Oligomerization of ferroportin and the mechanism of autosomal dominance in ferroportin disease

Ferroportin (Fpn)—the only known cellular iron‐export protein—plays a critical role in iron homeostasis by mediating iron export from macrophages and enterocytes. Rare, loss‐of‐function mutations in human Fpn (e.g. V160del) cause classical ferroportin disease, characterized by macrophage iron loading and iron‐restricted anemia. Ferroportin disease displays an autosomal–dominant inheritance pattern. Because all of the disease‐associated mutations are missense, the dominant–negative effect is most readily explained as resulting from oligomerization of Fpn protein. As a first step in testing this hypothesis, we have examined the functional activity of wildtype (wt) and mutant human Fpn co‐expressed in RNA‐injected Xenopus oocytes. We measured 55 Fe efflux from control oocytes, or oocytes expressing wtFpn or V160del GFP proteins with or without co‐expression of a wtFpn–mCherry fusion protein. The RNA dose eliciting half‐maximal iron‐export activity was 12.1 ± (SEM) 1.6 ng for wtFpn–GFP ( P = 0.005) and 11.7 ± 2.2 ng for wtFpn–mCherry ( P = 0.013), so we injected 12 ng RNA of each construct in subsequent co‐expression experiments. 55 Fe efflux activity resulting from co‐expression of wtFpn–GFP and wtFpn–mCherry constructs was additive (Fig 1). Whereas expression of V160del–GFP alone did not stimulate 55 Fe efflux, co‐expression of the mutant with wtFpn–mCherry stimulated 55 Fe efflux to roughly the same degree as either of the wildtype constructs alone. Our results therefore provide no evidence of a dominant–negative effect of V160del. Our next step will be to test co‐expression of wtFpn with other loss‐of‐function mutants. We will examine the oligomeric structure of wildtype and mutant Fpn by using freeze‐fracture electron microscopy to examine the interior ( p ) face of the inner lamella of the plasma membrane, as previously described for several membrane‐transport proteins [Eskandari S et al (1998) Proc Natl Acad Sci USA 95 , 11235–11240]. We anticipate that our study will further elucidate the structure–function of Fpn and help to explain the autosomal dominance of ferroportin disease. Support or Funding Information NIH–NIDDK grant R01 DK107309 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .