Structural Basis of Egg Coat-Sperm Recognition at Fertilization

Highlights

• Sperm-binding repeats of mollusk VERL and mouse ZP2 egg coat proteins fold similarly

• Structures of VERL/lysin complexes reveal the atomic basis of egg coat-sperm recognition

• A medium-affinity VERL repeat selectively binds lysin from the same species

• VERL/lysin recognition suggests a mechanism for sperm penetration through the egg coat

Summary

Recognition between sperm and the egg surface marks the beginning of life in all sexually reproducing organisms. This fundamental biological event depends on the species-specific interaction between rapidly evolving counterpart molecules on the gametes. We report biochemical, crystallographic, and mutational studies of domain repeats 1–3 of invertebrate egg coat protein VERL and their interaction with cognate sperm protein lysin. VERL repeats fold like the functionally essential N-terminal repeat of mammalian sperm receptor ZP2, whose structure is also described here. Whereas sequence-divergent repeat 1 does not bind lysin, repeat 3 binds it non-species specifically via a high-affinity, largely hydrophobic interface. Due to its intermediate binding affinity, repeat 2 selectively interacts with lysin from the same species. Exposure of a highly positively charged surface of VERL-bound lysin suggests that complex formation both disrupts the organization of egg coat filaments and triggers their electrostatic repulsion, thereby opening a hole for sperm penetration and fusion.