{"gene":"TBC1D21","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2010,"finding":"TBC1D21 (MgcRabGAP) is a testis-specific RabGAP protein expressed in elongating and elongated spermatids, localized at the acrosomal region, neck, and annulus during spermiogenesis; it co-localizes and physically interacts with β-actin, and co-localizes with its candidate substrate RAB3A at the acrosome/acroplaxome and neck regions, suggesting a role in acrosome/acroplaxome formation and cytoskeletal reorganization.","method":"Immunofluorescence, co-immunoprecipitation","journal":"International journal of andrology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — co-localization and single co-IP with β-actin and RAB3A, single lab, two partially orthogonal methods","pmids":["21128978"],"is_preprint":false},{"year":2017,"finding":"TBC1D21 (MGCRABGAP) exhibits GTPase-activating activity in vitro; RAB10, RAB5C, and RAP1 were identified as interactors/possible substrates by co-IP and mass spectrometry. RAB10 binding was confirmed by co-IP, and the TBC1D21–RAB10 complex co-localizes specifically in the manchette structure of spermatids and at the midpiece of mature spermatozoa.","method":"Co-immunoprecipitation, nano LC-MS/MS, GTPase-activating activity assay, immunofluorescence co-localization","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — enzymatic activity assay plus reciprocal Co-IP plus MS, single lab, multiple orthogonal methods","pmids":["28067790"],"is_preprint":false},{"year":2018,"finding":"TBC1D21 interacts with Rap1 (confirmed by co-IP) and overexpression of TBC1D21 inactivates Rap1 GTPase activity in a cell model; TBC1D21 and Rap1 co-localize at postacrosomal regions of spermatids and at the midpiece of mature sperm.","method":"Co-immunoprecipitation, Rap1 activation pull-down assay, immunofluorescence co-localization","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pull-down activity assay plus Co-IP, single lab, two orthogonal methods","pmids":["30360518"],"is_preprint":false},{"year":2020,"finding":"Loss of Tbc1d21 in mice causes male infertility with abnormal irregular mitochondrial arrangement, abnormal mitochondrial diameter, structural defects in the mitochondrial sheath, and severely disturbed axoneme structure in sperm tails. TBC1D21 physically co-localizes in vivo with TOMM20 (outer mitochondrial membrane translocase) and DNAH7 (inner arm dynein heavy chain); TOMM20 and DNAH7 detach from the axoneme in Tbc1d21-deficient sperm.","method":"Tbc1d21 knockout mouse model, electron microscopy, proteomic analysis, in vivo co-localization by immunofluorescence","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with defined ultrastructural phenotype, proteomic screen plus in vivo co-localization, replicated by independent lab","pmids":["32976492"],"is_preprint":false},{"year":2021,"finding":"TBC1D21 is essential for the interaction between ARMC12 and VDAC proteins (VDAC2 and VDAC3) in vivo; TBC1D21 functions cooperatively with ARMC12 during mitochondrial sheath formation, as demonstrated by Tbc1d21-null mice showing disrupted ARMC12–VDAC interaction. ARMC12 was confirmed to interact with TBC1D21 in testicular germ cells by co-immunoprecipitation from FLAG-tagged knock-in mice.","method":"Tbc1d21-null mouse, FLAG-tagged Armc12 knock-in co-immunoprecipitation, in vivo protein interaction assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in vivo (knockout ablates interaction), reciprocal tagged knock-in co-IP, independent lab replication context","pmids":["33536340"],"is_preprint":false},{"year":2022,"finding":"TBC1D21 functions as a scaffold protein rather than a canonical RabGAP: mice bearing mutations in the catalytic GAP residues (D125A R128K in the IxxDxxR motif) are fertile, indicating GTP hydrolysis activity is dispensable for its role in spermiogenesis. TBC1D21 localizes to the sperm mitochondrial sheath. RAB13 was identified as a novel TBC1D21 binding partner by co-IP screening of 34 Rab proteins. Immunoprecipitation-mass spectrometry identified interactions with ACTB, TPM3, SPATA19, and VDAC3.","method":"CRISPR/Cas9 catalytic residue knock-in mice (D125A R128K), co-immunoprecipitation with 34 Rab constructs, immunoprecipitation-mass spectrometry, FLAG knock-in localization, electron microscopy","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 1 / Strong — active-site