A NOVEL IRON UPTAKE MECHANISM MEDIATED BY GPI-ANCHORED HUMAN P97

The established process for iron uptake into mammalian cells involves transferrin and its receptor. Here, the role of the glycosyl-phosphati dylinositol (GPI)-linked transferrin homologue, melanotransferrin or p 97, was studied using CHO cell lines defective in the transferrin rece ptor (TR) and transfected with human TR and/or human p97. The presence of p97 doubled the iron uptake, which could be explained by the bindi ng of one atom of iron to one molecule of p97. The internalization of iron was shown to be temperature sensitive and saturated at a media ir on concentration of 2.5 mu g/ml with a V-max of 0.1 pmol Fe/10(6) cell /min and a K-m of 2.58 mu M for p97. Treatment of the cells with eithe r phosphatidylinositol-phospholipase C or monoclonal antibodies agains t p97 resulted in over a 50% reduction and a 47% increase in the iron uptake respectively, These data identify p97 as a unique cell surface GPI-anchored, iron binding protein involved in the transferrin-indepen dent uptake of iron in mammals.