The species-specific egg receptor for sea urchin sperm adhesion is EBR1, a novel ADAMTS protein

Species-specific adhesion of sperm to the egg during sea urchin fertilization involves the interaction of the sperm adhesive protein, bindin, and a complementary receptor on the egg surface, and serves to restrict the gene pool to individuals of the same species. We used PCR representation difference analysis to clone the species-specific egg receptor for bindin, EBR1, from Strongylocentrotus franciscanus (Sf) and S. purpuratus (Sp). Sf-EBR1 contains a novel ADAMTS-like N-terminal domain followed by similar to19 tandem EBR repeats consisting of alternating CUB and thrombospondin type 1 (TSP-1) domains where the last 10 EBR repeats are species-specific and highly conserved. Recombinant protein corresponding to the species-specific EBR repeat displays species-specific sperm adhesion and bindin-binding activity. The Sp-EBR1 ortholog has the same ADAMTS (a disintegrin and metalloprotease with thrombospondin type-1 modules) core region followed by eight and one-half tandem egg bindin receptor (EBR) repeats that share 88% identity with the Sf-EBR1 repeats, but has an entirely different species-specific domain consisting of hyalin-like (HYR) repeats. Thus, the species-specific domains of egg bindin receptor 1 (EBR1) from both species function as the egg surface receptor to mediate species-specific sperm adhesion.