mutagenesis in vivo with fertility readout plus IP-MS for interactors, multiple orthogonal methods in one study","pmids":["35403672"],"is_preprint":false},{"year":2024,"finding":"TBC1D21 interacts with TEKT1 (tektin-1), and loss of TBC1D21 causes abnormal accumulation of TEKT1 in the midpiece region with disrupted axonemal structures. TEKT1 also co-localizes with RAB10 during sperm tail formation, placing TBC1D21 in an axonemal transport system that regulates tektin protein localization.","method":"Comparative proteomics of wild-type vs Tbc1d21-null sperm, co-immunoprecipitation, immunofluorescence co-localization","journal":"Developmental dynamics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomic screen plus Co-IP plus localization, single lab, multiple orthogonal methods","pmids":["38822685"],"is_preprint":false}],"current_model":"TBC1D21 is a testis-specific scaffold protein expressed in elongating/elongated spermatids and localizing to the sperm mitochondrial sheath, where it coordinates mitochondrial sheath assembly by mediating interactions among ARMC12, VDAC2/3, ACTB, TPM3, SPATA19, and TOMM20; its GAP catalytic activity is dispensable for male fertility (catalytic-dead knock-in mice are fertile), yet it modulates RAB10, RAB13, and RAP1 binding and regulates TEKT1 localization in the axonemal transport system, with loss of TBC1D21 causing disorganized mitochondrial sheath, disrupted axoneme, and male infertility in mice."},"narrative":{"mechanistic_narrative":"TBC1D21 is a testis-specific protein expressed in elongating and elongated spermatids that orchestrates assembly of the sperm mitochondrial sheath and proper organization of the flagellar axoneme during spermiogenesis [PMID:21128978, PMID:32976492]. Although it was identified as a putative RabGAP with in vitro GTPase-activating activity and binds multiple small GTPases including RAB10, RAB5C, RAB13 and RAP1 [PMID:28067790, PMID:30360518, PMID:35403672], its catalytic activity is dispensable in vivo: mice carrying mutations in the GAP catalytic residues remain fertile, establishing that TBC1D21 acts as a scaffold rather than a canonical enzyme during spermiogenesis [PMID:35403672]. As a scaffold it nucleates a network of interactions at the mitochondrial sheath — bridging ARMC12 to VDAC2/VDAC3 and associating with ACTB, TPM3, SPATA19 and TOMM20 — and loss of TBC1D21 disrupts the ARMC12–VDAC interaction and produces irregular mitochondrial arrangement and structural sheath defects [PMID:32976492, PMID:33536340, PMID:35403672]. TBC1D21 additionally participates in axonemal transport, interacting with TEKT1 such that its loss causes abnormal TEKT1 accumulation in the midpiece together with detachment of TOMM20 and the dynein heavy chain DNAH7 from the axoneme [PMID:32976492, PMID:38822685]. Loss of TBC1D21 in mice causes male infertility with disorganized mitochondrial sheath and disrupted axoneme [PMID:32976492].","teleology":[{"year":2010,"claim":"Establishing where and when an uncharacterized testis RabGAP acts placed TBC1D21 in spermiogenesis and linked it to cytoskeletal reorganization.","evidence":"Immunofluorescence and co-immunoprecipitation in spermatids showing localization to acrosome/neck/annulus and interaction with β-actin and RAB3A","pmids":["21128978"],"confidence":"Medium","gaps":["Functional consequence of β-actin and RAB3A binding not tested by perturbation","GAP activity toward RAB3A not demonstrated"]},{"year":2017,"claim":"Demonstrating in vitro GTPase-activating activity and identifying RAB10/RAB5C/RAP1 as candidate substrates began to define a possible enzymatic mechanism.","evidence":"GTPase-activating assay, co-IP, nano LC-MS/MS, and co-localization of TBC1D21–RAB10 in manchette and midpiece","pmids":["28067790"],"confidence":"Medium","gaps":["In vitro activity not shown to be required in vivo","Substrate specificity among Rabs not resolved"]},{"year":2018,"claim":"Showing TBC1D21 inactivates Rap1 in a cell model extended its candidate regulatory targets beyond Rab GTPases.","evidence":"Co-IP, Rap1 activation pull-down assay, and immunofluorescence co-localization at postacrosomal region and midpiece","pmids":["30360518"],"confidence":"Medium","gaps":["Rap1 regulation shown only in overexpression cell model, not germ cells","Physiological relevance to sperm formation untested"]},{"year":2020,"claim":"Genetic knockout