Trophoblast-specific processing and phosphorylation of pregnancy-associated glycoprotein-1 in day 15 to 25 sheep placenta.

Bovine and ovine pregnancy-associated glycoproteins-1 (PAG-1) are products of binucleate trophoblast cells and belong to the aspartic proteinase gene family. Estimates of their relative molecular masses have varied considerably, from 47 to 90 kDa, even though the mature polypeptide has been inferred to be no more than 330 amino acids in length and that the glycosylated recombinant form synthesized in Chinese hamster ovary (CHO) or COS-1 cells had an apparent mass of 46 kDa. To establish the relationships among the various molecular forms, metabolic labeling, immunoprecipitation, and electrophoretic analysis were used to follow the biosynthesis of ovine PAG-1 (ovPAG-1) in placental explants. In time-course studies, ovPAG-1 could first be detected within 10 min as a 70-kDa form within the tissue. With time, forms of intermediate (53-61 kDa) and low (47 kDa) molecular mass began to accumulate. The latter predominated in medium after 6 h labeling. Pulse chase studies established that the 70-kDa forms were the precursors of the smaller species. Inhibition of glycosylation with tunicamycin or treatment with N-glycosidase F confirmed that ovPAG-1 contained N-linked oligosaccharide chains, but that this carbohydrate accounted for only a relatively small fraction (8-10 kDa) of the apparent mass. Consecutive treatment with neuraminidase and O-glycanase also reduced the apparent molecular mass of the precursor by approximately 11 kDa. OvPAG-1 incorporated 32P from [32P]orthophosphate into phosphoserine and phosphothreonine, but there was no incorporation of 35S from [35S]sulfate. The basis of the differences in molecular mass between the precursor and the final products remains to be elucidated, but the differences seem likely to be due to some unusual form of posttranslational modification introduced in the binucleate cell. The results of the study appear to explain the disparate size values that have been reported for these placenta-derived proteins.