The P(174)L mutation in the human hSCO1 gene affects the assembly of cytochrome c oxidase

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.

Copper metallochaperones

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

Copper metallochaperones

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

Copper metallochaperones

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

Human Sco1 functional studies and pathological implications of the P174L mutant

The pathogenic mutant (P174L) of human Sco1 produces respiratory chain deficiency associated with cytochrome c oxidase (CcO) assembly defects. The solution structure of the mutant in its Cu(I) form shows that Leu-174 prevents the formation of a well packed hydrophobic region around the metal-binding site and causes a reduction of the affinity of copper(I) for the protein. K(D) values for Cu(I)WT-HSco1 and Cu(I)P174L-HSco1 are approximately 10(-17) and approximately 10(-13), respectively. The reduction potentials of the two apo proteins are similar, but slower reduction/oxidation rates are found for the mutant with respect to the WT. The mitochondrial metallochaperone in the partially oxidized Cu(1)(I)Cox17(2S-S) form, at variance with the fully reduced Cu(4)(I)Cox17, interacts transiently with both WT-HSco1 and the mutant, forming the Cox17/Cu(I)/HSco1 complex, but copper is efficiently transferred only in the case of WT protein. Cu(1)(I)Cox17(2S-S) indeed has an affinity for copper(I) (K(D) approximately 10(-15)) higher than that of the P174L-HSco1 mutant but lower than that of WT-HSco1. We propose that HSco1 mutation, altering the structure around the metal-binding site, affects both copper(I) binding and redox properties of the protein, thus impairing the efficiency of copper transfer to CcO. The pathogenic mutation therefore could (i) lessen the Sco1 affinity for copper(I) and hence copper supply for CcO or (ii) decrease the efficiency of reduction of CcO thiols involved in copper binding, or both effects could be produced by the mutation.

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.

The P174L Mutation in Human Sco1 Severely Compromises Cox17-dependent Metallation but Does Not Impair Copper Binding

Sco1 is a metallochaperone that is required for copper delivery to the Cu(A) site in the CoxII subunit of cytochrome c oxidase. The only known missense mutation in human Sco1, a P174L substitution in the copper-binding domain, is associated with a fatal neonatal hepatopathy; however, the molecular basis for dysfunction of the protein is unknown. Immortalized fibroblasts from a SCO1 patient show a severe deficiency in cytochrome c oxidase activity that was partially rescued by overexpression of P174L Sco1. The mutant protein retained the ability to bind Cu(I) and Cu(II) normally when expressed in bacteria, but Cox17-mediated copper transfer was severely compromised both in vitro and in a yeast cytoplasmic assay. The corresponding P153L substitution in yeast Sco1 was impaired in suppressing the phenotype of cells harboring the weakly functional C57Y allele of Cox17; however, it was functional in sco1delta yeast when the wild-type COX17 gene was present. Pulse-chase labeling of mitochondrial translation products in SCO1 patient fibroblasts showed no change in the rate of CoxII translation, but there was a specific and rapid turnover of CoxII protein in the chase. These data indicate that the P174L mutation attenuates a transient interaction with Cox17 that is necessary for copper transfer. They further suggest that defective Cox17-mediated copper metallation of Sco1, as well as the subsequent failure of Cu(A) site maturation, is the basis for the inefficient assembly of the cytochrome c oxidase complex in SCO1 patients.

Involvement of SenC in assembly of cytochrome c oxidase in Rhodobacter capsulatus

SenC, a Sco1 homolog found in the purple photosynthetic bacteria, has been implicated in affecting photosynthesis and respiratory gene expression, as well as assembly of cytochrome c oxidase. In this study, we show that SenC from Rhodobacter capsulatus is involved in the assembly of a fully functional cbb(3)-type cytochrome c oxidase, as revealed by decreased cytochrome c oxidase activity in a senC mutant. We also show that a putative copper-binding site in SenC is required for activity and that a SenC deletion phenotype can be rescued by the addition of exogenous copper to the growth medium. In addition, we demonstrate that a SenC mutation has an indirect effect on gene expression caused by a reduction in cytochrome c oxidase activity. A model is proposed whereby a reduction in cytochrome c oxidase activity impedes the flow of electrons through the respiratory pathway, thereby affecting the oxidation/reduction state of the ubiquinone pool, leading to alterations of photosystem and respiratory gene expression.

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.

Cytochrome c oxidase biogenesis in a patient with a mutation in COX10 gene

We report a cytochrome c oxidase (COX)-deficient patient, clinically affected with Leigh-like disease, with a homozygous mutation in the COX10 start codon. Two-dimensional gel electrophoresis showed a decrease of fully assembled COX without the accumulation of partially assembled COX subcomplexes. Western blot analysis with antibodies directed to COX subunits I, II, and IV showed a decrease of these subunits in this patient compared with control. Overexpression of the COX10 protein in the patient's fibroblasts proved that the detected mutation was indeed the disease cause.

