The zona pellucida 'receptors' ZP1, ZP2 and ZP3

The male component that is necessary for successful reproduction depends on a large variety of biological processes working in concert. The sperm-egg interaction occurs through complementary molecules and is an obligatory process for successful fertilization. However, this complex phenomenon and its molecular mechanisms remain to be fully understood. The oocyte is protected by the zona pellucida, a network of various proteins which encloses the oocyte. Depending on the species, the zona pellucida consists of different glycoproteins that are proposed to function as 'receptors' for spermatozoa. In the mouse, ZP1 is the homodimeric filament crosslinker, held together by intermolecular disulphides. ZP2 is the 'secondary receptor', which is cleaved by egg proteases after egg activation. The mouse ZP3 protein appears to be the 'primary receptor', which is responsible for species-specific binding of spermatozoa to the oocyte and the induction of the acrosome reaction. To localize zona pellucida protein and to evaluate the function of ZP2 and ZP3, polyclonal antisera were raised against synthetic ZP2 or ZP3 peptides which are specific for human or for mouse zona pellucida proteins. It could be demonstrated that anti-synthetic peptide antisera detected their respective zona pellucida proteins in immunoblots, ovary sections and native hemizonae pellucidae. Functional assays with anti-ZP3 synthetic peptide antibodies revealed that the antisera did not inhibit sperm-zona pellucida binding, whereas one of the antisera against synthetic ZP2 peptides significantly inhibited binding of spermatozoa to the zona pellucida.