Copper trafficking to the mitochondrion and assembly of copper metalloenzymes

Methionine motifs of copper transport proteins provide general and flexible thioether-only binding sites for Cu(I) and Ag(I)

Cellular acquisition of copper in eukaryotic organisms is primarily accomplished through high-affinity copper transport proteins (Ctr). The extracellular N-terminal regions of both human and yeast Ctr1 contain multiple methionine residues organized in copper-binding Mets motifs. These motifs comprise combinations of methionine residues arranged in clusters of MXM and MXXM, where X can be one of several amino acids. Model peptides corresponding to 15 different Mets motifs were synthesized and determined to selectively bind Cu(I) and Ag(I), with no discernible affinity for divalent metal ions. These are rare examples of biological thioether-only metal binding sites. Effective dissociation constant (KD) values for the model Mets peptides and Cu(I) were determined by an ascorbic acid oxidation assay and validated through electrospray ionization mass spectrometry and range between 2 and 11 microM. Affinity appears to be independent of pH, the arrangement of the motif, and the composition of intervening amino acids, all of which reveal the generality and flexibility of the MX1-2MX1-2M domain. Circular dichroism spectroscopy, 1H-NMR spectroscopy, and X-ray absorption spectroscopy were also used to characterize the binding event. These results are intended to aid the development of the still unknown mechanism of copper transport across the cell membrane.

Assembly factors and ATP-dependent proteases in cytochrome c oxidase biogenesis

Eukaryotic cytochrome c oxidase (CcO), the terminal enzyme of the energy-transducing mitochondrial electron transport chain is a hetero-oligomeric, heme-copper oxidase complex composed of both mitochondrially and nuclear-encoded subunits. It is embedded in the inner mitochondrial membrane where it couples the transfer of electrons from reduced cytochrome c to molecular oxygen with vectorial proton translocation across the membrane. The biogenesis of CcO is a complicated sequential process that requires numerous specific accessory proteins, so-called assembly factors, which include translational activators, translocases, molecular chaperones, copper metallochaperones and heme a biosynthetic enzymes. Besides these CcO-specific protein factors, the correct biogenesis of CcO requires an even greater number of proteins with much broader substrate specificities. Indeed, growing evidence indicates that mitochondrial ATP-dependent proteases might play an important role in CcO biogenesis. Out of the four identified energy-dependent mitochondrial proteases, three were shown to be directly involved in proteolysis of CcO subunits. In addition to their well-established protein-quality control function these oligomeric proteolytic complexes with chaperone-like activities may function as molecular chaperones promoting productive folding and assembly of subunit proteins. In this review, we summarize the current knowledge of the functional involvement of eukaryotic CcO-specific assembly factors and highlight the possible significance for CcO biogenesis of mitochondrial ATP-dependent proteases.

Disparate pathways for the biogenesis of cytochrome oxidases in Bradyrhizobium japonicum

This work addresses the biogenesis of heme-copper terminal oxidases in Bradyrhizobium japonicum, the nitrogen-fixing root nodule symbiont of soybean. B. japonicum has four quinol oxidases and four cytochrome oxidases. The latter include the aa(3)- and cbb(3)-type oxidases. Although both have a Cu(B) center in subunit I, the subunit II proteins differ in having either a Cu(A) center (in aa(3)) or a covalently bound heme c (in cbb(3)). Two biogenesis factors were genetically studied here, the periplasmically exposed CoxG and ScoI proteins, which are the respective homologs of the mitochondrial copper-trafficking chaperones Cox11 and Sco1 for the formation of the Cu(B) center in subunit I and the Cu(A) center in subunit II of cytochrome aa(3). We could demonstrate copper binding to ScoI in vitro, a process for which the thiols of cysteine residues 74 and 78 in the ScoI polypeptide were shown to be essential. Knock-out mutations in the B. japonicum coxG and scoI genes led to loss of cytochrome aa(3) assembly and activity in the cytoplasmic membrane, whereas the cbb(3)-type cytochrome oxidase apparently remained unaffected. This suggests that subunit I of the cbb(3)-type oxidase obtains its copper cofactor via a different pathway than cytochrome aa(3). In contrast to the coxG mutation, the scoI mutation caused a decreased symbiotic nitrogen fixation activity. We hypothesize that a periplasmic B. japonicum protein other than any of the identified Cu(A) proteins depends on ScoI and is required for an effective symbiosis.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

Analysis of mouse models of cytochrome c oxidase deficiency owing to mutations in Sco2

Mutations in SCO2, a protein required for the proper assembly and functioning of cytochrome c oxidase (COX; complex IV of the mitochondrial respiratory chain), cause a fatal infantile cardioencephalomyopathy with COX deficiency. We have generated mice harboring a Sco2 knock-out (KO) allele and a Sco2 knock-in (KI) allele expressing an E-->K mutation at position 129 (E129K), corresponding to the E140K mutation found in almost all human SCO2-mutated patients. Whereas homozygous KO mice were embryonic lethals, homozygous KI and compound heterozygous KI/KO mice were viable, but had muscle weakness; biochemically, they had respiratory chain deficiencies as well as complex IV assembly defects in multiple tissues. There was a concomitant reduction in mitochondrial copper content, but the total amount of copper in examined tissues was not reduced. These mouse models should be of use in further studies of Sco2 function, as well as in testing therapeutic approaches to treat the human disorder.

