Insights into Egg Coat Assembly and Egg-Sperm Interaction from the X-Ray Structure of Full-Length ZP3

SummaryZP3, a major component of the zona pellucida (ZP) matrix coating mammalian eggs, is essential for fertilization by acting as sperm receptor. By retaining a propeptide that contains a polymerization-blocking external hydrophobic patch (EHP), we determined the crystal structure of an avian homolog of ZP3 at 2.0 Å resolution. The structure unveils the fold of a complete ZP domain module in a homodimeric arrangement required for secretion and reveals how EHP prevents premature incorporation of ZP3 into the ZP. This suggests mechanisms underlying polymerization and how local structural differences, reflected by alternative disulfide patterns, control the specificity of ZP subunit interaction. Close relative positioning of a conserved O-glycan important for sperm binding and the hypervariable, positively selected C-terminal region of ZP3 suggests a concerted role in the regulation of species-restricted gamete recognition. Alternative conformations of the area around the O-glycan indicate how sperm binding could trigger downstream events via intramolecular signaling.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (442 K)Download as PowerPoint slideHighlights► The structure of sperm receptor ZP3 reveals a homodimer essential for secretion ► A core β strand propeptide blocks premature assembly by intercalating between domains ► Disulfide clustering suggests how the specificity of egg coat assembly is regulated ► An O-linked glycan conserved from chicken to human is important for sperm binding