Identification of a Candida albicans ferrichrome transporter and its characterization by expression in Saccharomyces cerevisiae

Saccharomyces cerevisiae can accumulate iron through the uptake of siderophore-iron. Siderophore-iron uptake can occur through the reduction of the complex and the subsequent uptake of iron by the high affinity iron transporter Fet3p/Ftr1p. Alternatively, specific siderophore transporters can take up the siderophore-iron complex. The pathogenic fungus Candida albicans can also take up siderophore-iron. Here we identify a C. albicans siderophore transporter, CaArn1p, and characterize its activity. CaARN1 is transcriptionally regulated in response to iron. Through expression studies in S. cerevisiae strains lacking endogenous siderophore transporters, we demonstrate that CaArn1p specifically mediates the uptake of ferrichrome-iron. Iron-ferrichrome and gallium-ferrichrome, but not desferri-ferrichrome, could competitively inhibit the uptake of iron from ferrichrome. Uptake of siderophore-iron resulting from expression of CaARN1 under the control of the MET25-promoter in S. cerevisiae was independent of the iron status of the cells and of Aft1p, the iron-sensing transcription factor. These studies demonstrate that the expression of CaArn1p is both necessary and sufficient for the nonreductive uptake of ferrichrome-iron and suggests that the transporter may be the only required component of the siderophore uptake system that is regulated by iron and Aft1p.

Regulation of mitochondrial iron accumulation by Yfh1p, a putative homolog of frataxin

The gene responsible for Friedreich's ataxia, a disease characterized by neurodegeneration and cardiomyopathy, has recently been cloned and its product designated frataxin. A gene in Saccharomyces cerevisiae was characterized whose predicted protein product has high sequence similarity to the human frataxin protein. The yeast gene (yeast frataxin homolog, YFH1) encodes a mitochondrial protein involved in iron homeostasis and respiratory function. Human frataxin also was shown to be a mitochondrial protein. Characterizing the mechanism by which YFH1 regulates iron homeostasis in yeast may help to define the pathologic process leading to cell damage in Friedreich's ataxia.

Purification and characterization of Fet3 protein, a yeast homologue of ceruloplasmin

The FET3 gene product of Saccharomyces cerevisiae is an essential component of the high affinity iron transport system. Based on FET3 sequence homology to the multicopper oxidase family and iron oxidation studies in spheroplasts (De Silva, D. M., Askwith, C. C., Eide, D., and Kaplan, J. (1995) J. Biol. Chem. 270, 1098-1101), it was hypothesized that the Fet3 protein (Fet3p) was a cell surface ferroxidase. To further characterize the protein, we have isolated Fet3p from yeast membranes and purified the protein to apparent homogeneity. Consistent with its localization at the plasma membrane, Fet3p is a glycosylated protein. SDS-polyacrylamide gel electrophoresis analysis showed that the protein was present in two differentially glycosylated forms of approximately 120 and 100 kDa. Purified Fet3p is a copper-containing protein that is able to catalyze the oxidation of a variety of organic compounds in addition to ferrous iron. Azide and metal chelators strongly inhibited enzyme activity. Iron appeared to be the best substrate for the enzyme, and the apparent Km for ferrous oxidation was 2 microM. Interestingly, Fet3p was able to effectively catalyze the incorporation of iron onto apotransferrin. We conclude that Fet3p is a ferro-O2-oxidoreductase in yeast, homologous to the human plasma protein ceruloplasmin.

The FET3 gene of S. cerevisiae encodes a multicopper oxidase required for ferrous iron uptake

S. cerevisiae accumulate iron by a process requiring a ferrireductase and a ferrous transporter. We have isolated a mutant, fet3, defective for high affinity Fe(II) uptake. The wild-type FET3 gene was isolated by complementation of the mutant defect. Sequence analysis of the gene revealed the presence of an open reading frame coding for a protein with strong similarity to the family of blue multicopper oxidoreductases. Consistent with the role of copper in iron transport, growth of wild-type cells in copper-deficient media resulted in decreased ferrous iron transport. Addition of copper, but not other transition metals (manganese or zinc), to the assay media resulted in the recovery of Fe(II) transporter activity. We suggest that the catalytic activity of the Fet3 protein is required for cellular iron accumulation.

Regulation of iron uptake in Saccharomyces cerevisiae. The ferrireductase and Fe(II) transporter are regulated independently

Iron is required for the growth of Saccharomyces cerevisiae. High concentrations of iron, however, are toxic, forcing this yeast to tightly regulate its concentration of intracellular free iron. We demonstrate that S. cerevisiae accumulates iron through the combined action of a plasma membrane ferrireductase and an Fe(II) transporter. This transporter is highly selective for Fe(II). Several other transition metals did not inhibit iron uptake when these metals were present at a concentration 100-fold higher than the Km (0.15 microM) for iron transport. Pt(II) inhibited ferrireductase activity but not the ability of cells to transport iron that was chemically reduced to Fe(II). Incubation of cells in a synthetic iron-limited media resulted in the induction of both ferrireductase and Fe(II) transporter activities. In complex media, Fe(II) transport activity was regulated in response to media iron concentration, while the activity of the ferrireductase was not. When stationary phase cells were inoculated into fresh media, ferrireductase activity increased independent of the iron content of the media; in contrast, transporter activity varied inversely with iron levels. These results demonstrate that the ferrireductase and Fe(II) transporter are separately regulated and that iron accumulation may be limited by changes in either activity.

Inhibition of late endosome-lysosome fusion: studies on the mechanism by which isotonic-K+ buffers alter intracellular ligand movement

Incubation of alveolar macrophages or hepatocytes in media in which Na+ is replaced by K+ ("isotonic-K buffer") inhibited the movement of internalized ligand from late endosomes to lysosomes (Ward et al.: Journal of Cell Biology 110:1013-1022, 1990). In this study we investigate the mechanism responsible for the isotonic-K+ block in movement of ligand from late endosomes to lysosomes. We observed that iso-K+ inhibition of endosome-lysosome fusion is not unique to alveolar macrophages or hepatocytes but can be seen in a variety of cell types including J774 and Hela cells. The inhibition in intracellular ligand movement was time dependent with the maximum change occurring after 60 minutes. Once established the inhibition resulted in a prolonged and apparently permanent decrease in vesicle movement. Cells were able to recover from the effects of iso-K+ buffers over a time course of 5-10 minutes when placed back in Na(+)-containing media. The effect of iso-K+ buffers was independent of intracellular pH changes and appeared to involve cell swelling. When cells were incubated in iso-K+ buffers under conditions in which cell volume changes were reduced, intracellular ligand movement approached normal levels. Such conditions included replacing Cl- with the less permeant anion gluconate, and by addition of sucrose to isotonic-K+ buffers. Analysis of the mechanism by which changes in cell volume could alter intracellular movement ruled out changes in cyclic nucleotides, Ca2+, or microtubules. These results suggest that changes in cell shape or volume can alter intracellular transport systems by novel routes.