Metabolic crossroads of iron and copper

Interactions between the essential dietary metals, iron and copper, have been known for many years. This review highlights recent advances in iron-copper interactions with a focus on tissues and cell types important for regulating whole-body iron and copper homeostasis. Cells that mediate dietary assimilation (enterocytes) and storage and distribution (hepatocytes) of iron and copper are considered, along with the principal users (erythroid cells) and recyclers of red cell iron (reticuloendothelial macrophages). Interactions between iron and copper in the brain are also discussed. Many unanswered questions regarding the role of these metals and their interactions in health and disease emerge from this synopsis, highlighting extensive future research opportunities.

Visualizing ascorbate-triggered release of labile copper within living cells using a ratiometric fluorescent sensor

We present the synthesis, properties, and biological applications of Ratio-Coppersensor-1 (RCS1), a new water-soluble fluorescent sensor for ratiometric imaging of copper in living cells. RCS1 combines an asymmetric BODIPY reporter and thioether-based ligand receptor to provide high selectivity and sensitivity for Cu(+) over other biologically relevant metal ions, including Cu(2+) and Zn(2+), a ca. 20-fold fluorescence ratio change upon Cu(+) binding, and visible excitation and emission profiles compatible with standard fluorescence microscopy filter sets. Live-cell confocal microscopy experiments show that RCS1 is membrane-permeable and can sense changes in the levels of labile Cu(+) pools within living cells by ratiometric imaging, including expansion of endogenous stores of exchangeable intracellular Cu(+) triggered by ascorbate stimulation in kidney and brain cells.

Metal uptake by manganese superoxide dismutase

Manganese superoxide dismutase is an important antioxidant defense metalloenzyme that protects cells from damage by the toxic oxygen metabolite, superoxide free radical, formed as an unavoidable by-product of aerobic metabolism. Many years of research have gone into understanding how the metal cofactor interacts with small molecules in its catalytic role. In contrast, very little is presently known about how the protein acquires its metal cofactor, an important step in the maturation of the protein and one that is absolutely required for its biological function. Recent work is beginning to provide insight into the mechanisms of metal delivery to manganese superoxide dismutase in vivo and in vitro.

Nanovesicle trapping for studying weak protein interactions by single-molecule FRET

Protein-protein interactions are fundamental biological processes. While strong protein interactions are amenable to many characterization techniques including crystallography, weak protein interactions are challenging to study because of their dynamic nature. Single-molecule fluorescence resonance energy transfer (smFRET) can monitor dynamic protein interactions in real time, but are generally limited to strong interacting pairs because of the low concentrations needed for single-molecule detection. Here, we describe a nanovesicle trapping approach to enable smFRET study of weak protein interactions at high effective concentrations. We describe the experimental procedures, summarize the application in studying the weak interactions between intracellular copper transporters, and detail the single-molecule kinetic analysis of bimolecular interactions involving three states. Both the experimental approach and the theoretical analysis are generally applicable to studying many other biological processes at the single-molecule level.

Dual functions of Mss51 couple synthesis of Cox1 to assembly of cytochrome c oxidase in Saccharomyces cerevisiae mitochondria

Functional interactions of the translational activator Mss51 with both the mitochondrially encoded COX1 mRNA 5'-untranslated region and with newly synthesized unassembled Cox1 protein suggest that it has a key role in coupling Cox1 synthesis with assembly of cytochrome c oxidase. Mss51 is present at levels that are near rate limiting for expression of a reporter gene inserted at COX1 in mitochondrial DNA, and a substantial fraction of Mss51 is associated with Cox1 protein in assembly intermediates. Thus, sequestration of Mss51 in assembly intermediates could limit Cox1 synthesis in wild type, and account for the reduced Cox1 synthesis caused by most yeast mutations that block assembly. Mss51 does not stably interact with newly synthesized Cox1 in a mutant lacking Cox14, suggesting that the failure of nuclear cox14 mutants to decrease Cox1 synthesis, despite their inability to assemble cytochrome c oxidase, is due to a failure to sequester Mss51. The physical interaction between Mss51 and Cox14 is dependent upon Cox1 synthesis, indicating dynamic assembly of early cytochrome c oxidase intermediates nucleated by Cox1. Regulation of COX1 mRNA translation by Mss51 seems to be an example of a homeostatic mechanism in which a positive effector of gene expression interacts with the product it regulates in a posttranslational assembly process.

Translocation and assembly of mitochondrially coded Saccharomyces cerevisiae cytochrome c oxidase subunit Cox2 by Oxa1 and Yme1 in the absence of Cox18

