Zonadhesin (ZAN)

Zonadhesin (ZAN) is a sperm membrane protein that binds the zona pellucida of the egg in a species-specific manner. Once bound, the acrosomal reaction is triggered, involving exocytosis of the acrosome, creating an acrosomal shroud. Next, the sperm penetrates the egg zona pellucida, enters the perivitelline space between the zona pellucida and the plasma membrane, and then fuses with the plasma membrane. The zonadhesin protein, encoded by the ZAN gene, localizes to the anterior part of the sperm head and acts as a receptor to the zona pellucida matrix of the egg. Like other male reproductive proteins located in the acrosome or sperm head and proposed to be involved in species-specific binding to eggs, ZAN possesses high levels of divergence between closely related species.