De novo synthesis and secretion of prolactin-like protein-B by rat placental explants

Mid- to late-gestation rat placenta expresses three PRL-related mRNAs, rat placental lactogen-II (rPL-II), rat PRL-like protein-A (rPLP-A), and rat PRL-like protein-B (rPLP-B). The protein product of rPL-II mRNA has been characterized, and the protein products of the rPLP-A mRNA were recently identified. The mol wt of a nonsecreted nonglycosylated rPLP-B protein would be 27,145 based on the mRNA sequence. The present study is the first to report the identification of the rPLP-B protein. Antiserum was generated against a chemically synthesized oligopeptide inferred from a specific region of the rPLP-B cDNA. Three or four distinct proteins synthesized and secreted by rat basal zone explants (day 15 gestation) showed cross-reactivity with the rPLP-B antiserum. The relative mol wt of these immunoreactive proteins is approximately 30,000, with a pI varying from 6.1-6.6. De novo synthesized rPLP-B proteins were not secreted by the explant tissue in the presence of tunicamycin, suggesting that the proteins are glycosylated. These data are consistent with the presence of one potential N-glycosylation site derived from the rPLP-B mRNA sequence. The rPLP-B antiserum showed no cross-reactivity with proteins identified using antisera against rPLP-A, rPL-II, or human pregnancy-specific beta 1-glycoprotein. Immunocytochemical studies were carried out using paraffin sections from placentas of day 14 and 17 pregnant rats which were treated with anti-rPLP-B, followed by avidin-biotin-peroxidase complex. These experiments show perinuclear staining, which was localized in basophilic cytotrophoblast cells, confirming previous in situ mRNA hybridization studies. Although no physiological role has been established for rPLP-B, the synthesis and secretion of this protein by cells in contact only with maternal circulation suggest a hormonal role.