Members of the Oxa1/YidC/Alb3 family of protein translocases are essential for assembly of energy-transducing membrane complexes. In Saccharomyces cerevisiae, Oxa1 and its paralog, Cox18, are required for assembly of Cox2, a mitochondrially encoded subunit of cytochrome c oxidase. Oxa1 is known to be required for cotranslational export of the Cox2 N-terminal domain across the inner mitochondrial membrane, while Cox18 is known to be required for post-translational export of the Cox2 C-tail domain. We find that overexpression of Oxa1 does not compensate for the absence of Cox18 at the level of respiratory growth. However, it does promote some translocation of the Cox2 C-tail domain across the inner membrane and causes increased accumulation of Cox2, which remains unassembled. This result suggests that Cox18 not only translocates the C-tail, but also must deliver it in a distinct state competent for cytochrome oxidase assembly. We identified respiring mutants from a cox18Delta strain overexpressing OXA1, whose respiratory growth requires overexpression of OXA1. The recessive nuclear mutations allow some assembly of Cox2 into cytochrome c oxidase. After failing to identify these mutations by methods based on transformation, we successfully located them to MGR1 and MGR3 by comparative hybridization to whole-genome tiling arrays and microarray-assisted bulk segregant analysis followed by linkage mapping. While Mgr1 and Mgr3 are known to associate with the Yme1 mitochondrial inner membrane i-AAA protease and to participate in membrane protein degradation, their absence does not appear to stabilize Cox2 under these conditions. Instead, Yme1 probably chaperones the folding and/or assembly of Oxa1-exported Cox2 in the absence of Mrg1 or Mgr3, since respiratory growth and cytochrome c oxidase assembly in a cox18 mgr3 double-mutant strain overexpressing OXA1 is YME1 dependent.

The iron-sulfur clusters of dehydratases are primary intracellular targets of copper toxicity

Excess copper is poisonous to all forms of life, and copper overloading is responsible for several human pathologic processes. The primary mechanisms of toxicity are unknown. In this study, mutants of Escherichia coli that lack copper homeostatic systems (copA cueO cus) were used to identify intracellular targets and to test the hypothesis that toxicity involves the action of reactive oxygen species. Low micromolar levels of copper were sufficient to inhibit the growth of both WT and mutant strains. The addition of branched-chain amino acids restored growth, indicating that copper blocks their biosynthesis. Indeed, copper treatment rapidly inactivated isopropylmalate dehydratase, an iron-sulfur cluster enzyme in this pathway. Other enzymes in this iron-sulfur dehydratase family were similarly affected. Inactivation did not require oxygen, in vivo or with purified enzyme. Damage occurred concomitant with the displacement of iron atoms from the solvent-exposed cluster, suggesting that Cu(I) damages these proteins by liganding to the coordinating sulfur atoms. Copper efflux by dedicated export systems, chelation by glutathione, and cluster repair by assembly systems all enhance the resistance of cells to this metal.

The coiled coil-helix-coiled coil-helix proteins may be redox proteins

A number of nuclear encoded proteins are imported in to the intermembrane space of mitochondria where they adopt a coiled coil-helix-coiled coil-helix (CHCH) fold. Two disulfide bonds formed by twin CX(3)C or CX(9)C motifs stabilize this fold. Some of these proteins are also characterized at their N-termini by the presence of two additional cysteine residues which can perform oxidoreductase or metallochaperone functions or both. This fold represents the most 'minimal' oxidoreductase domain described so far.

Electron transfer and energy transduction in the terminal part of the respiratory chain - lessons from bacterial model systems

This review focuses on the terminal part of the respiratory chain where, macroscopically speaking, electron transfer (ET) switches from the two-electron donor, ubiquinol, to the single-electron carrier, cytochrome c, to finally reduce the four-electron acceptor dioxygen. With 3-D structures of prominent representatives of such multi-subunit membrane complexes known for some time, this section of the ET chain still leaves a number of key questions unanswered. The two relevant enzymes, ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase, appear as rather diverse modules, differing largely in their design for substrate interaction, internal ET, and moreover, in their mechanisms of energy transduction. While the canonical mitochondrial complexes have been investigated for almost five decades, the corresponding bacterial enzymes have been established only recently as attractive model systems to address basic reactions in ET and energy transduction. Lacking the intricate coding background and mitochondrial assembly pathways, bacterial respiratory enzymes typically offer a much simpler subunit composition, while maintaining all fundamental functions established for their complex "relatives". Moreover, related issues ranging from primary steps in cofactor insertion to supramolecular architecture of ET complexes, can also be favourably addressed in prokaryotic systems to hone our views on prototypic structures and mechanisms common to all family members.

Two Chlamydomonas CTR copper transporters with a novel cys-met motif are localized to the plasma membrane and function in copper assimilation

Inducible high-affinity copper uptake is key to copper homeostasis in Chlamydomonas reinhardtii. We generated cDNAs and updated gene models for four genes, CTR1, CTR2, CTR3, and COPT1, encoding CTR-type copper transporters in Chlamydomonas. The expression of CTR1, CTR2, and CTR3 increases in copper deficient cells and in response to hypoxia or Ni(2+) supplementation; this response depends on the transcriptional activator CRR1. A copper response element was identified by mutational analysis of the 5' upstream region of CTR1. Functional analyses identify CTR1 and CTR2 as the assimilatory transporters of Chlamydomonas based on localization to the plasma membrane and ability to rescue a Saccharomyces cerevisiae mutant defective in high-affinity copper transport. The Chlamydomonas CTRs contain a novel Cys-Met motif (CxxMxxMxxC-x(5/6)-C), which occurs also in homologous proteins in other green algae, amoebae, and pathogenic fungi. CTR3 appears to have arisen by duplication of CTR2, but CTR3 lacks the characteristic transmembrane domains found in the transporters, suggesting that it may be a soluble protein. Thus, Chlamydomonas CTR genes encode a distinct subset of the classical CTR family of Cu(I) transporters and represent new targets of CRR1-dependent signaling.

