Ovine Trophoblast Protein 1, an Early Secreted Blastocyst Protein, Binds Specifically to Uterine Endometrium and Affects Protein Synthesis*

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Abstract
Ovine trophoblastic protein-1 (oTP-1), an early secretory protein of the sheep blastocyst, was purified after culturing day 14-16 conceptuses for 24 h in vitro. The localization of oTP-1 in the pregnant day 16 sheep uterus was determined immunocytochemically. The protein was associated with trophectoderm cells of the elongated blastocyst and with the surface and upper glandular epithelium of the maternal uterus. Receptors that bound oTP-1 with high affinity (Kd = .apprx. 2 .times. 10-10 M) were present in crude membrane preparations derived from homogenates of endometria from day 12 nonpregnant and anestrous ewes. Uterine infusion of 125I-labeled oTP-1 into day 12 nonpregnant ewes showed that the majority of the radioactivity was retained in the uterus, and only very small amounts of intact protein appeared to enter the maternal vasculature. There was no significant association with the corpora lutea, ovaries or other tissues tested. oTP-1 failed to compete with ovine PRL [prolactin] for rabbit mammary gland receptors or with hCG [human chroionic gonadtorpin] or bovine LH [luteinizing hormone] for sheep luteal cell receptors, and the oTP-1 did not stimulate progesterone production by dispersed luteal cells from day 12 cycling ewes. Incubation of endometrial explants from day 12 nonpregnant ewes with 5 .mu.g/ml oTP-1 resulted in increased rates of protein release into the medium. Two-dimensional polyacrylamide gel electrophoresis revealed that the synthesis of 6 polypeptides was stimulated selectively by the presence of oTP-1. Evidently, oTP-1 acts on the maternal endometrium. The interaction of oTP-1 with uterine endometrium may elicit maternal responses which contribute to the maintenance of pregnancy in the sheep.