Affinage

SPAG16

Sperm-associated antigen 16 protein · UniProt Q8N0X2

Length
631 aa
Mass
70.8 kDa
Annotated
2026-04-28
11 papers in source corpus 6 papers cited in narrative 6 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPAG16 is a WD-repeat protein essential for the structural integrity and motility of the 9+2 axoneme, where its longer isoform (SPAG16L) localizes to the central apparatus and forms a stable complex with SPAG6 and SPAG17 that maintains intermicrotubule bridge architecture (PMID:9188098, PMID:12391165, PMID:17699735). Loss of SPAG16 in Chlamydomonas causes flagellar paralysis and complete absence of the central pair, while mammalian haploinsufficiency disrupts axonemal ultrastructure and depletes germ cells at the round spermatid stage (PMID:9188098, PMID:15328412). A shorter nuclear isoform (SPAG16S) localizes to nuclear speckles, binds the chromatin-associated protein MEIG1, and transcriptionally upregulates SPAG16L expression in male germ cells, establishing a regulatory link between the two isoforms (PMID:15328412, PMID:21655194).

Mechanistic history

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

    The founding question—what molecular components build the axonemal central apparatus—was answered by identification of PF20 (SPAG16 ortholog) as a WD-repeat protein essential for central pair assembly and flagellar motility.

    Evidence Insertional mutagenesis, cDNA rescue, and immunogold EM in Chlamydomonas

    PMID:9188098

    Open questions at the time
    • Mammalian ortholog function not yet tested
    • Direct binding partners within the central apparatus unknown
    • Mechanism by which loss of PF20 leads to complete central pair absence not defined
  2. 2002 High

    The question of whether SPAG16 functions through a defined protein complex was resolved by demonstrating direct interaction between SPAG16's WD-repeat domain and SPAG6, with SPAG6 required for SPAG16 stability in mammalian sperm.

    Evidence Yeast two-hybrid, GFP co-localization on microtubules in CHO cells, and Western blot of Spag6-knockout mouse sperm

    PMID:12391165

    Open questions at the time
    • Other central apparatus partners not yet identified
    • Whether the interaction is direct in vivo or mediated by additional factors not resolved
    • No structural data on the SPAG16–SPAG6 interface
  3. 2004 High

    The discovery of two SPAG16 isoforms resolved how one locus serves dual roles: SPAG16L is an axonemal structural component incorporated during meiosis, while the nuclear SPAG16S binds the chromatin protein MEIG1 and acts in postmeiotic germ cells.

    Evidence Gene targeting of WD-repeat exons in mice, immunocytochemistry, TEM, and protein interaction assays

    PMID:15328412

    Open questions at the time
    • Functional consequence of SPAG16S–MEIG1 interaction not defined
    • Whether SPAG16S has transcriptional activity not yet tested
    • Mechanism linking haploinsufficiency to germ-cell loss unclear
  4. 2007 Medium

    The biochemical basis of the central apparatus complex was clarified: SPAG16L, SPAG6, and a SPAG17 fragment form a mutually dependent tripartite complex whose stability collapses when SPAG16L is disrupted by heterozygous mutation.

    Evidence Freeze-thaw stability assay of human and mouse heterozygous mutant sperm with Western blot

    PMID:17699735

    Open questions at the time
    • Stoichiometry and order of assembly of the tripartite complex not determined
    • Whether complex disruption is the direct cause of motility loss or secondary to structural collapse not distinguished
    • Single-lab observation
  5. 2011 High

    The nuclear function of SPAG16S was established: it localizes to SC35-positive nuclear speckles and transcriptionally upregulates SPAG16L, revealing an autoregulatory circuit between the two isoforms.

    Evidence Immunofluorescence co-localization with SC35, lentiviral transduction, promoter-reporter assay, and SPAG16L-specific KO mice

    PMID:21655194

    Open questions at the time
    • Mechanism of SPAG16S transcriptional activation (direct DNA binding vs. co-activator) unknown
    • Whether SPAG16S regulates genes beyond SPAG16L not tested
    • Relevance of SPAG16S in non-germline tissues (e.g., ciliated epithelia) not established

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unknown whether SPAG16 plays functional roles on cytoplasmic microtubules outside the axoneme, what the structural basis of the SPAG16–SPAG6–SPAG17 complex is, and how SPAG16S mechanistically activates transcription from nuclear speckles.
  • No reconstituted or structural data for the central apparatus complex
  • Cytoplasmic microtubule role inferred only from overexpression co-localization without functional evidence
  • Transcriptional mechanism of SPAG16S entirely undefined at the molecular level

