In mammals,
natural-resistance-associated macrophage protein 1 (Nramp1) regulates macrophage activation and is associated with infectious and
autoimmune diseases. Nramp2 is associated with anaemia. Both belong to a highly conserved eukaryote/prokaryote
protein family. We used Xenopus oocytes to demonstrate that, like Nramp2, Nramp1 is a bivalent
cation (Fe2+, Zn2+ and Mn2+) transporter. Strikingly, however, where Nramp2 is a
symporter of H+ and
metal ions, Nramp1 is a highly pH-dependent
antiporter that fluxes
metal ions in either direction against a
proton gradient. At pH 9.0, oocytes injected with
cRNA from wild-type murine Nramp1 with a
glycine residue at position 169 (Nramp1(G169); P=3.22x10(-6)) and human NRAMP1 (P=3.87x10(-5)) showed significantly enhanced uptake of radiolabelled Zn2+ compared with water-injected controls. At pH 5.5, Nramp1(G169) (P=1.34x10(-13)) and NRAMP1 (P=1.09x10(-6)) oocytes showed significant efflux of Zn2+. Zn2+ transport was abolished when the
proton gradient was dissipated using
carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Using pre-acidified oocytes, currents of 130+/-57 nA were evoked by 100 microM Zn2+ at pH 7.5, and 139+/-47 nA by 100 microM Fe2+ at pH 7.0, in Nramp1(G169) oocytes; currents of 254+/-49 nA and 242+/-26 nA were evoked, respectively, in NRAMP1 oocytes. Steady-state currents evoked by increasing concentrations of Zn2+ were saturable, with apparent affinity constants of approx. 614 nM for Nramp1(G169) and approx. 562 nM for NRAMP1 oocytes, and a curvilinear voltage dependence of transporter activity (i.e. the data points approximate to a curve that approaches a linear asymptote). In the present study we propose a new model for
metal ion homoeostasis in macrophages. Under normal physiological conditions, Nramp2, localized to early endosomal membranes, delivers extracellularly acquired bivalent
cations into the cytosol. Nramp1, localized to late endosomal/lysosomal membranes, delivers bivalent
cations from the cytosol into this acidic compartment where they may directly affect antimicrobial activity.