Isolation and characterization of Cox17p from porcine heart by determining its survival-promoting activity in NIH3T3 cells

Roles of COMM-domain-containing 1 in stability and recruitment of the copper-transporting ATPase in a mouse hepatoma cell line

A novel function of COMMD1 {COMM [copper metabolism MURR1 (mouse U2af1-rs1 region 1)]-domain-containing 1}, a protein relevant to canine copper toxicosis, was examined in the mouse hepatoma cell line Hepa 1-6 with multi-disciplinary techniques consisting of molecular and cellular biological techniques, speciation and elemental imaging. To clarify the function of COMMD1, COMMD1-knockdown was accomplished by introducing siRNA (small interfering RNA) into the cells. Although COMMD1-knockdown did not affect copper incorporation, it inhibited copper excretion, resulting in copper accumulation, which predominantly existed in the form bound to MT (metallothionein). It is known that the liver copper transporter Atp7b (ATP-dependent copper transporter 7beta), localizes on the trans-Golgi network membrane under basal copper conditions and translocates to cytoplasmic vesicles to excrete copper when its concentration exceeds a certain threshold, with the vesicles dispersing in the periphery of the cell. COMMD1-knockdown reduced the expression of Atp7b, and abolished the relocation of Atp7b back from the periphery to the trans-Golgi network membrane when the copper concentration was reduced by treatment with a Cu(I) chelator. The same phenomena were observed during COMMD1-knockdown when another Atp7b substrate, cis-diamminedichloroplatinum, and its sequestrator, glutathione ethylester, were applied. These results suggest that COMMD1 maintains the amount of Atp7b and facilitates recruitment of Atp7b from cytoplasmic vesicles to the trans-Golgi network membrane, i.e. COMMD1 is required to shuttle Atp7b when the intracellular copper level returns below the threshold.

Mitochondrial copper(I) transfer from Cox17 to Sco1 is coupled to electron transfer

The human protein Cox17 contains three pairs of cysteines. In the mitochondrial intermembrane space (IMS) it exists in a partially oxidized form with two S-S bonds and two reduced cysteines (HCox17(2S-S)). HCox17(2S-S) is involved in copper transfer to the human cochaperones Sco1 and Cox11, which are implicated in the assembly of cytochrome c oxidase. We show here that Cu(I)HCox17(2S-S), i.e., the copper-loaded form of the protein, can transfer simultaneously copper(I) and two electrons to the human cochaperone Sco1 (HSco1) in the oxidized state, i.e., with its metal-binding cysteines forming a disulfide bond. The result is Cu(I)HSco1 and the fully oxidized apoHCox17(3S-S), which can be then reduced by glutathione to apoHCox17(2S-S). The HSco1/HCox17(2S-S) redox reaction is thermodynamically driven by copper transfer. These reactions may occur in vivo because HSco1 can be found in the partially oxidized state within the IMS, consistent with the variable redox properties of the latter compartment. The electron transfer-coupled metallation of HSco1 can be a mechanism within the IMS for an efficient specific transfer of the metal to proteins, where metal-binding thiols are oxidized. The same reaction of copper-electron-coupled transfer does not occur with the human homolog of Sco1, HSco2, for kinetic reasons that may be ascribed to the lack of a specific metal-bridged protein-protein complex, which is instead observed in the Cu(I)HCox17(2S-S)/HSco1 interaction.

A structural-dynamical characterization of human Cox17

Human Cox17 is a key mitochondrial copper chaperone responsible for supplying copper ions, through the assistance of Sco1, Sco2, and Cox11, to cytochrome c oxidase, the terminal enzyme of the mitochondrial energy transducing respiratory chain. A structural and dynamical characterization of human Cox17 in its various functional metallated and redox states is presented here. The NMR solution structure of the partially oxidized Cox17 (Cox17(2S-S)) consists of a coiled coil-helix-coiled coil-helix domain stabilized by two disulfide bonds involving Cys(25)-Cys(54) and Cys(35)-Cys(44), preceded by a flexible and completely unstructured N-terminal tail. In human Cu(I)Cox17(2S-S) the copper(I) ion is coordinated by the sulfurs of Cys(22) and Cys(23), and this is the first example of a Cys-Cys binding motif in copper proteins. Copper(I) binding as well as the formation of a third disulfide involving Cys(22) and Cys(23) cause structural and dynamical changes only restricted to the metal-binding region. Redox properties of the disulfides of human Cox17, here investigated, strongly support the current hypothesis that the unstructured fully reduced Cox17 protein is present in the cytoplasm and enters the intermembrane space (IMS) where is then oxidized by Mia40 to Cox17(2S-S), thus becoming partially structured and trapped into the IMS. Cox17(2S-S) is the functional species in the IMS, it can bind only one copper(I) ion and is then ready to enter the pathway of copper delivery to cytochrome c oxidase. The copper(I) form of Cox17(2S-S) has features specific for copper chaperones.

