Characterization of yeast genes involved in intracellular iron homeostatis

Iron is an essential element for all living organisms. It is crucial in oxygen transport, electron transfer, and enzyme activities. The process of iron absorption and circulation in the human body is highly organized and regulated. Many genetic disorders cause abnormal iron metabolism and threaten human life. In order to understand iron metabolism, baker's yeast Saccharomyces cerevisiae has been used as a research system. Evidence indicates that the mechanisms of iron metabolism are well conserved between yeast and human. For example, the yeast gene FET3 is homologous to human ceruloplasmin, yeast CCC2 is homologous to genes in Wilson and Menkes diseases, and the GEF1 of yeast is homologous to the human CLC5 gene in kidney disease. Although the plasma membrane iron transport system in Saccharomyces crevisiae has been well defined, little is known about intracellular iron metabolism. We first identified and characterized ERG25p, an iron containing enzyme involved in sterol synthesis. The enzyme has four putative histidine-iron-binding motifs with similarity to desaturase-hydroxylases and is located in the endoplasmic reticulum. A human homologue of ERG25 was cloned and mapped to chromosome 4q32-34. Mutant erg25 has a growth defect in low iron conditions. By screening multicopy suppressor genes to mutant erg25 in iron limited medium, MFTl and MFT2 were identified. They are mitochondrial membrane proteins and belong to a large family of metal transporters. Overexpression of MFT genes increases intracellular iron accumulation, especially in the mitochondria. Increased mitochondrial iron can then be mobilized for later use. The function of metal compartmentation or sequestration is one of a new chapter of cellular detoxification mechanism in study. We also demonstrated that defects of high affinity iron transport can cause metal sensitivity. This metal sensitivity is due to induced expression of nonspecific low affinity iron transporters such as FET4. This transporter accelerates other transition metals accumulation that result in toxicity and cell death. Characterization of these genes has led to progress in understanding intracellular iron metabolism in yeast. These results will help further characterization of corresponding human genes in normal and disease conditions.