Crystal structure of the second domain of the human copper chaperone for superoxide dismutase

Protein import by the mitochondrial disulfide relay in higher eukaryotes

The proteome of the mitochondrial intermembrane space (IMS) contains more than 100 proteins, all of which are synthesized on cytosolic ribosomes and consequently need to be imported by dedicated machineries. The mitochondrial disulfide relay is the major import machinery for soluble proteins in the IMS. Its major component, the oxidoreductase MIA40, interacts with incoming substrates, retains them in the IMS, and oxidatively folds them. After this reaction, MIA40 is reoxidized by the sulfhydryl oxidase augmenter of liver regeneration, which couples disulfide formation by this machinery to the activity of the respiratory chain. In this review, we will discuss the import of IMS proteins with a focus on recent findings showing the diversity of disulfide relay substrates, describing the cytosolic control of this import system and highlighting the physiological relevance of the disulfide relay machinery in higher eukaryotes.

A pH Switch Controls Zinc Binding in Tomato Copper-Zinc Superoxide Dismutase

Copper-zinc superoxide dismutase (SOD1) is a major antioxidant metalloenzyme that protects cells from oxidative damage by superoxide anions (O2-). Structural, biophysical, and other characteristics have in the past been compiled for mammalian SOD1s and for the highly homologous fungal and bovine SOD1s. Here, we characterize the biophysical properties of a plant SOD1 from tomato chloroplasts and present several of its crystal structures. The most unusual of these structures is a structure at low pH in which tSOD1 harbors zinc in the copper-binding site but contains no metal in the zinc-binding site. The side chain of D83, normally a zinc ligand, adopts an alternate rotameric conformation to form an unusual bidentate hydrogen bond with the side chain of D124, precluding metal binding in the zinc-binding site. This alternate conformation of D83 appears to be responsible for the previously observed pH-dependent loss of zinc from the zinc-binding site of SOD1. Titrations of cobalt into apo tSOD1 at a similar pH support the lack of an intact zinc-binding site. Further characterization of tSOD1 reveals that it is a weaker dimer relative to human SOD1 and that it can be activated in vivo through a copper chaperone for the SOD1-independent mechanism.

Bacterial evolutionary precursors of eukaryotic copper-zinc superoxide dismutases

Internalization of a bacteria by an archaeal cell expedited eukaryotic evolution. An important feature of the species that diversified into the great variety of eukaryotic life visible today was the ability to combat oxidative stress with a copper–zinc superoxide dismutase (CuZnSOD) enzyme activated by a specific, high-affinity copper chaperone. Adoption of a single protein interface that facilitates homodimerization and heterodimerization was essential; however, its evolution has been difficult to rationalize given the structural differences between bacterial and eukaryotic enzymes. In contrast, no consistent strategy for the maturation of periplasmic bacterial CuZnSODs has emerged. Here, 34 CuZnSODs are described that closely resemble the eukaryotic form but originate predominantly from aquatic bacteria. Crystal structures of a Bacteroidetes bacterium CuZnSOD portray both prokaryotic and eukaryotic characteristics and propose a mechanism for self-catalyzed disulfide maturation. Unification of a bacterial but eukaryotic-like CuZnSOD along with a ferredoxin-fold MXCXXC copper-binding domain within a single polypeptide created the advanced copper delivery system for CuZnSODs exemplified by the human copper chaperone for superoxide dismutase-1. The development of this system facilitated evolution of large and compartmentalized cells following endosymbiotic eukaryogenesis.

Role of Oxidative Stress in the Pathogenesis of Amyotrophic Lateral Sclerosis: Antioxidant Metalloenzymes and Therapeutic Strategies

Amyotrophic lateral sclerosis (ALS) affects motor neurons in the cerebral cortex, brainstem and spinal cord and leads to death due to respiratory failure within three to five years. Although the clinical symptoms of this disease were first described in 1869 and it is the most common motor neuron disease and the most common neurodegenerative disease in middle-aged individuals, the exact etiopathogenesis of ALS remains unclear and it remains incurable. However, free oxygen radicals (i.e., molecules containing one or more free electrons) are known to contribute to the pathogenesis of this disease as they very readily bind intracellular structures, leading to functional impairment. Antioxidant enzymes, which are often metalloenzymes, inactivate free oxygen radicals by converting them into a less harmful substance. One of the most important antioxidant enzymes is Cu²⁺Zn²⁺ superoxide dismutase (SOD1), which is mutated in 20% of cases of the familial form of ALS (fALS) and up to 7% of sporadic ALS (sALS) cases. In addition, the proper functioning of catalase and glutathione peroxidase (GPx) is essential for antioxidant protection. In this review article, we focus on the mechanisms through which these enzymes are involved in the antioxidant response to oxidative stress and thus the pathogenesis of ALS and their potential as therapeutic targets.

Copper Sources for Sod1 Activation

Copper ions (i.e., copper) are a critical part of several cellular processes, but tight regulation of copper levels and trafficking are required to keep the cell protected from this highly reactive transition metal. Cu, Zn superoxide dismutase (Sod1) protects the cell from the accumulation of radical oxygen species by way of the redox cycling activity of copper in its catalytic center. Multiple posttranslational modification events, including copper incorporation, are reliant on the copper chaperone for Sod1 (Ccs). The high-affinity copper uptake protein (Ctr1) is the main entry point of copper into eukaryotic cells and can directly supply copper to Ccs along with other known intracellular chaperones and trafficking molecules. This review explores the routes of copper delivery that are utilized to activate Sod1 and the usefulness and necessity of each.