MIA40 is an oxidoreductase that catalyzes oxidative protein folding in mitochondria

MIA40 has a key role in oxidative protein folding in the mitochondrial intermembrane space. We present the solution structure of human MIA40 and its mechanism as a catalyst of oxidative folding. MIA40 has a 66-residue folded domain made of an alpha-helical hairpin core stabilized by two structural disulfides and a rigid N-terminal lid, with a characteristic CPC motif that can donate its disulfide bond to substrates. The CPC active site is solvent-accessible and sits adjacent to a hydrophobic cleft. Its second cysteine (Cys55) is essential in vivo and is crucial for mixed disulfide formation with the substrate. The hydrophobic cleft functions as a substrate binding domain, and mutations of this domain are lethal in vivo and abrogate binding in vitro. MIA40 represents a thioredoxin-unrelated, minimal oxidoreductase, with a facile CPC redox active site that ensures its catalytic function in oxidative folding in mitochondria.

Deletion of hepatic Ctr1 reveals its function in copper acquisition and compensatory mechanisms for copper homeostasis

Copper is a vital trace element required for normal growth and development of many organisms. To determine the roles for copper transporter 1 (Ctr1) in hepatic copper metabolism and the contribution of the liver to systemic copper homeostasis, we have generated and characterized mice in which Ctr1 is deleted specifically in the liver. These mice express less than 10% residual Ctr1 protein in the liver and exhibit a small but significant growth retardation, which disappears with age. Hepatic copper concentrations and the activities of copper-requiring enzymes are reduced; however, mild copper deficiency relative to Ctr1 protein deficit indicates compensatory mechanisms for copper metabolism. Copper concentrations of other organs did not alter despite the defect in hepatic copper uptake. Whereas biliary copper excretion is reduced, urinary copper concentration in these mice is higher than that of control mice. Our data indicate that Ctr1 plays a critical role in copper acquisition in the liver, and, when Ctr1 expression is compromised, compensatory mechanisms facilitate copper uptake and/or retention in the liver and excretion of copper via urine.

Metallothionein-3, Zinc, and Copper in the Central Nervous System

Metallothionein-3 (MT-3), also known as the neuronal growth inhibitory factor, has been discovered by Uchida and coworkers in 1991 in their search for a cellular component responsible for antagonizing aberrant neuritic sprouting and increased survival of cultured neurons stimulated by Alzheimer's disease (AD) brain extract. Since this initial discovery further studies showed that MT-3 possesses peculiar structural and functional properties not shared by other members of the mammalian MT family. Several lines of evidence suggest that the metal-binding protein MT-3 plays a vital role in zinc and copper homeostasis in the brain. Although far from being understood, the unusual structural properties of MT-3 are responsible for its neuronal growth inhibitory activity, involvement in trafficking of zinc vesicles in the central nervous system, protection against copper-mediated toxicity in AD and in controlling abnormal metal-protein interactions in other neurodegenerative disorders.

Assembly of the oxidative phosphorylation system in humans: what we have learned by studying its defects

Assembly of the oxidative phosphorylation (OXPHOS) system in the mitochondrial inner membrane is an intricate process in which many factors must interact. The OXPHOS system is composed of four respiratory chain complexes, which are responsible for electron transport and generation of the proton gradient in the mitochondrial intermembrane space, and of the ATP synthase that uses this proton gradient to produce ATP. Mitochondrial human disorders are caused by dysfunction of the OXPHOS system, and many of them are associated with altered assembly of one or more components of the OXPHOS system. The study of assembly defects in patients has been useful in unraveling and/or gaining a complete understanding of the processes by which these large multimeric complexes are formed. We review here current knowledge of the biogenesis of OXPHOS complexes based on investigation of the corresponding disorders.

Copper transport activity of yeast Ctr1 is down-regulated via its C terminus in response to excess copper

Copper is an essential yet toxic trace element. The Ctr1 family of proteins plays a critical role for copper uptake in eukaryotes. However, the mechanisms of action of Ctr1 are largely unknown. Our previous data demonstrated that copper transport induces conformational changes in the cytosolic C terminus of the yeast Saccharomyces cerevisiae Ctr1. To define the physiological significance of this molecular event and gain better insights into the mechanism of Ctr1-mediated copper uptake, we have characterized the functional roles of the Ctr1 C terminus. A Ctr1 mutant lacking the entire C-terminal cytosolic tail is functional in high affinity copper uptake; however, yeast cells expressing this mutant are extremely sensitive to excess copper. Toxic copper uptake is not attributed to elevated expression or distinct subcellular localization of this mutant as compared with wild type Ctr1. Further characterization of the function of Ctr1 containing deletions or site-directed mutations at the C terminus indicates a structural role for the C terminus in controlling Ctr1 activities. In response to excess copper, Ctr1-mediated copper transport is rapidly blocked in a C terminus-dependent mechanism associated with direct binding of copper. We propose that conformational changes in the cytosolic tail of yeast Ctr1 by copper sensing within this domain lead to the inhibition of Ctr1-mediated copper transport. These data suggest a new regulatory mechanism by which yeast cells maintain homeostatic copper acquisition.

