Affinage

EQTN

Equatorin · UniProt Q9NQ60

Length
294 aa
Mass
32.8 kDa
Annotated
2026-06-09
50 papers in source corpus 12 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

Equatorin (EQTN/SPACA8/AFAF/C9orf11) is a sperm-specific type 1 transmembrane N,O-sialoglycoprotein that integrates into the acrosomal membrane during spermiogenesis and is essential for the acrosome reaction and sperm–egg interaction (PMID:9674989, PMID:19605790, PMID:24480441). The protein is first expressed in round spermatids and deposited onto nascent acrosomal membranes, where it associates with the electron-dense acrosomal matrix and participates in acrosome remodeling, though it is dispensable for acrosome biogenesis itself (PMID:16831425, PMID:23564009, PMID:24480441). On intact sperm equatorin is sequestered facing the acrosomal lumen and becomes exposed only after the acrosome reaction, during which it undergoes a staged redistribution from the anterior acrosome to the equatorial segment accompanied by a reduction in molecular mass (PMID:11673259, PMID:19605790, PMID:20032212). Mechanistically, equatorin acts upstream of acrosomal calcium efflux and physically interacts with the SNARE proteins SNAP25 and Syntaxin1a to support the membrane fusion underlying acrosomal exocytosis; in transfected cells it also modulates endosomal trafficking, down-regulating transferrin endocytosis (PMID:19285662, PMID:24480441). Its extracellular gamete-interaction domain is heavily O-glycosylated near threonine 138 by Galnt3-dependent mucin-type glycosylation, a modification required for the MN9 fertility-blocking epitope and for normal sperm–egg adhesion (PMID:19605790, PMID:23052838, PMID:37943980). Loss of Eqtn causes subfertility marked by defective induced acrosome exocytosis and reduced attachment of sperm to the oolemma, with aberrant behavior of the partner protein SPESP1 and a more severe phenotype in Eqtn/Spesp1 double knockouts, while fertility is restored by transgenic rescue (PMID:24480441, PMID:30328350).

Mechanistic history

Synthesis pass · year-by-year structured walk · 12 steps
  1. 1998 High

    Established that an equatorial-segment acrosomal molecule is specifically required for the gamete fusion step rather than earlier fertilization events, defining EQTN's functional niche.

    Evidence IVF inhibition with the MN9 monoclonal antibody, electron microscopy, and immunofluorescence in mouse

    PMID:9674989

    Open questions at the time
    • Molecular identity of the antigen not yet defined
    • Mechanism of how blocking the molecule prevents fusion unknown
  2. 2000 Low

    Identified the gene (C9orf11) encoding a testis-enriched 294-aa protein, providing the genomic and sequence framework for the antigen.

    Evidence cDNA cloning, sequence/expression analysis, and mutation screening on chromosome 9p21

    PMID:11118625

    Open questions at the time
    • No functional data beyond identification
    • Link to the equatorin/MN9 antigen not established in this study
  3. 2001 Medium

    Showed that equatorin is masked on intact sperm and exposed after the acrosome reaction, then tracked through fertilization, clarifying when and where it becomes accessible to act.

    Evidence Permeabilization immunofluorescence, IVF, sperm microinjection, and confocal microscopy

    PMID:11673259

    Open questions at the time
    • Biochemical basis of post-fertilization dissociation unknown
    • Functional consequence of inheritance pattern unclear
  4. 2006 Medium

    Defined the developmental timing and subcellular targeting of the protein, linking acrosome formation to plasma membrane and early endosome compartments.

    Evidence Cloning, immunofluorescence, and EEA1 co-localization in transfected HeLa cells

    PMID:16831425

    Open questions at the time
    • Endosomal role inferred from heterologous cells, not sperm
    • No direct mechanism linking trafficking to acrosome biogenesis
  5. 2009 Medium

    Connected equatorin to the exocytic machinery by demonstrating it acts upstream of acrosomal calcium efflux, interacts with SNAP25, and modulates endocytosis, providing a molecular mechanism for its role in exocytosis.

