Four methionine pairs (C, green; O, orange; N, blue; S, yellow) in each protomer of CusA form the path of Cu(I) or Ag(I) transport. Metal ions may enter from the cytoplasmic side of the channel, via M410-M501, or through the periplasmic cleft between the sub-domains PC1 and PC2, via the three-methionine binding site M573, M623 and M672. These pathways are illustrated in black.