A MAJOR GLYCOPROTEIN OF XENOPUS EGG VITELLINE ENVELOPE, GP41, IS A FROG HOMOLOG OF MAMMALIAN ZP3

A predominant glycoprotein in the vitelline envelope (VE) of the anura n Xenopus laevis is gp41, known to be proteolytically converted from g p43 of the coelomic egg envelope concomitant with the acquisition of e gg fertilizability, To characterize the protein core of gp41, purified gp41 from VE was digested with lysyl endopeptidase, and peptides isol ated from the digests were sequenced for amino acids to design degener ate primers for polymerase chain reaction. By reverse transcription-po lymerase chain reaction with a poly(A)(+) RNA from the ovary of an ovu lated female Xenopus, a specifically amplified band was obtained and s equenced. The upstream and downstream sequences of the sequenced regio n were completed by 5'- and 3'-rapid amplification of cDNA ends, respe ctively The cDN4, referred to as gp43 cDNA, comprises 1423 base pairs and contains one open reading frame with a sequence for 460 amino acid s. The predicted amino acid sequence of gp43 cDNA has a close similari ty with that of mammalian ZP3. Northern blot and in situ hybridization studies indicated that gp43 mRNA is expressed in oocytes, particularl y in the previtellogenic oocytes. A comparison of the N-terminal seque nces of gp41 and gp43 strongly suggested that gp41 is generated at lea st by processing of the N-terminal portion oi gp43 with oviductin.