Three cell wall mannoproteins facilitate the uptake of iron in Saccharomyces cerevisiae

Analysis of iron-regulated gene expression in Saccharomyces cerevisiae using cDNA microarrays has identified three putative cell wall proteins that are directly regulated by Aft1p, the major iron-dependent transcription factor in yeast. FIT1, FIT2, and FIT3 (for facilitator of iron transport) were more highly expressed in strains grown in low concentrations of iron and in strains in which AFT1-1(up), a constitutively active allele of AFT1, was expressed. Northern blot analysis confirmed that FIT1, FIT2, and FIT3 mRNA transcript levels were increased 60-230-fold in response to iron deprivation in an Aft1p-dependent manner. Fit1p was localized exclusively to the cell wall by indirect immunofluorescence. Deletion of the FIT genes, individually or in combination, resulted in diminished uptake of iron bound to the siderophores ferrioxamine B and ferrichrome, without diminishing the uptake of ferric iron salts, or the siderophores triacetylfusarinine C and enterobactin. FIT-deletion strains exhibited increased expression of Aft1p target genes as measured by a FET3-lacZ reporter gene or by Arn1p Western blotting, indicating that cells respond to the absence of FIT genes by up-regulating systems of iron uptake. Aft1p activation in FIT-deleted strains occurred when either ferrichrome or ferric salts were used as sources of iron during growth, suggesting that the FIT genes enhance uptake of iron from both sources. Enzymatic digestion of the cell wall resulted in the release of significant amounts of iron from cells, and the relative quantity of iron released was reduced in FIT-deletion strains. Fit1p, Fit2p, and Fit3p may function by increasing the amount of iron associated with the cell wall and periplasmic space.

Desferrioxamine-mediated iron uptake in Saccharomyces cerevisiae. Evidence for two pathways of iron uptake

In the yeast Saccharomyces cerevisiae, uptake of iron is largely regulated by the transcription factor Aft1. cDNA microarrays were used to identify new iron and AFT1-regulated genes. Four homologous genes regulated as part of the AFT1-regulon (ARN1-4) were predicted to encode members of a subfamily of the major facilitator superfamily of transporters. These genes were predicted to encode proteins with 14 membrane spanning domains and were from 26 to 53% identical at the amino acid level. ARN3 is identical to SIT1, which is reported to encode a ferrioxamine B permease. Deletion of ARN3 did not prevent yeast from using ferrioxamine B as an iron source; however, deletion of ARN3 and FET3, a component of the high affinity ferrous iron transport system, did prevent uptake of ferrioxamine-bound iron and growth on ferrioxamine as an iron source. The siderophore-mediated transport system and the high affinity ferrous iron transport system were localized to separate cellular compartments. Epitope-tagged Arn3p was expressed in intracellular vesicles that co-sediment with the endosomal protein Pep12. In contrast, Fet3p was expressed on the plasma membrane and was digested by extracellular proteases. These data indicate that S. cerevisiae has two pathways for ferrrioxamine-mediated iron uptake, one occurring at the plasma membrane and the other occurring in an intracellular compartment.

Cell-cycle arrest and inhibition of G1 cyclin translation by iron in AFT1-1(up) yeast

Although iron is an essential nutrient, it is also a potent cellular toxin, and the acquisition of iron is a highly regulated process in eukaryotes. In yeast, iron uptake is homeostatically regulated by the transcription factor encoded by AFT1. Expression of AFT1-1(up), a dominant mutant allele, results in inappropriately high rates of iron uptake, and AFT1-1(up) mutants grow slowly in the presence of high concentrations of iron. We present evidence that when Aft1-1(up) mutants are exposed to iron, they arrest the cell division cycle at the G1 regulatory point Start. This arrest is dependent on high-affinity iron uptake and does not require the activation of the DNA damage checkpoint governed by RAD9. The iron-induced arrest is bypassed by overexpression of a mutant G1 cyclin, cln3-2, and expression of the G1-specific cyclins Cln1 and Cln2 is reduced when yeast are exposed to increasing amounts of iron, which may account for the arrest. This reduction is not due to changes in transcription of CLN1 or CLN2, nor is it due to accelerated degradation of the protein. Instead, this reduction occurs at the level of Cln2 translation, a recently recognized locus of cell-cycle control in yeast.