A 42-kDa glycoprotein from chicken egg-envelope, an avian homolog of the ZPC family glycoproteins in mammalian zona pellucida

A glycoprotein with molecular mass of 42 kDa was identified as the major component of the chicken egg-envelope, the filamentous, extracellular matrix known as the perivitelline layer. By using a DNA probe amplified with degenerative primers derived from the protein’s partial amino acid sequences, a cDNA clone encoding the egg-envelope 42-kDa glycoprotein (gp42) was isolated from a hen’s ovary cDNA library. The gp42 open reading frame encoded 435 amino acid residues, including a putative signal peptide of 20 amino acids. The deduced amino acid sequence of gp42 showed significant similarity to egg-envelope glycoproteins of the ZPC family of several other vertebrate species, including human ZP3, mouse ZP3, Xenopus laevis gp43 and medaka (Oryzias latipes) ZI3 (LS-F), which play important roles for sperm–egg interaction. A single N-glycosylation site present in chicken gp42 is conserved among all five of these proteins: carbohydrate analysis of gp42 revealed the presence of a complex type glycan chain at this site. N-terminal sequence analysis of the mature polypeptide suggests that C-terminal processing of the pro-protein occurs during synthesis and secretion. The 1.4-kb gp42 transcript was detected only in follicles, and was found to be accumulated in granulosa cells in a manner dependent on ovarian follicular development. Furthermore, a metabolically radio-labeled gp42 was immunopreciptated from both cell lysate and culture supernatant of the granulosa cells with specific anti-gp42 antibody, suggesting granulosa cell-specific synthesis and secretion of the glycoprotein.