    Evidence Co-IP and co-localization with SNAP25, permeabilized acrosomal exocytosis and transferrin uptake assays, RNAi, and IVF

    PMID:19285662

    Open questions at the time
    • Single-lab co-IP without reciprocal validation
    • Direct vs indirect nature of SNAP25 interaction unresolved
  6. 2009 High

    Resolved the molecular identity and topology of equatorin as a glycosylated type 1 transmembrane N,O-sialoglycoprotein and mapped the gamete-interaction epitope to an O-glycosylation site near Thr138.

    Evidence cDNA cloning, mass spectrometry, glycosidase and mutagenesis assays, O-glycosylation inhibition, and immunogold EM

    PMID:19605790

    Open questions at the time
    • Identity of the egg-side binding partner not established
    • Functional role of N-glycans/sialylation not dissected
  7. 2009 Medium

    Mapped the staged redistribution and proteolytic/molecular-mass change of equatorin during the acrosome reaction on the zona, defining its dynamic behavior during exocytosis.

    Evidence MN9 immunofluorescence staging, PNA-FITC, immunogold EM, and Western blot of acrosome-reacted sperm

    PMID:20032212

    Open questions at the time
    • Enzyme(s) responsible for mass reduction unknown
    • Functional significance of hybrid-vesicle accumulation unclear
  8. 2012 Medium

    Identified Galnt3 as the glycosyltransferase generating the functional O-glycan epitope on equatorin, tying a specific glycosylation enzyme to the fertilization-relevant modification.

    Evidence Galnt3 knockout mice, immunohistochemistry, MN9 Western blot, and VVA lectin Tn-antigen detection

    PMID:23052838

    Open questions at the time
    • Whether Galnt3 loss impairs fertility through equatorin specifically not shown
    • Other glycosyltransferases acting on equatorin unidentified
  9. 2013 High

    Defined the precise expression timing, membrane integration, and nanoscale matrix association of equatorin during spermatogenesis at super-resolution.

    Evidence In situ hybridization, STED nanoscopy, immunogold EM, and GFP-tagged transgenic mice

    PMID:23564009

    Open questions at the time
    • Targeting machinery directing acrosomal integration unknown
    • Functional necessity at each developmental step not tested genetically here
  10. 2014 High

    Provided genetic proof via knockout that equatorin is dispensable for acrosome biogenesis but essential for the acrosome reaction, and confirmed interaction with both Syntaxin1a and SNAP25.

    Evidence Eqtn knockout mice, IVF and acrosome exocytosis assays, and co-IP with Syntaxin1a/SNAP25

    PMID:24480441

    Open questions at the time
    • How equatorin promotes SNARE-mediated fusion mechanistically unresolved
    • Residual fertility implies redundant pathways not identified
  11. 2018 High

    Localized the fertility defect specifically to reduced sperm–oolemma adhesion and revealed genetic interaction with SPESP1, refining the step at which EQTN acts during gamete interaction.

    Evidence Eqtn and Eqtn/Spesp1 double knockouts, transgenic rescue, zona-free oocyte binding assays, and immunofluorescence for IZUMO1/CD9/SPESP1

    PMID:30328350

    Open questions at the time
    • Molecular basis of EQTN-SPESP1 functional interaction unknown
    • Direct adhesion receptor on the oolemma not identified
  12. 2023 Medium

    Confirmed and extended the human O-glycoproteome of equatorin, establishing it as one of the most heavily core-1 O-glycosylated sperm proteins with multiple novel sites.

