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
protein