At fertilization, mammalian sperm binds to the extracellular zona pellucida that surrounds ovulated eggs. The zona pellucida is composed of three to four glycoproteins, ZP1, ZP2, ZP3, and ZP4 []. Fish eggs are also surrounded by an envelope, which consists of a few proteins homologous to mammalian ZP1, ZP3, and ZP4 [, ]. Glycoprotein ZP3 is essential for normal fertility []. It promotes fertilization by recognizing sperm binding and activating the acrosome reaction [, ]. It is also required for germinal vesicle breakdown in oocyte meiosis [].
The zona pellucida (ZP) domain is a protein polymerisation module of ~260amino acid module, which is found at the C terminus of many secretedeukaryotic glycoproteins that play fundamental roles in development, hearing,immunity, and cancer [, , , ].Proteins containing a ZP domain include:Sperm receptor proteins ZP2 and ZP3. Along with protein ZP1, proteins ZP2and ZP3 are responsible for sperm-adhesion to the zona pellucida. ZP3 firstbinds to specific sperm proteins, thus mediating sperm contacts with theoocyte. ZP2 acts as a second sperm receptor reinforcing the interactions.ZP1 cross-links the polymers formed by ZP2 and ZP3.Zona pellucida sperm-binding protein B (ZP-B) (also known as ZP-X in rabbitand ZP-3 alpha in pig).Glycoprotein GP2, the major component of pancreatic secretory granulemembranes.TGF-beta receptor type III (also known as betaglycan). This protein is aproteoglycan that binds to TGF-beta and could be involved in capturing andretaining TGF-beta for presentation to the signalling receptors.Uromodulin (also known as Tamm-Horsfall urinary glycoprotein). The functionof this protein, which is the most abundant in human urine, is not yetclear.Chicken beta-tectorin, a major glycoprotein of avian tectorial membrane.Most ZP domain proteins are synthesized as precursors with carboxy-terminaltransmembrane domains or glycosyl phosphatidylinositol (GPI) anchors [].The ZP domain contains eight strictly conserved cysteines, which formdisulphide bridges. The disulphide bonds within the ZP domains are divided intotwo groups, suggesting that the ZP domain consists of two subdomains. Inaddition to the conserved cysteines, only a few aromatic or hydrophobic aminoacids are absolutely invariant, probably as a result of structural rather thanfunctional constraints [, , ].
The zona pellucida (ZP) domain is a protein polymerisation module of ~260 amino acid module, which is found at the C terminus of many secreted eukaryotic glycoproteins that play fundamental roles in development, hearing, immunity, and cancer [, , , ].The ZP module is not only conserved in egg coat components but is also found in many other secreted eukaryotic proteins with variable architecture and biological function. It is responsible for the incorporation of ZP3 and other subunits into the ZP and consists of two domains, ZP-N and ZP-C, that are separated by a protease-sensitive linker. Whereas ZP-N is thought to constitute a basic building block of ZP filaments, ZP-C may mediate the specificity of interaction between subunits. These processes are controlled by an external hydrophobic patch (EHP) contained within the C-terminal propeptide of ZP component precursors and an internal hydrophobic patch (IHP) inside the ZP module.This superfamily represents the ZP-C domain.
The zona pellucida (ZP) domain is a protein polymerisation module of ~260amino acid module, which is found at the C terminus of many secretedeukaryotic glycoproteins that play fundamental roles in development, hearing,immunity, and cancer [, , , ].Proteins containing a ZP domain include:Sperm receptor proteins ZP2 and ZP3. Along with protein ZP1, proteins ZP2and ZP3 are responsible for sperm-adhesion to the zona pellucida. ZP3 firstbinds to specific sperm proteins, thus mediating sperm contacts with theoocyte. ZP2 acts as a second sperm receptor reinforcing the interactions.ZP1 cross-links the polymers formed by ZP2 and ZP3.Zona pellucida sperm-binding protein B (ZP-B) (also known as ZP-X in rabbitand ZP-3 alpha in pig).Glycoprotein GP2, the major component of pancreatic secretory granulemembranes.TGF-beta receptor type III (also known as betaglycan). This protein is aproteoglycan that binds to TGF-beta and could be involved in capturing andretaining TGF-beta for presentation to the signalling receptors.Uromodulin (also known as Tamm-Horsfall urinary glycoprotein). The functionof this protein, which is the most abundant in human urine, is not yetclear.Chicken beta-tectorin, a major glycoprotein of avian tectorial membrane.Most ZP domain proteins are synthesized as precursors with carboxy-terminaltransmembrane domains or glycosyl phosphatidylinositol (GPI) anchors [].The ZP domain contains eight strictly conserved cysteines, which formdisulphide bridges. The disulphide bonds within the ZP domainsare divided intotwo groups, suggesting that the ZP domain consists of two subdomains. Inaddition to the conserved cysteines, only a few aromatic or hydrophobic aminoacids are absolutely invariant, probably as a result of structural rather thanfunctional constraints [, , ].This entry represents a conserved site found within the zona pellucida domain, and includes 2 conserved Cys residues.