Cox17, a copper chaperone for cytochrome c oxidase: Expression, purification, and formation of mixed disulphide adducts with thiol reagents

Copper chaperone for cytochrome c oxidase (Cox17) is a 7 kDa copper-binding protein, which facilitates incorporation of copper ions into Cu(A) site of cytochrome c oxidase. Cox17 contains six conserved Cys residues and occurs in three different oxidative states, which display different metal-binding properties and stability. In the present study, we have elaborated technologies for production of partially oxidized human recombinant Cox17 in a bacterial expression system and purification of fully oxidized Cox17. For this purpose we used Escherichia coli Origami strain, which is deficient in thioredoxin and thioredoxin reductase systems and allows formation of disulfide bonds in cytoplasmic proteins. Fully oxidized Cox17 was purified by a simplified two-step procedure including gel filtration and cation exchange chromatography. By using mass spectrometry we demonstrated that application of 2-mercaptoethanol (2-ME) during purification leads to formation of its mixed disulfide adducts with Cox17. Moreover, partially reduced Cox17 can form mixed disulfide adducts also with the cellular reducing agent glutathione, which abolishes copper-binding ability of partially reduced Cox17.

Metal-binding mechanism of Cox17, a copper chaperone for cytochrome c oxidase

Cox17, a copper chaperone for cytochrome c oxidase, is an essential and highly conserved protein. The structure and mechanism of functioning of Cox17 are unknown, and even its metalbinding stoichiometry is elusive. In the present study, we demonstrate, using electrospray ionization-MS, that porcine Cox17 binds co-operatively four Cu+ ions. Cu4Cox17 is stable at pH values above 3 and fluorescence spectra indicate the presence of a solvent-shielded multinuclear Cu(I) cluster. Combining our results with earlier EXAFS results on yeast CuCox17, we suggest that Cu4Cox17 contains a Cu4S6-type cluster. At supramillimolar concentrations, dithiothreitol extracts metals from Cu4Cox17, and an apparent copper dissociation constant KCu=13 fM was calculated from these results. Charge-state distributions of different Cox17 forms suggest that binding of the first Cu+ ion to Cox17 causes a conformational change from an open to a compact state, which may be the rate-limiting step in the formation of Cu4Cox17. Cox17 binds non-co-operatively two Zn2+ ions, but does not bind Ag+ ions, which highlights its extremely high metal-binding specificity. We further demonstrate that porcine Cox17 can also exist in partly oxidized (two disulphide bridges) and fully oxidized (three disulphide bridges) forms. Partly oxidized Cox17 can bind one Cu+ or Zn2+ ion, whereas fully oxidized Cox17 does not bind metals. The metal-binding properties of Cox17 imply that, in contrast with other copper chaperones, Cox17 is designed for the simultaneous transfer of up to four copper ions to partner proteins. Metals can be released from Cox17 by non-oxidative as well as oxidative mechanisms.

A selective requirement for copper-dependent activation of cytochrome c oxidase by Cox17p

Cox17p is cloned from yeast as a chaperone to deliver copper to the mitochondria of assembly for cytochrome c oxidase (CCO). In mammals, CCO is a key enzyme for cellular respiration and a defect in its function is associated with severe neonatal or infantile lactic acidosis and early death. Recently, we found that Cox17p is not only required for mitochondrial oxidative phosphorylation but also is essential for embryonic growth and development in COX17 gene-deficient mice. To investigate its biochemical features, recombinant human Cox17p was overexpressed and purified without a purification tag. It specifically binds Cu(I) at a molar copper content of 3.3+/-0.04 under reduced conditions and significantly activates the mitochondrial CCO in vitro. Although the Cu-Cox17p complex was maintained between pH values from 5.0 to 7.7, Cu was completely released from Cox17p at pH 8.0. An acute exposure of excess amount of copper ion to mouse cells resulted in a significant reduction of Cox17p mRNA expression, whereas copper starvation maintained the Cox17p transcription level. These results suggest that the stringent selectivity of Cox17p for copper is required for CCO activation, to prevent copper overload, or promote the supply of copper.