Chaperone client proteins evolve slower than non-client proteins

Chaperones are important molecular machinery that assists proteins to attain their native three-dimensional structure crucial for function. Earlier studies using experimental evolution showed that chaperones impose a relaxation of sequence constraints on their "client" proteins, which may lead to the fixation of slightly deleterious mutations on the latter. However, we hypothesized that such a phenomenon might be harmful to the organism in a natural physiological condition. In this study, we investigated the evolutionary rates of chaperone client and non-client proteins in five model organisms from both prokaryotic and eukaryotic lineages. Our study reveals a slower evolutionary rate of chaperone client proteins in all five organisms. Additionally, the slower folding rate and lower aggregation propensity of chaperone client proteins reveal that the chaperone may play an essential role in rescuing the slightly disadvantageous effects due to random mutations and subsequent protein misfolding. However, the fixation of such mutations is less likely to be selected in the natural population.

Exploring the Extended Biological Functions of the Human Copper Chaperone of Superoxide Dismutase 1

The human copper chaperone of SOD1 (designated as CCS) was discovered more than two decades ago. It is an important copper binding protein and a homolog of Saccharomyces cerevisiae LYS7. To date, no studies have systematically or specifically elaborated on the functional development of CCS. This review summarizes the essential information about CCS, such as its localization, 3D structure, and copper binding ability. An emphasis is placed on its interacting protein partners and its biological functions in vivo and in vitro. Three-dimensional structural analysis revealed that CCS is composed of three domains. Its primary molecular function is the delivery of copper to SOD1 and activation of SOD1. It has also been reported to bind to XIAP, Mia40, and X11α, and other proteins. Through these protein partners, CCS is implicated in several vital biological processes in vivo, such as copper homeostasis, apoptosis, angiogenesis and oxidative stress. This review is anticipated to assist scientists in systematically understanding the latest research developments of CCS for facilitating the development of new therapeutics targeting CCS in the future.

Mutations in Superoxide Dismutase 1 (Sod1) Linked to Familial Amyotrophic Lateral Sclerosis Can Disrupt High-Affinity Zinc-Binding Promoted by the Copper Chaperone for Sod1 (Ccs)

Zinc (II) ions (hereafter simplified as zinc) are important for the structural and functional activity of many proteins. For Cu, Zn superoxide dismutase (Sod1), zinc stabilizes the native structure of each Sod1 monomer, promotes homo-dimerization and plays an important role in activity by "softening" the active site so that copper cycling between Cu(I) and Cu(II) can rapidly occur. Previously, we have reported that binding of Sod1 by its copper chaperone (Ccs) stabilizes a conformation of Sod1 that promotes site-specific high-affinity zinc binding. While there are a multitude of Sod1 mutations linked to the familial form of amyotrophic lateral sclerosis (fALS), characterizations by multiple research groups have been unable to realize strong commonalities among mutants. Here, we examine a set of fALS-linked Sod1 mutations that have been well-characterized and are known to possess variation in their biophysical characteristics. The zinc affinities of these mutants are evaluated here for the first time and then compared with the previously established value for wild-type Sod1 zinc affinity. Ccs does not have the same ability to promote zinc binding to these mutants as it does for the wild-type version of Sod1. Our data provides a deeper look into how (non)productive Sod1 maturation by Ccs may link a diverse set of fALS-Sod1 mutations.

Copper-only superoxide dismutase enzymes and iron starvation stress in Candida fungal pathogens

Copper (Cu)-only superoxide dismutases (SOD) represent a newly characterized class of extracellular SODs important for virulence of several fungal pathogens. Previous studies of the Cu-only enzyme SOD5 from the opportunistic fungal pathogen Candida albicans have revealed that the active-site structure and Cu binding of SOD5 strongly deviate from those of Cu/Zn-SODs in its animal hosts, making Cu-only SODs a possible target for future antifungal drug design. C. albicans also expresses a Cu-only SOD4 that is highly similar in sequence to SOD5, but is poorly characterized. Here, we compared the biochemical, biophysical, and cell biological properties of C. albicans SOD4 and SOD5. Analyzing the recombinant proteins, we found that, similar to SOD5, Cu-only SOD4 can react with superoxide at rates approaching diffusion limits. Both SODs were monomeric and they exhibited similar binding affinities for their Cu cofactor. In C. albicans cultures, SOD4 and SOD5 were predominantly cell wall proteins. Despite these similarities, the SOD4 and SOD5 genes strongly differed in transcriptional regulation. SOD5 was predominantly induced during hyphal morphogenesis, together with a fungal burst in reactive oxygen species. Conversely, SOD4 expression was specifically up-regulated by iron (Fe) starvation and controlled by the Fe-responsive transcription factor SEF1. Interestingly, Candida tropicalis and the emerging fungal pathogen Candida auris contain a single SOD5-like SOD rather than a pair, and in both fungi, this SOD was induced by Fe starvation. This unexpected link between Fe homeostasis and extracellular Cu-SODs may help many fungi adapt to Fe-limited conditions of their hosts.

