Mutational Analysis of the Cysteine-Rich Region of the Iron-Responsive GATA Factor Fep1. Role of Individual Cysteines as [2Fe–2S] Cluster Ligands

Glutaredoxins and iron-sulfur protein biogenesis at the interface of redox biology and iron metabolism

The physiological roles of the intracellular iron and redox regulatory systems are intimately linked. Iron is an essential trace element for most organisms, yet elevated cellular iron levels are a potent generator and amplifier of reactive oxygen species and redox stress. Proteins binding iron or iron-sulfur (Fe/S) clusters, are particularly sensitive to oxidative damage and require protection from the cellular oxidative stress protection systems. In addition, key components of these systems, most prominently glutathione and monothiol glutaredoxins are involved in the biogenesis of cellular Fe/S proteins. In this review, we address the biochemical role of glutathione and glutaredoxins in cellular Fe/S protein assembly in eukaryotic cells. We also summarize the recent developments in the role of cytosolic glutaredoxins in iron metabolism, in particular the regulation of fungal iron homeostasis. Finally, we discuss recent insights into the interplay of the cellular thiol redox balance and oxygen with that of Fe/S protein biogenesis in eukaryotes.

Biophysical characterization of the complex between the iron-responsive transcription factor Fep1 and DNA

Fep1 is an iron-responsive GATA-type transcriptional repressor present in numerous fungi. The DNA-binding domain of this protein is characterized by the presence of two zinc fingers of the Cys₂-Cys₂ type and a Cys-X₅-Cys-X₈-Cys-X₂-Cys motif located between the two zinc fingers, that is involved in binding of a [2Fe-2S] cluster. In this work, biophysical characterization of the DNA-binding domain of Pichia pastoris Fep1 and of the complex of the protein with cognate DNA has been undertaken. The results obtained by analytical ultracentrifugation sedimentation velocity, small-angle X-ray scattering and differential scanning calorimetry indicate that Fep1 is a natively unstructured protein that is able to bind DNA forming 1:1 and 2:1 complexes more compact than the individual partners. Complex formation takes place independently of the presence of a stoichiometric [2Fe-2S] cluster, suggesting that the cluster may play a role in recruiting other protein(s) required for regulation of transcription in response to changes in intracellular iron levels.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

Glutaredoxins with iron-sulphur clusters in eukaryotes - Structure, function and impact on disease

Among the thioredoxin superfamily of proteins, the observation that numerous glutaredoxins bind iron-sulphur (Fe/S) clusters is one of the more recent and major developments concerning their functional properties. Glutaredoxins are present in most organisms. All members of the class II subfamily (including most monothiol glutaredoxins), but also some members of the class I (mostly dithiol glutaredoxins) and class III (land plant-specific monothiol or dithiol glutaredoxins) are Fe/S proteins. In glutaredoxins characterised so far, the [2Fe2S] cluster is coordinated by two active-site cysteine residues and two molecules of non-covalently bound glutathione in homo-dimeric complexes bridged by the cluster. In contrast to dithiol glutaredoxins, monothiol glutaredoxins possess no or very little oxidoreductase activity, but have emerged as important players in cellular iron metabolism. In this review we summarise the recent developments of the most prominent Fe/S glutaredoxins in eukaryotes, the mitochondrial single domain monothiol glutaredoxin 5, the chloroplastic single domain monothiol glutaredoxin S14 and S16, the nuclear/cytosolic multi-domain monothiol glutaredoxin 3, and the mitochondrial/cytosolic dithiol glutaredoxin 2.

Iron-sulfur cluster signaling: The common thread in fungal iron regulation

Iron homeostasis in fungi involves balancing iron uptake and storage with iron utilization to achieve adequate, nontoxic levels of this essential nutrient. Extensive work in the nonpathogenic yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe has uncovered unique iron regulation networks for each organism that control iron metabolism via distinct molecular mechanisms. However, common themes have emerged from these studies. The activities of all fungal iron-sensing transcription factors characterized to date are regulated via iron-sulfur cluster signaling. Furthermore, glutaredoxins often play a key role in relaying the intracellular iron status to these DNA-binding proteins. Recent work with fungal pathogens, including Candida and Aspergillus species and Cryptococcus neoformans, has revealed novel iron regulation mechanisms, yet similar roles for iron-sulfur clusters and glutaredoxins in iron signaling have been confirmed. This review will focus on these recent discoveries regarding iron regulation pathways in both pathogenic and nonpathogenic fungi.

Genome-wide identification and analysis of the GATA transcription factor gene family in Ustilaginoidea virens

In filamentous fungi, the conserved transcription factors play important roles in multiple cellular and developmental processes. The GATA proteins, a family of GATA-binding zinc finger transcription factors, play diverse functions in fungi. Ustilaginoidea virens is an economically important pathogen-causing rice false smut worldwide. To gain additional insight into the cellular and molecular mechanisms of this pathogen, in this study, we identified and functionally characterized seven GATA proteins from the U. virens genome (UvGATA). Sequences analysis indicated that these GATA proteins are divided into seven clades. The proteins in each clade contained conserved clade-specific sequences and structures, thus leading to the same motif serving different purposes in various contexts. The expression profiles of UvGATA genes at different infection stages and under H₂O₂ stress were detected. Results showed that the majority of UvGATA genes performed functions at both processes, thereby confirming the roles of these genes in pathogenicity and reactive oxygen species stress tolerance. This study provided an important starting point to further explore the biological functions of UvGATA genes and increased our understanding of their potential transcriptional regulatory mechanisms in U. virens.