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 3 GO:0140110 transcription regulator activity 1
Localization
GO:0005929 cilium 3 GO:0005634 nucleus 2 GO:0005856 cytoskeleton 2 GO:0005654 nucleoplasm 1
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 4 R-HSA-1474165 Reproduction 2
Partners
MEIG1SPAG17SPAG6
Complex memberships
SPAG16L–SPAG6–SPAG17 central apparatus complex

Evidence

Reading pass · 6 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1997 PF20 (SPAG16 ortholog in Chlamydomonas) contains five contiguous WD repeats and localizes to the intermicrotubule bridges connecting the two central microtubules (C2 microtubule) of the axoneme; loss of PF20 causes paralyzed flagella and absence of the entire central apparatus in isolated axonemes. Insertional mutagenesis, cDNA cloning, rescue transformation, immunogold labeling of axonemes, axonemal ultrastructure analysis Molecular biology of the cell High 9188098
2002 Mammalian SPAG16 (Pf20) protein localizes to the axoneme central apparatus in sperm tails; the WD repeat domain of SPAG16 directly interacts with SPAG6 (mammalian PF16 ortholog), and SPAG16 is markedly reduced in sperm from Spag6-knockout mice, indicating SPAG6 is required for SPAG16 stability. Immunocytochemistry, electron microscope immunocytochemistry, yeast two-hybrid assay, co-localization of GFP-fusion proteins on microtubules in CHO cells, Western blot of Spag6-knockout sperm Molecular and cellular biology High 12391165
2004 The mouse Spag16 gene encodes two proteins: a 71-kDa isoform (SPAG16L) that is incorporated into the axoneme central apparatus during meiosis, and a 35-kDa isoform (SPAG16S) that accumulates in the nucleus of postmeiotic germ cells. The 35-kDa SPAG16S protein binds to MEIG1, a chromosome/chromatin-binding protein. Haploinsufficiency of the WD-repeat-encoding domains causes loss of germ cells at the round spermatid stage and disorganized sperm axoneme structure. Gene targeting (knockout of WD-repeat exons), Western blot, immunocytochemistry, transmission electron microscopy, protein-protein interaction assay (binding of 35-kDa PF20 to MEIG1) Proceedings of the National Academy of Sciences of the United States of America High 15328412
2007 A heterozygous frameshift mutation in exon 13 of SPAG16 reduces the biochemical stability of the sperm axoneme central apparatus; freeze-thaw cycling of sperm from heterozygous individuals causes loss of SPAG16L, SPAG6, and the 28-kDa fragment of SPAG17, demonstrating that these proteins form a stable complex dependent on SPAG16L integrity. Freeze-thaw stability assay of isolated sperm, Western blot, comparison of human mutation carriers and mouse heterozygous knockouts Biology of reproduction Medium 17699735
2011 SPAG16S (35-kDa isoform) localizes to nuclear speckles enriched in pre-mRNA splicing factors (co-localizing with SC35) in male germ cells. SPAG16S transduction into cultured male germ cells and bronchial epithelial cells increases SPAG16L mRNA and protein expression, and increases Spag16L promoter activity, demonstrating a nuclear gene-regulatory role for SPAG16S. Immunofluorescence co-localization with SC35, lentiviral transduction of SPAG16S into cultured cells, Western blot, promoter-reporter assay, SPAG16L-specific knockout mice PloS one High 21655194
2019 When EGFP-SPAG6 is overexpressed in cultured cells, it co-localizes with a subset of acetylated cytoplasmic microtubules, suggesting SPAG16/SPAG6 complex may have a role in stabilizing cytoplasmic microtubules beyond the axoneme. SPAG16 (small isoform) is detected in the nucleus of male germ cells and some neurons. Immunolocalization in multiple mouse tissues, EGFP-SPAG6 overexpression in cultured cells with acetylated tubulin co-staining, Western blot Journal of molecular histology Low 30911868

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 37 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 15 30911868
2015 Anti-SPAG16 antibodies in primary progressive multiple sclerosis are associated with an elevated progression index. European journal of neurology 12 26706657
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
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 1 36434627
2013 Association between a Tetranucleotide Repeat Polymorphism of SPAG16 Gene and Cataract in Male Children. Journal of biomarkers 1 26317022