Mitochondrial copper metabolism in yeast: mutational analysis of Sco1p involved in the biogenesis of cytochrome c oxidase

More than Just Bread and Wine: Using Yeast to Understand Inherited Cytochrome Oxidase Deficiencies in Humans

Inherited defects in cytochrome c oxidase (COX) are associated with a substantial subset of diseases adversely affecting the structure and function of the mitochondrial respiratory chain. This multi-subunit enzyme consists of 14 subunits and numerous cofactors, and it requires the function of some 30 proteins to assemble. COX assembly was first shown to be the primary defect in the majority of COX deficiencies 36 years ago. Over the last three decades, most COX assembly genes have been identified in the yeast Saccharomyces cerevisiae, and studies in yeast have proven instrumental in testing the impact of mutations identified in patients with a specific COX deficiency. The advent of accessible genome-wide sequencing capabilities has led to more patient mutations being identified, with the subsequent identification of several new COX assembly factors. However, the lack of genotype-phenotype correlations and the large number of genes involved in generating a functional COX mean that functional studies must be undertaken to assign a genetic variant as being causal. In this review, we provide a brief overview of the use of yeast as a model system and briefly compare the COX assembly process in yeast and humans. We focus primarily on the studies in yeast that have allowed us to both identify new COX assembly factors and to demonstrate the pathogenicity of a subset of the mutations that have been identified in patients with inherited defects in COX. We conclude with an overview of the areas in which studies in yeast are likely to continue to contribute to progress in understanding disease arising from inherited COX deficiencies.

Copper metabolism in Saccharomyces cerevisiae: an update

Copper is an essential element in all forms of life. It acts as a cofactor of some enzymes and is involved in forming proper protein conformations. However, excess copper ions in cells are detrimental as they can generate free radicals or disrupt protein structures. Therefore, all life forms have evolved conserved and exquisite copper metabolic systems to maintain copper homeostasis. The yeast Saccharomyces cerevisiae has been widely used to investigate copper metabolism as it is convenient for this purpose. In this review, we summarize the mechanism of copper metabolism in Saccharomyces cerevisiae according to the latest literature. In brief, bioavailable copper ions are incorporated into yeast cells mainly via the high-affinity transporters Ctr1 and Ctr3. Then, intracellular Cu⁺ ions are delivered to different organelles or cuproproteins by different chaperones, including Ccs1, Atx1, and Cox17. Excess copper ions bind to glutathione (GSH), metallothioneins, and copper complexes are sequestered into vacuoles to avoid toxicity. Copper-sensing transcription factors Ace1 and Mac1 regulate the expression of genes involved in copper detoxification and uptake/mobilization in response to changes in intracellular copper levels. Though numerous recent breakthroughs in understanding yeast's copper metabolism have been achieved, some issues remain unresolved. Completely elucidating the mechanism of copper metabolism in yeast helps decode the corresponding system in humans and understand how copper-related diseases develop.

Mitochondrial Sco proteins are involved in oxidative stress defense

Members of the evolutionary conserved Sco protein family have been intensively studied regarding their role in the assembly of the mitochondrial cytochrome c oxidase. However, experimental and structural data, specifically the presence of a thioredoxin-like fold, suggest that Sco proteins may also play a role in redox homeostasis.

In our study, we addressed this putative function of Sco proteins using Saccharomyces cerevisiae as a model system. Like many eukaryotes, this yeast possesses two SCO homologs (SCO1 and SCO2). Mutants bearing a deletion of either of the two genes are not affected in their growth under oxidative stress. However, the concomitant deletion of the SOD1 gene encoding the superoxide dismutase 1 resulted in a distinct phenotype: double deletion strains lacking SCO1 or SCO2 and SOD1 are highly sensitive to oxidative stress and show dramatically increased ROS levels.

The respiratory competent double deletion strain Δsco2Δsod1 paved the way to investigate the putative antioxidant function of SCO homologs apart from their role in respiration by complementation analysis. Sco homologs from Drosophila, Arabidopsis, human and two other yeast species were integrated into the genome of the double deletion mutant and the transformants were analyzed for their growth under oxidative stress. Interestingly, all homologs except for Kluyveromyces lactis K07152 and Arabidopsis thaliana HCC1 were able to complement the phenotype, indicating their role in oxidative stress defense. We further applied this complementation-based system to investigate whether pathogenic point mutations affect the putative antioxidant role of hSco2. Surprisingly, all of the mutant alleles failed to restore the ROS-sensitivity of the Δsco2Δsod1 strain.

In conclusion, our data not only provide clear evidence for the function of Sco proteins in oxidative stress defense but also offer a valuable tool to investigate this role for other homologous proteins.

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Concomitant deletion of SCO and SOD1 leads to a high ROS sensitivity.

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SCO homologs from higher organisms can rescue the oxidative stress sensitive phenotype of the double deletion mutant.

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Pathogenic human Sco2 mutations affect the antioxidant function of the protein.

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The role of the Sco proteins in oxidative stress defense is discussed.

Copper Acquisition and Utilization in Fungi

Fungal cells colonize and proliferate in distinct niches, from soil and plants to diverse tissues in human hosts. Consequently, fungi are challenged with the goal of obtaining nutrients while simultaneously elaborating robust regulatory mechanisms to cope with a range of availability of nutrients, from scarcity to excess. Copper is essential for life but also potentially toxic. In this review we describe the sophisticated homeostatic mechanisms by which fungi acquire, utilize, and control this biochemically versatile trace element. Fungal pathogens, which can occupy distinct host tissues that have their own intrinsic requirements for copper homeostasis, have evolved mechanisms to acquire copper to successfully colonize the host, disseminate to other tissues, and combat host copper bombardment mechanisms that would otherwise mitigate virulence.

