A Cu chaperone for SOD1 (CCS) is required for the incorporation of copper ion into the protein. To investigate the roles of the conserved metal-binding residues in CCS, we introduced amino acid substitutions into human CCS and examined the function of the mutant CCS by transforming a mutant yeast strain, SY2950, which lacks the lys7 gene, a yeast orthologue of the mammalian CCS. Mutant CCS in which amino acid residues His147 and Asp167 were substituted by Ala exhibited a decreased ability to complement the growth of SY2950 under Lys-deficient conditions. This is because the mutations made the human CCS function in a less efficient manner, especially under metal-restricted conditions, leaving Cu,Zn-SOD in an apo-form. Since the His and Asp residues are both responsible for binding Zn which would serve to maintain the folded structure, the structural integrity supported by the coordinated Zn ion would be essential for CCS function,