{"gene":"SPAG16","run_date":"2026-06-10T07:46:38","timeline":{"discoveries":[{"year":1997,"finding":"PF20 (SPAG16 ortholog in Chlamydomonas) encodes a 606-amino acid protein containing five contiguous WD repeats and localizes to the intermicrotubule bridges connecting the two central microtubules (C2 microtubule) of the flagellar central apparatus; loss of PF20 results in absence of the entire central apparatus and flagellar paralysis, and wild-type transformation rescues motility and ultrastructure.","method":"Insertional mutagenesis, gene cloning, mutant rescue by transformation, immunogold labeling of wild-type axonemes with anti-PF20 antibody","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (mutagenesis, rescue, immunogold localization) in a single rigorous study establishing the protein's location and necessity for central apparatus assembly","pmids":["9188098"],"is_preprint":false},{"year":2002,"finding":"Mammalian SPAG16 (Pf20) protein associates with SPAG6 (mammalian PF16 ortholog): the WD repeat domain of Pf20 directly interacts with Spag6 by yeast two-hybrid assay; when co-expressed in CHO cells, Pf20-GFP (which alone localizes to cytoplasm) co-localizes with Spag6 on polymerized microtubules. Pf20 protein is markedly reduced in sperm from Spag6-knockout mice.","method":"Yeast two-hybrid, co-expression in CHO cells with fluorescence microscopy, immunocytochemistry/electron microscopy of sperm axonemes, western blot of Spag6-KO sperm","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal yeast two-hybrid and co-localization experiments with multiple orthogonal methods establishing direct interaction and mutual dependence","pmids":["12391165"],"is_preprint":false},{"year":2004,"finding":"The mouse Spag16 gene encodes two proteins from alternative transcripts: a 71-kDa protein (SPAG16L) incorporated into the axonemal central apparatus and a 35-kDa protein (SPAG16S) that accumulates in the nucleus of postmeiotic germ cells. The 35-kDa SPAG16S binds to MEIG1 (meiosis-expressed gene 1), a chromosome/chromatin-binding protein. Targeted disruption of the WD-repeat-encoding domains causes haploinsufficiency, impairing spermatogenesis with loss of germ cells at the round spermatid stage and disorganized axoneme structure.","method":"Gene targeting/knockout in embryonic stem cells, chimeric mouse analysis, western blot, immunolocalization, protein-protein interaction assay (SPAG16S-MEIG1 binding)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function with defined cellular phenotype, subcellular localization, and protein interaction, multiple orthogonal methods in one study","pmids":["15328412"],"is_preprint":false},{"year":2007,"finding":"SPAG16L (71 kDa) is a central apparatus component whose stability depends on interactions with SPAG6 and the 28-kDa fragment of SPAG17; heterozygous frameshift mutation in SPAG16 (exon 13) causes selective loss of SPAG16L, SPAG6, and the SPAG17 fragment upon freeze-thaw stress of isolated sperm, demonstrating that SPAG16L is required for biochemical stability of its interacting central apparatus partners.","method":"Freeze-boil instability assay of sperm extracts from human and mouse heterozygous mutation carriers, western blot for SPAG16L, SPAG6, and SPAG17 fragments","journal":"Biology of reproduction","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional instability assay in both human and mouse samples, single lab but two species and multiple proteins assessed","pmids":["17699735"],"is_preprint":false},{"year":2011,"finding":"SPAG16S (35 kDa), the nuclear isoform, localizes specifically to nuclear speckles (co-localizing with SC35, a pre-mRNA splicing factor marker) in male germ cells. SPAG16S transduction into cultured male germ cells or bronchial epithelial cells increases SPAG16L mRNA expression and activates the Spag16L promoter, while having no effect on other axoneme component transcripts, indicating SPAG16S acts as a gene expression regulator specifically for the Spag16L transcript.","method":"Immunofluorescence co-localization with SC35, lentiviral transduction of SPAG16S into cultured germ cells and BEAS-2B cells, promoter-reporter assay, RT-PCR/western blot for axoneme components","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct nuclear localization experiment with functional consequence (promoter activation), multiple orthogonal methods, single lab","pmids":["21655194"],"is_preprint":false}],"current_model":"SPAG16 encodes two protein isoforms from the same locus: SPAG16L (71 kDa), a WD-repeat protein that localizes to the axonemal central apparatus (C2 microtubule intermicrotubule bridges) and is essential for flagellar/ciliary motility, where it directly interacts via its WD repeats with SPAG6 and stabilizes the SPAG6/SPAG17 central apparatus complex; and SPAG16S (35 kDa), a nuclear speckle-localized isoform expressed in male germ cells that binds MEIG1 and transcriptionally upregulates SPAG16L expression by activating the Spag16L promoter."