Plant Mitochondrial Inner Membrane Protein Insertion

During the biogenesis of the mitochondrial inner membrane, most nuclear-encoded inner membrane proteins are laterally released into the membrane by the TIM23 and the TIM22 machinery during their import into mitochondria. A subset of nuclear-encoded mitochondrial inner membrane proteins and all the mitochondrial-encoded inner membrane proteins use the Oxa machinery-which is evolutionarily conserved from the endosymbiotic bacterial ancestor of mitochondria-for membrane insertion. Compared to the mitochondria from other eukaryotes, plant mitochondria have several unique features, such as a larger genome and a branched electron transport pathway, and are also involved in additional cellular functions such as photorespiration and stress perception. This review focuses on the unique aspects of plant mitochondrial inner membrane protein insertion machinery, which differs from that in yeast and humans, and includes a case study on the biogenesis of Cox2 in yeast, humans, two plant species, and an algal species to highlight lineage-specific similarities and differences. Interestingly, unlike mitochondria of other eukaryotes but similar to bacteria and chloroplasts, plant mitochondria appear to use the Tat machinery for membrane insertion of the Rieske Fe/S protein.

Protein chaperones mediating copper insertion into the CuA site of the aa3-type cytochrome c oxidase of Paracoccus denitrificans

The biogenesis of the mitochondrial cytochrome c oxidase is a complex process involving the stepwise assembly of its multiple subunits encoded by two genetic systems. Moreover, several chaperones are required to recruit and insert the redox-active metal centers into subunits I and II, two a-type hemes and a total of three copper ions, two of which form the CuA center located in a hydrophilic domain of subunit II. The copper-binding Sco protein(s) have been implicated with the metallation of this site in various model organisms. Here we analyze the role of the two Sco homologues termed ScoA and ScoB, along with two other copper chaperones, on the biogenesis of the cytochrome c oxidase in the bacterium Paracoccus denitrificans by deleting each of the four genes individually or pairwise, followed by assessing the functionality of the assembled oxidase both in intact membranes and in the purified enzyme complex. Copper starvation leads to a drastic decrease of oxidase activity in membranes from strains involving the scoB deletion. This loss is shown to be of dual origin, (i) a severe drop in steady-state oxidase levels in membranes, and (ii) a diminished enzymatic activity of the remaining oxidase complex, traced back to a lower copper content, specifically in the CuA site of the enzyme. Neither of the other proteins addressed here, ScoA or the two PCu proteins, exhibit a direct effect on the metallation of the CuA site in P. denitrificans, but are discussed as potential interaction partners of ScoB.

Structural and mechanistic insights into an extracytoplasmic copper trafficking pathway in Streptomyces lividans

In Streptomyces lividans an extracytoplasmic copper-binding Sco protein plays a role in two unlinked processes: (i) initiating a morphological development switch and (ii) facilitating the co-factoring of the CuA domain of CcO (cytochrome c oxidase). How Sco obtains copper once secreted to the extracytoplasmic environment is unknown. In the present paper we report on a protein possessing an HX₆MX₂₁HXM motif that binds a single cuprous ion with subfemtomolar affinity. High-resolution X-ray structures of this extracytoplasmic copper chaperone-like protein (ECuC) in the apo- and Cu(I)-bound states reveal that the latter possesses a surface-accessible cuprous-ion-binding site located in a dish-shaped region of β-sheet structure. A cuprous ion is transferred under a favourable thermodynamic gradient from ECuC to Sco with no back transfer occurring. The ionization properties of the cysteine residues in the Cys⁸⁶xxxCys⁹⁰ copper-binding motif of Sco, together with their positional locations identified from an X-ray structure of Sco, suggests a role for Cys⁸⁶ in initiating an inter-complex ligand-exchange reaction with Cu(I)-ECuC. Generation of the genetic knockouts, Δsco, Δecuc and Δsco/ecuc, and subsequent in vivo assays lend support to the existence of a branched extracytoplasmic copper-trafficking pathway in S. lividans. One branch requires both Sco and to a certain extent ECuC to cofactor the CuA domain, whereas the other uses only Sco to deliver copper to a cuproenzyme to initiate morphological development.

Divergent functions of the Arabidopsis mitochondrial SCO proteins: HCC1 is essential for COX activity while HCC2 is involved in the UV-B stress response

The two related putative cytochrome c oxidase (COX) assembly factors HCC1 and HCC2 from Arabidopsis thaliana are Homologs of the yeast Copper Chaperones Sco1p and Sco2p. The hcc1 null mutation was previously shown to be embryo lethal while the disruption of the HCC2 gene function had no obvious effect on plant development, but increased the expression of stress-responsive genes. Both HCC1 and HCC2 contain a thioredoxin domain, but only HCC1 carries a Cu-binding motif also found in Sco1p and Sco2p. In order to investigate the physiological implications suggested by this difference, various hcc1 and hcc2 mutants were generated and analyzed. The lethality of the hcc1 knockout mutation was rescued by complementation with the HCC1 gene under the control of the embryo-specific promoter ABSCISIC ACID INSENSITIVE 3. However, the complemented seedlings did not grow into mature plants, underscoring the general importance of HCC1 for plant growth. The HCC2 homolog was shown to localize to mitochondria like HCC1, yet the function of HCC2 is evidently different, because two hcc2 knockout lines developed normally and exhibited only mild growth suppression compared with the wild type (WT). However, hcc2 knockouts were more sensitive to UV-B treatment than the WT. Complementation of the hcc2 knockout with HCC2 rescued the UV-B-sensitive phenotype. In agreement with this, exposure of wild-type plants to UV-B led to an increase of HCC2 transcripts. In order to corroborate a function of HCC1 and HCC2 in COX biogenesis, COX activity of hcc1 and hcc2 mutants was compared. While the loss of HCC2 function had no significant effect on COX activity, the disruption of one HCC1 gene copy was enough to suppress respiration by more than half compared with the WT. Therefore, we conclude that HCC1 is essential for COX function, most likely by delivering Cu to the catalytic center. HCC2, on the other hand, seems to be involved directly or indirectly in UV-B-stress responses.

Redox and reactive oxygen species regulation of mitochondrial cytochrome C oxidase biogenesis

SIGNIFICANCE

Cytochrome c oxidase (COX), the last enzyme of the mitochondrial respiratory chain, is the major oxygen consumer enzyme in the cell. COX biogenesis involves several redox-regulated steps. The process is highly regulated to prevent the formation of pro-oxidant intermediates.