Raman, IR, UV-vis and EPR characterization of two copper dioxolene complexes derived from L-dopa and dopamine

The anionic complexes [Cu(L(1-))3](1-), L(-)=dopasemiquinone or L-dopasemiquinone, were prepared and characterized. The complexes are stable in aqueous solution showing intense absorption bands at ca. 605 nm for Cu(II)-L-dopasemiquinone and at ca. 595 nm for Cu(II)-dopasemiquinone in the UV-vis spectra, that can be assigned to intraligand transitions. Noradrenaline and adrenaline, under the same reaction conditions, did not yield Cu-complexes, despite the bands in the UV region showing that noradrenaline and adrenaline were oxidized during the process. The complexes display a resonance Raman effect, and the most enhanced bands involve ring modes and particularly the nuCC+nuCO stretching mode at ca. 1384 cm(-1). The free radical nature of the ligands and the oxidation state of the Cu(II) were confirmed by the EPR spectra that display absorptions assigned to organic radicals with g=2.0005 and g=2.0923, and for Cu(II) with g=2.008 and g=2.0897 for L-dopasemiquinone and dopasemiquinone, respectively. The possibility that dopamine and L-dopa can form stable and aqueous-soluble copper complexes at neutral pH, whereas noradrenaline and adrenaline cannot, may be important in understanding how Cu(II)-dopamine crosses the cellular membrane as proposed in the literature to explain the role of copper in Wilson disease.

Stability of oxidized, reduced and copper bound forms of Bacillus subtilis Sco

Sco is an accessory protein required for assembly of the Cu(A) center of cytochrome c oxidase. Functions proposed for Sco include as a copper chaperone and as a thiol-disulfide exchange protein. Differential scanning calorimetry (DSC) is used here to assess the interaction between the Bacillis subtilis version of Sco (BsSco) and Cu(II). When BsSco binds Cu(II) its melting temperature increases by 23 degrees C, which corresponds to an equilibrium dissociation constant of 3.50 pM. In contrast BsSco exhibits a much weaker affinity for Cu(I) (K(D)=10 microM). BsSco-Cu(II) is stable over days indicating an extremely slow dissociation for BsSco-Cu(II). However, at high ionic strength in the presence of excess copper, BsSco-Cu(II) returns to its oxidized, disulfide-bonded state and loses its copper binding capacity with a half time of 100 s. DSC of BsSco at high ionic strength indicates an increase in stability of metal free, reduced BsSco combined with a small destabilization of BsSco-Cu(II). It is proposed that BsSco undergoes an ionic strength induced conformational change that promotes electron transfer from the thiol groups on BsSco to Cu(II) to effect copper release. Such a redox transformation could be an important aspect of the copper transfer role proposed for BsSco in Cu(A) assembly.

Direct metal transfer between periplasmic proteins identifies a bacterial copper chaperone

Transition metals require exquisite handling within cells to ensure that cells are not harmed by an excess of free metal species. In gram-negative bacteria, copper is required in only small amounts in the periplasm, not in the cytoplasm, so a key aspect of protection under excess metal conditions is to export copper from the periplasm. Additional protection could be conferred by a periplasmic chaperone to limit the free metal species prior to export. Using isothermal titration calorimetry, we have demonstrated that two periplasmic proteins, CusF and CusB, of the Escherichia coli Cu(I)/Ag(I) efflux system undergo a metal-dependent interaction. Through the development of a novel X-ray absorption spectroscopy approach using selenomethionine labeling to distinguish the metal sites of the two proteins, we have demonstrated transfer of Cu(I) occurs between CusF and CusB. The interaction between these proteins is highly specific, as a homologue of CusF with a 51% identical sequence and a similar affinity for metal, did not function in metal transfer. These experiments establish a metallochaperone activity for CusF in the periplasm of gram-negative bacteria, serving to protect the periplasm from metal-mediated damage.

Mechanism of Cu(A) assembly

Copper is essential for proper functioning of cytochrome c oxidases, and therefore for cellular respiration in eukaryotes and many bacteria. Here we show that a new periplasmic protein (PCu(A)C) selectively inserts Cu(I) ions into subunit II of Thermus thermophilus ba(3) oxidase to generate a native Cu(A) site. The purported metallochaperone Sco1 is unable to deliver copper ions; instead, it works as a thiol-disulfide reductase to maintain the correct oxidation state of the Cu(A) cysteine ligands.

Copper distributed by Atx1 is available to copper amine oxidase 1 in Schizosaccharomyces pombe

Copper amine oxidases (CAOs) have been proposed to be involved in the metabolism of xenobiotic and biogenic amines. The requirement for copper is absolute for their activity. In the fission yeast Schizosaccharomyces pombe, cao1(+) and cao2(+) genes are predicted to encode members of the CAO family. While both genes are expressed in wild-type cells, we determined that the expression of only cao1(+) but not cao2(+) results in the production of an active enzyme. Site-directed mutagenesis identified three histidine residues within the C-terminal region of Cao1 that are necessary for amine oxidase activity. By use of a cao1(+)-GFP allele that retained wild-type function, Cao1-GFP was localized in the cytosol (GFP is green fluorescent protein). Under copper-limiting conditions, disruption of ctr4(+), ctr5(+), and cuf1(+) produced a defect in amine oxidase activity, indicating that a functionally active Cao1 requires Ctr4/5-mediated copper transport and the transcription factor Cuf1. Likewise, atx1 null cells exhibited substantially decreased levels of amine oxidase activity. In contrast, deletion of ccc2, cox17, and pccs had no significant effect on Cao1 activity. Residual amine oxidase activity in cells lacking atx1(+) can be restored to normal levels by returning an atx1(+) allele, underscoring the critical importance of the presence of Atx1 in cells. Using two-hybrid analysis, we demonstrated that Cao1 physically interacts with Atx1 and that this association is comparable to that of Atx1 with the N-terminal region of Ccc2. Collectively, these results describe the first example of the ability of Atx1 to act as a copper carrier for a molecule other than Ccc2 and its critical role in delivering copper to Cao1.