Copper-zinc superoxide dismutase (Sod1) activation terminates interaction between its copper chaperone (Ccs) and the cytosolic metal-binding domain of the copper importer Ctr1

Copper-zinc superoxide dismutase (Sod1) is a critical antioxidant enzyme that rids the cell of reactive oxygen through the redox cycling of a catalytic copper ion provided by its copper chaperone (Ccs). Ccs must first acquire this copper ion, directly or indirectly, from the influx copper transporter, Ctr1. The three proteins of this transport pathway ensure careful trafficking of copper ions from cell entry to target delivery, but the intricacies remain undefined. Biochemical examination of each step in the pathway determined that the activation of the target (Sod1) regulates the Ccs·Ctr1 interaction. Ccs stably interacts with the cytosolic C-terminal tail of Ctr1 (Ctr1c) in a copper-dependent manner. This interaction becomes tripartite upon the addition of an engineered immature form of Sod1 creating a stable Cu(I)-Ctr1c·Ccs·Sod1 heterotrimer in solution. This heterotrimer can also be made by the addition of a preformed Sod1·Ccs heterodimer to Cu(I)-Ctr1c, suggestive of multiple routes to the same destination. Only complete Sod1 activation (i.e. active site copper delivery and intra-subunit disulfide bond formation) breaks the Sod1·Ccs·Ctr1c complex. The results provide a new and extended view of the Sod1 activation pathway(s) originating at cellular copper import.

The Role of Copper Homeostasis at the Host-Pathogen Axis: From Bacteria to Fungi

Copper is an essential trace element participating in many vital biological processes, however it becomes a toxic agent when in excess. Thus, precise and tight regulation of copper homeostasis processes, including transport, delivery, storage, detoxification, and efflux machineries, is important, ensuring that only the amount needed to sustain basic biological functions and simultaneously prevent copper toxicity in the cell is maintained. Numerous exciting studies have revealed that copper plays an indispensable role at the microbial pathogen-host axis for entities ranging from pathogenic bacteria to deadly fungal species. Analyses of copper homeostases in bacteria and fungi extensively demonstrate that copper is utilized by the host immune system as an anti-microbial agent. The expression of copper efflux and detoxification from microbial pathogens is induced to counteract the host's copper bombardment, which in turn disrupts these machineries, resulting in the attenuation of microbial survival in host tissue. We hereby review the latest work in copper homeostases in pathogenic bacteria and fungi and focus on the maintenance of a copper balance at the pathogen-host interaction axis.

Function, Structure, and Transport Aspects of ZIP and ZnT Zinc Transporters in Immune Cells

Zinc is an important trace metal in immune systems, and zinc transporters are involved in many immune responses. Recent advances have revealed the structural and biochemical bases for zinc transport across the cell membrane, with clinical implications for the regulation of zinc homeostasis in immune cells like dendritic cells, T cells, B cells, and mast cells. In this review, we discuss the function, structure, and transport aspects of two major mammalian zinc transporter types, importers and exporters. First, Zrt-/Irt-like proteins (ZIPs) mediate the zinc influx from the extracellular or luminal side into the cytoplasm. There are 14 ZIP family members in humans. They form a homo- or heterodimer with 8 transmembrane domains and extra-/intracellular domains of various lengths. Several ZIP members show specific extracellular domains composed of two subdomains, a helix-rich domain and proline-alanine-leucine (PAL) motif-containing domain. Second, ZnT (zinc transporter) was initially identified in early studies of zinc biology; it mediates zinc efflux as a counterpart of ZIPs in zinc homeostasis. Ten family members have been identified. They show a unique architecture characterized by a Y-shaped conformation and a large cytoplasmic domain. A precise, comprehensive understanding of the structures and transport mechanisms of ZIP and ZnT in combination with mice experiments would provide promising drug targets as well as a basis for identifying other transporters with therapeutic potential.

Copper-zinc superoxide dismutase is activated through a sulfenic acid intermediate at a copper ion entry site

Metallochaperones are a diverse family of trafficking molecules that provide metal ions to protein targets for use as cofactors. The copper chaperone for superoxide dismutase (Ccs1) activates immature copper-zinc superoxide dismutase (Sod1) by delivering copper and facilitating the oxidation of the Sod1 intramolecular disulfide bond. Here, we present structural, spectroscopic, and cell-based data supporting a novel copper-induced mechanism for Sod1 activation. Ccs1 binding exposes an electropositive cavity and proposed "entry site" for copper ion delivery on immature Sod1. Copper-mediated sulfenylation leads to a sulfenic acid intermediate that eventually resolves to form the Sod1 disulfide bond with concomitant release of copper into the Sod1 active site. Sod1 is the predominant disulfide bond-requiring enzyme in the cytoplasm, and this copper-induced mechanism of disulfide bond formation obviates the need for a thiol/disulfide oxidoreductase in that compartment.

Bio-Zombie: the rise of pseudoenzymes in biology

Pseudoenzymes are catalytically dead counterparts of enzymes. Despite their first description some 50 years ago, the importance and functional diversity of these 'fit-for-purpose' polypeptides is only now being appreciated. Pseudoenzymes have been identified throughout all the kingdoms of life and, owing to predicted deficits in enzyme activity due to the absence of catalytic residues, have been variously referred to as pseudoenzymes, non-enzymes, dead enzymes, prozymes or 'zombie' proteins. An important goal of the recent Biochemical Society Pseudoenzymes-focused meeting was to explore the functional and evolutionary diversity of pseudoenzymes and to begin to evaluate their functions in biology, including cell signalling and metabolism. Here, we summarise the impressive breadth of enzyme classes that are known to have pseudoenzyme counterparts and present examples of known cellular functions. We predict that the next decades will represent golden years for the analysis of pseudoenzymes.