RECENT ADVANCES

Regulation of COX assembly involves several reactive oxygen species and redox-regulated steps. These include: (i) Intricate redox-controlled machineries coordinate the expression of COX isoenzymes depending on the environmental oxygen concentration. (ii) COX is a heme A-copper metalloenzyme. COX copper metallation involves the copper chaperone Cox17 and several other recently described cysteine-rich proteins, which are oxidatively folded in the mitochondrial intermembrane space. Copper transfer to COX subunits 1 and 2 requires concomitant transfer of redox power. (iii) To avoid the accumulation of reactive assembly intermediates, COX is regulated at the translational level to minimize synthesis of the heme A-containing Cox1 subunit when assembly is impaired.

CRITICAL ISSUES

An increasing number of regulatory pathways converge to facilitate efficient COX assembly, thus preventing oxidative stress.

FUTURE DIRECTIONS

Here we will review on the redox-regulated COX biogenesis steps and will discuss their physiological relevance. Forthcoming insights into the precise regulation of mitochondrial COX biogenesis in normal and stress conditions will likely open future perspectives for understanding mitochondrial redox regulation and prevention of oxidative stress.

Regulation of cation balance in Saccharomyces cerevisiae

All living organisms require nutrient minerals for growth and have developed mechanisms to acquire, utilize, and store nutrient minerals effectively. In the aqueous cellular environment, these elements exist as charged ions that, together with protons and hydroxide ions, facilitate biochemical reactions and establish the electrochemical gradients across membranes that drive cellular processes such as transport and ATP synthesis. Metal ions serve as essential enzyme cofactors and perform both structural and signaling roles within cells. However, because these ions can also be toxic, cells have developed sophisticated homeostatic mechanisms to regulate their levels and avoid toxicity. Studies in Saccharomyces cerevisiae have characterized many of the gene products and processes responsible for acquiring, utilizing, storing, and regulating levels of these ions. Findings in this model organism have often allowed the corresponding machinery in humans to be identified and have provided insights into diseases that result from defects in ion homeostasis. This review summarizes our current understanding of how cation balance is achieved and modulated in baker's yeast. Control of intracellular pH is discussed, as well as uptake, storage, and efflux mechanisms for the alkali metal cations, Na(+) and K(+), the divalent cations, Ca(2+) and Mg(2+), and the trace metal ions, Fe(2+), Zn(2+), Cu(2+), and Mn(2+). Signal transduction pathways that are regulated by pH and Ca(2+) are reviewed, as well as the mechanisms that allow cells to maintain appropriate intracellular cation concentrations when challenged by extreme conditions, i.e., either limited availability or toxic levels in the environment.

Morphological development and cytochrome c oxidase activity in Streptomyces lividans are dependent on the action of a copper bound Sco protein

Copper has an important role in the life cycle of many streptomycetes, stimulating the developmental switch between vegetative mycelium and aerial hyphae concomitant with the production of antibiotics. In streptomycetes, a gene encoding for a putative Sco-like protein has been identified and is part of an operon that contains two other genes predicted to handle cellular copper. We report on the Sco-like protein from Streptomyces lividans (Sco^Sl) and present a series of experiments that firmly establish a role for Sco^Sl as a copper metallochaperone as opposed to a role as a thiol-disulphide reductase that has been assigned to other bacterial Sco proteins. Under low copper concentrations, a Δsco mutant in S. lividans displays two phenotypes; the development switch between vegetative mycelium and aerial hyphae stalls and cytochrome c oxidase (CcO) activity is significantly decreased. At elevated copper levels, the development and CcO activity in the Δsco mutant are restored to wild-type levels and are thus independent of Sco^Sl. A CcO knockout reveals that morphological development is independent of CcO activity leading us to suggest that Sco^Sl has at least two targets in S. lividans. We establish that one Sco^Sl target is the dinuclear Cu_A domain of CcO and it is the cupric form of Sco^Sl that is functionally active. The mechanism of cupric ion capture by Sco^Sl has been investigated, and an important role for a conserved His residue is identified.

Biogenesis and assembly of eukaryotic cytochrome c oxidase catalytic core

Eukaryotic cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial respiratory chain. COX is a multimeric enzyme formed by subunits of dual genetic origin which assembly is intricate and highly regulated. The COX catalytic core is formed by three mitochondrial DNA encoded subunits, Cox1, Cox2 and Cox3, conserved in the bacterial enzyme. Their biogenesis requires the action of messenger-specific and subunit-specific factors which facilitate the synthesis, membrane insertion, maturation or assembly of the core subunits. The study of yeast strains and human cell lines from patients carrying mutations in structural subunits and COX assembly factors has been invaluable to identify these ancillary factors. Here we review the current state of knowledge of the biogenesis and assembly of the eukaryotic COX catalytic core and discuss the degree of conservation of the players and mechanisms operating from yeast to human. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Plants contain two SCO proteins that are differentially involved in cytochrome c oxidase function and copper and redox homeostasis

Two Arabidopsis thaliana genes (HCC1 and HCC2), resulting from a duplication that took place before the emergence of flowering plants, encode proteins with homology to the SCO proteins involved in copper insertion during cytochrome c oxidase (COX) assembly in other organisms. Heterozygote HCC1 mutant plants produce 25% abnormal seeds with defective embryos arrested at the heart or torpedo stage. These embryos lack COX activity, suggesting that the requirement of HCC1 during the early stages of plant development is related with its COX assembly function. Homozygote HCC2 mutant plants develop normally and do not show changes in COX2 levels. These plants display increased sensitivity of root growth to increased copper and a higher expression of miR398 and other genes that respond to copper limitation, in spite of the fact that they have a higher copper content than the wild type. HCC2 mutant plants also show increased expression of stress-responsive genes. The results suggest that HCC1 is the protein involved in COX biogenesis and that HCC2, that lacks the cysteines and histidine putatively involved in copper binding, functions in copper sensing and redox homeostasis. In addition, plants that overexpress HCC1 have an altered response of root elongation to changes in copper in the growth medium and increased expression of two low-copper-responsive genes, suggesting that HCC1 may also have a role in copper homeostasis.