established TBC1D21 as essential for mitochondrial sheath and axoneme integrity, defining the loss-of-function phenotype.","evidence":"Tbc1d21 knockout mice, electron microscopy, proteomics, in vivo co-localization with TOMM20 and DNAH7","pmids":["32976492"],"confidence":"High","gaps":["Whether sheath and axoneme defects are independent or sequential consequences","Mechanism by which TOMM20/DNAH7 detach not defined"]},{"year":2021,"claim":"Genetic epistasis revealed TBC1D21 is required for the ARMC12–VDAC interaction, defining a specific molecular function in sheath assembly.","evidence":"Tbc1d21-null mice and FLAG-tagged Armc12 knock-in co-IP showing loss of ARMC12–VDAC interaction","pmids":["33536340"],"confidence":"High","gaps":["Whether TBC1D21 directly bridges ARMC12 and VDAC or acts indirectly","Stoichiometry/architecture of the complex unresolved"]},{"year":2022,"claim":"Catalytic-dead knock-in fertility reclassified TBC1D21 as a scaffold, decoupling its in vivo role from GAP activity, and expanded its interactome.","evidence":"CRISPR D125A R128K catalytic-residue knock-in mice (fertile), co-IP against 34 Rab constructs, IP-MS identifying ACTB/TPM3/SPATA19/VDAC3, EM","pmids":["35403672"],"confidence":"High","gaps":["Why an active RabGAP motif is conserved yet dispensable not explained","Direct versus indirect nature of IP-MS interactions not dissected"]},{"year":2024,"claim":"Linking TBC1D21 to TEKT1 placed it in an axonemal transport system regulating tektin localization, connecting sheath and axoneme phenotypes.","evidence":"Comparative proteomics of WT vs Tbc1d21-null sperm, co-IP, and immunofluorescence showing TEKT1 mislocalization and RAB10 co-localization","pmids":["38822685"],"confidence":"Medium","gaps":["Mechanism of TEKT1 transport regulation not defined","Role of RAB10 in this transport step untested by perturbation"]},{"year":null,"claim":"How a single scaffold simultaneously coordinates mitochondrial sheath assembly and axonemal transport, and why a catalytically functional but dispensable RabGAP fold is retained, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structural model of TBC1D21 within its interaction network","Direct binding hierarchy among partners not established","Human disease relevance not tested in the available corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[4,5]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,2]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,5]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[3,4,5]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,6]}],"pathway":[{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[3,4,5]}],"complexes":[],"partners":["ARMC12","VDAC2","VDAC3","RAB10","RAP1","TOMM20","TEKT1","ACTB"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8IYX1","full_name":"TBC1 domain family member 21","aliases":["Male germ cell Rab GTPase-activating protein"],"length_aa":336,"mass_kda":39.2,"function":"Acts as a GTPase-activating protein for Rab family protein(s) (PubMed:19077034, PubMed:28067790). Essential for the establishment of male fertility, and is required for both the production of normal sperm number and sperm function (By similarity). Plays an important role in the formation of intact mitochondria, outer dense fibers and axoneme within the sperm tail (By similarity). Essential for sperm mitochondrial sheath formation and for the interactions of ARMC12 with VDAC2 and VDAC3 (By similarity). May be involved in acrosome formation and cytoskeletal reorganization during spermiogenesis, possibly by regulating RAB3A activity (PubMed:21128978)","subcellular_location":"Cytoplasmic vesicle, secretory vesicle, acrosome; Cytoplasm, cytoskeleton","url":"https://www.uniprot.org/uniprotkb/Q8IYX1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TBC1D21","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TBC1D21","total_profiled":1310},"omim":[{"mim_id":"620387","title":"TBC1 DOMAIN FAMILY, MEMBER 21; TBC1D21","url":"https://www.omim.org/entry/620387"},{"mim_id":"620377","title":"ARMADILLO REPEAT-CONTAINING PROTEIN 12; ARMC12","url":"https://www.omim.org/entry/620377"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Calyx","reliability":"Approved"},{"location":"Mid piece","reliability":"Approved"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in single","driving_tissues":[{"tissue":"testis","ntpm":46.4}],"url":"https://www.proteinatlas.org/search/TBC1D21"},"hgnc":{"alias_symbol":["MGC34741","MgcRabGAP"],"prev_symbol":[]},"alphafold":{"accession":"Q8IYX1","domains":[{"cath_id":"-","chopping":"80-194","consensus_level":"medium","plddt":92.7378,"start":80,"end":194}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYX1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYX1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYX1-F1-predicted_aligned_error_v6.png","plddt_mean":93.