Wilbon, A. S. | bioRxiv Science

bioRxiv2022

Mechanism of Ca2+ transport by ferroportin

Ferroportin (Fpn) is a transporter that releases Fe2+ from cells and is important for iron homeostasis in circulation. Export of Fe2+ by Fpn is coupled to import of H+ to maintain charge balance. Although Ca2+ was shown to modulate Fe2+ transport in Fpn, transport of Ca2+ by Fpn has not been demonstrated. Here we show that human Fpn (HsFpn) mediates Ca2+ transport, and that the Ca2+ transport does not rely on the transport of other ions. We determine the structure of Ca2+-bound HsFpn and identify a single Ca2+ binding site distinct from the Fe2+ binding sites. Further studies validate the Ca2+ binding site and show that Ca2+ transport is inhibited in the presence of Fe2+ but not vice versa. Function of Fpn as a Ca2+ uniporter in the absence of Fe2+ provides a molecular basis for regulations of iron homeostasis by Ca2+.

biochemistry↗

bioRxiv2022

Structural basis of ferroportin inhibition by minihepcidin PR73

Ferroportin (Fpn) is the only known iron exporter in humans and is essential for maintaining iron homeostasis. Fpn activity is suppressed by hepcidin, an endogenous peptide hormone, which inhibits iron export and promotes endocytosis of Fpn. Hepcidin deficiency leads to hemochromatosis and iron-loading anemia. Previous studies have shown that small peptides that mimic the first few residues of hepcidin, i.e. minihepcidins, are more potent than hepcidin. However, the mechanism of enhanced inhibition by minihepcidins remains unclear. Here, we report the structure of human ferroportin in complex with a minihepcidin, PR73 that mimics the first 9 residues of hepcidin, at 2.7 [A] overall resolution. The structure reveals novel interactions that were not present between Fpn and hepcidin. We validate PR73-Fpn interactions through binding and transport assays. These results provide insights into how minihepcidins increase inhibition potency and will guide future developments of Fpn inhibitors.

biochemistry↗