Practice question

Post-acrosome reaction sperm protein Izumo1, Ig-superfamily type I transmembrane protein, becomes relocated to equatorial segment competent for fusion. On egg, cognate binding partner is Juno, GPI-anchored folate receptor 4 enriched on microvillar plasma membrane. High-resolution crystallography shows Izumo1 four-helix bundle interacts with Juno beta-propeller domains providing heterophilic adhesion interface bridging gametes before membrane merger. Association triggers Juno shedding via extracellular vesicles preventing polyspermy. CD9 organizes fusion synapse but core lock-key recognition is Izumo-Juno highly conserved across mammalian species, essential for fertility, monospermy enforcement and egg activation initiation.