SELECTIVE CLONING OF CDNA FOR SECRETORY PROTEINS OF EARLY EMBRYOS - IDENTIFICATION OF A TRANSIENTLY EXPRESSED KUNITZ DOMAIN PROTEIN FROM PREIMPLANTATION SHEEP TROPHOBLAST

The preimplantation ovine conceptus transiently secretes several prote ins, including a type I interferon (IFN-tau), that are likely involved in establishment of pregnancy. A method has been developed to identif y proteins produced simultaneously with IFN-tau. An antiserum against total ovine conceptus secretory proteins, from which IFN-tau and prote ins common to maternal uterine tract secretions had first been removed , was used to immunoscreen cDNA libraries created from mRNA of days 13 and 15 ovine conceptuses. This approach has allowed several unique cD NA to be identified, including one particularly abundant transcript fo r a novel member of the Kunitz family of serine protease inhibitors. T his cDNA encodes a 265-amino acid protein with a 20-amino acid signal sequence. A 64-amino acid Kunitz domain occupies the carboxyl terminus . It is preceded by two similar repeats of 84 residues that bear no ob vious similarity to any sequences present in the protein data banks. T he protein present in conceptus secretions (M(r) of similar to 14,000) represents only the carboxyl terminus of the molecule. The mRNA for t his putative proteinase inhibitor was confined to trophectoderm and wa s highly expressed for only a few days (similar to 13-18) of developme nt. A similar transcript was detected during the days 17-21 period in cattle embryos. Despite their high expression, no proteinase-inhibitor y activity can so far be ascribed to either the ovine or bovine protei ns. The P-1 residue, an asparagine, is not represented in any other kn own Kunitz inhibitors.