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TBC1D21","jax_strain_url":"https://www.jax.org/strain/search?query=TBC1D21"},"sequence":{"accession":"Q8IYX1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8IYX1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8IYX1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYX1"}},"corpus_meta":[{"pmid":"33536340","id":"PMC_33536340","title":"ARMC12 regulates spatiotemporal mitochondrial dynamics during spermiogenesis and is required for male fertility.","date":"2021","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/33536340","citation_count":65,"is_preprint":false},{"pmid":"32976492","id":"PMC_32976492","title":"Deficiency of the Tbc1d21 gene causes male infertility with morphological abnormalities of the sperm mitochondria and flagellum in mice.","date":"2020","source":"PLoS genetics","url":"https://pubmed.ncbi.nlm.nih.gov/32976492","citation_count":29,"is_preprint":false},{"pmid":"21128978","id":"PMC_21128978","title":"Identification and characterization of a novel Rab GTPase-activating protein in spermatids.","date":"2010","source":"International journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/21128978","citation_count":28,"is_preprint":false},{"pmid":"28067790","id":"PMC_28067790","title":"RAB10 Interacts with the Male Germ Cell-Specific GTPase-Activating Protein during Mammalian Spermiogenesis.","date":"2017","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/28067790","citation_count":24,"is_preprint":false},{"pmid":"24938310","id":"PMC_24938310","title":"Novel common variants and susceptible haplotype for exfoliation glaucoma specific to Asian population.","date":"2014","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/24938310","citation_count":22,"is_preprint":false},{"pmid":"35403672","id":"PMC_35403672","title":"TBC1D21 is an essential factor for sperm mitochondrial sheath assembly and male fertility‡.","date":"2022","source":"Biology of reproduction","url":"https://pubmed.ncbi.nlm.nih.gov/35403672","citation_count":14,"is_preprint":false},{"pmid":"30360518","id":"PMC_30360518","title":"TBC1D21 Potentially Interacts with and Regulates Rap1 during Murine Spermatogenesis.","date":"2018","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/30360518","citation_count":12,"is_preprint":false},{"pmid":"31192002","id":"PMC_31192002","title":"Multiple Gene Polymorphisms Associated with Exfoliation Syndrome in the Uygur Population.","date":"2019","source":"Journal of ophthalmology","url":"https://pubmed.ncbi.nlm.nih.gov/31192002","citation_count":12,"is_preprint":false},{"pmid":"37956402","id":"PMC_37956402","title":"Dopamine receptor D2 regulates genes involved in germ cell movement and sperm motility in rat testes†.","date":"2024","source":"Biology of reproduction","url":"https://pubmed.ncbi.nlm.nih.gov/37956402","citation_count":7,"is_preprint":false},{"pmid":"39907412","id":"PMC_39907412","title":"Microbial signatures in head and neck squamous cell carcinoma: an in silico study.","date":"2025","source":"Journal of applied oral science : revista FOB","url":"https://pubmed.ncbi.nlm.nih.gov/39907412","citation_count":5,"is_preprint":false},{"pmid":"25049720","id":"PMC_25049720","title":"The Possibility of TBC1D21 as a Candidate Gene for Teat Numbers in Pigs.","date":"2013","source":"Asian-Australasian journal of animal sciences","url":"https://pubmed.ncbi.nlm.nih.gov/25049720","citation_count":3,"is_preprint":false},{"pmid":"38822685","id":"PMC_38822685","title":"Proteomic profiling of TBC1 domain family member 21-null sperms reveals the critical roles of TEKT 1 in their tail defects.","date":"2024","source":"Developmental dynamics : an official publication of the American Association of Anatomists","url":"https://pubmed.ncbi.nlm.nih.gov/38822685","citation_count":3,"is_preprint":false},{"pmid":"36845001","id":"PMC_36845001","title":"Evaluation