Iron sulfur (Fe-S) clusters, preassembled on the ISCU scaffold, are transferred to target proteins or to intermediate scaffolds by a dedicated chaperone-cochaperone system. However, the molecular mechanisms that underlie substrate discrimination and guide delivery of nascent clusters to specific subsets of Fe-S recipients are poorly understood. Here, we identified interacting partners of the cochaperone HSC20 and discovered that LYR motifs are molecular signatures of specific recipient Fe-S proteins or accessory factors that assist Fe-S cluster delivery. In succinate dehydrogenase B, two LYR motifs engage the ISCU-HSC20-HSPA9 complex to aid incorporation of three Fe-S clusters within the final structure of complex II. Moreover, we show that members of the LYR motif family which assist assembly of complexes II or III, SDHAF1 and LYRM7, respectively, are HSC20 binding partners. Our studies unveil a network of interactions between HSC20 and LYR motif-containing proteins that are key to the assembly and function of complexes I, II, and III.

Profound metabolic changes are characteristic of macrophages during classical activation and have been implicated in this phenotype. Here we demonstrate that nitric oxide (NO) produced by murine macrophages is responsible for TCA cycle alterations and citrate accumulation associated with polarization. 13 C tracing and mitochondrial respiration experiments map NO-mediated suppression of metabolism to mitochondrial aconitase (ACO2). Moreover, we find that inflammatory macrophages reroute pyruvate away from pyruvate dehydrogenase (PDH) in an NO-dependent and hypoxia-inducible factor 1α (Hif1α)-independent manner, thereby promoting glutamine-based anaplerosis. Ultimately, NO accumulation leads to suppression and loss of mitochondrial electron transport chain (ETC) complexes. Our data reveal that macrophages metabolic rewiring, in vitro and in vivo, is dependent on NO targeting specific pathways, resulting in reduced production of inflammatory mediators. Our findings require modification to current models of macrophage biology and demonstrate that reprogramming of metabolism should be considered a result rather than a mediator of inflammatory polarization.

Neutrophils are a vital component of immune protection, yet in cancer they may promote tumour progression, partly by generating reactive oxygen species (ROS) that disrupts lymphocyte functions. Metabolically, neutrophils are often discounted as purely glycolytic. Here we show that immature, c-Kit+ neutrophils subsets can engage in oxidative mitochondrial metabolism. With limited glucose supply, oxidative neutrophils use mitochondrial fatty acid oxidation to support NADPH oxidase-dependent ROS production. In 4T1 tumour-bearing mice, mitochondrial fitness is enhanced in splenic neutrophils and is driven by c-Kit signalling. Concordantly, tumour-elicited oxidative neutrophils are able to maintain ROS production and T cell suppression when glucose utilisation is restricted. Consistent with these findings, peripheral blood neutrophils from patients with cancer also display increased immaturity, mitochondrial content and oxidative phosphorylation. Together, our data suggest that the glucose-restricted tumour microenvironment induces metabolically adapted, oxidative neutrophils to maintain local immune suppression.

Iron-sulfur (Fe-S) clusters are ancient, ubiquitous cofactors composed of iron and inorganic sulfur. The combination of the chemical reactivity of iron and sulfur, together with many variations of cluster composition, oxidation states and protein environments, enables Fe-S clusters to participate in numerous biological processes. Fe-S clusters are essential to redox catalysis in nitrogen fixation, mitochondrial respiration and photosynthesis, to regulatory sensing in key metabolic pathways (i. e. cellular iron homeostasis and oxidative stress response), and to the replication and maintenance of the nuclear genome. Fe-S cluster biogenesis is a multistep process that involves a complex sequence of catalyzed protein- protein interactions and coupled conformational changes between the components of several dedicated multimeric complexes. Intensive studies of the assembly process have clarified key points in the biogenesis of Fe-S proteins. However several critical questions still remain, such as: what is the role of frataxin? Why do some defects of Fe-S cluster biogenesis cause mitochondrial iron overload? How are specific Fe-S recipient proteins recognized in the process of Fe-S transfer? This review focuses on the basic steps of Fe-S cluster biogenesis, drawing attention to recent advances achieved on the identification of molecular features that guide selection of specific subsets of nascent Fe-S recipients by the cochaperone HSC20. Additionally, it outlines the distinctive phenotypes of human diseases due to mutations in the components of the basic pathway.

Fe-S cofactors are composed of iron and inorganic sulfur in various stoichiometries. A complex assembly pathway conducts their initial synthesis and subsequent binding to recipient proteins. In this minireview, we discuss how discovery of the role of the mammalian cytosolic aconitase, known as iron regulatory protein 1 (IRP1), led to the characterization of the function of its Fe-S cluster in sensing and regulating cellular iron homeostasis. Moreover, we present an overview of recent studies that have provided insights into the mechanism of Fe-S cluster transfer to recipient Fe-S proteins. Discovery of a regulatory role for an iron-sulfur cluster in iron regulatory protein 1Interest in understanding how mammalian cells regulated iron uptake and distribution in the 1980s led to the discovery of a post-transcriptional regulatory mechanism, which was found to be crucial for cellular and systemic iron homeostasis in vertebrates. It was known that the expression of a major iron storage protein, ferritin (Ft), 2 was primarily regulated at the translational rather than transcriptional level (1, 2). However, it was not possible to dissect how ferritin levels were controlled under different conditions of iron availability until the genes encoding H and L ferritin were cloned in 1984 (2). At that time, gel-shift assays were commonly used to demonstrate direct binding of specific transcription factors to DNA sequences. A similar approach revealed that one or more cytosolic proteins were bound to the 5Ј-untranslated (5Ј-UTR) region of ferritin transcripts in mammalian cells (3-5). Moreover, it appeared that the binding factors reflected the iron status of the cell, as ferritin translation and gel-shift binding activity were reduced in cells that were iron-deficient, while conversely increasing in cells that were iron-loaded. The region of the ferritin transcripts responsible for mediating translational regulation was identified and subsequently named the IRE, for iron-responsive element. The IRE was defined through mutagenesis and sequence homology as a short stem-loop structure located near the 5Ј-end of the ferritin transcript. Intensive efforts to identify cytosolic factors that bound to the IRE resulted in cloning of two major cytosolic regulatory proteins, iron regulatory proteins 1 and 2 (IRP1 and IRP2) (5). It was immediately apparent, upon inspection of its primary amino acid sequence, that IRP1 was remarkably similar to mitochondrial aconitase, which was a well-characterized Fe-S protein. A cubane [Fe 4 -S 4 ] cluster in the IRP1 active-site cleft was known to be critical for the reversible aconitase-mediated conversion of citrate to isocitrate (6), which is essential for cholesterol and fatty acid metabolism, as citrate is the substrate of ATP-citrate lyase, which generates acetyl-coenzyme A utilized for cholesterol and lipid biosynthesis. Additionally, citrate has important regulatory roles in glycolysis and fatty acid synthesis and oxidation (7). Isocitrate is also metabolized by the cytosolic NADP-dependent iso...