The iron transport protein NRAMP2 is an integral membrane glycoprotein that colocalizes with transferrin in recycling endosomes

The natural resistance associated macrophage protein (Nramp) gene family is composed of two members in mammals, Nramp1 and Nramp2. Nramp1 is expressed primarily in macrophages and mutations at this locus cause susceptibility to infectious diseases. Nramp2 has a much broader range of tissue expressio n and mutations at Nramp2 result in iron deficiency, indicating a role for Nramp2 in iron metabolism. To get further insight into the function and mec hanism of action of Nramp proteins, we have generated isoform specific anti -Nramp1 and anti-Nramp2 antisera. Immunoblotting experiments indicate that Nramp2 is present in a number of cell types, including hemopoietic precurso rs, and is coexpressed with Nramp1 in primary macrophages and macrophage ce ll lines. Nramp2 is expressed as a 90-100-kD integral membrane protein exte nsively modified by glycosylation (>40% of molecular mass). Subcellular loc alization studies by immunofluorescence and confocal microscopy indicate di stinct and nonoverlapping localization for Nramp1 and Nramp2. Nramp1 is exp ressed in the lysosomal compartment, whereas Nramp2 is not detectable in th e lysosomes but is expressed primarily in recycling endosomes and also, to a lower extent, at the plasma membrane, colocalizing with transferrin. Thes e findings suggest that Nramp2 plays a key role in the metabolism of transf errin-bound iron by transporting free Fe2+ across the endosomal membrane an d into the cytoplasm.