Purification, secretion and immunocytochemical localization of the uterine milk proteins, major progesterone-induced proteins in uterine secretions of the sheep.

Restriction of the conceptus to one uterine horn of the pregnant ewe results in the accumulation of fluid called uterine milk (UTM) in the contralateral horn. Two basic polypeptides, called the uterine milk proteins (UTM-proteins; Mr = 55,000 and 57,000 as determined by polyacrylamide-gel electrophoresis using sodium dodecyl sulfate), accounted for the majority of the protein in uterine milk. The two UTM-proteins were glycoproteins and were readily purified from uterine fluids by cation-exchange chromatography on carboxymethyl (CM)-cellulose followed by Sephacryl S-200 gel-filtration. The purified UIM-proteins had a weight-average molecular weight of 50,700 +/- 4,200, as determined by equilibrium sedimentation analysis. Endometrial explants from pregnant ewes were cultured in the presence of radioactive amino acids and released UTM-proteins into the medium as their major secretory products. The UTM-proteins were secreted into the uterine lumen of nonpregnant, ovariectomized ewes given daily injections of progesterone. Estrone alone was ineffective in inducing UTM-protein production. Immunocytochemical studies indicated that synthesis of the UTM-proteins was confined to the surface and glandular epithelium of the uterus.