A synthetic decapeptide from a conserved ZP3 protein domain induces the G protein-regulated acrosome reaction in bovine spermatozoa

In some animal species, the zona pellucida protein 3 (ZP3) plays a central role during fertilization, functioning as a specific receptor for sperm and as an inducer of the acrosome reaction. On the other hand, the zona pellucida protein 2 (ZP2) acts as a secondary receptor, binding to acrosome-reacted sperm. The objective of these studies was to identify ZP2 and ZP3 domains that may be of importance for the induction of the acrosome reaction. For this purpose, we synthesized a number of ZP2 and ZP3 peptides that were either conserved among species or that were species-specific according to their respective primary structures. We identified a defined, conserved ZP3 decapeptide (ZP3-6 peptide) that bound to the surface of the acrosomal region and induced the acrosome reaction in a concentration-dependent manner in capacitated bovine sperm; this effect was significant in the nanomolar range. Pertussis toxin inhibited the ZP3-6 peptide-induced acrosome reaction but had no effect on the progesterone-induced exocytotic event. Our data are in accordance with previous studies showing that progesterone induces acrosomal exocytosis via a different pathway than ZP3 and strengthen the hypothesis that the effect of ZP3-6 peptide upon acrosomal exocytosis is G protein regulated. Despite the commonly accepted idea that glycosylation of ZP proteins is required for successful sperm-oocyte interaction, we found that acrosomal exocytosis can be induced by a synthetic ZP3 peptide that is not glycosylated. The results presented in this study may be useful for the investigation of the molecular mechanisms of sperm-egg interaction in bovine and other species.