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 [ PUBMED:1313375 PUBMED:12021773 … expand

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 [ PUBMED:1313375 PUBMED:12021773 PUBMED:12878193 PUBMED:15079052 ]. Proteins containing a ZP domain include:

• Sperm receptor proteins ZP2 and ZP3. Along with protein ZP1, proteins ZP2 and ZP3 are responsible for sperm-adhesion to the zona pellucida. ZP3 first binds to specific sperm proteins, thus mediating sperm contacts with the oocyte. 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 rabbit and ZP-3 α in pig).
• Glycoprotein GP2, the major component of pancreatic secretory granule membranes.
• TGF-beta receptor type III (also known as betaglycan). This protein is a proteoglycan that binds to TGF-beta and could be involved in capturing and retaining TGF-beta for presentation to the signalling receptors.
• Uromodulin (also known as Tamm-Horsfall urinary glycoprotein). The function of this protein, which is the most abundant in human urine, is not yet clear.
• Chicken beta-tectorin, a major glycoprotein of avian tectorial membrane.

Most ZP domain proteins are synthesized as precursors with carboxy-terminal transmembrane domains or glycosyl phosphatidylinositol (GPI) anchors [ PUBMED:12021773 ].

The ZP domain contains eight strictly conserved cysteines, which form disulphide bridges. The disulphide bonds within the ZP domains are divided into two groups, suggesting that the ZP domain consists of two subdomains. In addition to the conserved cysteines, only a few aromatic or hydrophobic amino acids are absolutely invariant, probably as a result of structural rather than functional constraints [ PUBMED:1313375 PUBMED:12878193 PUBMED:15079052 ].