Porcine zona pellucida glycoprotein ZP4 is responsible for the sperm-binding activity of the ZP3/ZP4 complex.

The zona pellucida (ZP) is a transparent envelope that surrounds the mammalian oocyte and mediates species-selective sperm-egg interactions. Porcine and bovine ZPs consist of glycoproteins ZP2, ZP3, and ZP4. In both pig and bovine a heterocomplex consisting of ZP3 and ZP4 binds to sperm, however it is not clarified whether ZP3 or ZP4 in the complex is responsible for the sperm binding. Previously, we have established a baculovirus-Sf9 cell expression system for porcine ZP glycoproteins. A mixture of recombinant ZP3 (rZP3) and rZP4 displayed sperm-binding activity toward bovine sperm but not porcine sperm, probably due to differences in carbohydrate structure between the native and recombinant ZP glycoproteins. In this study, a mixture of porcine rZP3 and native ZP4 (nZP4) inhibited the binding of porcine sperm to the ZP. In contrast, a mixture of porcine nZP3 and rZP4 did not inhibit the binding of porcine sperm, although the mixture inhibited the binding of bovine sperm. The porcine rZP3/nZP4 mixture bound to the acrosomal region of porcine sperm, in a manner similar to that of the nZP3/nZP4 mixture. nZP3 was precipitated with rZP4, and nZP4 was precipitated with rZP3 by utilising the N-terminal tags on the recombinant proteins. These results indicated that nZP4, but not rZP4, is necessary for binding activity of porcine ZP3/ZP4 complex towards porcine sperm and further suggested that the carbohydrate structures of ZP4 in the porcine ZP3/ZP4 complex are responsible for porcine sperm-binding activity of the complex.