We have designed a genetic screen to identify human genes that alter iron homeostasis when expressed in yeast. Human genes for the iron-storage protein ferritin have been expressed in yeast, and although the proteins are stable and assemble into the proper 24-subunit oligomer, only a small amount of iron is deposited within the ferritin core. We hypothesize that cytosolic iron chaperones that facilitate iron loading into ferritin may be expressed in human cells, but not in yeast, which do not synthesize ferritin. We have isolated multiple human cDNAs which, when expressed in a yeast cell that contains human ferritin, increase the transfer of cytosolic iron into ferritin. Using siRNA in cultured human cell lines, we have also determined that depletion of poly-C binding protein 1 (PCBP1) results in a defect in ferritin iron loading. Co-immunoprecipitation experiments indicate that PCBP1 binds to ferritin in vivo, but only in the presence of reduced iron. PCBP1 binds ferrous iron in vitro with micromolar affinity and can donate iron to ferritin in vitro. We propose that PCBP1 can function in yeast and human cells as an iron chaperone, and enhance the loading of iron into ferritin. PCBP1 is an iron chaperone for ferritin, and we investigated whether other PCBP family members also act as ferritin iron chaperones. PCBP1 is a member of a family of four homologous RNA binding proteins42, and both PCBP1 and 2 are expressed at high levels in most cell types. Preliminary data indicate that PCBP2 also binds iron and can donate iron to ferritin in yeast. Current efforts indicate that additional iron-dependent enzymes require PCBPs for iron delivery and activation in vivo.
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