Endocrinology, Vol 129, 2746-2756, Copyright © 1991 by Endocrine Society

ARTICLES

Molecular cloning and expression of rat placental lactogen-Iv, a variant of rPL-I present in late pregnant rat placenta

MC Robertson, IC Schroedter and HG Friesen
Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Canada.

Late pregnant rat placenta was found to contain a messenger RNA (mRNA) that encodes an additional member of the prolactin-growth hormone family. This polypeptide was detected by hybridization of complementary DNA (cDNA) for rPL-I to a 1 kilobase mRNA transcript in late pregnant rat placenta. A cDNA clone for the new polypeptide was isolated from a phage lambda-gt10 library containing cDNA synthesized from day 18 rat placental mRNA. Sequencing of the day 18 cDNA clone revealed that it was most closely related structurally to rPL-I and at the amino acid terminal was identical to the rPL-I variant (rPL-Iv). The sequence of rPL-Iv (223 amino acids) is 85% homologous to rPL-I (230 amino acid). There are 71 base pair changes (85% nucleotide homology) between rPL-Iv and rPL-I which are not confined to any specific region of the molecule and which result in 36 amino acid changes. In vitro translation of rPL- Iv mRNA produced by transcription of the cDNA template yielded a 26 kilodalton polypeptide, the size of the expected precursor protein. In situ hybridization studies indicated that mRNA for rPL-Iv was present primarily in cytotrophoblasts of the basal zone as early as day 15 of gestation with some hybridization in a few giant cells as well.