Thiol-based copper handling by the copper chaperone Atox1

Human antioxidant protein 1 (Atox1) plays a crucial role in cellular copper homeostasis. Atox1 captures cytosolic copper for subsequent transfer to copper pumps in trans Golgi network, thereby facilitating copper supply to various copper-dependent oxidereductases matured within the secretory vesicles. Atox1 and other copper chaperones handle cytosolic copper using Cys thiols which are ideal ligands for coordinating Cu(I). Recent studies demonstrated reversible oxidation of these Cys residues in copper chaperones, linking cellular redox state to copper homeostasis. Highlighted in this review are unique redox properties of Atox1 and other copper chaperones. Also, summarized are the redox nodes in the cytosol which potentially play dominant roles in the redox regulation of copper chaperones. © 2016 IUBMB Life, 69(4):246-254, 2017.

Oxygen-dependent activation of Cu,Zn-superoxide dismutase-1

Copper zinc superoxide dismutase (Sod1) is a critical enzyme in limiting reactive oxygen species in both the cytosol and the mitochondrial intermembrane space.

Molecular features of copper binding proteins involved in copper homeostasis

Copper has a wide and important role in biological systems, determining conformation and activity of many metalloproteins and enzymes, such as cytochrome oxidase and superoxide dismutase . Furthermore, due to its possible reactivity with nonspecific proteins and toxic effects, elaborate systems of absorption, concentration buffering, delivery to specific protein sites and elimination, require a complex system including small carriers, chaperones and active transporters. The P-type copper ATPases ATP7A and ATP7B provide an important system for acquisition, active transport, distribution and elimination of copper. Relevance of copper metabolism to human diseases and therapy is already known. It is quite certain that further studies will reveal detailed and useful information on biochemical mechanisms and relevance to diseases. © 2016 IUBMB Life, 69(4):211-217, 2017.

Thioether Coordination Chemistry for Molecular Imaging of Copper in Biological Systems

Copper is an essential element in biological systems. Its potent redox activity renders it necessary for life, but at the same time, misregulation of its cellular pools can lead to oxidative stress implicated in aging and various disease states. Copper is commonly thought of as a static cofactor buried in protein active sites; however, evidence of a more loosely bound, labile pool of copper has emerged. To help identify and understand new roles for dynamic copper pools in biology, we have developed selective molecular imaging agents for this metal, drawing inspiration from both biological binding motifs and synthetic model complexes that reveal thioether coordination as a general design strategy for selective and sensitive copper recognition. In this review, we summarize some contributions, primarily from our own laboratory, on fluorescence- and magnetic resonance-based molecular-imaging probes for studying copper in living systems using thioether coordination chemistry.

A faulty interaction between SOD1 and hCCS in neurodegenerative disease

A proportion of Amyotrophic lateral sclerosis (ALS) cases result from impaired mutant superoxide dismutase-1 (SOD1) maturation. The copper chaperone for SOD1 (hCCS) forms a transient complex with SOD1 and catalyses the final stages of its maturation. We find that a neurodegenerative disease-associated hCCS mutation abrogates the interaction with SOD1 by inhibiting hCCS zinc binding. Analogously, SOD1 zinc loss has a detrimental effect on the formation, structure and disassociation of the hCCS-SOD1 heterodimer. This suggests that hCCS functionality is impaired by ALS mutations that reduce SOD1 zinc affinity. Furthermore, stabilization of wild-type SOD1 by chemical modification including cisplatination, inhibits complex formation. We hypothesize that drug molecules designed to stabilize ALS SOD1 mutants that also target the wild-type form will lead to characteristics common in SOD1 knock-outs. Our work demonstrates the applicability of chromatographic SAXS when studying biomolecules predisposed to aggregation or dissociation; attributes frequently reported for complexes involved in neurodegenerative disease.

Lipid-associated aggregate formation of superoxide dismutase-1 is initiated by membrane-targeting loops

Copper-Zinc superoxide dismutase 1 (SOD1) is a homodimeric enzyme that protects cells from oxidative damage. Hereditary and sporadic amyotrophic lateral sclerosis may be linked to SOD1 when the enzyme is destabilized through mutation or environmental stress. The cytotoxicity of demetallated or apo-SOD1 aggregates may be due to their ability to cause defects within cell membranes by co-aggregating with phospholipids. SOD1 monomers may associate with the inner cell membrane to receive copper ions from membrane-bound copper chaperones. But how apo-SOD1 interacts with lipids is unclear. We have used atomistic molecular dynamics simulations to reveal that flexible electrostatic and zinc-binding loops in apo-SOD1 dimers play a critical role in the binding of 1-octanol clusters and phospholipid bilayer, without any significant unfolding of the protein. The apo-SOD1 monomer also associates with phospholipid bilayer via its zinc-binding loop rather than its exposed hydrophobic dimerization interface. Our observed orientation of the monomer on the bilayer would facilitate its association with a membrane-bound copper chaperone. The orientation also suggests how membrane-bound monomers could act as seeds for membrane-associated SOD1 aggregation.

Copper in Eukaryotes

Copper is essential for normal growth and development of eukaryotic organisms. Numerous physiological processes rely on sufficient availability of copper: from indispensable reactions such as mitochondrial respiration to more highly specialized processes such as pigment development in a skin. Copper misbalance has been linked to a variety of metabolic and neurodegenerative disorders in humans. Complex cellular machinery has evolved to mediate copper uptake, compartmentalization and incorporation into target proteins. Extensive studies revealed a predominant utilization of methionines and histidines by copper handling molecules for copper capture at the extracellular surface and delivery to cuproenzymes in the lumen of cellular compartments, respectively. Cu(I) is a predominant form within the cell, and copper binding and distribution inside the cell at the cytosolic sites relies heavily on cysteines. The selectivity and directionality of copper transfer reactions is determined by thermodynamic and kinetic factors as well as spatial distribution of copper donors and acceptors. In this chapter, we review current structural and mechanistic data on copper transport and distribution in yeast and mammalian cells and highlight important issues and questions for future studies.

