Bovine placental lactogen: molecular cloning and protein structure

The bovine placenta secretes at least one hormone with prolactin-like and placental growth-hormone-like activity. The cDNA for bovine placental lactogen was isolated from a bovine fetal cDNA library by virtue of its nucleotide sequence homology to bovine prolactin (70%) and identified as such from amino acid sequence obtained from the amino terminus and internal tryptic fragments from the isolated hormone. The cDNA predicts a preprohormone of 236 amino acids, with a signal peptide of 36 amino acids. A single consensus site for N-glycosylation marks a probable site of carbohydrate addition. The encoded hormone is quite distinct from the pituitary hormones, as well as the primate and rodent placental lactogens and other predicted bovine placental hormones. It is 51% similar to bovine prolactin in amino acid sequence, 30% similar to the protein predicted by bovine prolactin-related cDNA I, about 30% similar to the rodent predicted placental hormones, and only about 20% similar to human placental lactogen and bovine growth hormone. Despite its greater similarity to bovine prolactin, sequence homology in the region of 5' flanking sequences and first exon to bovine prolactin-related cDNA I suggests that bovine placental lactogen may share a common evolutionary origin with this other placentally expressed member of the prolactin gene family.