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.