DIFFERENTIAL BINDING OF GOLD-LABELED ZONA-PELLUCIDA GLYCOPROTEINS MZP2 AND MZP3 TO MOUSE SPERM MEMBRANE COMPARTMENTS

Egg zona pellucida glycoproteins mZP3 and mZP2 serve as primary and secondary sperm receptors, respectively, during initial stages of fertilization in mice [Wassarman (1988) A. Rev. Biochem. 57, 415-442]. These receptors interact with complementary egg-binding proteins (EBPs) located on the sperm surface to support species-specific gamete adhesion. Results of whole-mount autoradiographic experiments suggest that purified egg mZP3 and mZP2 bind preferentially to acrosome-intact (AI) and acrosome-reacted (AR) sperm heads, respectively [Bleil and Wassarman (1986) J. Cell Biol. 102, 1363-1371]. Here, we used purified egg mZP2, egg mZP3 and fetuin, which were coupled directly to colloidal gold ('gold-probes'), to examine binding of these glycoproteins to membrane compartments of AI and AR sperm by transmission electron microscopy. mZP3 gold-probes were found associated primarily with plasma membrane overlying the acrosomal and postacrosomal regions of Al sperm heads. They were also found associated with plasma membrane overlying the post-acrosomal region of AR sperm heads. mZP2 gold-probes were found associated primarily with inner acrosomal membrane of AR sperm heads, although some gold was associated with outer acrosomal membrane of AI sperm that had holes in plasma membrane overlying the acrosome. Fetuin gold-probes, used to assess background levels of binding, were bound at relatively, low levels to plasma membrane and inner acrosomal membrane of AI and AR sperm, respectively None of the gold-probes exhibited significant binding to sperm tails, or to red blood cells and residual bodies present in sperm preparations. These results provide further evidence that mZP2 and mZP3 bind preferentially to heads of AR and AI sperm, respectively. Furthermore, they demonstrate that mZP2 and mZP3 bind preferentially to different sperm membrane compartments and permit quantitative assessment of the distribution and abundance of EBPs on sperm.