    Evidence Glycoproteomics (GlycoTCFM) and intact O-glycopeptide mass spectrometry of human sperm and seminal plasma

    PMID:37943980

    Open questions at the time
    • Functional role of individual human glycosites untested
    • Species conservation of site-level glycosylation not addressed

Open questions

Synthesis pass · forward-looking unresolved questions
  • The egg-side receptor that engages equatorin's O-glycosylated extracellular domain and the precise biochemical mechanism by which equatorin promotes SNARE-mediated acrosomal membrane fusion remain undefined.
  • No oolemmal binding partner identified
  • Direct biochemical role of equatorin in the fusion reaction not reconstituted

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 2
Localization
GO:0031410 cytoplasmic vesicle 3 GO:0005768 endosome 2 GO:0005886 plasma membrane 2
Pathway
R-HSA-1474165 Reproduction 3 R-HSA-5653656 Vesicle-mediated transport 2
Partners
SNAP25SPESP1STX1A

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1998 Equatorin (MN9 antigenic molecule) localized at the equatorial segment of the acrosome is required for sperm-oocyte fusion in mice. The MN9 antibody did not affect sperm motility, zona binding, or zona penetration, but significantly inhibited fertilization by blocking sperm-oocyte fusion, as evidenced by accumulation of sperm in the perivitelline space with unreleased cortical granules beneath the oolemma. In vitro fertilization inhibition assay with monoclonal antibody mMN9, electron microscopy, confocal laser scanning immunofluorescence microscopy Biology of reproduction High 9674989
2001 Equatorin in the posterior acrosome is not exposed on intact spermatozoa but becomes detectable after acrosome reaction (spontaneous or induced). After sperm-egg fusion during IVF, equatorin dissociates from the sperm head equatorial region, remains near decondensing male pronuclei, is pushed away during pronuclear apposition, and is inherited by one proembryonic cell after first cleavage, disappearing after the second cleavage. Immunofluorescence with permeabilization assays, in vitro fertilization, confocal microscopy, microinjection of sperm into MII oocytes Biology of reproduction Medium 11673259
2006 Afaf (EQTN/SPACA8), a novel membrane protein, is expressed abundantly in round spermatids and localizes to the inner and outer membranes of forming acrosomes during spermiogenesis, then declines in maturing acrosomes. In transfected HeLa cells, Afaf localizes to the plasma membrane and EEA1-positive early endosomes, suggesting involvement of early endosomes and plasma membrane in acrosome biogenesis. Cloning, immunofluorescence, subcellular fractionation/localization in transfected HeLa cells, co-localization with EEA1 marker FEBS letters Medium 16831425
2009 Afaf (EQTN) participates in calcium-triggered acrosomal exocytosis by acting upstream of calcium efflux from the acrosome interior. Afaf interacts with SNAP25 (a SNARE complex component critical for exocytosis and endosomal trafficking), as demonstrated by co-immunoprecipitation and co-localization. Afaf antibodies inhibit sperm penetration of eggs and reduce in vitro fertilization rates. Afaf also regulates endocytic pathway by down-regulating transferrin endocytosis in HeLa cells. Co-immunoprecipitation, co-localization, acrosomal exocytosis assay with streptolysin O permeabilization, transferrin uptake assay, RNAi, in vitro fertilization, sperm penetration assay Fertility and sterility Medium 19285662
2009 Equatorin is a highly glycosylated, sperm-specific type 1 transmembrane N,O-sialoglycoprotein. The gamete interaction-related domain recognized by the MN9 antibody is post-translationally modified near threonine 138, most likely via O-glycosylation. The MN9 epitope (N-terminus) localizes on the acrosomal membrane facing the acrosomal lumen as shown by immunogold electron microscopy. cDNA cloning, mass spectrometry, carbohydrate staining, glycosidase treatment, recombinant protein assays, amino acid substitution mutagenesis, dephosphorylation assay, O-glycosylation inhibitor assay, immunogold electron microscopy Biology of reproduction High 19605790
