Profile of a mammalian sperm receptor gene

ZP3, a glycoprotein present in the extracellular coat (zona pellucida) of the unfertilized egg, serves as a receptor for sperm and an inducer of the acrosome reaction (sperm exocytosis) during fertilization in mice. As such, ZP3 regulates the initial species-specific interactions between male and female mouse gametes. Recently, the organization and expression of the gene encoding ZP3 have been studied in some detail. These studies have led to some important findings, including the entire primary structure of the glycoprotein's polypeptide chain and the sequence of more than 11 kilobases of DNA at the ZP3 genomic locus. The latter includes the entire transcription unit for ZP3, as well as 5' and 3' flanking sequences. Of particular interest is the finding that the ZP3 gene is expressed at extremely high levels by growing oocytes and by no other cell type in the mouse. This oocyte-specific expression occurs only at a particular stage of oogenesis. Although the specific regulatory elements responsible for the highly restricted expression of ZP3 have not been identified as yet, certain organizational features of the ZP3 gene that have been described may be relevant in this connection. Further molecular analyses of ZP3 will provide additional insight into its synthesis, structure, and functions, and could have practical consequences in the context of human conception and contraception.