Crystal structure of human SCO1: implications for redox signaling by a mitochondrial cytochrome c oxidase "assembly" protein

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.