Fertilization in the sea urchin involves species-specific interaction between the ligand bindin on the surface of acrosome-reacted sperm and a receptor of high molecular weight on the surface of the egg. Efforts to understand this interaction and the resultant signal transduction events leading to egg activation have been limited because of the large size and extreme insolubility of the intact receptor on the egg surface. Earlier work suggested that an alternative strategy would be to isolate proteolytic fragments of the extracellular domain of this receptor. Consequently, we have treated S. purpuratus eggs with a specific protease, lysylendoproteinase C. This enzyme treatment abolished the ability of eggs to bind sperm and resulted in the release of proteolytic fragments that bound to sperm and showed inhibitory activity in a fertilization bioassay. One of these fragments, presumed to be a fragment of the extracellular domain of the receptor, was purified to homogeneity by gel filtration and anion exchange chromatography and shown to be a 70-kD glycosylated protein. Several lines of evidence support the contention that this fragment is derived from the receptor. First, the fragment inhibited fertilization species specifically. Second, species specific binding of the 70-kD glycoprotein to acrosome-reacted sperm was directly demonstrated by using 125I-labeled receptor fragment. Third, the fragment exhibited the same species specificity in binding to isolated bindin particles. Species specificity was abolished by Pronase digestion of the fragment. This observation supports the hypothesis that although binding is mediated by the carbohydrate moieties, species specificity is dependent on the polypeptide backbone. The availability of a structurally defined fragment of the receptor will facilitate further studies of the molecular basis of gamete interaction.

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