HCC1, the Arabidopsis homologue of the yeast mitochondrial copper chaperone SCO1, is essential for embryonic development

The Arabidopsis HCC1 gene is a homologue of the copper chaperone SCO1 from the yeast Saccharomyces cerevisiae. SCO1 (synthesis of cytochrome c oxidase 1) encodes a mitochondrial protein that is essential for the correct assembly of complex IV in the respiratory chain. GUS analyses showed HCC1 promoter activity in vascular tissue, guard cells, hydathodes, trichome support cells, and embryos. HCC1 function was studied in two hcc1 T-DNA insertion lines, hcc1-1 and hcc1-2. Gametophyte development was not affected by the disruption of HCC1, but homozygous hcc1-1 and hcc1-2 embryos became arrested at various developmental stages, mostly at the heart stage. Both the wild-type HCC1 gene and the modified gene coding for the C-terminally SNAP-tagged HCC1 were able to complement the embryo-lethal phenotype of the hcc1-1 line. Localization of the SNAP-tagged HCC1 in transgenic lines identified HCC1 as a mitochondrial protein. To determine if HCC1 is a functional homologue to Sco1p, the respiratory-deficient yeast sco1 mutant was transformed with chimeric constructs containing different combinations of HCC1 and SCO1 sequences. One of the resulting chimeric proteins restored respiration in the yeast mutant. This protein had the N-terminal mitochondrial targeting signal and the single transmembrane domain derived from Sco1p and the C-terminal half (including the copper-binding motif) derived from HCC1. Growth of the complemented yeast mutant was enhanced by the addition of copper to the medium. The data demonstrate that HCC1 is essential for embryo development in Arabidopsis, possibly due to its role in cytochrome c oxidase assembly.

The intermembrane space of mitochondria

Mitochondria contain two aqueous compartments: the matrix and the intermembrane space. Whereas many of the biologic functions of the matrix were well characterized in the past, it became clear very recently that the intermembrane space plays a pivotal role in the coordination of mitochondrial activities with other cellular processes. These activities include the exchange of proteins, lipids, or metal ions between the matrix and the cytosol, the regulated initiation of apoptotic cascades, signalling pathways that regulate respiration and metabolic functions, the prevention of reactive oxygen species produced by the respiratory chain, or the control of mitochondrial morphogenesis. We focus on the different biologic functions of the intermembrane space and discuss the relevance of this fascinating compartment for cellular physiology and human health.

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.

Reactive species and mitochondrial dysfunction: mechanistic significance of 4-hydroxynonenal

Mitochondrial dysfunction is a global term used in the context of "unhealthy" mitochondria. In practical terms, mitochondria are extremely complex and highly adaptive in structure, chemical and enzymatic composition, subcellular distribution and functional interaction with other components of cells. Consequently, altered mitochondrial properties that are used in experimental studies as measures of mitochondrial dysfunction often provide little or no distinction between adaptive and maladaptive changes. This is especially a problem in terms of generation of oxidant species by mitochondria, wherein increased generation of superoxide anion radical (O(2*)(-)) or hydrogen peroxide (H(2)O(2)) is often considered synonymously with mitochondrial dysfunction. However, these oxidative species are signaling molecules in normal physiology so that a change in production or abundance is not a good criterion for mitochondrial dysfunction. In this review, we consider generation of reactive electrophiles and consequent modification of mitochondrial proteins as a means to define mitochondrial dysfunction. Accumulated evidence indicates that 4-hydroxynonenal (HNE) modification of proteins reflects mitochondrial dysfunction and provides an operational criterion for experimental definition of mitochondrial dysfunction. Improved means to detect and quantify mitochondrial HNE-protein adduct formation could allow its use for environmental healthrisk assessment. Furthermore, application of improved mass spectrometry-based proteomic methods will lead to further understanding of the critical targets contributing to disease risk.

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.

The evolutionarily conserved leprecan gene: its regulation by Brachyury and its role in the developing Ciona notochord

In Ciona intestinalis, leprecan was identified as a target of the notochord-specific transcription factor Ciona Brachyury (Ci-Bra) (Takahashi, H., Hotta, K., Erives, A., Di Gregorio, A., Zeller, R.W., Levine, M., Satoh, N., 1999. Brachyury downstream notochord differentiation in the ascidian embryo. Genes Dev. 13, 1519-1523). By screening approximately 14 kb of the Ci-leprecan locus for cis-regulatory activity, we have identified a 581-bp minimal notochord-specific cis-regulatory module (CRM) whose activity depends upon T-box binding sites located at the 3'-end of its sequence. These sites are specifically bound in vitro by a GST-Ci-Bra fusion protein, and mutations that abolish binding in vitro result in loss or decrease of regulatory activity in vivo. Serial deletions of the 581-bp notochord CRM revealed that this sequence is also able to direct expression in muscle cells through the same T-box sites that are utilized by Ci-Bra in the notochord, which are also bound in vitro by the muscle-specific T-box activators Ci-Tbx6b and Ci-Tbx6c. Additionally, we created plasmids aimed to interfere with the function of Ci-leprecan and categorized the resulting phenotypes, which consist of variable dislocations of notochord cells along the anterior-posterior axis. Together, these observations provide mechanistic insights generally applicable to T-box transcription factors and their target sequences, as well as a first set of clues on the function of Leprecan in early chordate development.

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.

Suppression mechanisms of COX assembly defects in yeast and human: insights into the COX assembly process

Eukaryotic cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial respiratory chain. COX is a multimeric enzyme formed by subunits of dual genetic origin whose assembly is intricate and highly regulated. In addition to the structural subunits, a large number of accessory factors are required to build the holoenzyme. The function of these factors is required in all stages of the assembly process. They are relevant to human health because devastating human disorders have been associated with mutations in nuclear genes encoding conserved COX assembly factors. The study of yeast strains and human cell lines from patients carrying mutations in structural subunits and COX assembly factors has been invaluable to attain the current state of knowledge, even if still fragmentary, of the COX assembly process. After the identification of the genes involved, the isolation and characterization of genetic and metabolic suppressors of COX assembly defects, reviewed here, have become a profitable strategy to gain insight into their functions and the pathways in which they operate. Additionally, they have the potential to provide useful information for devising therapeutic approaches to combat human disorders associated with COX deficiency.

