Genetic analysis of cytosolic iron toxicity in yeast

Iron is an essential nutrient for all eukaryotes and involved in many biological processes such as energy production, oxygen transport and DNA synthesis. Both iron deficiency and iron overload lead to human diseases. All organisms from yeast to humans have no regulated iron excretory pathway. Consequently, once iron enters cells it is detoxified by iron storage. In Saccharomyces cerevisiae, iron is stored in the vacuolar compartment mediated by the vacuolar iron transporter Ccc1. Cells with a deletion of CCC1 are sensitive to high concentrations of iron. To understand the nature and origin of iron toxicity, we employed genetic screens to identify suppressors of high iron toxicity in Deltaccc1 cells. Our genetic analysis identified genes that reduced iron toxicity by decreasing cytosolic iron through increased iron sequestration in intracellular organelles. We identified that mutations in Zrc1, a vacuolar zinc and cobalt transporter, resulted in the ability to transport iron into the vacuole. We took advantage of these gain-of-function mutations to define amino acids and structural features important for substrate selection in the Zrc1 family of cation diffusion facilitators. We also identified that overexpression of mitochondrial iron transporters Mrs3 or Mrs4 protected Deltaccc1 from high concentrations of iron by storing iron in mitochondria. Our genetic screen also identified Rim2 as a homologue of Mrs3 and Mrs4 and showed that Rim2 could also affect mitochondrial iron transport. Our studies identified novel forms of regulation of Mrs3/Mrs4 mediated iron transport. These genetic results suggest that iron induced damage occurs in the cytosol and iron sequestration in organelles can alleviate the toxic effect of high concentrations of iron.