperm (single lab)","pmids":["39753944"],"confidence":"Low","gaps":["Single-lab IF without independent confirmation of mislocalization","Flagellar targeting signal/motif within FSIP2 not identified","Functional rescue experiments not performed"]},{"year":null,"claim":"The mechanism by which FSIP2 coordinates its dual roles at the acrosome and the fibrous sheath, the nature of any intraflagellar transport function, and the structural basis of its interactions with axonemal and acrosomal partners remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structural or domain-resolution mapping of FSIP2 interactions","No in vitro reconstitution of FSIP2 transport or scaffold activity","Relationship between FSIP2 dosage and mitochondrial sheath length not mechanistically explained"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,5]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,3,4,5]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,1,3]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[0,1,2]}],"complexes":[],"partners":["AKAP4","ACRV1","DPY19L2","SPACA1","HSP90B1","SPAG6","IFT20","CLTC"],"other_free_text":[]},"mechanistic_narrative":"FSIP2 is a testis-specific fibrous sheath protein that orchestrates multiple aspects of sperm structural assembly, including fibrous sheath formation, axonemal integrity, acrosome biogenesis, and mitochondrial sheath delimitation. Loss-of-function mutations cause complete fibrous sheath disorganization, central-pair microtubule and dynein arm defects, absence of AKAP4, SPAG6, and IFT20 from the flagellum, and acrosome malformation, resulting in the multiple morphological abnormalities of the flagella (MMAF) phenotype [PMID:30137358, PMID:40968718, PMID:36632462]. FSIP2 physically interacts with acrosome biogenesis factors DPY19L2, SPACA1, HSP90B1, and the acrosomal matrix protein ACRV1, and its expression level is dosage-dependent: loss-of-function truncates sperm tails while overexpression increases tail length [PMID:34125190, PMID:35654582]. Correct localization of FSIP2 to the flagellum is required for fibrous sheath assembly, and its loss additionally produces aberrant super-length mitochondrial sheaths with elevated TOMM20 and reduced mitochondrial ATP consumption [PMID:39753944, PMID:36632462]."},"prefetch_data":{"uniprot":{"accession":"Q5CZC0","full_name":"Fibrous sheath-interacting protein 2","aliases":[],"length_aa":6907,"mass_kda":780.6,"function":"Plays a role in spermatogenesis","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q5CZC0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FSIP2","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FSIP2","total_profiled":1310},"omim":[{"mim_id":"618153","title":"SPERMATOGENIC FAILURE 34; SPGF34","url":"https://www.omim.org/entry/618153"},{"mim_id":"615796","title":"FIBROUS SHEATH-INTERACTING PROTEIN 2; FSIP2","url":"https://www.omim.org/entry/615796"},{"mim_id":"615795","title":"FIBROUS SHEATH-INTERACTING PROTEIN 1; FSIP1","url":"https://www.omim.org/entry/615795"},{"mim_id":"258150","title":"SPERMATOGENIC FAILURE 1; SPGF1","url":"https://www.omim.org/entry/258150"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Microtubules","reliability":"Approved"},{"location":"Cytokinetic bridge","reliability":"Additional"},{"location":"Mitotic spindle","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"},{"location":"Mid piece","reliability":"Additional"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":1.3},{"tissue":"stomach 1","ntpm":2.4},{"tissue":"testis","ntpm":5.0}],"url":"https://www.proteinatlas.org/search/FSIP2"},"hgnc":{"alias_symbol":["FLJ34780"],"prev_symbol":[]},"alphafold":{"accession":"Q5CZC0","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5CZC0","model_url":"","pae_url":"","plddt_mean":null},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FSIP2","jax_strain_url":"https://www.jax.org/strain/search?query=FSIP2"},"sequence":{"accession":"Q5CZC0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5CZC0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5CZC0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5CZC0"}},"corpus_meta":[{"pmid":"30137358","id":"PMC_30137358","title":"Whole-exome sequencing identifies mutations in FSIP2 as a recurrent cause of multiple morphological abnormalities of the sperm flagella.","date":"2018","source":"Human reproduction (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/30137358","citation_count":94,"is_preprint":false},{"pmid":"34125190","id":"PMC_34125190","title":"Hypomorphic and hypermorphic mouse models of Fsip2 indicate its dosage-dependent roles in sperm tail and acrosome formation.","date":"2021","source":"Development (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/34125190","citation_count":24,"is_preprint":false},{"pmid":"36632462","id":"PMC_36632462","title":"Novel FSIP2 Variants Induce Super-Length Mitochondrial Sheath and Asthenoteratozoospermia in Humans.","date":"2023","source":"International journal of biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/36632462","citation_count":21,"is_preprint":false},{"pmid":"35654582","id":"PMC_35654582","title":"FSIP2 plays a role in the acrosome development during spermiogenesis.","date":"2022","source":"Journal of medical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35654582","citation_count":17,"is_preprint":false},{"pmid":"35672654","id":"PMC_35672654","title":"Novel Compound Heterozygous Mutation in FSIP2 Causes Multiple Morphological Abnormalities of the Sperm Flagella (MMAF) and Male Infertility.","date":"2022","source":"Reproductive sciences (Thousand Oaks, Calif.)","url":"https://pubmed.ncbi.nlm.nih.gov/35672654","citation_count":11,"is_preprint":false},{"pmid":"34935173","id":"PMC_34935173","title":"Successful outcomes of intracytoplasmic sperm injection-embryo transfer using ejaculated spermatozoa from two Chinese asthenoteratozoospermic brothers with a compound heterozygous FSIP2 mutation.","date":"2021","source":"Andrologia","url":"https://pubmed.ncbi.nlm.nih.gov/34935173","citation_count":8,"is_preprint":false},{"pmid":"34113648","id":"PMC_34113648","title":"Association Between FSIP2 Mutation and an Improved Efficacy of Immune Checkpoint Inhibitors in Patients With Skin Cutaneous Melanoma.","date":"2021","source":"Frontiers in molecular biosciences","url":"https://pubmed.ncbi.nlm.nih.gov/34113648","citation_count":4,"is_preprint":false},{"pmid":"39753944","id":"PMC_39753944","title":"Novel variants of FSIP2 and SPEF2 cause varying degrees of spermatozoa damage in MMAF patients and favorable ART outcomes.","date":"2025","source":"Journal of assisted reproduction and genetics","url":"https://pubmed.ncbi.nlm.nih.gov/39753944","citation_count":2,"is_preprint":false},{"pmid":"40968718","id":"PMC_40968718","title":"Novel mutations in FSIP2 cause male infertility through multiple morphological abnormalities of the sperm flagella.","date":"2025","source":"Asian journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/40968718","citation_count":0,"is_preprint":false},{"pmid":"40788209","id":"PMC_40788209","title":"[Clinical and genetic analysis of a patient with FSIP2 compound heterozygous variants causing multiple morphological abnormalities of sperm flagella].","date":"2025","source":"Zhonghua nan ke xue = National journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/40788209","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7408,"output_tokens":1576,"usd":0.022932},"stage2":{"model":"claude-opus-4-6","input_tokens":4813,"output_tokens":1982,"usd":0.110423},"total_usd":0.133355,"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\": 2018,\n      \"finding\": \"FSIP2 is a fibrous sheath (FS) protein specifically expressed in testis that is required for FS assembly and overall axonemal and flagellar biogenesis; loss-of-function mutations cause complete FS disorganization, axonemal defects (central-pair microtubules and inner/outer dynein arm abnormalities), and absence of AKAP4 in sperm.\",\n      \"method\": \"Whole-exome sequencing, Sanger sequencing, immunofluorescence (IF) on patient sperm, transmission electron microscopy (TEM), quantitative RT-PCR\",\n      \"journal\": \"Human reproduction (Oxford, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (TEM, IF, WES) in patient samples; replicated across 4 independent patients\",\n      \"pmids\": [\"30137358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FSIP2 protein localizes to the acrosome and physically interacts with Acrv1 (an acrosomal marker); its dosage-dependent expression regulates sperm tail length and acrosome formation, as demonstrated by knock-in (loss-of-function) mice showing MMAF and overexpression mice showing increased sperm tail length with no gross flagellar abnormality.\",\n      \"method\": \"Knock-in and overexpression mouse models, single-cell RNA sequencing, proteomics, co-immunoprecipitation, immunofluorescence\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vivo genetic models (KI and OE) combined with co-IP and proteomic validation in a single rigorous study\",\n      \"pmids\": [\"34125190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FSIP2 co-localizes with peanut agglutinin in the acrosome during spermatogenesis and physically interacts with acrosome biogenesis proteins DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC; its loss leads to downregulation of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2, and CSNK2A2, linking FSIP2 to acrosome development.