Affinage

ASTL

Astacin-like metalloendopeptidase · UniProt Q6HA08

Length
431 aa
Mass
45.9 kDa
Annotated
2026-04-28
23 papers in source corpus 12 papers cited in narrative 12 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

ASTL encodes ovastacin, an oocyte-specific astacin-family zinc metalloendoprotease that is essential for the post-fertilization block to polyspermy and early embryo development. Ovastacin is stored in cortical granules via a 7-amino-acid N-terminal targeting motif and, upon fertilization-triggered exocytosis, its N-terminal catalytic domain is secreted to cleave ZP2 in the zona pellucida while its C-terminal domain remains anchored at the oolemma, where it also serves as a sperm-binding partner for the acrosomal protein SLLP1 (PMID:22472438, PMID:28114310, PMID:28911209, PMID:22206759). Premature zona hardening by trace ovastacin leaking from unfertilized eggs is prevented by the plasma inhibitor fetuin-B, whose cystatin-like domains selectively neutralize astacin metalloproteinases (PMID:25205729, PMID:30679641). Bi-allelic loss-of-function variants in ASTL cause female infertility characterized by impaired ZP2 cleavage and reduced embryo developmental potential (PMID:37133443).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2004 High

    Establishing the enzymatic identity of ovastacin as a zinc metalloendoprotease answered the fundamental question of what catalytic activity the ASTL gene product possesses, placing it within the astacin family.

    Evidence Recombinant human ovastacin expressed in E. coli cleaved synthetic metalloproteinase substrates; activity abolished by metalloproteinase inhibitors

    PMID:15087446

    Open questions at the time
    • Physiological substrate unknown
    • No structural data for the catalytic domain
    • In vivo function not established
  2. 2011 High

    Demonstrating that ovastacin localizes to the oolemma and binds the sperm acrosomal protein SLLP1, with knockout females showing reduced fertility, established that ASTL has a direct functional role in gamete interaction beyond its protease activity.

    Evidence SPR, far-western, yeast two-hybrid, co-IP, and KO mouse fertility assay

    PMID:22206759

    Open questions at the time
    • Mechanism by which SLLP1–SAS1B interaction contributes to fertilization unclear
    • Relative contribution of oolemmal vs. cortical granule pools not resolved
  3. 2012 High

    Identifying ZP2 as the physiological substrate of ovastacin and showing that Astl-null eggs fail to undergo zona hardening resolved the decades-old question of which cortical granule protease mediates the post-fertilization block to polyspermy.

    Evidence Recombinant ovastacin cleaved ZP2 in native zonae; Astl-null mice lacked post-fertilization ZP2 cleavage and showed persistent sperm binding

    PMID:22472438

    Open questions at the time
    • Mechanism of ZP2 cleavage site recognition not defined
    • Role in embryo development beyond polyspermy block not examined
  4. 2014 High

    Discovering that fetuin-B neutralizes leaked ovastacin to maintain egg fertilizability explained how unfertilized oocytes avoid premature zona hardening despite constitutive low-level protease release.

    Evidence Fetuin-B-deficient mouse model plus biochemical inhibition assays

    PMID:25205729

    Open questions at the time
    • Stoichiometric balance between fetuin-B and ovastacin in vivo not quantified
    • Other potential extracellular regulators not explored
  5. 2017 High

    Three concurrent discoveries clarified the intracellular life cycle of ovastacin: a 7-amino-acid N-terminal motif directs cortical granule targeting; pro-ovastacin is activated before exocytosis; and the protein is proteolytically processed upon exocytosis so that the N-terminal domain is secreted while the C-terminal domain is retained at the membrane.

    Evidence CRISPR knock-in of targeting-motif deletion causing misdirection and premature ZP2 cleavage; immunoblot and domain-specific antibody localization with null controls; oocyte aging experiments linking mis-localization to premature ZP2 cleavage

    PMID:28114310 PMID:28137755 PMID:28911209

    Open questions at the time
    • Protease responsible for intracellular pro-ovastacin activation not identified
    • Membrane-anchoring mechanism of C-terminal domain unknown
    • Structural basis of the 7-aa targeting motif interaction with sorting machinery undefined
  6. 2019 High

    Mapping the inhibitory specificity of fetuin-B to its cystatin-like domains and showing selectivity for astacin-subfamily metalloproteinases defined the molecular basis of ovastacin regulation.

