Studi strutturali su ferroportina umana

Ferroportin is a polytopic membrane protein of 62.5 kDa that exports ferrous iron from specialized body cells into the bloodstream. To date, it is the only known iron exporter in mammals. At the systemic level, ferroportin is regulated with a negative post-translational mechanism operated by hepcidin, a peptide secreted by the liver in response to increased levels of body iron, which binds ferroportin causing its internalization and lysosomal degradation (Nemeth et al., 2004). Ferroportin is therefore an important regulator of intracellular and systemic iron. Mutations in the gene encoding ferroportin (SLC40A1) cause the iron overload disease type IV hereditary hemochromatosis (HH) or "ferroportin disease" characterized by autosomal dominant inheritance. Ferroportin disease usually presents as one of two different phenotypes. If the mutation results in a loss-of-function, patients display macrophage iron loading, high serum ferritin levels and normal to low transferrin iron saturation. In contrast to this classical phenotype, there are some gain-of-function mutations that do not affect the iron export ability of ferroportin, but result in a partial to complete resistance to hepcidin. Patients with mutations in this second category have a phenotype with features similar to those of the classical type of hemochromatosis. Specifically, the transferrin saturation is expected to be markedly elevated and iron accumulate mostly in parenchymal cells (Pietrangelo, 2006). The PhD work was focused on the study of ferroportin. To date, the ferroportin crystal structure has not yet been resolved and there is no information on the iron export mechanism. For the analysis of this complex membrane protein two different approaches were chosen: first, using bioinformatics techniques, a structural model of human ferroportin has been built and, on this basis, a mechanism of iron transport was hypothesized. The significance of the model was experimentally tested through iron export measurements in cells transfected with recombinant wild type and mutant ferroportin. In addition, a system for heterologous expression of ferroportin has been developed. The purpose was to produce enough protein, in a quantity higher than those so far obtained (Rice et al., 2009), which would provide the basis for the biochemical and structural characterization of ferroportin. Obviously, any attempt to disclose the three dimensional structure of ferroportin or the molecular mechanism of iron export, may be useful not only to explain the different pathological phenotypes associated to alterations of the transporter, but also to develop or improve the actual treatment of the ferroportin disease.