2009 During the acrosome reaction on zona pellucida, equatorin undergoes a defined progression: it spreads from the peripheral anterior acrosome to the equatorial segment in four stages (initial, early, advanced, final). Equatorin decreases in molecular mass from 40–60 kDa to 35 kDa as the acrosome reaction progresses, and accumulates on hybrid vesicles surrounded by amorphous substances at the advanced stage. MN9 antibody immunofluorescence staging, PNA-FITC staining, immunogold electron microscopy, Western blot of acrosome-reacted sperm Reproduction (Cambridge, England) Medium 20032212
2012 Galnt3-mediated mucin-type O-glycosylation of equatorin is required for the MN9 antibody epitope recognition. In Galnt3-deficient mice, the O-glycosylated moiety of equatorin (recognized by MN9 antibody, which inhibits sperm-egg interaction) was drastically reduced, linking Galnt3-catalyzed O-glycosylation of equatorin to the fertilization process. Galnt3 knockout mouse model, immunohistochemistry, Western blot with MN9 antibody, VVA lectin binding (Tn antigen detection) Histochemistry and cell biology Medium 23052838
2013 During spermatogenesis, equatorin mRNA is first detected in round spermatids but disappears in early elongating spermatids. The protein (~65 kDa in testis) is first detected on nascent acrosomal membrane at step 3 round spermatids, actively integrates into acrosomal membranes in subsequent steps, and participates in acrosome remodeling in elongating spermatids. Immunogold EM showed the epitope region lying 5–70 nm from the acrosomal membrane, associated with the electron-dense acrosomal matrix. In situ hybridization, super-resolution STED nanoscopy, immunogold electron microscopy, GFP-tagged transgenic mice, high-resolution fluorescence microscopy Cell and tissue research High 23564009
2014 Equatorin (Eqtn) is not required for acrosome biogenesis but is essential for the acrosome reaction. Eqtn-knockout males are subfertile (~50% of plugged females pregnant vs >90% for controls). Eqtn-deficient sperm have normal motility and morphology but dramatically reduced fertilization rates and induced acrosome exocytosis rates. Equatorin protein interacts with Syntaxin1a and SNAP25 (SNARE complex components), but loss of Eqtn does not affect protein levels of these partners. Gene knockout mouse model, in vitro fertilization assay, acrosome exocytosis assay, co-immunoprecipitation with Syntaxin1a and SNAP25, Western blot Biochemical and biophysical research communications High 24480441
2018 EQTN-knockout males have reduced fertility and reduced sperm-egg adhesion. Eqtn−/− sperm can travel to the oviduct and penetrate the zona pellucida but show significant reduction in sperm attached to zona-free oocytes. SPESP1 behaved aberrantly in Eqtn−/− sperm during the acrosome reaction. EQTN/SPESP1-double KO males showed more severe fertility impairment than single Eqtn−/− males. Fertility was rescued in Eqtn−/−-Tg(Eqtn) males. IZUMO1 and egg CD9 behaved normally in Eqtn−/− sperm. Eqtn knockout and double KO (Eqtn/Spesp1) mouse models, transgenic rescue (Eqtn−/−-Tg(Eqtn)), zona-free oocyte sperm binding assay, acrosome reporter (Acr-Egfp) transgenic mice, immunofluorescence for IZUMO1, CD9, and SPESP1 Reproduction (Cambridge, England) High 30328350
2000 C9orf11 (an alias for EQTN) encodes a protein of 294 amino acids (predicted 32.8 kDa) with a putative leucine zipper at the C-terminal end, organized in eight exons spanning ~13 kb on chromosome 9p21. Expression analysis showed C9orf11 is highly expressed in testis with minor expression in other tissues. cDNA cloning, sequence analysis, Northern blot/expression analysis, mutation screening in CMM families Biochimica et biophysica acta Low 11118625
2023 Equatorin (EQTN) is among the most highly O-glycosylated proteins in human sperm and seminal plasma, detected in a comprehensive O-glycoproteome map. EQTN was identified as one of three highly abundant, highly complex, and highly O-glycosylated proteins, with multiple novel O-glycosites identified for the first time, primarily modified by core 1 O-glycans. Glycoproteomics using two complementary fragmentation methods (GlycoTCFM), mass spectrometry-based intact O-glycopeptide analysis of human sperm and seminal plasma Journal of proteome research Medium 37943980

