Human Sco1 and Sco2 Function as Copper-binding Proteins

Abstract

Cytochrome c oxidase (CcO) 5 is the terminal enzyme of the energytransducing, electron transfer chain within the mitochondrial inner membrane. Mammalian CcO consists of 13 polypeptide subunits, three of which (CoxI-CoxIII) are encoded by the mitochondrial genome and the remaining 10 of which are encoded by the nuclear genome (1, 2). An additional 30 proteins are believed to be required for the assembly of the CcO complex (3). A number of these accessory proteins are important in the processing and translation of COXI-COXIII mRNA transcripts, in membrane insertion of subunits, and in either the synthesis or delivery of cofactors. The cofactors in CcO include two copper sites (Cu A and Cu B ), two heme A moieties, and a magnesium and zinc ion (4).The Cu A site is a binuclear, mixed valent copper center localized in the CoxII subunit, whereas the Cu B site consists of a single copper ion within a Cu-heme A binuclear center in the CoxI subunit (4). The CoxI and CoxII subunits are synthesized within the mitochondrion, so copper site insertion must occur during insertion of the nascent polypeptides into the inner membrane.Several proteins, including Cox11, Cox17, Cox19, Cox23, and Sco1, have been implicated in the assembly of the copper centers in CcO in yeast and all have human homologs (5). Cox17 is a soluble copper metallochaperone within the mitochondrial intermembrane space (IMS). Sco1 and Cox11 are inner membrane proteins tethered by a single transmembrane helix and are implicated in the assembly of the Cu A and Cu B centers, respectively (6, 7). Both proteins are copper-binding proteins, and mutations that abrogate Cu(I) binding attenuate in vivo assembly of . Recently, we demonstrated that Cox17 is capable of donating Cu(I) to both Sco1 and Cox11 (12). The prediction is that copper ions are only transiently bound to Sco1 and Cox11 in yeast prior to donation to CoxII and CoxI, respectively. Yeast Sco1 was shown to form a transient complex with CoxII (13).Assembly of CcO in mammalian cells has additional components, since two distinct Sco-like molecules are involved in the assembly process. Mutations in both human genes, SCO1 and SCO2, have been identified, and these result in respiratory chain deficiency associated with CcO assembly defects (14 -16). Although yeast also has a second Sco protein, designated Sco2, it has no function in CcO assembly (17). The human Sco1 and Sco2 molecules share the Cu(I) binding motif of yeast Sco1; however, nothing is currently known about the copper binding by human Sco1, whereas limited data exist on copper binding by human Sco2 (18,19). Neither human Sco1 nor Sco2 is able to rescue the respiratory defect of yeast sco1 null cells (20). However, a chimera containing the N-terminal segment of yeast Sco1 fused to the C-terminal segment of human Sco1 is functional in sco1 null cells (20). Two lines of evidence suggest that both human Sco1 and Sco2 proteins probably function in copper metallation of CcO. First, overexpression of COX17 partially rescues the CcO deficiency of SCO2, ...

A hint for the function of human Sco1 from different structures

“…However, we have prepared the Cu(II) and Ni(II)HSco1 derivatives of reduced HSco1 in a 1:1 metal͞protein ratio. The ultraviolet͞visible (UV͞VIS) and EPR spectra of Cu(II)HSco1 are identical to those recently reported (15). Similar UV͞VIS spectra also were reported for bacterial Sco homologues (12,14,17) and Cu(II) nitrosocyanin (21,22).…”

“…2), located in the seventh ␤-strand, as shown by 2 J NH coupling-based 1 H- 15 N HSQC experiments , and the soluble fragment of HSco1 are orange, blue, and yellow, respectively. The N terminus of HSco1 protruding into the mitochondrial matrix is green.…”

“…In NMR, 15 N relaxation rates are modulated by the correlation time for the protein tumbling ( m ), which is directly related to the molecular weight of the protein, thus monitoring its aggregation state (24). The correlation times of Cu(I)HSco1 and Ni(II)HSco1 proteins (at millimolar concentrations) are 14.5 Ϯ 1.1 ns and 15.6 Ϯ 1.2, respectively, as expected for a protein of this size in a monomeric state.…”

“…The structure revealed a potential metal binding site constituted by two Cys residues present in a conserved motif CPXXCP and a fully conserved His residue, in agreement with earlier extended x-ray absorption fine structure investigations of yeast Cu(I)Sco1 (13). Besides extended x-ray absorption fine structure data many other spectroscopic data on the human, yeast, and B. subtilis Sco proteins have confirmed that the protein binds Cu(I), Cu(II), and other metal ions (14)(15)(16)(17). The fold of apoSco1 protein, which contains four ␣-helices and seven ␤-strands organized in two ␤-sheets (12), is atypical for a metal chaperone given that it resembles the fold of thioredoxins, which are enzymes specialized for the reduction of protein disulfides (18).…”

A stabilizing factor for mitochondrial respiratory supercomplex assembly regulates energy metabolism in muscle

“…For example, SURF1 is responsible for Leigh syndrome 9,10 , SCO1 for neonatal-onset hepatic failure and encephalopathy 11 , SCO2 for fatal infantile cardioencephalomyopathy 12 , and COX15 for fatal hypertrophic cardiomyopathy 13 . SCO1 and SCO2 are copper metallochaperones that are necessary for copper insertion into the catalytic core of COX assembly 14 . The latter has a particular role in the regulation of cellular copper homoeostasis as the overexpression of SCO2 can suppress cellular copper deficiency in fibroblasts from SCO2, SCO1 and COX15 mutation patients 15 .…”