    Evidence Domain truncation analysis with recombinant proteins and cross-family protease panel

    PMID:30679641

    Open questions at the time
    • No co-crystal structure of fetuin-B–ovastacin complex
    • Binding interface residues not mapped
  7. 2022 High

    Solving the crystal structure of a Fab–SAS1B C-terminal peptide complex provided the first structural insight into ovastacin's unique C-terminal domain, revealing an amphipathic α-helix at the antibody epitope.

    Evidence X-ray crystallography at 2.75 Å with SPR affinity measurement

    PMID:35503210

    Open questions at the time
    • Full-length ovastacin structure still unavailable
    • Function of the C-terminal domain beyond membrane retention not resolved
  8. 2023 High

    Global substrate profiling defined ovastacin's cleavage-site specificity and identified additional fertilization-relevant substrates beyond ZP2, while patient-derived ASTL variants were shown to impair enzymatic activity and cause female infertility in knock-in mice.

    Evidence N-TAILS mass spectrometry profiling against MEF secretome; patient variant characterization by ZP2 cleavage assay and knock-in mouse models

    PMID:37133443 PMID:37690456

    Open questions at the time
    • Biological significance of non-ZP2 substrates in fertilization not validated in vivo
    • Genotype–phenotype correlation across diverse ASTL variants incomplete
    • Mechanism by which impaired ZP2 cleavage reduces embryo developmental potential unclear

Open questions

Synthesis pass · forward-looking unresolved questions
  • A full-length ovastacin structure, the identity of the protease that activates pro-ovastacin intracellularly, and the in vivo relevance of newly identified non-ZP2 substrates remain to be determined.
  • No full-length crystal or cryo-EM structure
  • Intracellular activating protease unknown
  • Functional validation of non-ZP2 substrates in vivo lacking

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 4 GO:0016787 hydrolase activity 2
Localization
GO:0005886 plasma membrane 3 GO:0031410 cytoplasmic vesicle 3 GO:0005576 extracellular region 2
Pathway
R-HSA-1474165 Reproduction 4
Partners
FETUBSLLP1ZP2

