Disease relevance of FRE1

• Fre1p Cu2+ reduction and Fet3p Cu1+ oxidation modulate copper toxicity in Saccharomyces cerevisiae [1].
• Two yeast genes, FRE1 and FRE2 (encoding Fe(III) reductases) were placed under the control of the cauliflower mosaic virus 35S promoter and introduced into tobacco (Nicotiana tabacum L.) via Agrobacterium tumefaciens-mediated transformation [2].

High impact information on FRE1

• FRE1 and FRE2 encode plasma membrane ferric reductases, obligatory for ferric iron assimilation, and FET3 encodes a copper-dependent membrane-associated oxidase required for ferrous iron uptake [3].
• These effects may be explained by the failure of iron to repress transcription of FRE1, FRE2 and FET3 [3].
• First, MAC1 is involved in basal level transcription of FRE1, encoding a plasma membrane component associated with both Cu(II) and Fe(III) reduction [4].
• Cells harboring the MAC1(up1) allele fail to attenuate FRE1 and CTR1 expression in a Cu-dependent manner [5].
• An 85-base-pair segment of FRE1 5' noncoding sequence contains a RAP1 binding site and a repeated sequence, TTTTTGCTCAYC; this segment is sufficient to confer iron-repressible transcriptional activity on heterologous downstream promoter elements [6].

Biological context of FRE1

• Similarly, a gain-of-function mutation, MAC1up1, which causes elevated and unregulated transcription from FRE1 and elevated Fe(III) reduction and 59Fe uptake exhibits a similar phenotype with respect to Cu(II) reduction and 64Cu uptake [7].
• We demonstrate that the CTR1, CTR3, and FRE1 genes involved in high affinity copper uptake share a common promoter element, TTTGCTC, which is necessary for both copper repression and copper-starvation activation of gene expression [8].
• An identical sequence appears as a direct repeat in the FRE1 promoter [9].
• Despite its sequence homology with S. cerevisiae FRE1, CFL1 mRNA was not induced by iron deprivation, and CFL1 did not complement the slow growth of a S. cerevisiae fre1 delta null mutant in the absence of iron, suggesting that CFL1 is functionally distinct from S. cerevisiae FRE1 [10].
• The predicted FRE1 protein appears to be a membrane protein and shows homology to the beta-subunit of the human respiratory burst oxidase [11].

Anatomical context of FRE1

• Fre1p is a metalloreductase in the yeast plasma membrane that is essential to uptake of environmental Cu2+ and Fe3+ [1].
• Potentiometric titrations indicated that FRE1, like neutrophil NADPH oxidase, has an unusually low redox potential, in the region of -250 mV, and binds CO [12].
• Vacuole deficiency led to constitutively high ferrireductase activity slightly induced by Cu limitation, and to high levels of FRE1 expression further inducible by Fe or Cu deprivation [13].
• Despite structural similarities between gp91phox and FRE1, flavocytochrome b558 does not mediate iron uptake by myeloid cells [14].

Associations of FRE1 with chemical compounds

• FRE1 and FRE2 encode cell surface metalloreductases that are required for reduction and uptake of free ferric iron [15].
• Cell-surface reductases, FRE1 and FRE2, provide ferrous iron for both systems [16].
• The high affinity uptake systems for iron and copper ions in Saccharomyces cerevisiae involve metal-specific permeases and two known cell surface Cu(II) and Fe(III) metalloreductases, Fre1 and Fre2 [17].
• Enterobactin, a catecholate siderophore, was not a substrate for Fre3p, and reductive uptake required either Fre1p or Fre2p [15].
• Fre3p was required for the reduction and uptake of ferrioxamine B-iron and for growth on ferrioxamine B, ferrichrome, triacetylfusarinine C, and rhodotorulic acid in the absence of Fre1p and Fre2p [15].

Other interactions of FRE1

• In this communication, we provide evidence that Fre2p has also cupric reductase activity, as has been previously shown for Fre1p (Hassett, R., and Kosman, D.J. (1995) J. Biol. Chem. 270, 128-134) [18].
• Overexpression of FRE1 did not lead to an increased ferrireductase activity of the cells when NCP1 was repressed [19].
• In addition, the downstream genes encoded the surface reductases FRE1 and FRE2 and the iron regulatory protein AFT1 [20].
• These include the FRE1-encoded Cu2+/Fe3+ reductase and the CTR1 and CTR3-encoded membrane-associated copper transport proteins [8].
• The gene product of MAC1, which is required for regulation of FRE1 transcription, is also required for expression of Cu(II) reduction activity [7].

Analytical, diagnostic and therapeutic context of FRE1

• Four critical histidine residues in the FRE1 protein of the iron reductase were identified by site-directed mutagenesis [21].
• Northern-blot analyses revealed mRNA of two different sizes in FRE1 lines, whereas all FRE2 lines had mRNA only of the expected length [2].

References