Copper metallochaperone essential for the assembly of the mitochondrial respiratory chain complex IV (CIV), also known as cytochrome c oxidase. Binds two copper ions and delivers them to the metallochaperone SCO1 which transports the copper ions to the Cu(A) site on the cytochrome c oxidase subunit II (MT-CO2/COX2). Interacts with COA1. Interacts with the chaperone CHCHD4; this is important for correct folding and the formation of disulfide bonds that stabilize the structure. Belongs to the COX17 family.