Conserved Furin Cleavage Site Not Essential for Secretion and Integration of ZP3 into the Extracellular Egg Coat of Transgenic Mice

Abstract

The extracellular zona pellucida surrounding mammalian eggs is formed by interactions of the ZP1, ZP2, and ZP3 glycoproteins. Female mice lacking ZP2 or ZP3 do not form a stable zona matrix and are sterile. The three zona proteins are synthesized in growing oocytes and secreted prior to incorporation into the zona pellucida. A well-conserved furin site upstream of a transmembrane domain near the carboxyl terminus of each has been implicated in the release of the zona ectodomains from oocytes. However, mutation of the furin site (RNRR 3 ANAA) does not affect the intracellular trafficking or secretion of an enhanced green fluorescent protein (EGFP)-ZP3 fusion protein in heterologous somatic cells. After transient expression in growing oocytes, normal EGFP-ZP3 and mutant EGFP-ZP3 associate with the inner aspect of the zona pellucida, which is distinct from the plasma membrane. These in vitro results are confirmed in transgenic mice expressing EGFP-ZP3 with or without the mutant furin site. In each case, EGFP-ZP3 is incorporated throughout the width of the zona pellucida and the transgenic mice are fertile. These results indicate that the zona matrix accrues from the inside out and, unexpectedly, suggest that cleavage at the furin site is not required for formation of the extracellular zona pellucida surrounding mouse eggs.Extracellular matrices provide assorted roles in biology, and the mechanisms of their formation appear quite disparate. The matrix that surrounds vertebrate eggs and early embryos is variously known as the vitelline envelope, the perivitelline membrane, or the zona pellucida. The component parts are most commonly synthesized and secreted from female germ cells, but in some vertebrates they are produced in the somatic compartment of the ovary or in the liver, which requires both transport to the egg and assembly on its surface. Although the molecular biology of the mouse egg coat has been well studied, little is known about the intracellular protein trafficking of individual components or the mechanisms by which they are secreted to form the insoluble extracellular matrix required for fertilization and embryonic development.The mouse zona pellucida consists of three major sulfated glycoproteins (ZP1, ZP2, and ZP3), each of which is encoded by a single-copy gene in the mouse genome. Although the primary structures of ZP1 (623 amino acids, 68 kDa), ZP2 (713 amino acids, 80 kDa), and ZP3 (424 amino acids, 46 kDa) are distinct, they share certain motifs among themselves, as well as with homologues in other mammals (29). Each zona protein has an N-terminal signal peptide, a signature zona box (260 amino acids with eight conserved cysteine residues, (3), and a transmembrane domain near its carboxyl terminus. The conservation of zona proteins among mammals suggests that the three-dimensional structures of the proteins in each class of zona proteins are similar and that the interactions among classes that effect the supramolecular structure of the zona pellucida may be preserved as well. This hypothesis ha...

Zona pellucida glycoprotein ZP3 and fertilization in mammals

“…To determine whether the portion of mZP3 encoded by exon-8 was present in secreted EC-IgG(Fc)/mZP3(7), immunoblots were also stained with anti-mZP3 specific for mZP3(8) (polyclonal rabbit antibody against an mZP3 peptide encompassing amino acids 369-388 encoded by exon-8). As reported earlier (Litscher et al, 1999;Qi et al, 2002;Zhao et al, 2002), polypeptide encoded by exon-8 of mZP3 is removed by proteolysis during secretion of nascent native mZP3. Similarly, we did not detect any immunopositive stain for the C-terminal portion of mZP3, suggesting that the same is true for EC-IgG(Fc)/mZP3(7).…”

Insights into the molecular basis of sperm–egg recognition in mammals

“…ANAA or ! RNGE) does not preclude secretion or incorporation into the zona pellucida even in transgenic mice suggests that alternative cleavage sites are available (Kiefer & Saling 2002, Zhao et al 2002.…”

“…The mechanisms by which the glycosylated ectodomains of the zona proteins are released from the anchoring transmembrane domain remain to be determined. Although the mouse zona proteins contain a proprotein convertase (RX(K/R)R) site N-terminal to their transmembrane domains, they remain associated with the plasma membrane (Rankin et al 1996, Zhao et al 2002 despite their obligatory passage through the trans-Golgi network wherein resides the endoprotease (Thomas 2002). The C-termini of mouse ZP1, ZP2 and ZP3 isolated from native zonae pellucidae end immediately upstream of a dibasic motif that is part of, but distinct from, the convertase cleavage site .…”

Structural Characterization of Native Mouse Zona Pellucida Proteins Using Mass Spectrometry

“…A conserved hydrophobic patch upstream of the transmembrane domain is required for progression to the cell surface 2 and a consensus cleavage site (RX(K/R)R2) for the proprotein convertase furin is present upstream of the transmembrane domain. Although this site has been implicated in the release of the zona ectodomain (13)(14)(15), mutations (RNRR3 ANAA, or RNRR3 ANGE), do not prevent incorporation of reporter-ZP3 proteins into the zona pellucida in growing oocytes (12,16) or transgenic mice (12) and secretion of recombinant human ZP3 with a similar mutation (RNRR3 ANAA) is not prevented (17).…”

“…During oocyte growth, ZP1, ZP2, and ZP3 traffick through the growing oocyte, and their ectodomains are released from a transmembrane domain at the surface of the cell (11,12). A conserved hydrophobic patch upstream of the transmembrane domain is required for progression to the cell surface 2 and a consensus cleavage site (RX(K/R)R2) for the proprotein convertase furin is present upstream of the transmembrane domain.…”