\",\n      \"method\": \"Whole-exome sequencing, Western blot, immunofluorescence, co-immunoprecipitation, liquid chromatography-tandem mass spectrometry proteomics\",\n      \"journal\": \"Journal of medical genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus proteomics and IF with multiple orthogonal methods in one study\",\n      \"pmids\": [\"35654582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FSIP2 variants cause fibrous sheath disassembly, axonemal defects, and aberrant 'super-length' mitochondrial sheaths with increased TOMM20 levels and decreased mitochondrial ATP consumption; FSIP2 is proposed to function as an intra-flagellar transporter involved in axonemal assembly, mitochondrial selection, and termination of mitochondrial sheath extension during spermatogenesis.\",\n      \"method\": \"Exome sequencing, immunofluorescence, transmission electron microscopy, mitochondrial ATP consumption assay, Western blot for TOMM20 and SEPT4\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in patient samples; proposed intra-flagellar transport role is mechanistically inferred, not directly reconstituted\",\n      \"pmids\": [\"36632462\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss of FSIP2 in patient sperm leads to co-absence of AKAP4, SPAG6, IFT20, and ACTL7A proteins, indicating FSIP2 is required for the flagellar localization or stability of these interacting or downstream proteins, including intraflagellar transport component IFT20.\",\n      \"method\": \"Immunofluorescence staining, RT-PCR, transmission electron microscopy on patient sperm\",\n      \"journal\": \"Asian journal of andrology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single-lab IF in patient samples without direct interaction assay; consistent with prior reports of AKAP4 loss\",\n      \"pmids\": [\"40968718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Missense variants in FSIP2 cause reduced protein levels or mislocalization of FSIP2 to the spermatozoa head rather than the flagellum, demonstrating that correct FSIP2 localization to the flagellum is required for normal fibrous sheath assembly.\",\n      \"method\": \"Whole-exome sequencing, mini-gene assay, immunofluorescence staining\",\n      \"journal\": \"Journal of assisted reproduction and genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single IF method for localization finding\",\n      \"pmids\": [\"39753944\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FSIP2 is a testis-specific fibrous sheath protein that is required for fibrous sheath assembly, axonemal integrity (including central-pair microtubules and dynein arms), acrosome biogenesis (via direct interaction with DPY19L2, SPACA1, HSP90B1, and others), and the flagellar localization/stability of AKAP4, SPAG6, and IFT20; its expression is dosage-dependent, controlling sperm tail length and acrosome formation, and loss-of-function mutations cause the MMAF phenotype with associated mitochondrial sheath and annulus defects.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FSIP2 is a testis-specific fibrous sheath protein that orchestrates multiple aspects of sperm structural assembly, including fibrous sheath formation, axonemal integrity, acrosome biogenesis, and mitochondrial sheath delimitation. Loss-of-function mutations cause complete fibrous sheath disorganization, central-pair microtubule and dynein arm defects, absence of AKAP4, SPAG6, and IFT20 from the flagellum, and acrosome malformation, resulting in the multiple morphological abnormalities of the flagella (MMAF) phenotype [PMID:30137358, PMID:40968718, PMID:36632462]. FSIP2 physically interacts with acrosome biogenesis factors DPY19L2, SPACA1, HSP90B1, and the acrosomal matrix protein ACRV1, and its expression level is dosage-dependent: loss-of-function truncates sperm tails while overexpression increases tail length [PMID:34125190, PMID:35654582]. Correct localization of FSIP2 to the flagellum is required for fibrous sheath assembly, and its loss additionally produces aberrant super-length mitochondrial sheaths with elevated TOMM20 and reduced mitochondrial ATP consumption [PMID:39753944, PMID:36632462].\",\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"The first demonstration that FSIP2 is essential for fibrous sheath assembly and axonemal integrity established it as a structural organizer of the sperm flagellum, explaining how its loss produces the MMAF phenotype with complete FS disorganization, dynein arm abnormalities, and absence of AKAP4.