A structural view of egg coat architecture and function in fertilization

Area of Science:

• Reproductive Biology
• Structural Biology
• Biochemistry

Background:

• Fertilization involves species-specific recognition between egg and sperm, mediated by the egg's extracellular coat, the zona pellucida (ZP) in mammals.
• The ZP is composed of glycoprotein subunits featuring a conserved ZP domain, crucial for polymerization and egg coat formation.

Purpose of the Study:

• To elucidate the atomic-level structural organization of the ZP domain protein ZP3, a key sperm receptor.
• To understand the molecular basis of gamete recognition and egg coat assembly.

Main Methods:

• X-ray crystallography of ZP3, a ZP domain protein.
• Analysis of conserved structural features, including immunoglobulin-like domains (ZP-N, ZP-C), disulfide bonds, and the external hydrophobic patch (EHP).

Main Results:

• The crystal structures reveal ZP3's modular organization, with ZP-N and ZP-C domains linked by a loop containing a conserved O-glycan.
• The EHP is integrated at the interface of the ZP-C domain, influencing polymerization.
• The structures reconcile variable disulfide patterns and multiple gamete interaction determinants in ZP3 homologues.

Conclusions:

• The ZP3 structure provides a molecular basis for gamete recognition and species-specific fertilization.
• Findings impact understanding of ZP biogenesis, egg coat assembly, post-fertilization hardening, and the evolution of egg coats.