PART A
ZONA PELLUCIDA DOMAIN PROTEINS

A.1 NATURE OF THE ZONA PELLUCIDA DOMAIN

In 1992, Peer Bork and Chris Sander coined the phrase "zona pellucida domain" (ZPD) to define a structural element present in proteins of the zona pellucida (ZP), an extracellular coat that surrounds all mammalian eggs and also present in transforming growth factor type-III receptor and some other receptor-like proteins. The location of the ZPD in these proteins suggested to Bork and Sander that the domain might play a common biological role. The new family of ZPD proteins was defined by pattern-based sequence analysis and it was suggested that this type of domain has a common tertiary structure.

A ZPD consists of ?260 amino acids (aa) and has eight conserved Cys residues that participate in four intramolecular disulfides. The ZPD is composed of two sub-domains, referred to as ZP-N (?120 aa) and ZP-C (?130 aa), that are separated by a short protease sensitive region (Fig. A.1.1). Each sub-domain has four conserved Cys residues. However, the ZP-C sub-domain of some ZPD proteins may have additional Cys residues.

FIGURE A.1.1 Schematic representation of a ZPD. Each ZPD consists of ?260 aa and the ZP-N and ZP-C sub-domains are connected by a short protease-sensitive linker region.

Since its identification more than 20?years ago, a ZPD or a ZP-N sub-domain has been found in hundreds of proteins of diverse functions in a wide variety of organs (e.g., ovary, ear, kidney, heart, liver, brain, pancreas, uterus, etc.; Table C.12.1) and organisms (e.g., jellyfish, sea urchins, worms, mollusks, fruit flies, tunicates, fish, amphibians, reptiles, birds, and mammals; Table E.3). ZPD proteins are frequently glycosylated and often display a mosaic architecture since they can consist of a combination of different structural and functional modules (Tables C.12.2 and E.3). ZPD proteins can be secreted into the extracellular space or sometimes be anchored to the cell's plasma membrane by a glycosylphosphatidylinositol (GPI) linkage. ZPD proteins function as structural components of egg coats and other tissues, and as receptors, mechanical transducers, and antimicrobials. They can also play vital roles during differentiation, morphogenesis, and signaling. ZPD proteins are present at the apical surface of many epithelia and participate in the functioning of the senses, including taste and smell. Mutations in genes encoding ZPD proteins can result in severe human pathologies, including deafness, vascular disease, renal disease, cancer, and possibly infertility (Table C.12.3).

FURTHER READING

Bork P, Sander C. A large domain common to sperm receptors (Zp2 and Zp3) and TGF-beta type III receptor. FEBS Lett 300, 237-240 (1992).

Jovine L, Darie CC, Litscher ES, Wassarman PM. Zona pellucida domain proteins. Annu Rev Biochem 74, 83-114 (2005).

Monné M, Han L, Jovine L. Tracking down the ZP domain: from the mammalian zona pellucida to the molluscan vitelline envelope. Semin Reprod Med 24, 204-216 (2006).

Plaza S, Chanut-Delalande H, Fernandes I, Wassarman PM, Payre F. From A to Z: apical structures and zona pellucida-domain proteins. Trends Cell Biol 20, 524-532 (2010).

A.2 MOUSE ZP PROTEINS

Much of what is known today about ZPD proteins has its origins in early biochemical and molecular genetic studies of the mouse oocyte's ZP. A ZP surrounds all mammalian oocytes, ovulated eggs, and embryos up to the early blastocyst stage of development when embryos hatch from the ZP and implant in the uterus. In mice, the ZP first appears around growing oocytes during the final stages of oogenesis while oocytes are arrested in first meiotic prophase. The ZP increases in thickness as oocytes increase in diameter from ?12 to ?80?µm. The ZP of fully-grown oocytes is ?6?µm thick and contains ?3.5?ng of protein. Overall, the ZP is a very porous (e.g., permeable to antibodies and viruses) and relatively elastic matrix that is composed of long, interconnected fibrils (Fig. A.2.1).

FIGURE A.2.1 Light and electron micrographs of the mouse ZP. (a) Light micrograph of sperm bound to the mouse egg's ZP. Bar ?13?µm. (b) Scanning electron micrograph of the mouse egg's ZP. Bar ?200?nm. © Journal of Biological Chemistry.

