A Third Prolactin-Like Protein Expressed by the Developing Rat Placenta: Complementary Deoxyribonucleic Acid Sequence and Partial Structure of the Gene

We have previously reported on the isolation and characterization of two PRL-related cDNA clones, rat placental lactogen II and rat PRL-like protein A which are among the more abundant mRNAs expressed int he late term rat placenta. In this paper we report the isolation and characterization of cDNA clones of a third abundant placental protein, the predicted amino acid sequence of which shows a 44% homology to rPRL. This protein which we have called rPRL-like protein B (rPLP-B) is different from, but related to, the other PRL related proteins which have been identified in the rat, mouse, and bovine placentas. Two rPLP-B mRNA transcripts of 0.9 and 1.2 kilobases are strongly expressed in essentially equal amounts from day 14 of pregnancy until term. Nucleotide sequence and hybrid select translation data predict one secreted, potentially glycosylated protein of approximately mol wt 27,000. Hybridization and primer extension studies show that the two transcripts differ in their 5'-untranslated regions. One of the cDNA clones isolated represents a portion of the unprocessed rPLP-B mRNA. All intron/exon boundaries in this clone are of the same splice class and occur in identical locations within the coding region as in the rPRL gene.