Mammalian copper chaperone Cox17p has an essential role in activation of cytochrome C oxidase and embryonic development

Cox17p is essential for the assembly of functional cytochrome c oxidase (CCO) and for delivery of copper ions to the mitochondrion for insertion into the enzyme in yeast. Although this small protein has already been cloned or purified from humans, mice, and pigs, the function of Cox17p in the mammalian system has not yet been elucidated. In vitro biochemical data for mammalian Cox17p indicate that the copper binds to the sequence -KPCCAC-. Although mouse embryos homozygous for COX17 disruption die between embryonic days E8.5 and E10, they develop normally until E6.5. This phenotype is strikingly similar to embryos of Ctr1(-/-), a cell surface copper transporter, in its lethality around the time of gastrulation. COX17-deficient embryos exhibit severe reductions in CCO activity at E6.5. Succinate dehydrogenase activity and immunoreactivities for anti-COX subunit antibodies were normal in the COX17(-/-) embryos, indicating that this defect was not caused by the deficiency of other complexes and/or subunits but was caused by impaired CCO activation by Cox17p. Since other copper chaperone (Atox1 and CCS)-deficient mice show a more moderate defect, the disruption of the COX17 locus causes the expression of only the phenotype of Ctr1(-/-). We found that the activity of lactate dehydrogenase was also normal in E6.5 embryos, implying that the activation of CCO by Cox17p may not be essential to the progress of embryogenesis before gastrulation.

Characterization and identification of promoter elements in the mouse COX17 gene

Cox17p, essential for the assembly of functional cytochrome c oxidase (CCO) in Saccharomyces cerevisiae, has been believed to deliver copper ions to the mitochondrion for insertion into the enzyme. We have recently isolated an approximately 20 kb genomic fragment of the mouse COX17. Reporter assay experiments have shown that most of the promoter activity was restricted to a 0.85 kb fragment flanking the first exon. Further intensive deletion and detailed mutation analysis suggested that the minimal essential region for transactivation was located at bases -155 to -70. This 5'-flanking region did not possess a TATA box, but contained putative Sp1, NRF-1 and NRF-2 binding sites. COX17 basal promoter activity was abrogated by site-directed mutagenesis of Sp1, NRF-1 and NRF-2 binding sites. Electrophoretic mobility shift assays with AtT-20 and NIH3T3 cell nuclear extract revealed that this region binds both a Sp1-like protein and NRF-1 transcription factors. These results indicated that Sp1, NRF-1 and NRF-2 are involved in basal transcription of the COX17 gene, similar to the transcription mechanism of other CCO-related genes.

Genomic structure of mouse copper chaperone, COX17

Coxl7p was first cloned as a cytoplasmic copper chaperone from yeast mutant and recent works suggested the existence of mammalian homologues. Previous report has shown that a gel filtration fraction of heart extract containing porcine Coxl7p peptide promoted the survival of NIH3T3 fibroblast cells. In the present study, we first cloned DNA fragments of the mouse COX17 gene. The mouse COX17 spans approximately 6kb and consists of three exons. It was mapped to the center of chromosome 16, using a radiation hybrid-mapping panel. The major transcription start site is 80 bp upstream of the ATG initiation codon as determined by rapid amplification of cDNA ends (5'-RACE) analysis. Two potential polyadenylation sites are 3233 and 3293 bp downstream of the termination codon, respectively. Transient transfection of reporter plasmids containing portions of the mouse COX17 5'-flanking region into AtT-20 and NIH3T3 cells allowed the localization of the essential promoter to a 0.8 kb region upstream of the transcription starting site. Furthermore, the transfected luciferase activity was much higher in AtT-20 than NIH3T3. According to sequence analysis of the approximately 0.8kb 5'-flanking region, GC rich segments including consensus sequences for binding of the transcription factor Sp1, but no TATA/CAAT boxes, exist in the region of the transcription start site. Besides the GC box, binding sites for NRF-1 and 2 known as specific transcription factors for COX subunits are also localized around the transcription starting site.

The expression of Cox17p in rodent tissues and cells

Previous works have reported the isolation of a novel polypeptide from porcine heart. Structural analysis has shown that it is a mammalian homologue of Cox17p, believed essential for the assembly of functional cytochrome c oxidase and delivery of copper ions to the mitochondrion for insertion into the enzyme in yeast. Although the human, mouse and porcine homologs of this small protein have already been cloned or purified, the function of Cox17p in the mammalian system has not yet been elucidated. To investigate the physiological function of Cox17p in mammals, we performed Northern blot analysis using probes containing the mouse and rat sequences obtained by RT-PCR. The hybridization signals were detected in all mouse tissues, but notably intense signals were observed in heart, brain and kidney RNA samples. Some of the neuroendocrine and endocrine cell lines showed higher expression levels than fibroblasts. The highest expression level of Cox17p mRNA in mouse brain was observed in the pituitary sample. While in rat heart, Cox17p mRNA expression was detected from early development, in rat brain, embryonic and postnatal changes in the expression were observed. Immunocytochemical analysis showed that Cox17p immunoreactivity was strong in the pituitary cell line, AtT-20. These findings suggested that Cox17p is not only part of the respiratory chain but also involved in brain and endocrine functions.