An Aspartic Proteinase Expressed in the Equine Placenta

Abstract

The pregnancy-associated glycoproteins (PAG) are a large and diverse group of proteins produced by the placenta of cattle, sheep, pigs, and presumably most other species within the Artiodactyla order.^1,2 PAG are members of the aspartic proteinase gene family having greatest sequence identity (~50%) with pepsinogens.¹ However, analysis of the putative amino acid sequence of several PAG cDNA revealed that most of these molecules are probably not proteinases due to mutations in key residues involved directly or indirectly in the catalytic mechanism.³ Neither native or recombinant PAG have detectable enzymatic activity in proteinase assays with ¹⁴C-hemoglobin as substrate. However, these changes within the active site do not interfere with their ability to bind peptide ligands since both native and recombinant PAG can be isolated by affinity chromatography with pepstatin agarose (JG, SX, RMR; unpublished).