Human copper chaperone for superoxide dismutase 1 mediates its own oxidation-dependent import into mitochondria

Oxidative stress is counteracted by various cellular systems, including copper-zinc superoxide dismutase 1 (SOD1) and its activating chaperone, that is, the copper chaperone for SOD1 (CCS1). Both enzymes are structurally related, and both localize to the cytosol and the mitochondrial intermembrane space where they specifically counteract mitochondria-derived superoxide. The mechanism by which human CCS1 is transported into mitochondria is largely unclear. Here we show that CCS1 import depends on the presence of mature CCS1 in the mitochondria. During import, a disulphide bond is formed in CCS1 in a CCS1-dependent reaction. We demonstrate that oxidation and import depend on the presence of cysteine residues at positions 227 and 141/144 in CCS1. Notably, CCS1 import parallels SOD1 import that also depends on CCS1. Our observations suggest that CCS1 serves as a specialized import receptor in mitochondria that facilitates the import and folding of SOD1 and CCS1, thereby extending the substrate spectrum of oxidation-dependent protein import in the mitochondrial intermembrane space.

Regulation of the chloroplastic copper chaperone (CCS) and cuprozinc superoxide dismutase (CSD2) by alternative splicing and copper excess in Glycine max

Metal homeostasis is an important aspect of plant physiology, and the copper transport into the chloroplast and its fate after delivery is of special relevance for plants. In this work, the regulation of the chloroplastic copper chaperone for the cuprozinc superoxide dismutase (GmCCS) and its target, the cuprozinc superoxide dismutase (GmCSD2), was investigated in photosynthetic cell suspensions and entire plants from Glycine max (L.) Merr. Both genes were expressed in cell suspensions and in all plant tissues analysed, and their RNAs matured by alternative splicing with intron retention (IntronR). This mechanism generated a spliced and three non-spliced mRNAs in the case of GmCCS but only a spliced and a non-spliced mRNAs in GmCSD2. Copper excess strongly upregulated the expression of both fully spliced mRNAs but mostly unaffected the non-spliced forms. In entire plants, some tissue specificity was also observed depending on copper content status. At the protein level, the GmCCS was mostly unaffected but the GmCSD2 was strongly induced under copper excess in all subcellular fractions analysed, suggesting a post-transcriptional regulation for the former. This different protein regulation of the chaperone and its target may indicate some additional function for the CSD2 protein. In addition to its well-known superoxide dismutase (SOD) activity, it may also function as a metal sink in copper excess availability to avoid metal cell damage. Furthermore, the GmCCS seems to be present in the stroma only but the GmCSD2 was present in both stroma and thylakoids despite the general idea that the SOD enzymes are typically soluble stroma proteins. The presence of the SOD enzyme on the surface of the thylakoid membranes is reasonable considering that the superoxide radical (O2-) is preferentially formed at the acceptor side of the PSI.

The importance of Zn(ii) binding by the human copper metallochaperone for Cu,Zn-superoxide dismutase

Zn(ii) removal converts dimeric human CCS to a monomer that weakens its interaction with Cu,Zn-superoxide dismutase and may be disease causing.

Species-specific activation of Cu/Zn SOD by its CCS copper chaperone in the pathogenic yeast Candida albicans

Candida albicans is a pathogenic yeast of important public health relevance. Virulence of C. albicans requires a copper and zinc containing superoxide dismutase (SOD1), but the biology of C. albicans SOD1 is poorly understood. To this end, C. albicans SOD1 activation was examined in baker's yeast (Saccharomyces cerevisiae), a eukaryotic expression system that has proven fruitful for the study of SOD1 enzymes from invertebrates, plants, and mammals. In spite of the 80% similarity between S. cerevisiae and C. albicans SOD1 molecules, C. albicans SOD1 is not active in S. cerevisiae. The SOD1 appears incapable of productive interactions with the copper chaperone for SOD1 (CCS1) of S. cerevisiae. C. albicans SOD1 contains a proline at position 144 predicted to dictate dependence on CCS1. By mutation of this proline, C. albicans SOD1 gained activity in S. cerevisiae, and this activity was independent of CCS1. We identified a putative CCS1 gene in C. albicans and created heterozygous and homozygous gene deletions at this locus. Loss of CCS1 resulted in loss of SOD1 activity, consistent with its role as a copper chaperone. C. albicans CCS1 also restored activity to C. albicans SOD1 expressed in S. cerevisiae. C. albicans CCS1 is well adapted for activating its partner SOD1 from C. albicans, but not SOD1 from S. cerevisiae. In spite of the high degree of homology between the SOD1 and CCS1 molecules in these two fungal species, there exists a species-specific barrier in CCS-SOD interactions which may reflect the vastly different lifestyles of the pathogenic versus the noninfectious yeast.

Copper chaperones. The concept of conformational control in the metabolism of copper

Copper chaperones compose a specific class of proteins assuring safe handling and specific delivery of potentially harmful copper ions to a variety of essential copper proteins. Copper chaperones are structurally heterogeneous and can exist in multiple metal-loaded as well as oligomeric forms. Moreover, many copper chaperones can exist in various oxidative states and participate in redox catalysis, connected with their functioning. This review is focused on the analysis of the structural and functional properties of copper chaperones and their partners, which allowed us to define specific regulatory principles in copper metabolism connected with copper-induced conformational control of copper proteins.

