Purification and properties of placental prolactin-related protein-I

We used sucrose density gradient centrifugation, size exclusion chromatography, and high-pressure reversed-phase chromatography in the purification of bovine prolactin-related protein-I (bPRP-I) to homogeneity from a secretory granule-enriched fraction of fetal cotyledon. Amino terminal sequence was unambiguous, consistent with the nucleic acid sequence of the cDNA 50 codons distal to the initial AUG in the open reading frame, and began with the residues: RKSFTDRFMNAASLSHDFY. This is distinct from the signal peptide cleavage site predicted by the algorithm of von Heijne (1986) as well as that expected by comparison with other members of the growth hormone/prolactin family of hormones. The level of bPRP-I in uterine fluid was sufficient to detect by Western blot of unfractionated material and estimated as at least 0.65 microM. In contrast, bPRP-I was undetectable in the serum by this method. Interaction of [125I]-bPRP-I with high molecular weight serum components interfered with its measurement by radio-immunoassay, and could be replicated with purified alpha 2-macroglobulin with an apparent KD of about 0.41 microM. Thus, the bPRP-I gene product is processed secreted and distributed in a manner consistent with a paracrine action at the materno-fetal interface.