Maternal effect genes produce mRNA or proteins that accumulate in the egg during oogenesis. We show here that Mater, a mouse oocyte protein dependent on the maternal genome, is essential for embryonic development beyond the two-cell stage. Females lacking the maternal effect gene Mater are sterile. Null males are fertile.

We reported previously that Mater is a maternal effect gene that is required for early embryonic development beyond the two-cell stage in mice. Here we show the expressional profile of Mater and its protein during oogenesis and embryogenesis as well as its subcellular localization in oocytes. Mater mRNA was detectable earliest in oocytes of type 2 follicles, whereas MATER protein appeared earliest in oocytes of type 3a primary follicles. Both mRNA and protein accumulated during oocyte growth. In situ hybridization showed that Mater mRNA appeared progressively less abundant in oocytes beyond type 5a primary follicles. By ribonuclease protection assay, Mater mRNA was abundant in germinal vesicle oocytes, but was undetectable in all stages of preimplantation embryos. In contrast, the protein persisted throughout preimplantation development. Immunogold electron microscopic analysis revealed that MATER was located in oocyte mitochondria and nucleoli, and close to nuclear pores. Taken together, our data indicate that Mater gene transcription and protein translation are active during oogenesis, but appear inactive during early embryogenesis. Thus, Mater and its protein are expressed in a manner typical of maternal effect genes. The presence of MATER protein in mitochondria and nucleoli suggests that it may participate in both cytoplasmic and nuclear events during early development.

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...