Affinage

SPAG16

Sperm-associated antigen 16 protein · UniProt Q8N0X2

Length
631 aa
Mass
70.8 kDa
Annotated
2026-06-10
11 papers in source corpus 5 papers cited in narrative 5 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

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).

Mechanistic history

Synthesis pass · year-by-year structured walk · 5 steps
  1. 1997 High

    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

    PMID:9188098

    Open questions at the time
    • Did not address mammalian isoform diversity
    • Mechanism by which PF20 stabilizes the central apparatus not defined at the molecular partner level
  2. 2002 High

    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

    PMID:12391165

    Open questions at the time
    • Did not establish the full composition of the central apparatus complex
    • Stoichiometry and binding interface not resolved
  3. 2004 High

    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

    PMID:15328412

    Open questions at the time
    • Functional role of the SPAG16S-MEIG1 interaction not defined here
    • Mechanism of haploinsufficiency not resolved
  4. 2007 Medium

    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

    PMID:17699735

    Open questions at the time
    • Single lab, stress-based assay rather than in vivo stability measurement
    • Does not define which interactions are direct versus indirect within the complex
  5. 2011 Medium

    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

    PMID:21655194

    Open questions at the time
    • Direct DNA-binding or splicing-factor mechanism of promoter activation not established
    • Role of MEIG1 binding in this regulatory activity not tested

Open questions

Synthesis pass · forward-looking unresolved questions
  • 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.
  • No defined DNA-binding or transcriptional effector activity for SPAG16S
  • No structural model of the SPAG16L/SPAG6/SPAG17 central apparatus complex

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 3 GO:0008092 cytoskeletal protein binding 1 GO:0140110 transcription regulator activity 1
Localization
GO:0005654 nucleoplasm 2
Pathway
R-HSA-1474165 Reproduction 1
Partners
MEIG1SPAG17SPAG6
Complex memberships
axonemal central apparatus

Evidence

Reading pass · 5 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1997 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. Insertional mutagenesis, gene cloning, mutant rescue by transformation, immunogold labeling of wild-type axonemes with anti-PF20 antibody Molecular biology of the cell High 9188098
2002 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. Yeast two-hybrid, co-expression in CHO cells with fluorescence microscopy, immunocytochemistry/electron microscopy of sperm axonemes, western blot of Spag6-KO sperm Molecular and cellular biology High 12391165
2004 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. Gene targeting/knockout in embryonic stem cells, chimeric mouse analysis, western blot, immunolocalization, protein-protein interaction assay (SPAG16S-MEIG1 binding) Proceedings of the National Academy of Sciences of the United States of America High 15328412
2007 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. Freeze-boil instability assay of sperm extracts from human and mouse heterozygous mutation carriers, western blot for SPAG16L, SPAG6, and SPAG17 fragments Biology of reproduction Medium 17699735
2011 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. 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 PloS one Medium 21655194

Source papers

Stage 0 corpus · 11 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
1997 PF20 gene product contains WD repeats and localizes to the intermicrotubule bridges in Chlamydomonas flagella. Molecular biology of the cell 90 9188098
2002 A sperm-associated WD repeat protein orthologous to Chlamydomonas PF20 associates with Spag6, the mammalian orthologue of Chlamydomonas PF16. Molecular and cellular biology 73 12391165
2004 Haploinsufficiency for the murine orthologue of Chlamydomonas PF20 disrupts spermatogenesis. Proceedings of the National Academy of Sciences of the United States of America 57 15328412
2007 A heterozygous mutation disrupting the SPAG16 gene results in biochemical instability of central apparatus components of the human sperm axoneme. Biology of reproduction 38 17699735
2002 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. American journal of respiratory cell and molecular biology 29 11867345
2011 Spag16, an axonemal central apparatus gene, encodes a male germ cell nuclear speckle protein that regulates SPAG16 mRNA expression. PloS one 19 21655194
2019 Distribution of sperm antigen 6 (SPAG6) and 16 (SPAG16) in mouse ciliated and non-ciliated tissues. Journal of molecular histology 16 30911868
2015 Anti-SPAG16 antibodies in primary progressive multiple sclerosis are associated with an elevated progression index. European journal of neurology 13 26706657
2022 Genetic variant in SPAG16 is associated with the susceptibility of ACPA-positive rheumatoid arthritis possibly via regulation of MMP-3. Journal of orthopaedic surgery and research 3 36434627
2012 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. BMC urology 2 22963137
2013 Association between a Tetranucleotide Repeat Polymorphism of SPAG16 Gene and Cataract in Male Children. Journal of biomarkers 1 26317022

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