Molecular cloning and characterization of a new member of the rat placental prolactin (PRL) family, PRL-like protein D (PLP-D)

The rat placental PRL family consists of molecules structurally similar to PRL and GH, and to date, seven members have been identified. During investigation of pregnancy stage-specific placental factors by the differential display method, we obtained a complementary DNA (cDNA) fragment (199 bp) encoding a peptide homologous to PRL-like protein (PLP)-C. By using the 3' and 5' rapid amplification of cDNA ends method, a full-length cDNA was cloned and tentatively named PLP-D. The cDNA encoded a mature protein of 240 amino acids, including a 29-amino acid signal sequence. PLP-D contains one putative N-glycosylation site and six cysteine residues that are highly conserved in the placental PRL family. Sequence comparison between PLP-D and other members of the placental PRL family showed that PLP-D is highly homologous to PLP-C (80%) and decidual PRL-related protein (73%). Northern blot analysis revealed that PLP-D messenger RNA (mRNA) first appeared at day 14 of pregnancy, and that its expression increased until term. In situ hybridization analysis indicated that PLP-D mRNA was specifically expressed in spongiotrophoblast cells and trophoblast giant cells of the placental junctional zone. Differentiated Rcho-1 cells also expressed PLP-D mRNA, whereas undifferentiated Rcho-1 cells did not.