\",\n      \"evidence\": \"WES, Sanger sequencing, TEM, and IF across four independent MMAF patients\",\n      \"pmids\": [\"30137358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No direct protein-protein interactions were identified\",\n        \"Mechanism by which FSIP2 recruits or stabilizes AKAP4 unknown\",\n        \"Role limited to flagellum; acrosomal involvement not examined\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Genetic mouse models revealed that FSIP2 function is dosage-dependent and extends beyond the fibrous sheath to acrosome biogenesis, physically interacting with ACRV1 and localizing to the acrosomal region — establishing FSIP2 as a dual-function protein bridging flagellar and head assembly.\",\n      \"evidence\": \"Knock-in (loss-of-function) and overexpression mouse models, scRNA-seq, proteomics, and co-IP\",\n      \"pmids\": [\"34125190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism linking dosage to tail length control unresolved\",\n        \"Whether FSIP2 acrosomal function is cell-autonomous versus intercellular not tested\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of a broader FSIP2 acrosomal interactome — including DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC — positioned FSIP2 as a scaffold for acrosome biogenesis and explained how its loss leads to coordinate downregulation of multiple acrosomal proteins.\",\n      \"evidence\": \"Reciprocal co-IP, LC-MS/MS proteomics, WB, and IF in human spermatozoa\",\n      \"pmids\": [\"35654582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct versus indirect nature of each interaction not resolved\",\n        \"Structural basis of FSIP2–DPY19L2 interaction unknown\",\n        \"Whether FSIP2 acts as a vesicular trafficking adaptor for acrosome formation not tested\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstration that FSIP2 loss produces aberrant super-length mitochondrial sheaths with increased TOMM20 and decreased mitochondrial ATP consumption extended its functional scope to mitochondrial sheath boundary control and flagellar energy metabolism.\",\n      \"evidence\": \"TEM, IF, mitochondrial ATP consumption assay, and WB for TOMM20/SEPT4 in patient sperm\",\n      \"pmids\": [\"36632462\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Proposed intraflagellar transport role inferred but not reconstituted biochemically\",\n        \"Whether FSIP2 directly regulates annulus positioning or SEPT ring assembly not tested\",\n        \"Single study; mitochondrial phenotype not replicated in animal models\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showing co-absence of SPAG6, IFT20, and ACTL7A alongside AKAP4 in FSIP2-null sperm broadened the set of dependent flagellar proteins and linked FSIP2 to intraflagellar transport machinery stability.\",\n      \"evidence\": \"IF staining, RT-PCR, and TEM on patient sperm\",\n      \"pmids\": [\"40968718\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No direct interaction assay for FSIP2–IFT20 or FSIP2–SPAG6; co-absence could be indirect\",\n        \"Whether FSIP2 is an IFT cargo or IFT regulator remains unresolved\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of missense variants causing FSIP2 mislocalization to the sperm head instead of the flagellum demonstrated that correct flagellar targeting of FSIP2 is essential for its structural functions.\",\n      \"evidence\": \"WES, mini-gene splicing assay, and IF in patient sperm (single lab)\",\n      \"pmids\": [\"39753944\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Single-lab IF without independent confirmation of mislocalization\",\n        \"Flagellar targeting signal/motif within FSIP2 not identified\",\n        \"Functional rescue experiments not performed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The mechanism by which FSIP2 coordinates its dual roles at the acrosome and the fibrous sheath, the nature of any intraflagellar transport function, and the structural basis of its interactions with axonemal and acrosomal partners remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No structural or domain-resolution mapping of FSIP2 interactions\",\n        \"No in vitro reconstitution of FSIP2 transport or scaffold activity\",\n        \"Relationship between FSIP2 dosage and mitochondrial sheath length not mechanistically explained\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 3, 4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"AKAP4\", \"ACRV1\", \"DPY19L2\", \"SPACA1\", \"HSP90B1\", \"SPAG6\", \"IFT20\", \"CLTC\"],\n    \"other_free_text\": []\n  }\n}\n```"}