of the efficacy of creatine chemical exchange saturation transfer imaging in assessing testicular maturity.","date":"2023","source":"Reproductive medicine and biology","url":"https://pubmed.ncbi.nlm.nih.gov/36845001","citation_count":2,"is_preprint":false},{"pmid":"39961484","id":"PMC_39961484","title":"Comparative proteomics and phosphoproteomics analysis reveals differential sperm motility in Mediterranean buffalo semen.","date":"2025","source":"Journal of proteomics","url":"https://pubmed.ncbi.nlm.nih.gov/39961484","citation_count":1,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.10.31.25337980","title":"Bi-allelic variants in  <i>TBC1D8</i>  result in non-obstructive azoospermia in both humans and mice","date":"2025-11-06","source":"bioRxiv","url":"https://doi.org/10.1101/2025.10.31.25337980","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10407,"output_tokens":2076,"usd":0.03118,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9002,"output_tokens":2681,"usd":0.056017,"stage2_stop_reason":"end_turn"},"total_usd":0.087197,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2010,\n      \"finding\": \"TBC1D21 (MgcRabGAP) is a testis-specific RabGAP protein expressed in elongating and elongated spermatids, localized at the acrosomal region, neck, and annulus during spermiogenesis; it co-localizes and physically interacts with β-actin, and co-localizes with its candidate substrate RAB3A at the acrosome/acroplaxome and neck regions, suggesting a role in acrosome/acroplaxome formation and cytoskeletal reorganization.\",\n      \"method\": \"Immunofluorescence, co-immunoprecipitation\",\n      \"journal\": \"International journal of andrology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — co-localization and single co-IP with β-actin and RAB3A, single lab, two partially orthogonal methods\",\n      \"pmids\": [\"21128978\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"TBC1D21 (MGCRABGAP) exhibits GTPase-activating activity in vitro; RAB10, RAB5C, and RAP1 were identified as interactors/possible substrates by co-IP and mass spectrometry. RAB10 binding was confirmed by co-IP, and the TBC1D21–RAB10 complex co-localizes specifically in the manchette structure of spermatids and at the midpiece of mature spermatozoa.\",\n      \"method\": \"Co-immunoprecipitation, nano LC-MS/MS, GTPase-activating activity assay, immunofluorescence co-localization\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — enzymatic activity assay plus reciprocal Co-IP plus MS, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"28067790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TBC1D21 interacts with Rap1 (confirmed by co-IP) and overexpression of TBC1D21 inactivates Rap1 GTPase activity in a cell model; TBC1D21 and Rap1 co-localize at postacrosomal regions of spermatids and at the midpiece of mature sperm.\",\n      \"method\": \"Co-immunoprecipitation, Rap1 activation pull-down assay, immunofluorescence co-localization\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pull-down activity assay plus Co-IP, single lab, two orthogonal methods\",\n      \"pmids\": [\"30360518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Loss of Tbc1d21 in mice causes male infertility with abnormal irregular mitochondrial arrangement, abnormal mitochondrial diameter, structural defects in the mitochondrial sheath, and severely disturbed axoneme structure in sperm tails. TBC1D21 physically co-localizes in vivo with TOMM20 (outer mitochondrial membrane translocase) and DNAH7 (inner arm dynein heavy chain); TOMM20 and DNAH7 detach from the axoneme in Tbc1d21-deficient sperm.\",\n      \"method\": \"Tbc1d21 knockout mouse model, electron microscopy, proteomic analysis, in vivo co-localization by immunofluorescence\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with defined ultrastructural phenotype, proteomic screen plus in vivo co-localization, replicated by independent lab\",\n      \"pmids\": [\"32976492\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TBC1D21 is essential for the interaction between ARMC12 and VDAC proteins (VDAC2 and VDAC3) in vivo; TBC1D21 functions cooperatively with ARMC12 during mitochondrial sheath formation, as demonstrated by Tbc1d21-null mice showing disrupted ARMC12–VDAC interaction. ARMC12 was confirmed to interact with TBC1D21 in testicular germ cells by co-immunoprecipitation from FLAG-tagged knock-in mice.