Practice question

Fertilin heterodimer ADAM1/ADAM2 resides on sperm surface after epididymal processing, relocating to equatorial segment ready for fusion. Its disintegrin loop interacts with integrin alpha6beta1 on egg microvilli, stabilizing adhesion immediately after zona penetration and prior to Izumo1-Juno binding. This interaction lowers energy barrier for lipid bilayer merging, promoting fusion pore expansion and cytoplasmic continuity. While acrosin and hyaluronidase mediate zona penetration and acrosome formation occurs earlier, fertilin specifically mediates membrane apposition step. Knockout studies demonstrate reduced fertilization rates due to fusion failure despite intact motility and zona binding, underscoring role in gamete membrane unification.