Functional properties of a prototypical member of the NRAMP superfamily of metal-ion transporters

Human NRAMP1 (SLC11A1) and DMT1 (SLC11A2) metal-ion transporters play nonredundant roles in host defense against infection and in iron homeostasis, respectively. Both are archetypical members of a superfamily extending to plants, yeasts and bacteria. The existence of prototypical and archetypical NRAMP homologs in simple eukaryotic organisms provides the opportunity to explore functional divergence and evolution in this family. We are characterizing a prototype NRAMP homolog (PppNRAMP) from an ancestral green moss, Physcomitrella patens. Disk-diffusion assays upon expression of PppNRAMP in E. coli revealed increased cytotoxicity in the presence of Co and Cd, but not Fe, consistent with bacterial 109Cd2+ uptake. In Xenopus oocytes expressing PppNRAMP, we observed at alkaline pHo large constitutive inward currents that were reversed in the presence of divalent metal ions, ranked Zn2+, Cd2+ (K0.5 ≈ 0.3 mM), Pb2+ > Ni2+, Co2+, Mn2+, Sn2+ >> Sr2+, Fe2+. Radiotracer assays revealed limited capacity for 109Cd2+ ...