The molecular basis of sperm-egg interaction in the sea urchin

In echinoderms the binding of sperm to eggs is a highly species-specific adhesive process. This adhesion is mediated by the interaction between bindin, a protein that coats the sperm acrosomal process, and a high Mr, carbohydrate-rich component of the egg surface. Previous results have shown that sperm binding is destroyed by treatment of the egg surface with proteases. Such treatment results in the release of a carbohydrate-rich fragment that can bind to sperm and inhibit their ability to fertilize eggs. Recent studies have focused on identifying and purifying the sperm receptor from the surface of the eggs of Strongylocentrotus purpuratus and Arbacia punctulata. Purified bindin has been used as a probe to purify an egg surface glycoprotein of very high Mr that can bind to bindin. When this component was added to acrosome-reacted sperm it species-specifically inhibited their ability to fertilize eggs. Antibodies (or Fab fragments of the antibodies) to the glycoprotein receptor coated the egg surface and species-specifically inhibited fertilization. Exhaustive proteolytic digestion of the receptor from S. purpuratus yielded a carbohydrate-rich fragment of high Mr that was highly sulphated. Similar proteolytic treatment of the receptor from A. punctulata yielded an active glycopeptide(s) of much lower Mr that was uncharged. The carbohydrate-rich fragments from both S. purpuratus and A. punctulata bind to acrosome-reacted sperm and inhibit fertilization in nanomolar concentrations. Unlike the respective intact receptors, the fragments do not act species-specifically. From these findings it is concluded that the intact receptor in these two species contains two domains. The carbohydrate-rich domain appears to serve as the adhesive element, whereas the protein component in some way controls accessibility to this adhesive element.