Sea Urchin Egg Receptor for Sperm: Sequence Similarity of Binding Domain and hsp70

Fertilization depends on cell surface recognition proteins that interact and thereby mediate binding and subsequent fusion of the sperm and egg. Overlapping complementary DNA's encoding the egg plasma membrane receptor for sperm from the sea urchin Strongylocentrotus purpuratus were cloned and sequenced. Analysis of the deduced primary structure suggests that the receptor is a transmembrane protein with a short cytoplasmic domain. This domain showed no sequence similarity to known protein sequences. In contrast, the extracellular, sperm binding domain of the receptor did show sequence similarity to the heat shock protein 70 (hsp70) family of proteins. Recombinant protein representing this portion of the receptor bound to the sperm protein, bindin, and also inhibited fertilization in a species-specific manner; beads coated with the protein became specifically bound to acrosome-reacted sperm. These data provide a basis for detailed investigations of molecular interactions that occur in gamete recognition and egg activation.