},"narrative":{"mechanistic_narrative":"SPAG16 encodes a structural component of the axonemal central apparatus that is essential for flagellar and ciliary motility, established originally through its Chlamydomonas ortholog PF20, a WD-repeat protein localized to the intermicrotubule bridges of the C2 central microtubule whose loss eliminates the entire central apparatus and paralyzes flagella [PMID:9188098]. In mammals the locus produces two isoforms with distinct roles [PMID:15328412]. The 71-kDa SPAG16L is incorporated into the central apparatus, where its WD-repeat domain directly binds SPAG6 and, together with a SPAG17 fragment, anchors a mutually stabilizing central apparatus complex: SPAG16L is reduced in Spag6-null sperm, and loss of SPAG16L destabilizes SPAG6 and SPAG17 under stress, demonstrating reciprocal dependence among these partners [PMID:12391165, PMID:17699735]. The 35-kDa SPAG16S accumulates in nuclear speckles of postmeiotic germ cells, binds the chromatin-associated protein MEIG1, and acts as a transcriptional regulator that specifically activates the Spag16L promoter to upregulate the axonemal isoform [PMID:15328412, PMID:21655194]. Genetic disruption of the WD-repeat-encoding domains causes haploinsufficiency that impairs spermatogenesis with germ cell loss at the round spermatid stage and disorganized axonemes [PMID:15328412].","teleology":[{"year":1997,"claim":"Established that the SPAG16 ortholog PF20 is a WD-repeat protein physically located at the C2 central microtubule and necessary for assembly of the entire flagellar central apparatus, defining the gene's foundational role in motility.","evidence":"Insertional mutagenesis, gene cloning, transformation rescue, and immunogold localization in Chlamydomonas","pmids":["9188098"],"confidence":"High","gaps":["Did not address mammalian isoform diversity","Mechanism by which PF20 stabilizes the central apparatus not defined at the molecular partner level"]},{"year":2002,"claim":"Identified a direct molecular partner of SPAG16L, showing its WD-repeat domain binds SPAG6 and that the two co-localize on microtubules and depend on each other for accumulation.","evidence":"Yeast two-hybrid, CHO co-expression with fluorescence microscopy, and western blot of Spag6-KO sperm","pmids":["12391165"],"confidence":"High","gaps":["Did not establish the full composition of the central apparatus complex","Stoichiometry and binding interface not resolved"]},{"year":2004,"claim":"Revealed that the locus produces two functionally distinct isoforms—axonemal SPAG16L and a nuclear SPAG16S that binds MEIG1—and that WD-repeat disruption impairs spermatogenesis via haploinsufficiency.","evidence":"Gene targeting in ES cells, chimeric mouse analysis, immunolocalization, and protein interaction assay","pmids":["15328412"],"confidence":"High","gaps":["Functional role of the SPAG16S-MEIG1 interaction not defined here","Mechanism of haploinsufficiency not resolved"]},{"year":2007,"claim":"Demonstrated that SPAG16L is required for the biochemical stability of its central apparatus partners, integrating it into a mutually dependent SPAG6/SPAG17 complex.","evidence":"Freeze-boil instability assay on human and mouse heterozygous-carrier sperm extracts with western blotting","pmids":["17699735"],"confidence":"Medium","gaps":["Single lab, stress-based assay rather than in vivo stability measurement","Does not define which interactions are direct versus indirect within the complex"]},{"year":2011,"claim":"Assigned a regulatory function to the nuclear SPAG16S isoform, showing it localizes to nuclear speckles and specifically activates the Spag16L promoter to upregulate the axonemal isoform.","evidence":"SC35 