Expression, purification, and characterization of BsSco, an accessory protein involved in the assembly of cytochrome c oxidase in Bacillus subtilis

The studies described here were performed to characterize further the plasma membrane associated protein BsSco, which is the product of the gene ypmQ, in Bacillus subtilis. BsSco is a member of the Sco family of proteins found in the inner mitochondrial membrane of yeast and humans and implicated as an accessory protein in the assembly of the Cu(A) site of cytochrome c oxidase. We have cloned the gene expressing BsSco, placed a six-histidine tag on its C-terminus, and over-expressed this protein in B. subtilis. Recombinant BsSco with the his-tag has been purified from Triton X-100-solubilized plasma membranes by nickel metal affinity chromatography. Mass spectral analysis of the purified protein is consistent with processing of BsSco by signal peptidase II removing an N-terminal putative transmembrane sequence to leave an acyl-glyceryl moiety at cysteine residue 19. Antibodies, raised against purified, recombinant BsSco, were used to characterize the timing of the level of native BsSco in batch cultures of wild-type B. subtilis. There is a marked lag in the level of native BsSco, but it does appear prior to cytochrome c oxidase, which is expressed in late stage growth. This work supports a role for BsSco in the assembly of the Cu(A) site of cytochrome c oxidase and its functional relationship to the Sco proteins found in eukaryotic cells.

Copper chaperones for cytochrome c oxidase and human disease

Biological processes in living cells are compartmentalized between lipid membranes. Integral membrane proteins often confer specific functions to these compartments and as such have a critical role in cellular metabolism and function. Cytochrome c oxidase is a macromolecular metalloprotein complex essential for the respiratory function of the cell. Elucidating the mechanisms of assembly of cytochrome c oxidase within the inner mitochondrial membrane represents a unique challenge for understanding metalloprotein biosynthesis. Elegant genetic experiments in yeast have defined several proteins required for copper delivery to cytochrome c oxidase. While the precise role of each of these proteins in copper incorporation remains unclear, recent studies have revealed that inherited mutations in two of these proteins can result in severe pathology in human infants in association with cytochrome c oxidase deficiency. Characterization of the molecular pathogenesis of these disorders offers new insights into the mechanisms of cellular copper metabolism and the role of these cytochrome c oxidase copper chaperones in human disease.

Molecular characterization of Saccharomyces cerevisiae Sco2p reveals a high degree of redundancy with Sco1p

The Saccharomyces cerevisiae gene SCO1 has been shown to play an essential role in the transfer of copper to the Cu(A)-centre of the mitochondrial cytochrome c oxidase subunit Cox2p. By contrast, the function of Sco2p, the gene product of the highly homologous SCO2 gene, remains to be elucidated. Deletion of the SCO2 gene does not affect growth on a variety of carbon sources, including glycerol, lactate and ethanol. We report here, that Sco2p is anchored in the mitochondrial membrane by a single transmembrane segment and displays a similar tripartite structure as Sco1p. Most parts of Sco1p can be replaced by the homologous parts of Sco2p without loss of function. A short stretch of 13 amino acids, immediately adjacent to the transmembrane region, is crucial for Sco1p function and cannot be replaced by its Sco2p counterpart. We propose that this region is relevant for the correct spatial orientation of the C-terminal part of the protein. Immunoprecipitation and in vitro binding assays show that Sco2p interacts with the C-terminal portion of Cox2p. This interaction is neither dependent on bound copper ions nor on the presence of Sco1p. Furthermore we report on in vitro binding assays which show that Sco2p can form homomeric complexes, but also heteromeric complexes with Sco1p. Our data suggest that Sco2p is involved in the transfer of copper to Cox2p, but that this activity is insufficient for oxidative growth and not able to substitute for Sco1p activity.

Cytochrome oxidase in health and disease

Yeast and bovine cytochrome c oxidases (COX) are composed of 12 and 13 different polypeptides, respectively. In both cases, the three subunits constituting the catalytic core are encoded by mitochondrial DNA. The other subunits are all products of nuclear genes that are translated on cytoplasmic ribosomes and imported through different transport routes into mitochondria. Biogenesis of the functional complex depends on the expression of all the structural and more than two dozen COX-specific genes. The latter impinge on all aspects of the biogenesis process. Here we review the current state of information about the functions of the COX-specific gene products and of their relationship to human COX deficiencies.

Mitochondrial oxidative phosphorylation system assembly in man: recent achievements

The human oxidative phosphorylation system consists of five multi-subunit complexes of which the individual subunits, with the exception of complex II, are encoded either by mitochondrial or nuclear DNA. Consequently, a deficient enzyme activity of one or more of the complexes can be caused by mitochondrial or nuclear DNA mutations. In the past 5 years numerous mutations have been found in structural nuclear oxidative phosphorylation system genes. However, in a substantial number of patients with oxidative phosphorylation system complex deficiencies, despite extensive investigations, no mutations in the mitochondrial DNA or the structural nuclear genes have been found. Genetic defects in such patients are therefore suspected at the transcriptional, translational, post-translational level or in gene products involved in the assembly of the oxidative phosphorylation system. The latter is a complicated process, as the proteins encoded by the two genomes have to be brought together in a proper stoichiometric way to form five functional complexes. In the past year substantial progress in the knowledge of the human oxidative phosphorylation assembly process has been made. Several human assembly genes have been identified, and mutations in these genes responsible for human oxidative phosphorylation system complex-related diseases have been found. In this review, we summarize our current knowledge about human oxidative phosphorylation system assembly genes in health and disease.