Copper chaperone for superoxide dismutase-1 transfers copper to mitochondria but does not affect cytochrome c oxidase activity.

Copper chaperone for superoxide dismutase-1 (CCS-1) is present in the cytosol and in the intermembrane space of mitochondria. It transfers copper ions to superoxide dismutase 1 in the cytosol, but its function in the mitochondria is not clear. The present study was undertaken to test the hypothesis that CCS-1 functions in mitochondrial copper homeostasis. Mitochondria were isolated from human umbilical vein endothelial cells and copper concentrations in the mitochondria were measured in the CCS-1 deficient cells made by siRNA targeting the protein. Copper concentrations in the mitochondria were about 10 fold higher than its total concentrations in the cell and the CCS-1 deficiency significantly reduced the copper level in the mitochondria. However, this decrease in the mitochondrial copper concentration did not affect cytochrome c oxidase (CCO) activity. On the other hand, siRNA targeting COX17, a copper chaperone for the CCO, significantly increased the mitochondrial copper concentration, but suppressed the CCO activity. This study thus demonstrates that CCS-1 facilitates copper trafficking to the mitochondria, but does not affect the transfer of copper to the CCO. In addition, the COX17 not only functions in the copper shuttling to the CCO, but also may participate in the copper efflux from the mitochondria.