Characterization of placental prolactin-like protein-A in intracellular and extracellular compartments

Prolactin-like protein-A (PLP-A) was purified from medium conditioned by junctional zone explants dissected from the rat chorioallantoic placenta via concanavalin A affinity chromatography, sodium dodecyl sulfate-gel electrophoresis and two-dimensional gel electrophoresis. Monomeric PLP-A was found to be heterogeneous and consisted of two major molecular sizes, 29 and 33 kDa. The charge and size heterogeneity attributed to monomeric PLP-A could be accounted for by the addition of N-linked carbohydrate moieties to the PLP-A structure. Monomeric PLP-A lacked lactogenic bioactivities and several attempts to sequence 29 and 33 kDa PLP-A monomers proved unsuccessful. Biochemical characterization of PLP-A species present in various intracellular and extracellular compartments indicated that PLP-A is normally secreted as high molecular weight complexes and that PLP-A species were also targeted to the nucleus. PLP-A species isolated from placental cytosol or from serum of pregnant rats predominantly circulated as disulfide-linked high molecular weight complexes. PLP-A antipeptide antisera showed limited but specific reactivity with the high molecular weight PLP-A species. We have specifically identified a 29 kDa protein species in placental cell nuclei with three antibodies directed to three different regions of the predicted amino acid sequence of PLP-A. The 29 kDa nuclear immunoreactive protein had an equivalent electrophoretic mobility to the 29 kDa PLP-A protein of junctional zone cytosol. The 33 kDa PLP-A protein was not identified in the nucleus. The biological significance of the circulating high molecular weight PLP-A species or the nuclear PLP-A species remains to be determined.