co-localization, lentiviral SPAG16S transduction into germ and BEAS-2B cells, promoter-reporter assay, and transcript profiling","pmids":["21655194"],"confidence":"Medium","gaps":["Direct DNA-binding or splicing-factor mechanism of promoter activation not established","Role of MEIG1 binding in this regulatory activity not tested"]},{"year":null,"claim":"The molecular mechanism by which nuclear SPAG16S activates the Spag16L promoter—and whether it does so directly or via splicing/nuclear speckle factors—remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No defined DNA-binding or transcriptional effector activity for SPAG16S","No structural model of the SPAG16L/SPAG6/SPAG17 central apparatus complex"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,3]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[1]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[4]}],"localization":[{"term_id":"GO:0005654","term_label":"nucleoplasm","supporting_discovery_ids":[2,4]}],"pathway":[{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[2]}],"complexes":["axonemal central apparatus"],"partners":["SPAG6","SPAG17","MEIG1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8N0X2","full_name":"Sperm-associated antigen 16 protein","aliases":["Pf20 protein homolog"],"length_aa":631,"mass_kda":70.8,"function":"Necessary for sperm flagellar function. Plays a role in motile ciliogenesis. May help to recruit STK36 to the cilium or apical surface of the cell to initiate subsequent steps of construction of the central pair apparatus of motile cilia (By similarity)","subcellular_location":"Cytoplasm; Cytoplasm, cytoskeleton, flagellum axoneme; Cytoplasm, cytoskeleton, cilium axoneme; Cell projection, cilium, flagellum","url":"https://www.uniprot.org/uniprotkb/Q8N0X2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SPAG16","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SPAG16","total_profiled":1310},"omim":[{"mim_id":"616554","title":"SPERM-ASSOCIATED ANTIGEN 17; SPAG17","url":"https://www.omim.org/entry/616554"},{"mim_id":"612173","title":"SPERM-ASSOCIATED ANTIGEN 16; SPAG16","url":"https://www.omim.org/entry/612173"},{"mim_id":"301059","title":"SPERMATOGENIC FAILURE, X-LINKED, 3; SPGFX3","url":"https://www.omim.org/entry/301059"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"},{"location":"Equatorial segment","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"},{"location":"Plasma membrane","reliability":"Additional"},{"location":"Acrosome","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SPAG16"},"hgnc":{"alias_symbol":["PF20","FLJ22724","DKFZp666P1710","WDR29"],"prev_symbol":[]},"alphafold":{"accession":"Q8N0X2","domains":[{"cath_id":"2.130.10.10","chopping":"344-631","consensus_level":"medium","plddt":90.7851,"start":344,"end":631},{"cath_id":"1.20.5","chopping":"191-222","consensus_level":"medium","plddt":93.4347,"start":191,"end":222}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N0X2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N0X2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N0X2-F1-predicted_aligned_error_v6.png","plddt_mean":76.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPAG16","jax_strain_url":"https://www.jax.org/strain/search?query=SPAG16"},"sequence":{"accession":"Q8N0X2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8N0X2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8N0X2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N0X2"}},"corpus_meta":[{"pmid":"9188098","id":"PMC_9188098","title":"PF20 gene product contains WD repeats and localizes to the intermicrotubule bridges in Chlamydomonas flagella.","date":"1997","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/9188098","citation_count":90,"is_preprint":false},{"pmid":"12391165","id":"PMC_12391165","title":"A sperm-associated WD repeat protein orthologous to Chlamydomonas PF20 associates with Spag6, the mammalian orthologue of Chlamydomonas PF16.