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#Izumo protein

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Fertilization in mammals-ll

Izumo protein on sperm binds to egg membrane protein:

Izumo1 is sperm-specific immunoglobulin superfamily transmembrane protein that becomes exposed on equatorial segment after acrosome reaction. Its cognate receptor on egg plasma membrane is Juno, also known as folate receptor 4, enriched on microvillar oolemma. Structural studies show Izumo1 helical bundle binds Juno with nanomolar affinity, providing essential adhesion preceding fusion. CD9 organizes membrane microdomains facilitating clustering. After fertilization Juno is rapidly shed in extracellular vesicles, preventing additional sperm attachment. Izumo-Juno interaction is conserved, indispensable for mammalian fertilization success, monospermy enforcement and initiation of egg activation cascade.

Ref: Bianchi et al., Nature 2014, Izumo1-Juno interaction: essential sperm-egg adhesion pair in mammalian fertilization.

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Fertilization in mammals-iv

Izumo interacts with egg protein:

Izumo1 is conserved immunoglobulin-like transmembrane protein exposed on inner acrosomal membrane after acrosome reaction in mouse and human sperm. Its essential egg counterpart is Juno, a GPI-anchored folate receptor family member enriched on oolemma microvilli. Izumo1-Juno binding mediates tight adhesion between gamete membranes, preceding CD9-organized fusion complex formation. Genetic deletion of either protein results in sperm accumulation in perivitelline space without fusion, proving interaction indispensability. CD9 is tetraspanin facilitating fusion but not direct Izumo ligand, while ZP3 and fertilin participate in zona adhesion and earlier steps, not final fusion recognition.

Ref: Satouh et al., Science 2012 & Inoue et al., Nature 2005: Izumo1-Juno interaction essential for sperm-egg membrane adhesion and fusion.

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