Role of copper in human neurological disorders

Copper is a trace element present in all tissues and is required for cellular respiration, peptide amidation, neurotransmitter biosynthesis, pigment formation, and connective tissue strength. Copper is a cofactor for numerous enzymes and plays an important role in central nervous system development; low concentrations of copper may result in incomplete development, whereas excess copper maybe injurious. Copper may be involved in free radical production, via the Haber-Weiss reaction, that results in mitochondrial damage, DNA breakage, and neuronal injury. Evidence of abnormal copper transport and aberrant copper-protein interactions in numerous human neurological disorders supports the critical importance of this trace metal for proper neurodevelopment and neurological function. The biochemical phenotypes of human disorders that involve copper homeostasis suggest possible biomarkers of copper status that may be applicable to general populations.

Elevated glutathione levels confer cellular sensitization to cisplatin toxicity by up-regulation of copper transporter hCtr1

Previous studies have demonstrated that treating cultured cells with cisplatin (CDDP) up-regulated the expression of glutathione (GSH) and its de novo rate-limiting enzyme glutamate-cysteine ligase (GCL), which consists of a catalytic (GCLC) and a modifier (GCLM) subunit. It has also been shown that many CDDP-resistant cell lines exhibit high levels of GCLC/GCLM and GSH. Because the GSH system is the major intracellular regulator of redox conditions that serve as an important detoxification cytoprotector, these results have been taken into consideration that elevated levels of GCL/GSH are responsible for the CDDP resistance. In contrast to this context, we demonstrated here that overexpression of GSH by transfection with an expression plasmid containing the GCLC cDNA conferred sensitization to CDDP through up-regulation of human copper transporter (hCtr) 1, which is also a transporter for CDDP. Depleting GSH levels in these transfected cells reversed CDDP sensitivity with concomitant reduction of hCtr1 expression. Although rates of copper transport were also up-regulated in the transfected cells, these cells exhibited biochemical signature of copper deficiency, suggesting that GSH functions as an intracellular copper-chelator and that overexpression of GSH can alter copper metabolism. More importantly, our results reveal a new role of GSH in the regulation of CDDP sensitivity. Overproduction of GSH depletes the bioavailable copper pool, leading to up-regulation of hCtr1 and sensitization of CDDP transport and cell killing. These findings also have important implications in that modulation of the intracellular copper pool may be a novel strategy for improving chemotherapeutic efficacy of platinum-based antitumor agents.

Cytochrome c oxidase biogenesis: new levels of regulation

Eukaryotic cytochrome c oxidase (COX), the last enzyme of the mitochondrial respiratory chain, is a multimeric enzyme of dual genetic origin, whose assembly is a complicated and highly regulated process. COX displays a concerted accumulation of its constitutive subunits. Data obtained from studies performed with yeast mutants indicate that most catalytic core unassembled subunits are posttranslationally degraded. Recent data obtained in the yeast Saccharomyces cerevisiae have revealed another contribution to the stoichiometric accumulation of subunits during COX biogenesis targeting subunit 1 or Cox1p. Cox1p is a mitochondrially encoded catalytic subunit of COX which acts as a seed around which the full complex is assembled. A regulatory mechanism exists by which Cox1p synthesis is controlled by the availability of its assembly partners. The unique properties of this regulatory mechanism offer a means to catalyze multiple-subunit assembly. New levels of COX biogenesis regulation have been recently proposed. For example, COX assembly and stability of the fully assembled enzyme depend on the presence in the mitochondrial compartments of two partners of the oxidative phosphorylation system, the mobile electron carrier cytochrome c and the mitochondrial ATPase. The different mechanisms of regulation of COX assembly are reviewed and discussed.

Mitochondrial copper metabolism and delivery to cytochrome c oxidase

Metals are essential elements of all living organisms. Among them, copper is required for a multiplicity of functions including mitochondrial oxidative phosphorylation and protection against oxidative stress. Here we will focus on describing the pathways involved in the delivery of copper to cytochrome c oxidase (COX), a mitochondrial metalloenzyme acting as the terminal enzyme of the mitochondrial respiratory chain. The catalytic core of COX is formed by three mitochondrially-encoded subunits and contains three copper atoms. Two copper atoms bound to subunit 2 constitute the Cu(A) site, the primary acceptor of electrons from ferrocytochrome c. The third copper, Cu(B), is associated with the high-spin heme a(3) group of subunit 1. Recent studies, mostly performed in the yeast Saccharomyces cerevisiae, have provided new clues about 1) the source of the copper used for COX metallation; 2) the roles of Sco1p and Cox11p, the proteins involved in the direct delivery of copper to the Cu(A) and Cu(B) sites, respectively; 3) the action mechanism of Cox17p, a copper chaperone that provides copper to Sco1p and Cox11p; 4) the existence of at least four Cox17p homologues carrying a similar twin CX(9)C domain suggestive of metal binding, Cox19p, Cox23p, Pet191p and Cmc1p, that could be part of the same pathway; and 5) the presence of a disulfide relay system in the intermembrane space of mitochondria that mediates import of proteins with conserved cysteines motifs such as the CX(9)C characteristic of Cox17p and its homologues. The different pathways are reviewed and discussed in the context of both mitochondrial COX assembly and copper homeostasis.

