Ctr4 has a Cys- and Trp-rich sequence in the N-terminal extracellular domain.

10">Cu(II) is reduced and stabilized as Cu(I) by the Cys/Trp-motif.

15">Trp protects the thiolate-Cu(I) center against oxidation via cation–π interaction.

This sequence may be a novel Cu(I)-binding motif in an oxidizing environment.