Egg zona pellucida, egg vitelline envelope, and related extracellular glycoproteins

Abstract

Abstract Vertebrate eggs are surrounded by an extracellular coat that performs vital functions during development, especially during fertilization. In mammalian and non-mammalian animals the egg coat is called a zona pellucida (ZP) and vitelline envelope (VE), respectively. The ZP of mammalian eggs, from mice to humans, and VE of fish, bird, and amphibian eggs, consists of only a few glycoproteins that are related to each other and often possess several recognizable elements (e.g., a signal sequence, ZP domain, consensus furin cleavage-site, and transmembrane domain). In most instances, ZP and VE glycoproteins are synthesized in the ovary by growing oocytes and surrounding follicle cells, but in some cases (e.g., in fish and birds) certain VE glycoproteins are synthesized in the liver and transported to the ovary. Many other families of extracellular proteins, found in vertebrates and invertebrates, share a conserved ZP domain (∼260 amino acids and 8 conserved Cys residues) with ZP and VE glycoproteins. The ZP domain enables these proteins to multimerize and perform their various functions, for example, as receptors and mechanotransducers. It is likely that the list of proteins containing a ZP domain will grow considerably in coming years and that our understanding of structural-functional relationships for such proteins will be clarified.