Two variants of the assembly factor Surf1 target specific terminal oxidases in Paracoccus denitrificans

Biogenesis of cytochrome c oxidase (COX) relies on a large number of assembly proteins, one of them being Surf1. In humans, the loss of Surf1 function is associated with Leigh syndrome, a fatal neurodegenerative disorder. In the soil bacterium Paracoccus denitrificans, homologous genes specifying Surf1 have been identified and located in two operons of terminal oxidases: surf1q is the last gene of the qox operon (coding for a ba(3)-type ubiquinol oxidase), and surf1c is found at the end of the cta operon (encoding subunits of the aa(3)-type cytochrome c oxidase). We introduced chromosomal single and double deletions for both surf1 genes, leading to significantly reduced oxidase activities in membrane. Our experiments on P. denitrificans surf1 single deletion strains show that both Surf1c and Surf1q are functional and act independently for the aa(3)-type cytochrome c oxidase and the ba(3)-type quinol oxidase, respectively. This is the first direct experimental evidence for the involvement of a Surf1 protein in the assembly of a quinol oxidase. Analyzing the heme content of purified cytochrome c oxidase, we conclude that Surf1, though not indispensable for oxidase assembly, is involved in an early step of cofactor insertion into subunit I.

Mapping the functional interaction of Sco1 and Cox2 in cytochrome oxidase biogenesis

Sco1 is implicated in the copper metallation of the Cu(A) site in Cox2 of cytochrome oxidase. The structure of Sco1 in the metallated and apo-conformers revealed structural dynamics primarily in an exposed region designated loop 8. The structural dynamics of loop 8 in Sco1 suggests it may be an interface for interactions with Cox17, the Cu(I) donor and/or Cox2. A series of conserved residues in the sequence motif (217)KKYRVYF(223) on the leading edge of this loop are shown presently to be important for yeast Sco1 function. Cells harboring Y219D, R220D, V221D, and Y222D mutant Sco1 proteins failed to restore respiratory growth or cytochrome oxidase activity in sco1Delta cells. The mutant proteins are stably expressed and are competent to bind Cu(I) and Cu(II) normally. Specific Cu(I) transfer from Cox17 to the mutant apo-Sco1 proteins proceeds normally. In contrast, using two in vivo assays that permit monitoring of the transient Sco1-Cox2 interaction, the mutant Sco1 molecules appear compromised in a function with Cox2. The mutants failed to suppress the respiratory defect of cox17-1 cells unlike wild-type SCO1. In addition, the mutants failed to suppress the hydrogen peroxide sensitivity of sco1Delta cells. These studies implicate different surfaces on Sco1 for interaction or function with Cox17 and Cox2.

Pet191 is a cytochrome c oxidase assembly factor in Saccharomyces cerevisiae

The twin-Cx(9)C motif protein Pet191 is essential for cytochrome c oxidase maturation. The motif Cys residues are functionally important and appear to be present in disulfide linkages within a large oligomeric complex associated with the mitochondrial inner membrane. The import of Pet191 differs from that of other twin-Cx(9)C motif class of proteins in being independent of the Mia40 pathway.

"Pulling the plug" on cellular copper: the role of mitochondria in copper export

Mitochondria contain two enzymes, Cu,Zn superoxide dismutase (Sod1) and cytochrome c oxidase (CcO), that require copper as a cofactor for their biological activity. The copper used for their metallation originates from a conserved, bioactive pool contained within the mitochondrial matrix, the size of which changes in response to either genetic or pharmacological manipulation of cellular copper status. Its dynamic nature implies molecular mechanisms exist that functionally couple mitochondrial copper handling with other, extramitochondrial copper trafficking pathways. The recent finding that mitochondrial proteins with established roles in CcO assembly can also effect changes in cellular copper levels by modulating copper efflux from the cell supports a mechanistic link between organellar and cellular copper metabolism. However, the proteins and molecular mechanisms that link trafficking of copper to and from the organelle with other cellular copper trafficking pathways are unknown. This review documents our current understanding of copper trafficking to, and within, the mitochondrion for metallation of CcO and Sod1; the pathways by which the two copper centers in CcO are formed; and, the interconnections between mitochondrial function and the regulation of cellular copper homeostasis.

Isolated cytochrome c oxidase deficiency in G93A SOD1 mice overexpressing CCS protein

G93A SOD1 transgenic mice overexpressing CCS protein develop an accelerated disease course that is associated with enhanced mitochondrial pathology and increased mitochondrial localization of mutant SOD1. Because these results suggest an effect of mutant SOD1 on mitochondrial function, we assessed the enzymatic activities of mitochondrial respiratory chain complexes in the spinal cords of CCS/G93A SOD1 and control mice. CCS/G93A SOD1 mouse spinal cord demonstrates a 55% loss of complex IV (cytochrome c oxidase) activity compared with spinal cord from age-matched non-transgenic or G93A SOD1 mice. In contrast, CCS/G93A SOD1 spinal cord shows no reduction in the activities of complex I, II, or III. Blue native gel analysis further demonstrates a marked reduction in the levels of complex IV but not of complex I, II, III, or V in spinal cords of CCS/G93A SOD1 mice compared with non-transgenic, G93A SOD1, or CCS/WT SOD1 controls. With SDS-PAGE analysis, spinal cords from CCS/G93A SOD1 mice showed significant decreases in the levels of two structural subunits of cytochrome c oxidase, COX1 and COX5b, relative to controls. In contrast, CCS/G93A SOD1 mouse spinal cord showed no reduction in levels of selected subunits from complexes I, II, III, or V. Heme A analyses of spinal cord further support the existence of cytochrome c oxidase deficiency in CCS/G93A SOD1 mice. Collectively, these results establish that CCS/G93A SOD1 mice manifest an isolated complex IV deficiency which may underlie a substantial part of mutant SOD1-induced mitochondrial cytopathy.

