Isolation of a novel prolactin-like cDNA clone from developing rat placenta.

The mid- to late-term rat placenta produces several moderately abundant proteins in a specific temporal manner, one of which we have identified as rat placental lactogen II (rPLII) (Duckworth, M. L., Kirk, K. L., and Friesen, H. G. (1986) J. Biol. Chem. 261, 10871-10878). In this paper, we describe the isolation and characterization of cDNA clones of another of these proteins, called rat prolactin-like protein A (rPLP-A) because of its homology with the prolactins. The single mRNA transcript corresponding to rPLP-A is 1 kilo-base in length and first appears at Day 14 of pregnancy, 2 days later than rPLII mRNA, and then increases and remains at high levels until term. The mRNA hybridizing to rPLP-A cDNA clones translates in vitro to a protein of 25,000 daltons which is processed by dog pancreatic microsomes to 27,000 daltons. The amino acid sequence deduced from the nucleotide sequence of the rPLP-A clones suggests that rPLP-A is a secreted protein of 196 amino acids with two potential glycosylation signals. The sequence is more than 40% homologous at the amino acid level to rat and human prolactins and rPLII. This study provides clear evidence for the existence of considerable amounts of undescribed prolactin-like proteins in late term rat placenta.