Isolation of the PEP8 (peptidase-deficient) mutant in yeast revealed 44% identity with the Hp58 gene in mouse. The defect in yeast was not lethal and overexpression of PEP8 did not affect cell viability. Analysis of the protein distribution showed immunolocalization to the yeast vacuole (equivalent of the mammalian lysosome). Domain swap experiments between the carboxyl terminus of PEP8 and Hp58 complemented the pep8 mutation, suggesting that Hp58 is associated with the lysosome, either as a hydrolytic enzyme or in the trafficking of proteins to and from the organelle. However, our immunolocalization studies show that rat Hp58 localizes to the nucleus as well as the cytoplasm, suggesting that the mammalian protein may perform additional or alternative functions to the yeast equivalent. This interpretation is consistent with the failure of chorioallantoic placental development and death in Hp58 null mice, while in Hp58 mutant yeast, there were no effects on viability. Further studies on the role of Hp58 in the placenta, and in particular its potential importance in regulating events necessary for erythropoiesis, are required to obtain additional insight into its specific function in chorioallantoic placental development.
In the current study, expression of Hp58 was similar when HRP/LRP placental cell lines were maintained under conditions that promote proliferation. The labyrinthine cell lines utilized in this study are postulated to contain tropho-blast stem cells capable of differentiating into syncytial tro-phoblast and labyrinthine trophoblast giant cells. Our results suggest that HRP/LRP cell lines may provide a valuable system for studying the steps involved in trophoblast differentiation and for identifying the factors that promote stem cell specification along particular cell lineage pathways. Taken together, the results obtained from this study indicate that erythroid stem cells express a protein that has sequence similarity to a yeast vacuolar protein. Further characterization of Hp58 and its function in fetal erythroid and placental cells will provide new insight into the regulatory networks that control hematopoietic development. Improved understanding about the regulation of circulatory development and hematopoiesis in the fetal placenta should lead to novel approaches to reduce perinatal mortality and intrauterine growth retardation.