| Nramp genes : roles in resistance to infection and in iron metabolism (1999) | |||||||||||||||||
Abstract | |||||||||||||||||
| The Nramp1 locus controls the innate resistance or susceptibility of mice to infection with a group of unrelated intracellular parasites which includes Salmonella, Leishmania, and Mycobacterium. Nramp1 is expressed exclusively in professional phagocytes and encodes an integral membrane protein that shares structural characteristics with ion channels and transporters. Its mechanism of action is not known. The intracellular localization of the Nramp1 protein was analyzed in control 129/sv and mutant Nramp1-/- macrophages. Nramp1 was localized to the late endosomal/lysosomal compartments. Immunofluorescence studies and direct purification of latex bead-containing phagosomes demonstrated that upon phagocytosis, Nramp1 is recruited to the membrane of the phagosome. cDNA clones corresponding to a second mouse Nramp gene, Nramp2, were isolated. Nucleotide and predicted amino acid sequence analyses of full length cDNA clones for Nramp2 indicate that this novel protein is closely related to Nramp1, and that the two genes form part of a small family. The two Nramp proteins share 63% identical residues and are predicted to have very similar secondary structures, including identical hydropathy profiles and predicted membrane organization. Analysis of the distribution of Nramp2 mRNA transcripts in normal mouse tissues by Northern blotting revealed that in contrast to the previously described macrophage-specific Nramp1 gene, Nramp2 mRNAs are expressed ubiquitously. Specific antiserum was generated and used to detect Nramp2 in a number of cell types. Nramp2 is expressed as a 90--100 kDa integral membrane protein that is extensively modified by glycosylation. Subcellular localization studies indicate distinct and non-overlapping localizations for Nramp1 and Nramp2. Nramp2 is expressed primarily in recycling endosomes and also to a lower extent at the plasma membrane, colocalizing with transferrin. Together with the concurrent reports that mutations in Nramp2 result in iron deficiency, these results support the hypothesis that Nramp2 plays a key role in the metabolism of transferrin-bound iron, transporting free Fe2+ across the endosomal membrane and into the cytoplasm. The identification of a divalent cation transport function for the Nramp family, together with the localization of Nramp1 to the phagosomal membrane suggests that Nramp1 may act by sequestering iron and other divalent cations away from microbes during infection. | |||||||||||||||||
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