Egg extracellular coat proteins: From fish to mammals

Abstract

The extracellular coat surrounding fish (vitelline envelope; VE) and mammalian (zona pellucida; ZP) eggs is composed of long, interconnected filaments. Fish VE and mammalian ZP proteins that make up the filaments are highly conserved groups of proteins that are related to each other, as well as to their amphibian and avian egg counterparts. The rainbow trout (O. mykiss) egg VE is composed of 3 proteins, called VEa (~58 kDa), VEß (~54 kDa), and VEg (~47 kDa). The mouse (M. musculus) egg ZP also is composed of 3 proteins, called ZP1 (~200 kDa), ZP2 (~120 kDa), and ZP3 (~83 kDa). Overall, trout VE and mouse ZP proteins share ~25% sequence identity and have features in common; these include an N-terminal signal sequence, a ZP domain, a consensus furin cleavage-site, and a C-terminal tail. VEa, VEß, and ZP1 also have a trefoil or P-type domain upstream of the ZP domain. VEa and VEß are very similar in sequence (~65% sequence identity) and are related to ZP1 and ZP2, whereas VEg is related to ZP3 (~25% sequence identity). Mouse ZP proteins are synthesized and secreted exclusively by growing oocytes in the ovary. Trout VE proteins are synthesized by the liver under growing oocytes in the ovary. The trout VE is assembled from VEa/g and VEß/g heterodimers. The mouse ZP is assembled from ZP2/3 heterodimers and crosslinked by ZP1. Despite ~400 million years separating the appearance of trout and mice, and the change from external to internal fertilization and development, trout VE and mouse ZP proteins have many common structural features; as do avian and amphibian egg VE proteins. However, the site of synthesis of trout and mouse egg extracellular coat proteins has changed over time from the liver to the ovary, necessitating some changes in the C-terminal region of the polypeptides that regulates processing, secretion, and assembly of the proteins