Molecular basis of egg coat cross-linking sheds light on ZP1-associated female infertility

Interaction between sperm and the egg zona pellucida (ZP) is the first step of mammalian fertilization, and ZP component ZP1 is important for fertility by covalently cross-linking ZP filaments into a matrix. Like ZP4, a structurally-related subunit absent in the mouse, ZP1 is predicted to contain an N-terminal ZP-N domain of unknown function. Characterization of ZP1 proteins carrying mutations from infertile patients suggests that, unlike in the mouse, filament cross-linking by ZP1 is crucial for human ZP assembly. We map the function of ZP1 to its ZP-N1 domain and determine crystal structures of ZP-N1 homodimers from a chicken homolog of ZP1. These reveal that ZP filament cross-linking is highly plastic and can be modulated by ZP1 fucosylation and, potentially, zinc sparks. Moreover, we show that ZP4 ZP-N1 forms non-covalent homodimers in chicken but not human. Together, these data identify human ZP1 cross-links as a promising target for non-hormonal contraception.