"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.

Function and redox state of mitochondrial localized cysteine-rich proteins important in the assembly of cytochrome c oxidase

The cytochrome c oxidase (CcO) complex of the mitochondrial respiratory chain exists within the mitochondrial inner membrane (IM). The biogenesis of the complex is a multi-faceted process requiring multiple assembly factors that function on both faces of the IM. Formation of the two copper centers of CcO occurs within the intermembrane space (IMS) and is dependent on assembly factors with critical cysteinyl thiolates. Two classes of assembly factors exist, one group being soluble IMS proteins and the second class being proteins tethered to the IM. A common motif in the soluble assembly factors is a duplicated Cx(9)C sequence motif. Since mitochondrial respiration is a major source of reactive oxygen species, control of the redox state of mitochondrial proteins is an important process. This review documents the role of these cysteinyl CcO assembly factors within the IMS and the necessity of redox control in their function.

Translocation of mitochondrially synthesized Cox2 domains from the matrix to the intermembrane space

The N-terminal and C-terminal domains of mitochondrially synthesized cytochrome c oxidase subunit II, Cox2, are translocated through the inner membrane to the intermembrane space (IMS). We investigated the distinct mechanisms of N-tail and C-tail export by analysis of epitope-tagged Cox2 variants encoded in Saccharomyces cerevisiae mitochondrial DNA. Both the N and C termini of a truncated protein lacking the Cox2 C-terminal domain were translocated to the IMS via a pathway dependent upon the conserved translocase Oxa1. The topology of this Cox2 variant, accumulated at steady state, was largely but not completely unaffected in mutants lacking proteins required for export of the C-tail domain, Cox18 and Mss2. C-tail export was blocked by truncation of the last 40 residues from the C-tail domain, indicating that sequence and/or structural features of this domain are required for its translocation. Mss2, a peripheral protein bound to the inner surface of the inner membrane, coimmunoprecipitated with full-length newly synthesized Cox2, whose leader peptide had already been cleaved in the IMS. Our data suggest that the C-tail domain is recognized posttranslationally by a specialized translocation apparatus after the N-tail has been translocated by Oxa1.

Copper trafficking to the mitochondrion and assembly of copper metalloenzymes

Copper is required within the mitochondrion for the function of two metalloenzymes, cytochrome c oxidase (CcO) and superoxide dismutase (Sod1). Copper metallation of these two enzymes occurs within the mitochondrial intermembrane space and is mediated by metallochaperone proteins. Cox17 is a key copper donor to two accessory proteins, Sco1 and Cox11, to form the two copper centers in the mature CcO complex. Ccs1 is the necessary metallochaperone for the copper metallation of Sod1 in the IMS as well as within the cytoplasm where the bulk of Sod1 resides. Copper ions used in the metallation of CcO and Sod1 appear to be provided by a novel copper pool within the mitochondrial matrix. This review documents copper ion shuttling within the mitochondrion and the proteins that mediate assembly of active CcO and Sod1.

Assembly of mitochondrial cytochrome c-oxidase, a complicated and highly regulated cellular process

Cytochrome c-oxidase (COX), the terminal enzyme of the mitochondrial respiratory chain, plays a key role in the regulation of aerobic production of energy. Biogenesis of eukaryotic COX involves the coordinated action of two genomes. Three mitochondrial DNA-encoded subunits form the catalytic core of the enzyme, which contains metal prosthetic groups. Another 10 subunits encoded in the nuclear DNA act as a protective shield surrounding the core. COX biogenesis requires the assistance of >20 additional nuclear-encoded factors acting at all levels of the process. Expression of the mitochondrial-encoded subunits, expression and import of the nuclear-encoded subunits, insertion of the structural subunits into the mitochondrial inner membrane, addition of prosthetic groups, assembly of the holoenzyme, further maturation to form a dimer, and additional assembly into supercomplexes are all tightly regulated processes in a nuclear-mitochondrial-coordinated fashion. Such regulation ensures the building of a highly efficient machine able to catalyze the safe transfer of electrons from cytochrome c to molecular oxygen and ultimately facilitate the aerobic production of ATP. In this review, we will focus on describing and analyzing the present knowledge about the different regulatory checkpoints in COX assembly and the dynamic relationships between the different factors involved in the process. We have used information mostly obtained from the suitable yeast model, but also from bacterial and animal systems, by means of large-scale genetic, molecular biology, and physiological approaches and by integrating information concerning individual elements into a cellular system network.

Defenses against oxidative stress in Neisseria gonorrhoeae: a system tailored for a challenging environment

Neisseria gonorrhoeae is a host-adapted pathogen that colonizes primarily the human genitourinary tract. This bacterium encounters reactive oxygen and reactive nitrogen species as a consequence of localized inflammatory responses in the urethra of males and endocervix of females and also of the activity of commensal lactobacilli in the vaginal flora. This review describes recent advances in the understanding of defense systems against oxidative stress in N. gonorrhoeae and shows that while some of its defenses have similarities to the paradigm established with Escherichia coli, there are also some key differences. These differences include the presence of a defense system against superoxide based on manganese ions and a glutathione-dependent system for defense against nitric oxide which is under the control of a novel MerR-like transcriptional regulator. An understanding of the defenses against oxidative stress in N. gonorrhoeae and their regulation may provide new insights into the ways in which this bacterium survives challenges from polymorphonuclear leukocytes and urogenital epithelial cells.