Structural analysis of tissues affected by cytochrome C oxidase deficiency due to mutations in the SCO2 gene

Structural and histochemical studies carried out in a series of seven cases (from five families) with isolated cytochrome c oxidase (COX) deficiency caused by mutations in the SCO2 gene (1, 2) disclosed changes concentrated in the nervous system, skeletal muscle and myocardium. In five patients homozygous for the E140K mutation, the phenotype was predominantly neuromuscular and the average life span ranged between 9 and 15 months. In two cases, the course was more rapid (death at 7 and 11 weeks of life) and featured marked cardiac hypertrophy (3- and 4-fold increase in heart weight). This predominantly cardiomyopathic phenotype was associated with compound heterozygosity (E140K with another nonsense mutation) in the SCO2 gene. Polioencephalopathy with neurodegeneration and neuronal drop out was present in all cases with evidence that retinal neurons might be seriously affected too. Involvement of spinal motoneurons together with cytochrome c oxidase deficiency in muscle represents a "double hit" for the skeletal muscle. The mitochondrial population was not found to be significantly increased or structurally altered, with the exception of two compound heterozygotes in which the cardiac mitochondria were increased in number and size. Our report extends knowledge of the pathology of COX deficiency caused by mutations in the SCO2 gene.

The genetics of essential metal homeostasis during development

The essential metals copper, zinc, and iron play key roles in embryonic, fetal, and postnatal development in higher eukaryotes. Recent advances in our understanding of the molecules involved in the intricate control of the homeostasis of these metals and the availability of natural mutations and targeted mutations in many of the genes involved have allowed for elucidation of the diverse roles of these metals during development. Evidence suggests that the ability of the embryo to control the homeostasis of these metals becomes essential at the blastocyst stage and during early morphogenesis. However, these metals play unique roles throughout development and exert pleiotropic, metal-specific, and often cell-specific effects on morphogenesis, growth, and differentiation. Herein, we briefly review the major players known to be involved in the homeostasis of each of these essential metals and their known roles in development.

Transcriptional activators HAP/NF-Y rescue a cytochrome c oxidase defect in yeast and human cells

Cell survival and energy production requires a functional mitochondrial respiratory chain. Biogenesis of cytochrome c oxidase (COX), the last enzyme of the mitochondrial respiratory chain, is a very complicated process and requires the assistance of a large number of accessory factors. Defects in COX assembly alter cellular respiration and produce severe human encephalomyopathies. Mutations in SURF1, a COX assembly factor of exact unknown function, produce Leigh's syndrome (LS), the most frequent cause of COX deficiency in infants. In the yeast Saccharomyces cerevisiae, deletion of the SURF1 homologue SHY1 results in a similar COX deficiency. In order to identify genetic modifiers of the shy1 mutant phenotype, we have explored for genetic interactions involving SHY1. Here we report that overexpression of Hap4p, the catalytic subunit of the CCAAT binding transcriptional activator Hap2/3/4/5p complex, suppresses the respiratory defect of yeast shy1 mutants by increasing the expression of nuclear-encoded COX subunits that interact with the mitochondrially encoded Cox1p. Analogously, overexpression of the Hap complex human homologue NF-YA/B/C transcription complex in SURF1-deficient fibroblasts from an LS patient efficiently rescues their COX deficiency.

A place for thioether chemistry in cellular copper ion recognition and trafficking

Over the last decade, cysteine thiolate ligands have been shown to be critical to the Cu(I) (cuprous) binding chemistry of many cytosolic metallochaperone and metalloregulatory proteins involved in copper physiology. More recently, the thioether group of methionine has begun to emerge as an important Cu(I) ligand for trafficking proteins in more oxidizing cellular environments.

[The molecular bases for copper uptake and distribution: lessons from yeast]

Copper exists in two oxidation states, cuprous (Cu1+) and cupric (Cu2+), which, respectively, can donate or accept electrons. The fact that copper has two readily interconvertible redox states makes it a catalytic co-factor for many important enzymes. Over the past years, work in a number of laboratories has clearly demonstrated that studies in yeast have served as a springboard for identifying cellular components and processes involved in copper uptake and distribution. In several cases, it has been shown that mammalian proteins are capable of functionally replacing yeast proteins, thereby revealing their remarkable functional conservation. For high-affinity copper transport into cells, it has been shown that copper transporters of the Ctr family are required. Upon entering the cell, copper is partitioned to different proteins and into different compartments within the cell. Given the potential toxicity of copper, specialized proteins bind copper after it enters the cell and subsequently donate the bound copper to their corresponding recipient proteins. Three copper-binding proteins, Ccs1, Cox17, and Atx1, have been identified that serve as "copper chaperones" to deliver copper. double dagger.