The major chicken egg envelope protein ZP1 is different from ZPB and is synthesized in the liver

The extracellular matrix surrounding vertebrate oocytes is called the zona pellucida in mammals and perivitelline membrane (pvm) in birds. We have analyzed this structure in chicken follicles and laid eggs and have identified a 95-kDa component of the pvm, which, by protein sequencing, shows homology to mammalian zona pellucida proteins. Surprisingly, we could not detect this protein in ovarian granulosa cells or oocytes but instead found high levels in the liver of the laying hen. In contrast, it is absent in rooster liver but can be efficiently induced by estrogen treatment of the animal. An immunoscreen of a liver lambda-ZAP library yielded a cDNA coding for a protein of 934 amino acids. It displayed significant homology to members of the ZP1/ZPB family from other species, notably to mouse and rat ZP1, and was therefore designated chkZP1. It is clearly different from a protein designated chkZPB that had been deposited in the data base previously. Alignment of the known members of the ZP1/ZPB family demonstrated the existence of at least three subgroups, with representatives of both the ZP1 and the ZPB sequence homology group occurring in vertebrates. Northern blot analysis of liver extracts revealed the presence of a single 3. 2-kilobase mRNA coding for chkZP1, distinct from the chkZPB transcript detectable in follicles. Immunohistochemical analysis of follicle sections demonstrates that chkZP1 can be found in the blood vessels of the theca cell layer as well as in the pvm surrounding the oocyte. Thus, in the chicken, at least one of the major pvm components is synthesized in the liver and is transported via the bloodstream to the follicle.