Purification of a distinct placental lactogen receptor, a new member of the growth hormone/prolactin receptor family.
Recent findings from this laboratory suggest that the biological actions of placental lactogen (PL) in mammalian fetal tissues are mediated through binding of the hormone to a distinct and unique PL receptor. We have now purified this receptor from fetal and maternal sheep liver, characterized its binding to PL, growth hormone (GH), and prolactin (PRL), and determined its molecular weight by SDS-PAGE and by affinity cross-linking techniques. Soluble extracts containing specific, high-affinity (Kd 0.5 nM) PL binding activity were prepared by incubating ovine fetal and maternal liver microsomes with 1% Triton X-100. The detergent solubilized PL receptor was purified two- to threefold by ion-exchange chromatography and an additional twofold by gel exclusion chromatography on Sepharose 6B. The PL receptor was then purified 75,000- to 125,000-fold by affinity chromatography using a column of ovine PL (oPL) coupled to Affi-Gel 10. The molecular weight of the oPL receptor as determined by SDS-PAGE and by cross-linking techniques was 44,000 +/- 2,000 (range 40,000-48,000). The purified receptor bound 125I-oPL specifically and with high affinity (Kd 0.5 nM) but did not bind either radiolabeled ovine GH or ovine PRL. In addition, in competition studies using 125I-oPL as the radioligand, the purified PL receptor bound unlabeled oPL with a potency 30-50 times greater than that of ovine GH and 500-1,000 times greater than that of ovine PRL. These findings demonstrate the presence of a specific PL receptor in fetal and maternal sheep liver. The PL receptor, together with the GH and PRL receptors, constitute a family of distinct but related hormone receptors that differ in their relative affinities for PL, GH, and PRL. Changes in the expression of the three receptors may mediate changes in the hormonal control of growth during the transition from fetal to postnatal life.
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