Required for mitochondrial cytochrome c oxidase (COX) assembly and respiration. May be involved in copper trafficking and distribution to mitochondrial COX and SOD1. Interacts with CMC1. Imported into the mitochondria via the mitochondrial MIA40-ERV1 machinery. The twin Cx9C motifs are involved in the recognition by the mitochondrial MIA40-ERV1 disulfide relay system and the subsequent transfer of disulfide bonds by dithiol/disulfide exchange reactions to the newly imported protein. Present with 279 molecules/cell in log phase SD medium. Belongs to the CMC family.