Redox pathways of the mitochondrion

The mitochondrion houses a variety of redox pathways, utilized for protection from oxidative damage and assembly of the organelle. The glutathione/glutaredoxin and thioredoxin systems function in the mitochondrial matrix. The intermembrane space is protected from oxidative damage via superoxide dismutase and glutathione. Subunits in the cytochrome bc (1) complex utilize disulfide bonds for enzymatic activity, whereas cytochrome oxidase relies on disulfide linkages for copper acquisition. A redox pathway (Mia40p and Erv1p) mediates the import of intermembrane space proteins such as the small Tim proteins, Cox17p, and Cox19p, which have disulfide bonds. Many of the candidate proteins with disulfide bridges possess a twin CX3C motif or CX9C motif and utilize both metal binding and disulfide linkages for function. It may seem surprising that the intermembrane space has developed redox pathways, considering that the buffered environment should be reducing like the cytosol. However, the prokaryotic origin of the mitochondrion suggests that the intermembrane space may be akin to the oxidative environment of the bacterial periplasm. Although the players forming disulfide bonds are not conserved between mitochondria and prokaryotes, the mitochondrion may have maintained redox chemistry as an assembly mechanism in the intermembrane space for the import of proteins and metals and enzymatic activity.

Crystal structure of yeast Sco1

The Sco family of proteins are involved in the assembly of the dinuclear CuA site in cytochrome c oxidase (COX), the terminal enzyme in aerobic respiration. These proteins, which are found in both eukaryotes and prokaryotes, are characterized by a conserved CXXXC sequence motif that binds copper ions and that has also been proposed to perform a thiol:disulfide oxidoreductase function. The crystal structures of Saccharomyces cerevisiae apo Sco1 (apo-ySco1) and Sco1 in the presence of copper ions (Cu-ySco1) were determined to 1.8- and 2.3-A resolutions, respectively. Yeast Sco1 exhibits a thioredoxin-like fold, similar to that observed for human Sco1 and a homolog from Bacillus subtilis. The Cu-ySco1 structure, obtained by soaking apo-ySco1 crystals in copper ions, reveals an unexpected copper-binding site involving Cys181 and Cys216, cysteine residues present in ySco1 but not in other homologs. The conserved CXXXC cysteines, Cys148 and Cys152, can undergo redox chemistry in the crystal. An essential histidine residue, His239, is located on a highly flexible loop, denoted the Sco loop, and can adopt positions proximal to both pairs of cysteines. Interactions between ySco1 and its partner proteins yeast Cox17 and yeast COX2 are likely to occur via complementary electrostatic surfaces. This high-resolution model of a eukaryotic Sco protein provides new insight into Sco copper binding and function.

Tellurite effects on Rhodobacter capsulatus cell viability and superoxide dismutase activity under oxidative stress conditions

Cells of the facultative photosynthetic bacterium Rhodobacter capsulatus (MT1131 strain) incubated with 10 microg ml-1 of the toxic oxyanion tellurite (TeO2-(3)) exhibited an increase in superoxide dismutase activity. The latter effect was also seen upon incubation with sublethal amounts of paraquat, a cytosolic generator of superoxide anions (O2-), in parallel with a strong increase in tellurite resistance (TeR). A mutant strain (CW10) deficient in SenC, a protein with similarities to peroxiredoxin/thiol:disulfide oxidoreductases and a homologue of mitochondrial Sco proteins, was constructed by interposon mutagenesis via the gene transfer agent system. Notably, the absence of SenC affected R. capsulatus resistance to periplasmic O2- generated by xanthine/xanthine oxidase but not to cytosolic O2- produced by paraquat. Further, the absence of SenC did not affect R. capsulatus tellurite resistance. We conclude that: (1) cytosolic-generated O2- enhances TeR of this bacterial species; (2) small amounts of tellurite increase SOD activity so as to mimic the early cell response to oxidative stress; (3) SenC protein is required in protection of R. capsulatus against periplasmic oxidative stress; and finally, (4) SenC protein is not involved in TeR, possibly because tellurite does not generate O-2 at the periplasmic space level.

Inactivation of homocitrate synthase causes lysine auxotrophy in copper/zinc-containing superoxide dismutase-deficient yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe lacking copper/zinc-containing superoxide dismutase (CuZn-SOD) is auxotrophic for lysine and sulfurous amino acids under aerobic growth conditions. A multicopy suppressor gene (phx1+) that restored the growth of CuZn-SOD-deficient cells on minimal medium was isolated. It encodes a putative DNA-binding protein with a conserved homeobox domain. Overproduction of Phx1 increased the amount of several proteins, and one of those turned out to be a putative homocitrate synthase (HCS) encoded by the lys4+ gene in S. pombe as judged by mass spectrometric analysis. Consistent with this observation, overexpression of the lys4+ gene increased HCS enzyme activity and was sufficient to suppress the lysine requirement of the CuZn-SOD-deficient cells. Enzyme activity and Western blot analyses revealed that the activity and protein level of HCS were dramatically reduced upon depletion of CuZn-SOD. Treatment of exponentially growing S. pombe cells with paraquat, a superoxide generator, caused a decrease in the amount of Lys4 protein as expected. These results led us to conclude that HCS, the first enzyme in the alpha-aminoadipate-mediated pathway for lysine synthesis common in fungi and some bacteria, is a labile target of oxidative stress caused by CuZn-SOD depletion and that its synthesis is positively regulated by the putative transcriptional regulator Phx1.

Biogenesis of cytochrome c oxidase

Cytochrome c oxidase (COX), the terminal enzyme of electron transport chains in some prokaryotes and in mitochondria, has been characterized in detail over many years. Recently, a number of new data on structural and functional aspects as well as on COX biogenesis emerged. COX biogenesis includes a variety of steps starting from translation to the formation of the mature complex. Each step involves a set of specific factors that assist translation of subunits, their translocation across membranes, insertion of essential cofactors, assembly and final maturation of the enzyme. In this review, we focus on the organization and biogenesis of COX.