Oxidative folding in the mitochondrial intermembrane space in human health and disease

Oxidative folding in the mitochondrial intermembrane space (IMS) is a key cellular event associated with the folding and import of a large and still undetermined number of proteins. This process is catalyzed by an oxidoreductase, Mia40 that is able to recognize substrates with apparently little or no homology. Following substrate oxidation, Mia40 is reduced and must be reoxidized by Erv1/Alr1 that consequently transfers the electrons to the mitochondrial respiratory chain. Although our understanding of the physiological relevance of this process is still limited, an increasing number of pathologies are being associated with the impairment of this pathway; especially because oxidative folding is fundamental for several of the proteins involved in defense against oxidative stress. Here we review these aspects and discuss recent findings suggesting that oxidative folding in the IMS is modulated by the redox state of the cell.

Human superoxide dismutase 1 (hSOD1) maturation through interaction with human copper chaperone for SOD1 (hCCS)

Copper chaperone for superoxide dismutase 1 (SOD1), CCS, is the physiological partner for the complex mechanism of SOD1 maturation. We report an in vitro model for human CCS-dependent SOD1 maturation based on the study of the interactions of human SOD1 (hSOD1) with full-length WT human CCS (hCCS), as well as with hCCS mutants and various truncated constructs comprising one or two of the protein's three domains. The synergy between electrospray ionization mass spectrometry (ESI-MS) and NMR is fully exploited. This is an in vitro study of this process at the molecular level. Domain 1 of hCCS is necessary to load hSOD1 with Cu(I), requiring the heterodimeric complex formation with hSOD1 fostered by the interaction with domain 2. Domain 3 is responsible for the catalytic formation of the hSOD1 Cys-57-Cys-146 disulfide bond, which involves both hCCS Cys-244 and Cys-246 via disulfide transfer.

Molecular and biochemical characterization of a unique mutation in CCS, the human copper chaperone to superoxide dismutase

Copper (Cu) is a trace metal that readily gains and donates electrons, a property that renders it desirable as an enzyme cofactor but dangerous as a source of free radicals. To regulate cellular Cu metabolism, an elaborate system of chaperones and transporters has evolved, although no human Cu chaperone mutations have been described to date. We describe a child from a consanguineous family who inherited homozygous mutations in the SLC33A1, encoding an acetyl CoA transporter, and in CCS, encoding the Cu chaperone for superoxide dismutase. The CCS mutation, p.Arg163Trp, predicts substitution of a highly conserved arginine residue at position 163, with tryptophan in domain II of CCS, which interacts directly with superoxide dismutase 1 (SOD1). Biochemical analyses of the patient's fibroblasts, mammalian cell transfections, immunoprecipitation assays, and Lys7Δ (CCS homolog) yeast complementation support the pathogenicity of the mutation. Expression of CCS was reduced and binding of CCS to SOD1 impaired. As a result, this mutation causes reduced SOD1 activity and may impair other mechanisms important for normal Cu homeostasis. CCS-Arg163Trp represents the primary example of a human mutation in a gene coding for a Cu chaperone.

Cu(I) affinities of the domain 1 and 3 sites in the human metallochaperone for Cu,Zn-superoxide dismutase

The delivery of copper by the human metallochaperone CCS is a key step in the activation of Cu,Zn-superoxide dismutase (SOD1). CCS is a three-domain protein with Cu(I)-binding CXXC and CXC motifs in domains 1 and 3, respectively. A detailed analysis of the binding of copper to CCS, including variants in which the Cys residues from domains 1 and 3 have been mutated to Ser, and also using separate domain 1 and 3 constructs, demonstrates that CCS is able to bind 1 equiv of Cu(I) in both of these domains. The Cu(I) affinity of domain 1 is approximately 5 × 10(17) M(-1) at pH 7.5, while that of domain 3 is at least 1 order of magnitude weaker. The CXXC site will therefore be preferentially loaded with Cu(I), suggesting that domain 1 plays a role in the acquisition of the metal. The delivery of copper to the target occurs via domain 3 whose structural flexibility and ability to be transiently metalated during copper delivery appear to be more important than the Cu(I) affinity of its CXC motif. The Cu(I) affinity of domain 1 of CCS is comparable to that of HAH1, another cytosolic copper metallochaperone. CCS and HAH1 readily exchange Cu(I), providing a mechanism whereby cross-talk can occur between copper trafficking pathways.

The structural plasticity of the human copper chaperone for SOD1: insights from combined size-exclusion chromatographic and solution X-ray scattering studies

The incorporation of copper into biological macromolecules such as SOD1 (Cu,Zn superoxide dismutase) is essential for the viability of most organisms. However, copper is toxic and therefore the intracellular free copper concentration is kept to an absolute minimum. Several proteins, termed metallochaperones, are charged with the responsibility of delivering copper from membrane transporters to its intracellular destination. The CCS (copper chaperone for SOD1) is the major pathway for SOD1 copper loading. We have determined the first solution structure of hCCS (human CCS) by SAXS (small-angle X-ray scattering) in conjunction with SEC (size-exclusion chromatography). The findings of the present study highlight the importance of this combined on-line chromatographic technology with SAXS, which has allowed us to unambiguously separate the hCCS dimer from other oligomeric and non-physiological aggregated states that would otherwise adversely effect measurements performed on bulk solutions. The present study exposes the dynamic molecular conformation of this multi-domain chaperone in solution. The metal-binding domains known to be responsible for the conveyance of copper to SOD1 can be found in positions that would expedite this movement. Domains I and III of a single hCCS monomer are able to interact and can also move into positions that would facilitate initial copper binding and ultimately transfer to SOD1. Conversely, the interpretation of our solution studies is not compatible with an interaction between these domains and their counterparts in an hCCS dimer. Overall, the results of the present study reveal the plasticity of this multi-domain chaperone in solution and are consistent with an indispensable flexibility necessary for executing its dual functions of metal binding and transfer.

