A progesterone-modulated, low-molecular-weight protein from the uterus of the sheep is associated with crystalline inclusion bodies in uterine epithelium and embryonic trophectoderm.

A protein of Mr 14,000 (14K protein) has been identified in the medium after culturing sheep conceptuses in vitro. However, it was also a component of uterine flushes of nonpregnant ewes during the luteal phase of the estrous cycle. The 14K protein, which consists of several isoforms, was purified from conceptus culture medium by a simple three-step procedure, involving ammonium sulfate precipitation, high-performance anion-exchange chromatography, and gel filtration on Sephacryl S-200. Western blotting procedures with an anti-14K antiserum showed that endometrium from ovariectomized ewes began to release the 14K protein after 6 to 14 days of progesterone treatment. Immunostaining of tissue from Day 16 pregnant ewes and from ewes treated with progesterone for 14 or 30 days showed the protein to be confined to the surface and upper glandular epithelium of the endometrium and to be absent from the deep glands. It was also present in trophectoderm of Day 16 conceptuses. Immunogold labeling in conjunction with electron microscopy revealed that within trophectoderm the 14K protein was localized to large, membrane-bound rhomboidal or needle-shaped crystal structures, and it seems likely that the protein was accumulated by the conceptus as a result of uptake of uterine histotroph. In the uterine epithelium, immunogold label was again most strongly concentrated over crystal-like structures but was also uniformly present over the cytoplasm and nucleoplasm. It was absent over mitochondria, over lipid droplets, and over non-epithelial types of endometrial cell. Such a cellular distribution for a secretory protein is novel, and it remains unclear whether the protein is synthesized by these cells.