Reproduced with permission from Wassarman (2008).

The mouse ZP is composed of three proteins, called mZP1-3. Together, mZP2 and 3 account for more than 80% of the mass of the ZP and are present in roughly equimolar amounts. mZP1 is the least abundant protein component of the ZP. A fourth ZP protein, mZP4, is missing from the ZP as it is encoded by a pseudogene (pseudogenes are dysfunctional relatives of genes that have lost their protein-coding ability or are no longer expressed). mZP1-3 are heterogeneously glycosylated with asparagine (N) and serine/threonine- (O-) linked oligosaccharides and the oligosaccharides are sialylated and sulfated making the proteins relatively acidic. mZP1, 2, and 3 possess 4, 6, and 5 N-linked oligosaccharides, respectively, and at least two O-linked oligosaccharides are present on mZP3. Under nonreducing conditions, mZP2 and 3 migrate on SDS-PAGE as ?120 and ?83?kD MW monomers, respectively, whereas mZP1 migrates as a ?200?kD MW disulfide-linked homodimer. mZP1 crosslinks individual fibrils that consist of mZP2 and 3 and thereby ensures the structural integrity of the ZP matrix. mZP2 and 3 serve as building blocks of ZP fibrils and also as sperm receptors during fertilization. Modification of both mZP2 and 3 following fertilization renders the ZP refractory to sperm binding.

mZP1-3 are prototypical ZPD proteins. Their nascent precursor polypeptides consist of an N-terminal signal sequence (SS), a ZPD, a C-terminal propeptide (CTP) that has a consensus furin cleavage site (CFCS), a transmembrane domain (TMD), and a cytoplasmic tail (CT). The SS is a ?25-30 aa peptide, almost always present at the N-terminus of the polypeptide, that directs proteins to the secretory pathway where the SS is removed. The CFCS is a short aa sequence, frequently R-X-X-R or R-X-R/K-R, that is recognized and cleaved by a member of the furin-like family of pro-protein convertases. The TMD is ?20 aa in length, consists primarily of hydrophobic aa, and represents a stable structure, either an a- or ß-helix when inserted in membrane.

The precursor polypeptide of mZP1 is ?69?kD MW and has a ZPD that is preceded by a trefoil (P) domain (a 45 aa sequence characterized by six Cys residues that form three intramolecular disulfides linked 1,5, 2,4, and 3,6) and a single extra ZP-N sub-domain (Fig. A.2.2). The precursor polypeptide of mZP2 is ?79?kD MW and has a ZPD that is preceded by three extra copies of the ZP-N sub-domain (Fig. A.2.2). The precursor polypeptide of mZP3 is ?47?kD MW, the smallest of the three mouse ZP proteins, and consists primarily of a single ZPD (Fig. A.2.2).

FIGURE A.2.2 Schematic representation of the organization of mZP1, 2 and 3. In each case, the polypeptide contains an SS at the N-terminus, a ZPD, and a CFCS, TMD, and CT at the CTP. mZP1 also has a trefoil (P) domain adjacent to the ZPD and an extra ZP-N sub-domain close to the N-terminus of the polypeptide. mZP2 has three extra ZP-N sub-domains between the ZPD and N-terminus of the polypeptide. mZP3, the smallest of the three proteins, consists primarily of a ZPD.

mZP1 polypeptide aa sequence and domain organization:

The polypeptide of ZP1 consists of 623 aa residues and has an SS (aa 1-20; highlighted), a trefoil domain (aa 225-266; italicized) with six Cys residues (aa 228, 237, 247, 252, 253, 262; capitalized and underlined), a ZPD (aa 271-540; highlighted) with 10 Cys residues (aa 272, 306, 325, 368, 449, 470, 522, 527, 535, 539; capitalized and underlined), followed by a CFCS (aa 545-548, RRRR; highlighted and underlined) and a TMD (aa 591-611; highlighted). Cys residues 272, 306, 325, and 368 are in the ZP-N sub-domain and Cys residues 449, 470, 522, 527, 535, and 539 are in the ZP-C sub-domain of the...