Mitochondrial Ccs1 contains a structural disulfide bond crucial for the import of this unconventional substrate by the disulfide relay system

The Mia40/Erv1 disulfide relay system forms a structural disulfide bond in Ccs1, an unconventional substrate of this system. Thereby it promotes import of Ccs1 into mitochondria and controls its cellular distribution. Thus this system is unexpectedly able to form single disulfide bonds in complex multidomain proteins.

The copper chaperone for superoxide dismutase 1 (Ccs1) provides an important cellular function against oxidative stress. Ccs1 is present in the cytosol and in the intermembrane space (IMS) of mitochondria. Its import into the IMS depends on the Mia40/Erv1 disulfide relay system, although Ccs1 is, in contrast to typical substrates, a multidomain protein and lacks twin Cx_nC motifs. We report on the molecular mechanism of the mitochondrial import of Saccharomyces cerevisiae Ccs1 as the first member of a novel class of unconventional substrates of the disulfide relay system. We show that the mitochondrial form of Ccs1 contains a stable disulfide bond between cysteine residues C27 and C64. In the absence of these cysteines, the levels of Ccs1 and Sod1 in mitochondria are strongly reduced. Furthermore, C64 of Ccs1 is required for formation of a Ccs1 disulfide intermediate with Mia40. We conclude that the Mia40/Erv1 disulfide relay system introduces a structural disulfide bond in Ccs1 between the cysteine residues C27 and C64, thereby promoting mitochondrial import of this unconventional substrate. Thus the disulfide relay system is able to form, in addition to double disulfide bonds in twin Cx_nC motifs, single structural disulfide bonds in complex protein domains.

Computational modeling of laminin N-terminal domains using sparse distance constraints from disulfide bonds and chemical cross-linking

Basement membranes are thin extracellular protein layers, which separate endothelial and epithelial cells from the underlying connecting tissue. The main noncollagenous components of basement membranes are laminins, trimeric glycoproteins, which form polymeric networks by interactions of their N-terminal (LN) domains; however, no high-resolution structure of laminin LN domains exists so far. To construct models for laminin β(1) and γ(1) LN domains, 14 potentially suited template structures were determined using fold recognition methods. For each target/template-combination comparative models were created with Rosetta. Final models were selected based on their agreement with experimentally obtained distance constraints from natural cross-links, that is, disulfide bonds as well as chemical cross-links obtained from reactions with two amine-reactive cross-linkers. We predict that laminin β(1) and γ(1) LN domains share the galactose-binding domain-like fold.

Copper metallochaperones

The current state of knowledge on how copper metallochaperones support the maturation of cuproproteins is reviewed. Copper is needed within mitochondria to supply the Cu(A) and intramembrane Cu(B) sites of cytochrome oxidase, within the trans-Golgi network to supply secreted cuproproteins and within the cytosol to supply superoxide dismutase 1 (Sod1). Subpopulations of copper-zinc superoxide dismutase also localize to mitochondria, the secretory system, the nucleus and, in plants, the chloroplast, which also requires copper for plastocyanin. Prokaryotic cuproproteins are found in the cell membrane and in the periplasm of gram-negative bacteria. Cu(I) and Cu(II) form tight complexes with organic molecules and drive redox chemistry, which unrestrained would be destructive. Copper metallochaperones assist copper in reaching vital destinations without inflicting damage or becoming trapped in adventitious binding sites. Copper ions are specifically released from copper metallochaperones upon contact with their cognate cuproproteins and metal transfer is thought to proceed by ligand substitution.

Copper metallochaperones

The current state of knowledge on how copper metallochaperones support the maturation of cuproproteins is reviewed. Copper is needed within mitochondria to supply the Cu(A) and intramembrane Cu(B) sites of cytochrome oxidase, within the trans-Golgi network to supply secreted cuproproteins and within the cytosol to supply superoxide dismutase 1 (Sod1). Subpopulations of copper-zinc superoxide dismutase also localize to mitochondria, the secretory system, the nucleus and, in plants, the chloroplast, which also requires copper for plastocyanin. Prokaryotic cuproproteins are found in the cell membrane and in the periplasm of gram-negative bacteria. Cu(I) and Cu(II) form tight complexes with organic molecules and drive redox chemistry, which unrestrained would be destructive. Copper metallochaperones assist copper in reaching vital destinations without inflicting damage or becoming trapped in adventitious binding sites. Copper ions are specifically released from copper metallochaperones upon contact with their cognate cuproproteins and metal transfer is thought to proceed by ligand substitution.

Copper metallochaperones

The current state of knowledge on how copper metallochaperones support the maturation of cuproproteins is reviewed. Copper is needed within mitochondria to supply the Cu(A) and intramembrane Cu(B) sites of cytochrome oxidase, within the trans-Golgi network to supply secreted cuproproteins and within the cytosol to supply superoxide dismutase 1 (Sod1). Subpopulations of copper-zinc superoxide dismutase also localize to mitochondria, the secretory system, the nucleus and, in plants, the chloroplast, which also requires copper for plastocyanin. Prokaryotic cuproproteins are found in the cell membrane and in the periplasm of gram-negative bacteria. Cu(I) and Cu(II) form tight complexes with organic molecules and drive redox chemistry, which unrestrained would be destructive. Copper metallochaperones assist copper in reaching vital destinations without inflicting damage or becoming trapped in adventitious binding sites. Copper ions are specifically released from copper metallochaperones upon contact with their cognate cuproproteins and metal transfer is thought to proceed by ligand substitution.

