Identification of Proteins Immunochemically Related to Human Pregnancy-Specific β1-Glycoprotein in the Rat Placenta*

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Abstract
Human pregnancy-specific .beta.1-glycoprotein (hPS.beta.G) consists of a set of glycoproteins present in placenta and maternal serum. This study characterized proteins in rat placenta that show immunological cross-reactivity with antisera to hPS.beta.G. Immunocytochemical studies using two independent preparations of anti-hPS.beta.G showed intense specific staining within basophilic cytotrophoblast cells of the basal zone of the gestation day 15 rat placenta. In contrast, basophilic cytotrophoblasts located in the labyrinth did not stain. Subsequent experiments used gel electrophoresis and immunoblot analysis to compare PS.beta.G in human placenta and serum with immunoreactive proteins in rat placenta and serum. A set of two or three proteins was detected in human villous tissue and pregnancy serum with apparent mol wt (Mr) ranging from 54,000-76,000. In contrast rat placenta showed a major immunoreactive protein with 120,000 Mr, while rat serum contained bands of 48,000 64,000 and 69,000 Mr. Explant cultures of rat basal zone tissue secreted two [35S]methionine-labeled proteins that were immunoreactive, a major 120,000 Mr species and a minor 76,000 Mr form, with pI values of 4.6-5.5; tunicamycin inhibited the secretion of both species. Thus, a 120,000 Mr glycoprotein appears to be the major tissue and secreted form of rat PS.beta.G analog in day 15 placenta. Finally, the cytochemical localization of PS.beta.G-like proteins in rat placenta showed a progressive gestational shift from giant trophoblast cells in the parietal yolk sac placenta on day 12 to the basal zone cytotrophoblast cells by day 15. Data indicate that the pregnant rat may provide an animal model for investigation of the biological function of PS.beta.G during late gestation.
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