Isolation and identification of porcine embryonic basic protein as a fragment of pregnancy-associated glycoprotein-2

Between days 11 and 12 of gestation, the porcine conceptus undergoes a metamorphosis from a spherical blastocyst to an elongate thread-like form. During this process, the conceptus secretes a variety of products. One of these products is protein previously referred to as porcine embryonic basic protein (BP). This protein has been shown to be a major secreted product between days 13 and 18. In this study, we report a simple two-step procedure to isolate BP from day 15 porcine conceptus conditioned medium, utilized ion-exchange chromatography and reverse-phase HPLC. Purified BP was subjected to Edman degradation amino-terminal sequencing and a 25 amino acid residue sequence was obtained. Comparing the N-terminal sequence of BP to sequences in the GenBank database determined that BP shared amino acid homology with porcine pregnancy-associated glycoprotein-2 (PAG-2). The region of identity corresponded to an internal site of PAG-2, suggesting BP was a proteolytic fragment of PAG-2. The purified protein was confirmed to be BP by Western blot using a previously characterized anti-BP antiserum. Also, the BP was immunolocalized with the trophectoderm of day 11 blastocysts. Staining intensity was diminished in spherical blastocysts compared to elongated blastocysts. Although the function of PAG-2 and its cleavage product BP are unknown, the large quantity produced by the porcine conceptus and its sequence conservation across species may indicate a necessary role in early pregnancy.