Human ferroportin mediates proton-coupled active transport of iron.

As the sole iron exporter in humans, ferroportin controls systemic iron homeostasis through exporting iron into the blood plasma. The molecular mechanism of how ferroportin exports iron under various physiological settings remains unclear. Here we found that purified ferroportin incorporated into liposomes preferentially transports Fe2+ and exhibits lower affinities of transporting other divalent metal ions. The iron transport by ferroportin is facilitated by downhill proton gradients at the same direction. Human ferroportin is also capable of transporting protons, and this activity is tightly coupled to the iron transport. Remarkably, ferroportin can conduct active transport uphill against the iron gradient, with favorable charge potential providing the driving force. Targeted mutagenesis suggests that the iron translocation site is located at the pore region of human ferroportin. Together, our studies enhance the mechanistic understanding by which human ferroportin transports iron and suggest that a combination of electrochemical gradients regulates iron export.