A novel family of progesterone-induced, retinol-binding proteins from uterine secretions of the pig

Two-dimensional polyacrylamide gel electrophoresis has revealed the presence of a group of relatively acidic proteins of molecular weight about 22,000 in the uterine flushings of pseudopregnant pigs. The proteins have been purified by a combination of gel filtration chromatography and high performance anion-exchange chromatography and shown to bind both [3H] retinol and [3H]retinoic acid. At least four protein peaks that bound retinoids could be detected in the uterine secretions of a single pig. The ion-exchange procedure also allowed the retinol-free apoproteins to be separated from the holoforms that had associated ligand. Amino acid sequencing of the NH2 termini of polypeptides within three of the peaks revealed the presence of proteins with some degree of sequence identity to serum retinol-binding proteins (RBP). The most basic polypeptides showed the least similarity (about 30% identity), while the most acidic isoform analyzed shared about 70% sequence identity with the NH2 terminus of human serum RBP. Western blotting procedures employing an antiserum raised against the most basic isoforms showed that the amount of retinol-binding protein in uterine secretions increased markedly in ovariectomized animals in response to long term progesterone treatment. These proteins appear to form part of the uterine histotroph thought to be essential for nourishment of the conceptuses during pregnancy. A simple three-step procedure for purifying retinol-binding protein from pig serum is also described. The NH2-terminal sequence of this RBP is similar to that of human RBP but different from those of the uterine forms. The study suggests that a family of RBP, distinct from the serum form, is secreted by the uterine endometrium of the pig in response to progesterone.