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12391165","citation_count":73,"is_preprint":false},{"pmid":"15328412","id":"PMC_15328412","title":"Haploinsufficiency for the murine orthologue of Chlamydomonas PF20 disrupts spermatogenesis.","date":"2004","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/15328412","citation_count":57,"is_preprint":false},{"pmid":"17699735","id":"PMC_17699735","title":"A heterozygous mutation disrupting the SPAG16 gene results in biochemical instability of central apparatus components of the human sperm axoneme.","date":"2007","source":"Biology of reproduction","url":"https://pubmed.ncbi.nlm.nih.gov/17699735","citation_count":38,"is_preprint":false},{"pmid":"11867345","id":"PMC_11867345","title":"Isolation and expression of the human hPF20 gene orthologous to Chlamydomonas PF20: evaluation as a candidate for axonemal defects of respiratory cilia and sperm flagella.","date":"2002","source":"American journal of respiratory cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/11867345","citation_count":29,"is_preprint":false},{"pmid":"21655194","id":"PMC_21655194","title":"Spag16, an axonemal central apparatus gene, encodes a male germ cell nuclear speckle protein that regulates SPAG16 mRNA expression.","date":"2011","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/21655194","citation_count":19,"is_preprint":false},{"pmid":"30911868","id":"PMC_30911868","title":"Distribution of sperm antigen 6 (SPAG6) and 16 (SPAG16) in mouse ciliated and non-ciliated tissues.","date":"2019","source":"Journal of molecular histology","url":"https://pubmed.ncbi.nlm.nih.gov/30911868","citation_count":16,"is_preprint":false},{"pmid":"26706657","id":"PMC_26706657","title":"Anti-SPAG16 antibodies in primary progressive multiple sclerosis are associated with an elevated progression index.","date":"2015","source":"European journal of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/26706657","citation_count":13,"is_preprint":false},{"pmid":"36434627","id":"PMC_36434627","title":"Genetic variant in SPAG16 is associated with the susceptibility of ACPA-positive rheumatoid arthritis possibly via regulation of MMP-3.","date":"2022","source":"Journal of orthopaedic surgery and research","url":"https://pubmed.ncbi.nlm.nih.gov/36434627","citation_count":3,"is_preprint":false},{"pmid":"22963137","id":"PMC_22963137","title":"Genetic variation in SPAG16 regions encoding the WD40 repeats is not associated with reduced sperm motility and axonemal defects in a population of infertile males.","date":"2012","source":"BMC urology","url":"https://pubmed.ncbi.nlm.nih.gov/22963137","citation_count":2,"is_preprint":false},{"pmid":"26317022","id":"PMC_26317022","title":"Association between a Tetranucleotide Repeat Polymorphism of SPAG16 Gene and Cataract in Male Children.","date":"2013","source":"Journal of biomarkers","url":"https://pubmed.ncbi.nlm.nih.gov/26317022","citation_count":1,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.12.16.628556","title":"Modulation of Common Synaptic Inputs and Motor Unit Recruitment Threshold in Triceps Surae Muscles: Effects of Ankle Position","date":"2024-12-17","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.16.628556","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8971,"output_tokens":1664,"usd":0.025937,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8410,"output_tokens":2150,"usd":0.0479,"stage2_stop_reason":"end_turn"},"total_usd":0.073837,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"PF20 (SPAG16 ortholog in Chlamydomonas) encodes a 606-amino acid protein containing five contiguous WD repeats and localizes to the intermicrotubule bridges connecting the two central microtubules (C2 microtubule) of the flagellar central apparatus; loss of PF20 results in absence of the entire central apparatus and flagellar paralysis, and wild-type transformation rescues motility and ultrastructure.\",\n      \"method\": \"Insertional mutagenesis, gene cloning, mutant rescue by transformation, immunogold labeling of wild-type axonemes with anti-PF20 antibody\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (mutagenesis, rescue, immunogold localization) in a single rigorous study establishing the protein's location and necessity for central apparatus assembly\",\n      \"pmids\": [\"9188098\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Mammalian SPAG16 (Pf20) protein associates with SPAG6 (mammalian PF16 ortholog): the WD repeat domain of Pf20 directly interacts with Spag6 by yeast two-hybrid assay; when co-expressed in CHO cells, Pf20-GFP (which alone localizes to cytoplasm) co-localizes with Spag6 on polymerized microtubules. Pf20 protein is markedly reduced in sperm from Spag6-knockout mice.