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2004 Human and mouse ovastacin (encoded by ASTL) is a metalloendoprotease of the astacin family with signal sequence, propeptide, and metalloproteinase domain plus a unique C-terminal domain. Recombinant human ovastacin expressed in E. coli hydrolyzed synthetic metalloproteinase substrates, and this activity was abolished by metalloproteinase inhibitors but not inhibitors of other protease classes, establishing its zinc metalloendoprotease catalytic activity. Recombinant protein expression, synthetic substrate cleavage assay, protease inhibitor panel The Journal of biological chemistry High 15087446
2011 Ovastacin (SAS1B) localizes to the microvillar oolemma of ovulated MII oocytes and functions as an oolemmal binding partner for the intra-acrosomal sperm protein SLLP1. Binding was demonstrated by surface plasmon resonance, far-western blot, yeast two-hybrid, and co-IP with both recombinant and native proteins. SAS1B has protease activity, and anti-SAS1B antibody or protein significantly inhibited fertilization; SAS1B knockout females showed 34% reduced fertility. Surface plasmon resonance, far-western blot, yeast two-hybrid, co-IP, immunofluorescence, KO mouse fertility assay, fertilization inhibition assay Developmental biology High 22206759
2012 Ovastacin, stored in egg cortical granules, is released upon fertilization and directly cleaves ZP2 in the zona pellucida to prevent polyspermy. Recombinant ovastacin cleaved ZP2 in native zonae pellucidae in vitro. Female Astl-null mice failed to cleave ZP2 after fertilization, and sperm bound equally well to null two-cell embryos as to normal eggs, establishing ovastacin as the cortical granule protease responsible for the post-fertilization block to sperm binding. Recombinant protein cleavage assay, Astl knockout mouse, sperm binding assay, immunofluorescence, subcellular fractionation The Journal of cell biology High 22472438
2013 Ovastacin (SAS1B) protein expression is temporally and spatially restricted to growing oocytes beginning at the primary-to-secondary follicle transition; immunoelectron microscopy localized SAS1B within cortical granules of MII oocytes, and a population was found on the microvillar oolemma anti-podal to the nucleus in ovulated oocytes, consistent with its dual role as a cortical granule protease and oolemmal receptor. Immunohistochemistry, immunoelectron microscopy, immunofluorescence across seven eutherian species Developmental dynamics Medium 24038607
2014 The plasma protein fetuin-B inhibits ovastacin activity to keep unfertilized eggs fertilizable; trace amounts of ovastacin that leak from unfertilized eggs are neutralized by fetuin-B, preventing premature zona hardening. Once sperm penetrates the egg, bulk ovastacin released from cortical granules overrides fetuin-B and triggers zona hardening. Genetic (fetuin-B deficient mouse), biochemical inhibition assays, zona hardening functional assay Biological chemistry High 25205729
2017 A unique 7-amino-acid motif near the N-terminus of ovastacin is necessary and sufficient for its trafficking to cortical granules during oogenesis. CRISPR/Cas9 deletion of this motif (AstlΔ) misdirects ovastacin to the endomembrane system, causing premature ZP2 cleavage, poor sperm binding, and sub-fertility. Deletion mutant analysis, AstlmCherry transgenic rescue, CRISPR/Cas9 knock-in, immunofluorescence, sperm binding assay, fertility assay PLoS genetics High 28114310
2017 Ovastacin undergoes intracellular activation before exocytosis: MII oocytes contain both inactive pro-ovastacin and activated ovastacin, and partial ZP2 cleavage occurs before fertilization coinciding with germinal vesicle breakdown. Upon exocytosis, the C-terminal domain of ovastacin remains attached to the plasma membrane while the N-terminal active domain is secreted, reconciling its roles as an oolemmal receptor (SAS1B) and a secreted ZP2 protease. Immunoblot, ZP digestion assay (α-chymotrypsin), immunofluorescence with domain-specific antibodies, comparison of wild-type and Astl-null oocytes Molecular human reproduction High 28911209
2017 Post-ovulatory oocyte aging causes mis-localization and premature exocytosis of ovastacin with consequent premature ZP2 cleavage, disrupting the sperm recognition domain and reducing sperm binding. Melatonin rescues ovastacin localization and abundance in aged oocytes, maintaining fertilization competence. Immunofluorescence, western blot, sperm binding assay, in-vitro fertilization, melatonin treatment Human reproduction Medium 28137755
2019 Fetuin-B selectively inhibits astacin metalloproteinases including ovastacin and meprins, but not tolloid-subfamily astacins or other protease classes. The cystatin-like domains of fetuin-B alone are necessary and sufficient for this inhibition. Fetuin-A shows no inhibitory activity toward any tested protease. Recombinant protein expression in multiple cell systems, protease inhibition assays, domain truncation analysis Scientific reports High 30679641
2022 Crystal structure of the Fab of monoclonal antibody 7H2.2 in complex with a 15-residue fragment from the C-terminal domain of SAS1B (ovastacin) was determined at 2.75 Å resolution. The antigen forms an amphipathic α-helix bound through hydrophilic residues, providing the first structural insight into the C-terminal subdomain architecture of ovastacin. Single-crystal X-ray crystallography at 2.75 Å, SPR binding affinity measurement Acta crystallographica. Section D, Structural biology High 35503210
2023 N-terminal amine isotopic labeling of substrates (N-TAILS) profiling of ovastacin against the secretome of mouse embryonic fibroblasts revealed the physicochemical properties governing substrate recognition, the precise cleavage site specificity, and characteristics distinguishing ovastacin from meprins and tolloid. Additional fertilization-relevant substrates beyond ZP2 were identified. N-TAILS substrate profiling, mass spectrometry, recombinant enzyme cleavage assays The FEBS journal High 37690456
2023 Bi-allelic missense variants in ASTL from infertile patients impair ovastacin's enzymatic activity (ZP2 cleavage assay in mouse eggs in vitro), while frameshift variants reduce protein quantity. Three knock-in mouse lines corresponding to patient missense variants display subfertility due to reduced embryo developmental potential. In vitro ZP2 cleavage assay, western blot, knock-in mouse models, embryo development assay Human molecular genetics High 37133443

