Cloning of rainbow trout egg envelope proteins: Members of a unique group of structural proteins

All vertebrate eggs are surrounded by an extracellular envelope that protec ts the egg and is vital for a successful fertilization. The terminology and functions of the egg envelope vary in different vertebrate groups, but the envelope itself is consistently composed of a few major proteins that are deposited around the oocyte during oocyte growth. Here, we describe the ded uced amino acid sequences and tissue expression patterns of the three major egg envelope proteins for rainbow trout (Oncorhynchus mykiss). All three v itelline envelope proteins (VEPs) are expressed in the livers of both male and female fish, with higher expression in females. In addition, VEP gamma mRNA is also detected in the female gonads. To our knowledge, this is the f irst time that expression of a VEP protein gene has been demonstrated to oc cur in more than one organ. Sequence comparison reveals that all three VEP proteins share distinct homology with their amphibian, avian, and mammalian counterparts. Whereas mammalian zona pellucida protein 3 isoforms contain two conserved serines needed for sperm binding, these are not conserved in teleost species, in which sperm entry is restricted to the micropyle. Besid es the difference in VEP gamma sperm-binding function, the high sequence ho mology suggests that the egg envelope proteins from these distinct vertebra te groups share a common ancestry and form a unique group of structural pro teins.