Porcine pregnancy-associated glycoproteins: New members of the aspartic proteinase gene family expressed in trophectoderm

Abstract

Pregnancy-associated glycoproteins (PAG) are members of the aspartyl proteinase gene family that were initially identified in cattle (bPAG) and sheep (oPAG) as placenta-specific antigens in maternal blood. The objective of this study was to determine whether PAG a re expressed in pig trophoblast. A porcine conceptus cDNA library was screened with 32P-labeled ovine and bovine PAG cDNA. Of the ~104 plaques that were initially screened, a very high number (~5.3%) were positive. Two distinct types were identified, and full-length clones representing each type (1371 bp, pPAG1; 1378 bp, pPAG2) were fully sequenced in both directions. Their open reading frames coded polypeptides of 389 and 387 amino acids, respectively, including 15 amino acid signal peptides. Each had several potential sites for N-glycosylation. Both were members of the aspartic proteinase gone family, with ~50% amino acid sequence identity to porcine pepsinogen and 64% to each other. They were only distantly related to PAG of ruminant species (53% and 49% identity in amino acid sequence to oPAG1 and bPAG1, respectively). Interestingly, pPAG1 had amino acid substitutions within its catalytic center (Gly→Ala81, domain 1; Thr→Ser263, Thr→Ser265, Ser→Ala266, domain 2) that together were likely to render it enzymatically inactive, whereas pPAG2 retained sequences identical to pepsin in these regions. Western blotting of secretory products of porcine trophoblast with anti OPAG1 and anti-bPAG2 antisera indicated that pPAG, like PAG from ruminants, had an unexpectedly high M(r) (~70 000). Northern blot analysis revealed abundant mRNA for both pPAG (~1.7 kb) as early as Day 15, which persisted throughout pregnancy. In situ hybridization localized pPAG mRNA to chorionic cells tightly adherent to the maternal luminal epithelium within the deep folds of endometrium. In conclusion, PAG are not restricted to species with a synepitheliochorial placenta, and their function may not depend upon their potential for peptide bond cleavage.