Functional analysis of iron transporter Nramp2 (SLC11A2) from Dictyostelium discoideum

Iron plays a leading role in many essential cellular functions, but it is also potentially toxic being able to generate reactive oxygen species (ROS) that can damage cell components. The social amoeba Dictyostelium discoideum is a model organism for studying cellular iron homeostasis having a subcellular localization of iron transporters (Nramps) that resembles that of macrophages. The ortholog of Nramp1 (SLC11A1) transporter is expressed in phago-lysosomes and is involved in resistance to bacterial infection mediating the efflux of iron and manganese from the phago-lysosomes to the cytosol; Nramp2 (SLC11A2) ortholog is localized in the contractile vacuole, that may have a role of reservoir of iron in a cell lacking ferritin. Nramp2 was expressed on the membrane of Xenopus laevis oocytes, therefore, the N- and C- termini of the protein were replaced with the corresponding regions of murine transporter DMT1. The cRNA was injected and classical radiochemical techniques and a fluorophore indicator (calcein) quenched by the entrance in the cytoplasm of divalent ions, were used for studying Nramp2 activity. Our experiments showed that Nramp2 iron transport is Na+-dependent and proton independent. It prefers ferrous iron and it can transports, Cd++, Co++, Mn++ and Zn++. Dictyostelium Nramp2, in synergy with Nramp1, appears to contribute to the regulation of the iron homeostasis of the cell allowing the efflux of the ion from the vacuole to the cytosol.