\",\n      \"method\": \"Tbc1d21-null mouse, FLAG-tagged Armc12 knock-in co-immunoprecipitation, in vivo protein interaction assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in vivo (knockout ablates interaction), reciprocal tagged knock-in co-IP, independent lab replication context\",\n      \"pmids\": [\"33536340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TBC1D21 functions as a scaffold protein rather than a canonical RabGAP: mice bearing mutations in the catalytic GAP residues (D125A R128K in the IxxDxxR motif) are fertile, indicating GTP hydrolysis activity is dispensable for its role in spermiogenesis. TBC1D21 localizes to the sperm mitochondrial sheath. RAB13 was identified as a novel TBC1D21 binding partner by co-IP screening of 34 Rab proteins. Immunoprecipitation-mass spectrometry identified interactions with ACTB, TPM3, SPATA19, and VDAC3.\",\n      \"method\": \"CRISPR/Cas9 catalytic residue knock-in mice (D125A R128K), co-immunoprecipitation with 34 Rab constructs, immunoprecipitation-mass spectrometry, FLAG knock-in localization, electron microscopy\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — active-site mutagenesis in vivo with fertility readout plus IP-MS for interactors, multiple orthogonal methods in one study\",\n      \"pmids\": [\"35403672\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TBC1D21 interacts with TEKT1 (tektin-1), and loss of TBC1D21 causes abnormal accumulation of TEKT1 in the midpiece region with disrupted axonemal structures. TEKT1 also co-localizes with RAB10 during sperm tail formation, placing TBC1D21 in an axonemal transport system that regulates tektin protein localization.\",\n      \"method\": \"Comparative proteomics of wild-type vs Tbc1d21-null sperm, co-immunoprecipitation, immunofluorescence co-localization\",\n      \"journal\": \"Developmental dynamics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomic screen plus Co-IP plus localization, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"38822685\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TBC1D21 is a testis-specific scaffold protein expressed in elongating/elongated spermatids and localizing to the sperm mitochondrial sheath, where it coordinates mitochondrial sheath assembly by mediating interactions among ARMC12, VDAC2/3, ACTB, TPM3, SPATA19, and TOMM20; its GAP catalytic activity is dispensable for male fertility (catalytic-dead knock-in mice are fertile), yet it modulates RAB10, RAB13, and RAP1 binding and regulates TEKT1 localization in the axonemal transport system, with loss of TBC1D21 causing disorganized mitochondrial sheath, disrupted axoneme, and male infertility in mice.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TBC1D21 is a testis-specific protein expressed in elongating and elongated spermatids that orchestrates assembly of the sperm mitochondrial sheath and proper organization of the flagellar axoneme during spermiogenesis [#0, #3]. Although it was identified as a putative RabGAP with in vitro GTPase-activating activity and binds multiple small GTPases including RAB10, RAB5C, RAB13 and RAP1 [#1, #2, #5], its catalytic activity is dispensable in vivo: mice carrying mutations in the GAP catalytic residues remain fertile, establishing that TBC1D21 acts as a scaffold rather than a canonical enzyme during spermiogenesis [#5]. As a scaffold it nucleates a network of interactions at the mitochondrial sheath — bridging ARMC12 to VDAC2/VDAC3 and associating with ACTB, TPM3, SPATA19 and TOMM20 — and loss of TBC1D21 disrupts the ARMC12–VDAC interaction and produces irregular mitochondrial arrangement and structural sheath defects [#3, #4, #5]. TBC1D21 additionally participates in axonemal transport, interacting with TEKT1 such that its loss causes abnormal TEKT1 accumulation in the midpiece together with detachment of TOMM20 and the dynein heavy chain DNAH7 from the axoneme [#3, #6]. Loss of TBC1D21 in mice causes male infertility with disorganized mitochondrial sheath and disrupted axoneme [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Establishing where and when an uncharacterized testis RabGAP acts placed TBC1D21 in spermiogenesis and linked it to cytoskeletal reorganization.