\",\n      \"method\": \"Yeast two-hybrid, co-expression in CHO cells with fluorescence microscopy, immunocytochemistry/electron microscopy of sperm axonemes, western blot of Spag6-KO sperm\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal yeast two-hybrid and co-localization experiments with multiple orthogonal methods establishing direct interaction and mutual dependence\",\n      \"pmids\": [\"12391165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The mouse Spag16 gene encodes two proteins from alternative transcripts: a 71-kDa protein (SPAG16L) incorporated into the axonemal central apparatus and a 35-kDa protein (SPAG16S) that accumulates in the nucleus of postmeiotic germ cells. The 35-kDa SPAG16S binds to MEIG1 (meiosis-expressed gene 1), a chromosome/chromatin-binding protein. Targeted disruption of the WD-repeat-encoding domains causes haploinsufficiency, impairing spermatogenesis with loss of germ cells at the round spermatid stage and disorganized axoneme structure.\",\n      \"method\": \"Gene targeting/knockout in embryonic stem cells, chimeric mouse analysis, western blot, immunolocalization, protein-protein interaction assay (SPAG16S-MEIG1 binding)\",\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 loss-of-function with defined cellular phenotype, subcellular localization, and protein interaction, multiple orthogonal methods in one study\",\n      \"pmids\": [\"15328412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SPAG16L (71 kDa) is a central apparatus component whose stability depends on interactions with SPAG6 and the 28-kDa fragment of SPAG17; heterozygous frameshift mutation in SPAG16 (exon 13) causes selective loss of SPAG16L, SPAG6, and the SPAG17 fragment upon freeze-thaw stress of isolated sperm, demonstrating that SPAG16L is required for biochemical stability of its interacting central apparatus partners.\",\n      \"method\": \"Freeze-boil instability assay of sperm extracts from human and mouse heterozygous mutation carriers, western blot for SPAG16L, SPAG6, and SPAG17 fragments\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional instability assay in both human and mouse samples, single lab but two species and multiple proteins assessed\",\n      \"pmids\": [\"17699735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SPAG16S (35 kDa), the nuclear isoform, localizes specifically to nuclear speckles (co-localizing with SC35, a pre-mRNA splicing factor marker) in male germ cells. SPAG16S transduction into cultured male germ cells or bronchial epithelial cells increases SPAG16L mRNA expression and activates the Spag16L promoter, while having no effect on other axoneme component transcripts, indicating SPAG16S acts as a gene expression regulator specifically for the Spag16L transcript.\",\n      \"method\": \"Immunofluorescence co-localization with SC35, lentiviral transduction of SPAG16S into cultured germ cells and BEAS-2B cells, promoter-reporter assay, RT-PCR/western blot for axoneme components\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct nuclear localization experiment with functional consequence (promoter activation), multiple orthogonal methods, single lab\",\n      \"pmids\": [\"21655194\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SPAG16 encodes two protein isoforms from the same locus: SPAG16L (71 kDa), a WD-repeat protein that localizes to the axonemal central apparatus (C2 microtubule intermicrotubule bridges) and is essential for flagellar/ciliary motility, where it directly interacts via its WD repeats with SPAG6 and stabilizes the SPAG6/SPAG17 central apparatus complex; and SPAG16S (35 kDa), a nuclear speckle-localized isoform expressed in male germ cells that binds MEIG1 and transcriptionally upregulates SPAG16L expression by activating the Spag16L promoter.