Source papers

Stage 0 corpus · 50 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2010 Association of left atrial endothelin-1 with atrial rhythm, size, and fibrosis in patients with structural heart disease. Circulation. Arrhythmia and electrophysiology 132 20495015
2008 Genetic architecture of transcript-level variation in humans. American journal of human genetics 113 18439551
2015 Discovery of predictive biomarkers for litter size in boar spermatozoa. Molecular & cellular proteomics : MCP 93 25693803
1998 An MN9 antigenic molecule, equatorin, is required for successful sperm-oocyte fusion in mice. Biology of reproduction 73 9674989
2015 Increased male fertility using fertility-related biomarkers. Scientific reports 69 26489431
1994 Nucleotide sequence of the afimbrial-adhesin-encoding afa-3 gene cluster and its translocation via flanking IS1 insertion sequences. Journal of bacteriology 68 8002584
1996 The afimbrial adhesive sheath encoded by the afa-3 gene cluster of pathogenic Escherichia coli is composed of two adhesins. Molecular microbiology 65 8820639
2018 Genetic architecture underlying the lignin biosynthesis pathway involves noncoding RNAs and transcription factors for growth and wood properties in Populus. Plant biotechnology journal 50 29947466
2006 Afaf, a novel vesicle membrane protein, is related to acrosome formation in murine testis. FEBS letters 34 16831425
2012 Galnt3 deficiency disrupts acrosome formation and leads to oligoasthenoteratozoospermia. Histochemistry and cell biology 31 23052838
2001 Exposure of sperm head equatorin after acrosome reaction and its fate after fertilization in mice. Biology of reproduction 31 11673259
2014 Equatorin is not essential for acrosome biogenesis but is required for the acrosome reaction. Biochemical and biophysical research communications 26 24480441
1996 The clp (CS31A) operon is negatively controlled by Lrp, ClpB, and L-alanine at the transcriptional level. Molecular microbiology 24 8858583
2018 Deletion of Eqtn in mice reduces male fertility and sperm-egg adhesion. Reproduction (Cambridge, England) 23 30328350
2009 Equatorin: identification and characterization of the epitope of the MN9 antibody in the mouse. Biology of reproduction 23 19605790
2023 Enhanced genome-wide association reveals the role of YABBY11-NGATHA-LIKE1 in leaf serration development of Populus. Plant physiology 22 36535002
2009 A model of the acrosome reaction progression via the acrosomal membrane-anchored protein equatorin. Reproduction (Cambridge, England) 22 20032212
2000 The afa-related gene cluster in necrotoxigenic and other Escherichia coli from animals belongs to the afa-8 variant. Veterinary microbiology 22 10946147
2023 Bull Sperm SWATH-MS-Based Proteomics Reveals Link between High Fertility and Energy Production, Motility Structures, and Sperm-Oocyte Interaction. Journal of proteome research 21 37782577
2021 Membrane lipid replacement with nano-micelles in human sperm cryopreservation improves post-thaw function and acrosome protein integrity. Reproductive biomedicine online 21 34256996
2019 Linkage-linkage disequilibrium dissection of the epigenetic quantitative trait loci (epiQTLs) underlying growth and wood properties in Populus. The New phytologist 21 31560799
2020 Optimization of sperm RNA processing for developmental research. Scientific reports 20 32665575
2009 Acrosome formation-associated factor is involved in fertilization. Fertility and sterility 20 19285662
2022 Establishment of a male fertility prediction model with sperm RNA markers in pigs as a translational animal model. Journal of animal science and biotechnology 18 35794675
2021 Miniature inverted repeat transposable elements cis-regulate circular RNA expression and promote ethylene biosynthesis, reducing heat tolerance in Populus tomentosa. Journal of experimental botany 16 33258949
2003 Leucine-responsive regulatory protein-mediated repression of clp (encoding CS31A) expression by L-leucine and L-alanine in Escherichia coli. Journal of bacteriology 16 12618452
