Practice question

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.