\",\n      \"evidence\": \"Immunofluorescence and co-immunoprecipitation in spermatids showing localization to acrosome/neck/annulus and interaction with \\u03b2-actin and RAB3A\",\n      \"pmids\": [\"21128978\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of \\u03b2-actin and RAB3A binding not tested by perturbation\", \"GAP activity toward RAB3A not demonstrated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrating in vitro GTPase-activating activity and identifying RAB10/RAB5C/RAP1 as candidate substrates began to define a possible enzymatic mechanism.\",\n      \"evidence\": \"GTPase-activating assay, co-IP, nano LC-MS/MS, and co-localization of TBC1D21\\u2013RAB10 in manchette and midpiece\",\n      \"pmids\": [\"28067790\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vitro activity not shown to be required in vivo\", \"Substrate specificity among Rabs not resolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing TBC1D21 inactivates Rap1 in a cell model extended its candidate regulatory targets beyond Rab GTPases.\",\n      \"evidence\": \"Co-IP, Rap1 activation pull-down assay, and immunofluorescence co-localization at postacrosomal region and midpiece\",\n      \"pmids\": [\"30360518\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Rap1 regulation shown only in overexpression cell model, not germ cells\", \"Physiological relevance to sperm formation untested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Genetic knockout established TBC1D21 as essential for mitochondrial sheath and axoneme integrity, defining the loss-of-function phenotype.\",\n      \"evidence\": \"Tbc1d21 knockout mice, electron microscopy, proteomics, in vivo co-localization with TOMM20 and DNAH7\",\n      \"pmids\": [\"32976492\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether sheath and axoneme defects are independent or sequential consequences\", \"Mechanism by which TOMM20/DNAH7 detach not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Genetic epistasis revealed TBC1D21 is required for the ARMC12\\u2013VDAC interaction, defining a specific molecular function in sheath assembly.\",\n      \"evidence\": \"Tbc1d21-null mice and FLAG-tagged Armc12 knock-in co-IP showing loss of ARMC12\\u2013VDAC interaction\",\n      \"pmids\": [\"33536340\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TBC1D21 directly bridges ARMC12 and VDAC or acts indirectly\", \"Stoichiometry/architecture of the complex unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Catalytic-dead knock-in fertility reclassified TBC1D21 as a scaffold, decoupling its in vivo role from GAP activity, and expanded its interactome.\",\n      \"evidence\": \"CRISPR D125A R128K catalytic-residue knock-in mice (fertile), co-IP against 34 Rab constructs, IP-MS identifying ACTB/TPM3/SPATA19/VDAC3, EM\",\n      \"pmids\": [\"35403672\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why an active RabGAP motif is conserved yet dispensable not explained\", \"Direct versus indirect nature of IP-MS interactions not dissected\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Linking TBC1D21 to TEKT1 placed it in an axonemal transport system regulating tektin localization, connecting sheath and axoneme phenotypes.\",\n      \"evidence\": \"Comparative proteomics of WT vs Tbc1d21-null sperm, co-IP, and immunofluorescence showing TEKT1 mislocalization and RAB10 co-localization\",\n      \"pmids\": [\"38822685\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of TEKT1 transport regulation not defined\", \"Role of RAB10 in this transport step untested by perturbation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How a single scaffold simultaneously coordinates mitochondrial sheath assembly and axonemal transport, and why a catalytically functional but dispensable RabGAP fold is retained, remains unresolved.\",\n      \"evidence\": null,\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural model of TBC1D21 within its interaction network\", \"Direct binding hierarchy among partners not established\", \"Human disease relevance not tested in the available corpus\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [4, 5]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [3, 4, 5]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [3, 4, 5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ARMC12\", \"VDAC2\", \"VDAC3\", \"RAB10\", \"RAP1\", \"TOMM20\", \"TEKT1\", \"ACTB\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}