Copper accumulation and compartmentalization in mouse fibroblast lacking metallothionein and copper chaperone, Atox1

Copper (Cu) is the active center of some enzymes because of its redox-active property, although that property could have harmful effects. Because of this, cells have strict regulation/detoxification systems for this metal. In this study, multi-disciplinary approaches, such as speciation and elemental imaging of Cu, were applied to reveal the detoxification mechanisms for Cu in cells bearing a defect in Cu-regulating genes. Although Cu concentration in metallothionein (MT)-knockout cells was increased by the knockdown of the Cu chaperone, Atox1, the concentrations of the Cu influx pump, Ctr1, and another Cu chaperone, Ccs, were paradoxically increased; namely, the cells responded to the Cu deficiency despite the fact that cellular Cu concentration was actually increased. Cu imaging showed that the elevated Cu was compartmentalized in cytoplasmic vesicles. Together, the results point to the novel roles of MT and cytoplasmic vesicles in the detoxification of Cu in mammalian cells.

Selenocysteine positional variants reveal contributions to copper binding from cysteine residues in domains 2 and 3 of human copper chaperone for superoxide dismutase

The human copper chaperone for superoxide dismutase binds copper both in an Atx1-like MTCQSC motif in domain 1 and via a multinuclear cluster formed by two CXC motifs at the D3 dimer interface. The composition of the Cu(I) cluster has been investigated previously by mutagenesis of the CXC motif, and by construction of a CXU selenocysteine derivative, which has permitted XAS studies at both Cu and Se absorption edges. Here, we report the semisynthesis and spectroscopic characterization of a series of derivatives with the sequences 243-CACA, 243-CAUA, 243-UACA, and 243-UAUA in the D1 double mutant (C22AC25A) background, prepared by expressed protein ligation of Sec-containing tetrapeptides to an hCCS-243 truncation. By varying the position of the Se atom in the CXC motif, we have been able to show that Se is always bridging (2 Se-Cu) rather than terminal (1 Se-Cu). Substitution of both D3 Cys residues by Sec in the UAUA variant does not eliminate the Cu-S contribution, confirming our previous description of the cluster as most likely a Cu(4)S(6) species, and suggesting that D2 Cys residues contribute to the cluster. As predicted by this model, when Cys residues C141, C144, and C227 are mutated to alanine either individually or together as a triple mutant, the cluster nuclearity is dramatically attenuated. These data suggest that Cys residues in D2 of hCCS are involved in the formation, stability, and redox potential of the D3 cluster. The significance of these finding to the SOD1 thiol/disulfide oxidase activity are discussed in terms of a model in which a similar multinuclear cluster may form in the CCS-SOD heterodimer.

A selenocysteine variant of the human copper chaperone for superoxide dismutase. A Se-XAS probe of cluster composition at the domain 3-domain 3 dimer interface

We report the semisynthesis of a selenocysteine (Sec) derivative of the human copper chaperone for superoxide dismutase, substituted with Sec at the C-terminal C246 residue. Measurements of hCCS-induced SOD1 activation were used to show that the C-terminal CXC sequence is both necessary and sufficient for EZn-SOD maturation. Therefore, an active CAU variant carrying Sec as the terminal amino acid was prepared by expressed protein ligation of a single selenocysteine amino acid to a 243-CA truncation. This reaction proceeded in high yield and generated the desired 243-CAX (X = C or U) protein with the expected mass. Se-edge XAS of the apoprotein indicated that both Se-S and Se-Se interactions were present in a 0.3:0.7 ratio, indicating an equilibrium between species with either a selenosulfide or a diselenide cross-link. After reduction on immobilized TCEP, the ligated Cys and Sec apoproteins bound up to 2.5 Cu(I) ions per hCCS monomer with both Cu and Se as constituent atoms of the cluster which forms at the domain 3 interface of a hCCS dimer. Merging of XAS data at the Cu and Se K-absorption edges provided additional details of the cluster composition, specifically the fact that both Se atoms occupied bridging positions between two Cu(I) atoms. Further, the requirement for identical Cu-Se bond lengths and Debye-Waller factors at each absorption edge allowed us to rule out simple models for the cluster composition such as a bis-Cys(Sec)-bridged dinuclear cluster and was indicative of a more complex cluster with a nuclearity of >or=3.

Ctr1 drives intestinal copper absorption and is essential for growth, iron metabolism, and neonatal cardiac function

The trace element copper (Cu) is a cofactor for biochemical functions ranging from energy generation to iron (Fe) acquisition, angiogenesis, and free radical detoxification. While Cu is essential for life, the molecules that mediate dietary Cu uptake have not been identified. Ctr1 is a homotrimeric protein, conserved from yeast to humans, that transports Cu across the plasma membrane with high affinity and specificity. Here we describe the generation of intestinal epithelial cell-specific Ctr1 knockout mice. These mice exhibit striking neonatal defects in Cu accumulation in peripheral tissues, hepatic Fe overload, cardiac hypertrophy, and severe growth and viability defects. Consistent with an intestinal Cu absorption block, the growth and viability defects can be partially rescued by a single postnatal Cu administration, indicative of a critical neonatal metabolic requirement for Cu that is provided by intestinal Ctr1. These studies identify Ctr1 as the major factor driving intestinal Cu absorption in mammals.