2021 Integration of genome wide association studies and co-expression networks reveal roles of PtoWRKY 42-PtoUGT76C1-1 in trans-zeatin metabolism and cytokinin sensitivity in poplar. The New phytologist 15 33999454
2019 Finding New Cell Wall Regulatory Genes in Populus trichocarpa Using Multiple Lines of Evidence. Frontiers in plant science 15 31649710
2013 Integration of the mouse sperm fertilization-related protein equatorin into the acrosome during spermatogenesis as revealed by super-resolution and immunoelectron microscopy. Cell and tissue research 14 23564009
2023 GlycoTCFM: Glycoproteomics Based on Two Complementary Fragmentation Methods Reveals Distinctive O-Glycosylation in Human Sperm and Seminal Plasma. Journal of proteome research 12 37943980
2022 The vertebrate- and testis- specific transmembrane protein C11ORF94 plays a critical role in sperm-oocyte membrane binding. Molecular biomedicine 11 36050562
2016 Novel Sperm and Gonadotropin-releasing Hormone-based Recombinant Fusion Protein: Achievement of 100% Contraceptive Efficacy by Co-immunization of Male and Female Mice. Molecular reproduction and development 10 27676172
2014 Leucine-responsive regulatory protein Lrp and PapI homologues influence phase variation of CS31A fimbriae. Journal of bacteriology 10 24914179
2024 afila, the origin and nature of a major innovation in the history of pea breeding. The New phytologist 9 38837425
2023 High-throughput proteomic characterization of seminal plasma from bulls with contrasting semen quality. 3 Biotech 8 36714547
2025 Dahuang-Gancao decoction ameliorates testosterone-induced androgenetic alopecia in mice. Journal of ethnopharmacology 7 39800247
2014 Preoperative hepatic CT perfusion as an early predictor for the recurrence of esophageal squamous cell carcinoma: initial clinical results. Oncology reports 7 24452736
2023 Cryostress induces fragmentation and alters the abundance of sperm transcripts associated with fertilizing competence and reproductive processes in buffalo. Cell and tissue research 6 37079096
2021 Genetic Architecture Underlying the Metabolites of Chlorogenic Acid Biosynthesis in Populus tomentosa. International journal of molecular sciences 6 33673666
2014 Analysis of the complexity of the sperm acrosomal membrane by super-resolution stimulated emission depletion microscopy compared with transmission electron microscopy. Microscopy (Oxford, England) 6 25430742
2000 Isolation and characterisation of a novel human gene (C9orf11) on chromosome 9p21, a region frequently deleted in human cancer. Biochimica et biophysica acta 6 11118625
2019 Transcription factors involved in the regulatory networks governing the Calvin-Benson-Bassham cycle. Tree physiology 5 30941430
2024 Exploring the Biological Effects of Polystyrene Nanoplastics on Spermatogenesis: Insights From Transcriptomic Analysis in Mouse Spermatocytes. International journal of toxicology 3 39648428
2023 Allelic variations of WAK106-E2Fa-DPb1-UGT74E2 module regulate fibre properties in Populus tomentosa. Plant biotechnology journal 3 37988335
2019 A multimodal attempt to follow-up linkage regions using RNA expression, SNPs and CpG methylation in schizophrenia and bipolar disorder kindreds. European journal of human genetics : EJHG 3 31695175
2018 [Relationship between FGF23/FGFR4 expression in atrial tissue and atrial fibrosis in patients with atrial fibrillation]. Zhonghua yi xue za zhi 3 29690710
2022 Transcriptome and association mapping revealed functional genes respond to drought stress in Populus. Frontiers in plant science 2 35968119
2006 Analysis of alcohol-related phenotypes in F2 progeny derived from FH/Wjd and ACI/N rat strains reveals independent measures and sex differences. Behavioural brain research 2 17161877
2025 Identification of Anoikis-Related Genes in Gastric Cancer: Bioinformatics and Experimental Validation. Cancer medicine 0 40263929
2023 Transitions experienced by mothers of children/adolescents with sickle cell disease in the context of the COVID-19 pandemic. Revista gaucha de enfermagem 0 37283433

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