Human Sco1 and Sco2 function as copper-binding proteins

The function of human Sco1 and Sco2 is shown to be dependent on copper ion binding. Expression of soluble domains of human Sco1 and Sco2 either in bacteria or the yeast cytoplasm resulted in the recovery of copper-containing proteins. The metallation of human Sco1, but not Sco2, when expressed in the yeast cytoplasm is dependent on the co-expression of human Cox17. Two conserved cysteines and a histidyl residue, known to be important for both copper binding and in vivo function in yeast Sco1, are also critical for in vivo function of human Sco1 and Sco2. Human and yeast Sco proteins can bind either a single Cu(I) or Cu(II) ion. The Cu(II) site yields S-Cu(II) charge transfer transitions that are not bleached by weak reductants or chelators. The Cu(I) site exhibits trigonal geometry, whereas the Cu(II) site resembles a type II Cu(II) site with a higher coordination number. To identify additional potential ligands for the Cu(II) site, a series of mutant proteins with substitutions in conserved residues in the vicinity of the Cu(I) site were examined. Mutation of several conserved carboxylates did not alter either in vivo function or the presence of the Cu(II) chromophore. In contrast, replacement of Asp238 in human or yeast Sco1 abrogated the Cu(II) visible transitions and in yeast Sco1 attenuated Cu(II), but not Cu(I), binding. Both the mutant yeast and human proteins were nonfunctional, suggesting the importance of this aspartate for normal function. Taken together, these data suggest that both Cu(I) and Cu(II) binding are critical for normal Sco function.

Functional Analysis of the Domains in Cox11

Cox11 is an intrinsic mitochondrial membrane protein essential for the assembly of an active cytochrome c oxidase complex. Cox11 is tethered to the mitochondrial inner membrane by a single transmembrane helix. Domain mapping was carried out to determine the functional segments of the Cox11 protein. The C-terminal 189 residue Cu(I)-binding domain is shown to be exposed within the mitochondrial intermembrane space. This orientation was demonstrated by the proteolytic susceptibility of a C-terminal Myc epitope tag in mitoplasts but not intact mitochondria. Fusion of the N terminus of Cox11 to the matrix ribosomal protein Rsm22 results in a functional protein capable of suppressing the respiratory defect of both Deltacox11 cells and Deltarsm22 cells. The functionality of the fusion protein suggests that the Cox11 N terminus projects into the matrix. The fusion of the C-terminal segment of Cox11 to Rsm22 resembles a naturally occurring fusion of Cox11 in Schizosaccharomyces pombe to a sequence homologous to the Saccharomyces cerevisiae Rsm22. Studies on a series of SCO1/COX11 chimeras reveal that the matrix domain of Cox11 lacks a specific function, whereas the Cu(I) binding/donating function requires the yeast Cox11 sequence. The Cu(I)-binding domain from human Cox11 cannot functionally replace the yeast sequence. The copper domain of Cox11 may be an important docking motif for Cox1 or a Cox1-associated protein.

Crystal Structure of Human SCO1

Human SCO1 and SCO2 are copper-binding proteins involved in the assembly of mitochondrial cytochrome c oxidase (COX). We have determined the crystal structure of the conserved, intermembrane space core portion of apo-hSCO1 to 2.8 A. It is similar to redox active proteins, including thioredoxins (Trx) and peroxiredoxins (Prx), with putative copper-binding ligands located at the same positions as the conserved catalytic residues in Trx and Prx. SCO1 does not have disulfide isomerization or peroxidase activity, but both hSCO1 and a sco1 null in yeast show extreme sensitivity to hydrogen peroxide. Of the six missense mutations in SCO1 and SCO2 associated with fatal mitochondrial disorders, one lies in a highly conserved exposed surface away from the copper-binding region, suggesting that this region is involved in protein-protein interactions. These data suggests that SCO functions not as a COX copper chaperone, but rather as a mitochondrial redox signaling molecule.

Solution structure of Sco1: a thioredoxin-like protein Involved in cytochrome c oxidase assembly

Sco1, a protein required for the proper assembly of cytochrome c oxidase, has a soluble domain anchored to the cytoplasmic membrane through a single transmembrane segment. The solution structure of the soluble part of apoSco1 from Bacillus subtilis has been solved by NMR and the internal mobility characterized. Its fold places Sco1 in a distinct subgroup of the functionally unrelated thioredoxin proteins. In vitro Sco1 binds copper(I) through a CXXXCP motif and possibly His 135 and copper(II) in two different species, thus suggesting that copper(II) is adventitious more than physiological. The Sco1 structure represents the first structure of this class of proteins, present in a variety of eukaryotic and bacterial organisms, and elucidates a link between copper trafficking proteins and thioredoxins. The availability of the structure has allowed us to model the homologs Sco1 and Sco2 from S. cerevisiae and to discuss the physiological role of the Sco family.

The Schizosaccharomyces pombe Pccs protein functions in both copper trafficking and metal detoxification pathways

Because copper is both an essential cofactor and a toxic metal, different strategies have evolved to appropriately regulate its homeostasis as a function of changing environmental copper levels. In this report, we describe a metallochaperone-like protein from Schizosaccharomyces pombe that maintains the delicate balance between essentiality and toxicity. This protein, designated Pccs, has four distinct domains. SOD activity assays reveal that the first three domains of Pccs are necessary and sufficient to deliver copper to its target, copper-zinc superoxide dismutase (SOD1). Pccs domain IV, which is absent in Saccharomyces cerevisiae CCS1, contains seventeen cysteine residues, eight pairs of which are in a potential metal coordination arrangement, Cys-Cys. We show that S. cerevisiae ace1Delta mutant cells expressing the full-length Pccs molecule are resistant to copper toxicity. Furthermore, we demonstrate that the Pccs domain IV enhances copper resistance of the ace1Delta cells by an order of magnitude compared with that observed in the same strain expressing a pccs+ I-II-III allele encoding Pccs domains I-III. We consistently found that S. pombe cells disrupted in the pccs+ gene exhibit an increased sensitivity to copper and cadmium. Furthermore, we demonstrate that overexpression of pccs+ is associated with increased copper resistance in fission yeast cells. Taken together, our findings suggest that Pccs activates apo-SOD1 under copper-limiting conditions through the use of its first three domains and protects cells against metal ion toxicity via its fourth domain.