Source papers

Stage 0 corpus · 23 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2012 Ovastacin, a cortical granule protease, cleaves ZP2 in the zona pellucida to prevent polyspermy. The Journal of cell biology 234 22472438
2017 Melatonin improves the fertilization ability of post-ovulatory aged mouse oocytes by stabilizing ovastacin and Juno to promote sperm binding and fusion. Human reproduction (Oxford, England) 83 28137755
2004 Identification and characterization of human and mouse ovastacin: a novel metalloproteinase similar to hatching enzymes from arthropods, birds, amphibians, and fish. The Journal of biological chemistry 68 15087446
2017 A Unique Egg Cortical Granule Localization Motif Is Required for Ovastacin Sequestration to Prevent Premature ZP2 Cleavage and Ensure Female Fertility in Mice. PLoS genetics 43 28114310
2011 Oocyte specific oolemmal SAS1B involved in sperm binding through intra-acrosomal SLLP1 during fertilization. Developmental biology 41 22206759
2019 Mammalian plasma fetuin-B is a selective inhibitor of ovastacin and meprin metalloproteinases. Scientific reports 39 30679641
2017 Intracellular activation of ovastacin mediates pre-fertilization hardening of the zona pellucida. Molecular human reproduction 31 28911209
2021 ASTL is mutated in female infertility. Human genetics 25 34704130
2014 Mammalian gamete fusion depends on the inhibition of ovastacin by fetuin-B. Biological chemistry 25 25205729
2013 SAS1B protein [ovastacin] shows temporal and spatial restriction to oocytes in several eutherian orders and initiates translation at the primary to secondary follicle transition. Developmental dynamics : an official publication of the American Association of Anatomists 19 24038607
2015 Membrane associated cancer-oocyte neoantigen SAS1B/ovastacin is a candidate immunotherapeutic target for uterine tumors. Oncotarget 12 26327203
2023 Bi-allelic variants in ASTL cause abnormal fertilization or oocyte maturation defects. Human molecular genetics 7 37133443
2023 Ovastacin: An oolemma protein that cleaves the zona pellucida to prevent polyspermy. Clinical and experimental reproductive medicine 6 37643828
2023 Novel biallelic ASTL variants are associated with polyspermy and female infertility: A successful live birth following ICSI treatment. Gene 5 37640117
2020 A Primary Evaluation of Potential Small-Molecule Inhibitors of the Astacin Metalloproteinase Ovastacin, a Novel Drug Target in Female Infertility Treatment. ChemMedChem 5 32946206
2023 In silico identification and characterization of the SNPs in the human ASTL gene and their probable role in female infertility. Frontiers in cell and developmental biology 3 37363721
2023 Substrate profiling of the metalloproteinase ovastacin uncovers specific enzyme-substrate interactions and discloses fertilization-relevant substrates. The FEBS journal 3 37690456
2018 Evaluation of SAS1B as a target for antibody-drug conjugate therapy in the treatment of pancreatic cancer. Oncotarget 3 29507667
2025 Crossing the barrier or how regulation of ovastacin controls fertilization and translates into clinical phenotypes. iScience 1 40697826
2025 Novel homozygous variants in ASTL and WEE2 responsible for female infertility characterized by abnormal fertilization. Journal of assisted reproduction and genetics 1 40830301
2024 Cancer-oocyte SAS1B protein is expressed at the cell surface of multiple solid tumors and targeted with antibody-drug conjugates. Journal for immunotherapy of cancer 1 38485187
2022 Monoclonal antibody 7H2.2 binds the C-terminus of the cancer-oocyte antigen SAS1B through the hydrophilic face of a conserved amphipathic helix corresponding to one of only two regions predicted to be ordered. Acta crystallographica. Section D, Structural biology 0 35503210
2021 The detection of SAS1B in serum provides clues for early diagnosis of thyroid cancer. European review for medical and pharmacological sciences 0 33629311