Introduction

• The recognition of an egg’s extracellular coat by a sea urchin sperm is a critical species-specific binding event that involves multiple stages.

Species-specific Recognition Events

• The first set of species-specific recognition events includes sperm attraction, activation, and acrosome reaction when the sperm comes into contact with the egg’s jelly coat.

Binding of Sperm and Egg

• Once the sperm penetrates the egg jelly and its acrosomal process contacts the surface of the egg, a second critical species-specific binding event occurs.
• The acrosomal protein responsible for this recognition is called bindin, a 30,500 Da protein that is insoluble and isolated from the acrosome of Strongylocentrotus purpuratus.
• Sperm bindin is capable of binding to the dejellied eggs of the same species, while not binding to those of a different species.

Biochemical Studies on Bindin

• Biochemical studies have confirmed that bindins of closely related sea urchin species have different protein sequences, indicating the existence of species-specific bindin receptors on the egg vitelline envelope.
• A 350 kDa glycoprotein that serves as a bindin receptor has been isolated from sea urchin eggs and is thought to be aggregated into complexes on the vitelline envelope.
• The bindin receptor on the egg vitelline envelope recognizes the protein portion of bindin on the acrosome in a species-specific manner, resulting in species-specific recognition of sea urchin gametes.
• Closely related sea urchin species have divergent bindin receptors, and eggs will only adhere to the bindin of their own species.

Evolution of Gamete Recognition Proteins

• Bindin and other gamete recognition proteins are among the fastest evolving proteins known and may have diverged significantly, even when closely related urchin species have near-identity of every other protein.

Conclusion

• The recognition of the egg’s extracellular coat by a sea urchin sperm involves multiple species-specific binding events that occur at the levels of sperm attraction, activation, acrosome reaction, and sperm adhesion to the egg surface. Bindin plays a critical role in this process and is a fast evolving protein.