Yeast contain a non-proteinaceous pool of copper in the mitochondrial matrix

The yeast mitochondrion is shown to contain a pool of copper that is distinct from that associated with the two known mitochondrial cuproenzymes, superoxide dismutase (Sod1) and cytochrome c oxidase (CcO) and the copper-binding CcO assembly proteins Cox11, Cox17, and Sco1. Only a small fraction of mitochondrial copper is associated with these cuproproteins. The bulk of the remainder is localized within the matrix as a soluble, anionic, low molecular weight complex. The identity of the matrix copper ligand is unknown, but the bulk of the matrix copper fraction is not protein-bound. The mitochondrial copper pool is dynamic, responding to changes in the cytosolic copper level. The addition of copper salts to the growth medium leads to an increase in mitochondrial copper, yet the expansion of this matrix pool does not induce any respiration defects. The matrix copper pool is accessible to a heterologous cuproenzyme. Co-localization of human Sod1 and the metallochaperone CCS within the mitochondrial matrix results in suppression of growth defects of sod2Delta cells. However, in the absence of CCS within the matrix, the activation of human Sod1 can be achieved by the addition of copper salts to the growth medium.

CtaG is required for formation of active cytochrome c oxidase in Bacillus subtilis

The Gram-positive bacteriumBacillus subtiliscontains two respiratory oxidases of the haem-copper superfamily: cytochromeaa3, which is a quinol oxidase, and cytochromecaa3, which is a cytochromecoxidase. Cytochromecoxidase uniquely contains a di-copper centre, CuA.B. subtilisCtaG is a membrane protein encoded by the same gene cluster as that which encodes the subunits of cytochromecoxidase. The role ofB. subtilisCtaG and orthologous proteins present in many other Gram-positive bacteria has remained unexplored. The sequence of CtaG is unrelated to that of CtaG/Cox11p of proteobacteria and eukaryotic cells. This study shows thatB. subtilisCtaG is essential for the formation of active cytochromecaa3but is not required for assembly of the core subunits I and II with haem in the membrane and it has no role in the synthesis of active cytochromeaa3.B. subtilisYpmQ, a homologue to Sco1p of eukaryotic cells, is also a membrane-bound cytochromecoxidase-specific assembly factor. Properties of CtaG- and YpmQ-deficient mutants were compared. Cells lacking YpmQ showed a low cytochromecoxidase activity and this defect was suppressed by the supplementation of the growth medium with copper ions. It has previously been proposed that YpmQ/Sco1p is involved in synthesis of the CuAcentre. The results of this study are consistent with this proposal but the exact role of YpmQ in assembly of cytochromecoxidase remains to be elucidated.

Cox17 Is Functional When Tethered to the Mitochondrial Inner Membrane

Cox17 is an essential protein in the assembly of cytochrome c oxidase within the mitochondrion. Cox17 is implicated in providing copper ions for formation of CuA and CuB sites in the oxidase complex. To address whether Cox17 is functional in shuttling copper ions to the mitochondrion, Cox17 was tethered to the mitochondrial inner membrane by a fusion to the transmembrane domain of the inner membrane protein, Sco2. The copper-binding domain of Sco2 that projects into the inter-mitochondrial membrane space was replaced with Cox17. The Sco2/Cox17 fusion protein containing the mitochondrial import sequence and transmembrane segment of Sco2 is exclusively localized within the mitochondrion. The Sco2/Cox17 protein restores respiratory growth and normal cytochrome oxidase activity in cox17Delta cells. These studies suggest that the function of Cox17 is confined to the mitochondrial intermembrane space. Domain mapping of yeast Cox17 reveals that the carboxyl-terminal segment of the protein has a function within the intermembrane space that is independent of copper ion binding. The essential C-terminal function of Cox17 maps to a candidate amphipathic helix that is important for mitochondrial uptake and retention of the Cox17 protein. This motif can be spatially separated from the N-terminal copper-binding functional motif. Possible roles of the C-terminal motif are discussed.

The many highways for intracellular trafficking of metals

Metal ions such as copper and manganese represent a unique problem to living cells in that these ions are not only essential co-factors for metalloproteins, but are also potentially toxic. To aid in the homeostatic balance of essential but toxic metals, cells have evolved with a complex network of metal trafficking pathways. The object of such pathways is two-fold: to prevent accumulation of the metal in the freely reactive form (metal detoxification pathways) and to ensure proper delivery of the ion to target metalloproteins (metal utilization pathways). Much of what we currently know regarding these complex pathways of metal trafficking has emerged from molecular genetic studies in baker's yeast, Saccharomyces cerevisiae. In this review, we shall briefly highlight the current understanding of factors that function in the trafficking and handling of copper, including copper detoxification factors, copper transporters and copper chaperones. In addition, very recent findings on the players involved in manganese trafficking will be presented. The goal is to provide a paradigm for the intracellular handling of metals that may be applied in a more general sense to metals that serve essential functions in biology.

A Sco homologue plays a role in defence against oxidative stress in pathogenic Neisseria

Sco proteins are found in mitochondria and in a variety of oxidase positive bacteria. Although Sco is required for the formation of the Cu(A) centre in a cytochrome oxidase of the aa(3) type, it was observed that oxidases with a Cu(A) centre are not present in many bacteria that contain a Sco homologue. Two bacteria of this type are the pathogens Neisseria meningitidis and Neisseria gonorrhoeae. The sco genes of N. gonorrhoeae strain 1291 and N. meningitidis strain MC58 were cloned, inactivated by inserting a kanamycin resistance cassette and used to make knockout mutants by allelic exchange. Both N. gonorrhoeae and N. meningitidis sco mutants were highly sensitive to oxidative killing by paraquat, indicating that Sco is involved in protection against oxidative stress in these bacteria.

Assembly of cytochrome c oxidase within the mitochondrion

Cytochrome c oxidase (CcO) is an oligomeric complex localized within the mitochondrial inner membrane. Assembly of the active oxidase complex requires the coordinate assembly of subunits synthesized in both the cytoplasm and the mitochondrion. In addition, assembly is dependent on the insertion of five types of cofactors, including two hemes, three copper ions, and one Zn, Mg, and Na ion. A series of accessory proteins are critical for synthesis of the heme A cofactor and insertion of the copper ions. This Account will focus on the steps in the coordinate assembly of CcO subunits, the formation of heme A, and the delivery and insertion of copper ions.