Isolation and characterisation of a cDNA encoding a zona pellucida protein (ZPB) from the marsupial Trichosurus vulpecula (brushtail possum)

We have cloned a cDNA containing the entire coding sequence of a marsupial (the brushtail possum, Trichosurus vulpecula) zona pellucida protein (ZPB). The open reading frame of 1,581 nt is predicted to encode a ZPB polypeptide of 527 amino acids which contains 20 cysteine residues, 7 potential N‐linked glycosylation sites, a potential N‐terminal signal peptide and a potential C‐terminal trans‐membrane domain, preceded by a furin proteolytic processing signal. Sequence comparisons between possum ZPB and orthologous polypeptides from 7 eutherian species and from Xenopus laevis, reveal the existence of a high degree of sequence similarity, particularly in the central portion of the molecule. Cysteine residues are highly conserved, and all nine species possess potential N‐terminal signal peptide sequences and C‐terminal trans‐membrane domains of approximately the same length. In situ hybridisation revealed that expression of ZPB was restricted to oocytes of primordial and primary follicles of adult possums; no expression was detected in the surrounding granulosa cells. The broad conservation of ZPB sequence, structure and expression over a wide range of mammalian species, revealed by our studies, makes it unlikely that these features account for the different properties of the marsupial and eutherian zona pellucidae. Mol. Reprod. Dev. 52:174–182, 1999. © 1999 Wiley‐Liss, Inc.