Crucial function of vertebrate glutaredoxin 3 (PICOT) in iron homeostasis and hemoglobin maturation.

The mechanisms by which eukaryotic cells handle and distribute the essential micronutrient iron within the cytosol and other cellular compartments are only beginning to emerge. The yeast monothiol multidomain glutaredoxins (Grx) 3 and 4 are essential for both transcriptional iron regulation and intracellular iron distribution. Despite the fact that the mechanisms of iron metabolism differ drastically in fungi and higher eukaryotes, the glutaredoxins are conserved, yet their precise function in vertebrates has remained elusive. Here we demonstrate a crucial role of the vertebrate-specific monothiol multidomain Grx3 (PICOT) in cellular iron homeostasis. During zebrafish embryonic development, depletion of Grx3 severely impairs the maturation of hemoglobin, the major iron-consuming process. Silencing of human Grx3 expression in HeLa cells decreases the activities of several cytosolic Fe/S proteins, for example, iron-regulatory protein 1, a major component of posttranscriptional iron regulation. As a consequence, Grx3-depleted cells show decreased levels of ferritin and increased levels of transferrin receptor, features characteristic of cellular iron starvation. Apparently, Grx3-deficient cells are unable to efficiently use iron, despite unimpaired cellular iron uptake. These data suggest an evolutionarily conserved role of cytosolic monothiol multidomain glutaredoxins in cellular iron metabolism pathways, including the biogenesis of Fe/S proteins and hemoglobin maturation.

The multidomain thioredoxin-monothiol glutaredoxins represent a distinct functional group.

Monothiol glutaredoxins (Grxs) with a noncanonical CGFS active site are found in all kingdoms of life. They include members with a single domain and thioredoxin-Grx fusion proteins. In Saccharomyces cerevisiae, the multidomain Grx3 and Grx4 play an essential role in intracellular iron trafficking. This crucial task is mediated by an essential Fe/S cofactor. This study shows that this unique physiological role cannot be executed by single domain Grxs, because the thioredoxin domain is indispensable for function in vivo. Mutational analysis revealed that a CPxS active site motif is fully compatible with Fe/S cluster binding on Grx4, while a dithiol active site results in cofactor destabilization and a moderate impairment of in vivo function. These requirements for Fe/S cofactor stabilization on Grx4 are virtually the opposite of those previously reported for single domain Grxs. Grx4 functions as iron sensor for the iron-sensing transcription factor Aft1 in S. cerevisiae. We found that Aft1 binds to a conserved binding site at the C-terminus of Grx4. This interaction is essential for the regulation of Aft1. Collectively, our analysis demonstrates that the multidomain monothiol Grxs form a unique protein family distinct from that of the single domain Grxs.

Redox atlas of the mouse. Immunohistochemical detection of glutaredoxin-, peroxiredoxin-, and thioredoxin-family proteins in various tissues of the laboratory mouse.

BACKGROUND: Oxidoreductases of the thioredoxin family of proteins have been thoroughly studied in numerous cellular and animal models mimicking human diseases. Despite of their well documented role in various disease conditions, no systematic information on the presence of these proteins is available. METHODS: Here, we have systematically analyzed the presence of some of the major constituents of the glutaredoxin (Grx)-, peroxiredoxin (Prx)-, and thioredoxin (Trx)-systems, i.e. Grx1, Grx2, Grx3 (TXNL-2/PICOT), Grx5, nucleoredoxin (Nrx), Prx1, Prx2, Prx3, Prx4, Prx5, Prx6, Trx1, thioredoxin reductase 1 (TrxR1), Trx2, TrxR2, and γ-glutamyl cysteine synthetase (γ-GCS) in various tissues of the mouse using immunohistochemistry. RESULTS: The identification of the Trx family proteins in the central nervous system, sensory organs, digestive system, lymphatic system, reproductive system, urinary system, respiratory system, endocrine system, skin, heart, and muscle revealed a number of significant differences between these proteins with respect to their distribution in these tissues. CONCLUSION: Our results imply more specific functions and interactions between the proteins of this family than previously assumed. GENERAL SIGNIFICANCE: Crucial functions of Trx family proteins have been demonstrated in various disease conditions. A detailed overview on their distribution in various tissues will be helpful to fully comprehend their potential role and the interactions of these proteins in the most thoroughly studied model for human diseases-the laboratory mouse. This article is part of a Special Issue entitled Human and Murine Redox Protein Atlases.

Characterization of the human monothiol glutaredoxin 3 (PICOT) as iron-sulfur protein.

Mammalian glutaredoxin 3 (Grx3/PICOT) is an essential protein involved in the regulation of signal transduction, for instance during immune cell activation and development of cardiac hypertrophy, presumably in response to redox signals. This function requires the sensing of such stresses by a hitherto unknown mechanism. Here, we characterized Grx3/PICOT as iron-sulfur protein. The protein binds two bridging [2Fe-2S] clusters in a homodimeric complex with the active site cysteinyl residues of its two monothiol glutaredoxin domains and glutathione bound non-covalently to the Grx domains. Co-immunoprecipitation of 55-iron with Grx3/PICOT from Jurkat cells suggested the presence of these cofactors under physiological conditions. The [2Fe-2S]2+ clusters were not redox active, instead they were lost upon treatment of the holo protein with ferricyanide or S-nitroso glutathione. This redox-induced dissociation of the Grx3/PICOT holo complex may be a mechanism of Grx3/PICOT activation in response to reactive oxygen and nitrogen species.