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

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Abstract
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.

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