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SPAG16 encodes a structural component of the axonemal central apparatus that is essential for flagellar and ciliary motility, established originally through its Chlamydomonas ortholog PF20, a WD-repeat protein localized to the intermicrotubule bridges of the C2 central microtubule whose loss eliminates the entire central apparatus and paralyzes flagella [#0]. In mammals the locus produces two isoforms with distinct roles [#2]. The 71-kDa SPAG16L is incorporated into the central apparatus, where its WD-repeat domain directly binds SPAG6 and, together with a SPAG17 fragment, anchors a mutually stabilizing central apparatus complex: SPAG16L is reduced in Spag6-null sperm, and loss of SPAG16L destabilizes SPAG6 and SPAG17 under stress, demonstrating reciprocal dependence among these partners [#1, #3]. The 35-kDa SPAG16S accumulates in nuclear speckles of postmeiotic germ cells, binds the chromatin-associated protein MEIG1, and acts as a transcriptional regulator that specifically activates the Spag16L promoter to upregulate the axonemal isoform [#2, #4]. Genetic disruption of the WD-repeat-encoding domains causes haploinsufficiency that impairs spermatogenesis with germ cell loss at the round spermatid stage and disorganized axonemes [#2].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established that the SPAG16 ortholog PF20 is a WD-repeat protein physically located at the C2 central microtubule and necessary for assembly of the entire flagellar central apparatus, defining the gene's foundational role in motility.\",\n      \"evidence\": \"Insertional mutagenesis, gene cloning, transformation rescue, and immunogold localization in Chlamydomonas\",\n      \"pmids\": [\"9188098\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address mammalian isoform diversity\", \"Mechanism by which PF20 stabilizes the central apparatus not defined at the molecular partner level\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identified a direct molecular partner of SPAG16L, showing its WD-repeat domain binds SPAG6 and that the two co-localize on microtubules and depend on each other for accumulation.\",\n      \"evidence\": \"Yeast two-hybrid, CHO co-expression with fluorescence microscopy, and western blot of Spag6-KO sperm\",\n      \"pmids\": [\"12391165\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the full composition of the central apparatus complex\", \"Stoichiometry and binding interface not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Revealed that the locus produces two functionally distinct isoforms—axonemal SPAG16L and a nuclear SPAG16S that binds MEIG1—and that WD-repeat disruption impairs spermatogenesis via haploinsufficiency.\",\n      \"evidence\": \"Gene targeting in ES cells, chimeric mouse analysis, immunolocalization, and protein interaction assay\",\n      \"pmids\": [\"15328412\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of the SPAG16S-MEIG1 interaction not defined here\", \"Mechanism of haploinsufficiency not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrated that SPAG16L is required for the biochemical stability of its central apparatus partners, integrating it into a mutually dependent SPAG6/SPAG17 complex.\",\n      \"evidence\": \"Freeze-boil instability assay on human and mouse heterozygous-carrier sperm extracts with western blotting\",\n      \"pmids\": [\"17699735\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, stress-based assay rather than in vivo stability measurement\", \"Does not define which interactions are direct versus indirect within the complex\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Assigned a regulatory function to the nuclear SPAG16S isoform, showing it localizes to nuclear speckles and specifically activates the Spag16L promoter to upregulate the axonemal isoform.\",\n      \"evidence\": \"SC35 co-localization, lentiviral SPAG16S transduction into germ and BEAS-2B cells, promoter-reporter assay, and transcript profiling\",\n      \"pmids\": [\"21655194\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct DNA-binding or splicing-factor mechanism of promoter activation not established\", \"Role of MEIG1 binding in this regulatory activity not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular mechanism by which nuclear SPAG16S activates the Spag16L promoter—and whether it does so directly or via splicing/nuclear speckle factors—remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No defined DNA-binding or transcriptional effector activity for SPAG16S\", \"No structural model of the SPAG16L/SPAG6/SPAG17 central apparatus complex\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005930\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"complexes\": [\"axonemal central apparatus\"],\n    \"partners\": [\"SPAG6\", \"SPAG17\", \"MEIG1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}