Characterization of human zona pellucida glycoproteins

The human egg may only be fertilized by one spermatozoon to prevent polyplo idy. In most mammals, the primary block to polyspermy occurs at the zona pe llucida (ZP), Little is known of the human ZP and the changes occurring fol lowing fertilization to prevent polyploidy. Using antibodies directed again st synthetic peptides predicted from the human ZP2 and ZP3 cDNA, we identif ied ZP3 as a 53-60 kDa glycoprotein and ZP2 as a 90-110 kDa glycoprotein in prophase-l oocytes. Characterization of the ZP from metaphase II arrested eggs (inseminated-unfertilized and fertilized-uncleaved), shows no visible modification of ZP3, but demonstrates that ZP2 undergoes limited proteolysi s in the amino terminal domain, to a 60-73 kDa species, denoted ZP2,, which remains linked to the proteolysed fragments by intramolecular disulphide b onds. A lack of ZP2 proteolytic activity in acrosomal supernatants is consi stent with an oocyte origin for the protease. The ZP2-specific protease may be released during cortical granule exocytosis which occurs during meiotic maturation and following sperm-egg fusion as part of the block to polysper my. Since mouse ZP2 acts as a secondary sperm receptor, it is possible that intact ZP2 binds a secondary egg binding protein, whereas cleaved ZP2 does not, suggesting a possible mechanism for the block to polyspermy.