Structure of hepcidin-bound ferroportin reveals iron homeostatic mechanisms

Abstract

The serum iron level in humans is tightly controlled by the action of the hormone hepcidin on the iron efflux transporter ferroportin. Hepcidin regulates iron absorption and recycling by inducing ferroportin internalization and degradation 1 . Aberrant ferroportin activity can lead to diseases of iron overload, like hemochromatosis, or iron limitation anemias 2 . Here, we determined cryogenic electron microscopy (cryo-EM) structures of ferroportin in lipid nanodiscs, both in the apo state and in complex with cobalt, an iron mimetic, and hepcidin. These structures and accompanying molecular dynamics simulations identify two metal binding sites within the N- and C-domains of ferroportin. Hepcidin binds ferroportin in an outward-open conformation and completely occludes the iron efflux pathway to inhibit transport. The carboxy-terminus of hepcidin directly contacts the divalent metal in the ferroportin C-domain. We further show that hepcidin binding to ferroportin is coupled to iron binding, with an 80-fold increase in hepcidin affinity in the presence of iron. These results suggest a model for hepcidin regulation of ferroportin, where only iron loaded ferroportin molecules are targeted for degradation. More broadly, our structural and functional insights are likely to enable more targeted manipulation of the hepcidin-ferroportin axis in disorders of iron homeostasis.

Isolation and thermal stabilization of mouse ferroportin

“…In addition, we have made selective deletion constructs that exhibit improved thermal stability in solution and largely retain iron‐transport activity in vitro . During the review process of this manuscript, the structure of human Fpn was published [25]. Whilst this provides an important advancement, the detailed transport mechanism remains unresolved.…”

COVID-19 and iron dysregulation: distant sequence similarity between hepcidin and the novel coronavirus spike glycoprotein

“…These remain to be investigated experimentally, but of note, Rishi and colleagues recently reported on the intracellular localization of ferroportin dimers, and concluded that both the carboxy-and amino-termini of the protein are intracellular [109]. As cited earlier, the details of hepcidin's own interaction sites with ferroportin remain the subject of different structural determination projects [12,18,108]. Relatedly, and of interest, Neves and colleagues, using experiments on iron overload in European bass (Dicentrarchus labrax), have discussed the functional partnership between hepcidin and ferroportin from an evolutionary perspective and suggested that this may "open new possibilities for the pharmaceutical use of selected fish […] hepcidins during infections, with no impact on iron homeostasis" [90,91].…”

“…Finally, hepcidin binds to and mediates the degradation of ferroportin (encoded by the SLC40A1 / FPN1 gene), the only known cellular iron exporter. The structural details of this interaction are being mapped and studied in ever more detail [18,88,108].…”

“…Li points out that "the primordial form of coronavirus spikes might contain S2 only" [75], and the cytoplasmic motif features highlighted in the current report do not appear to be present in other class I viral membrane fusion proteins (which include the influenza virus hemagglutinin [20,66]), although we have not performed an exhaustive search. However, a number of questions might then arise under this scenario, such as the difference between the primary localizations of hepcidin (considering its putative interaction with extracellular and transmembrane regions of ferroportin [12,18,108]) versus the CT domain of the spike protein. Alternatively, it might be argued that perhaps a case of convergent sequence evolution is at play.…”

Iron Metabolism in the Tumor Microenvironment—Implications for Anti-Cancer Immune Response

“…FPN activity is decreased by the liver-derived hormone hepcidin. Recent studies highlight that hepcidin degrades FPN loaded with iron by binding to a specific cavity located between the N and C domains, thus blocking iron efflux and inhibiting its transport [ 69 ]. This results in an overall reduction of iron in the bloodstream [ 70 ].…”