Cysteine-to-serine mutants of a maltose binding protein fusion with the human copper chaperone for superoxide dismutase (hCCS) were studied with respect to (i) their ability to transfer Cu to E,Zn superoxide dismutase (SOD) and (ii) their Zn and Cu binding and X-ray absorption spectroscopic (XAS) properties. Previous work has established that Cu(I) binds to four cysteine residues, two of which, C22 and C25, reside within an Atox1-like N-terminal domain (DI) and two of which, C244 and C246, reside in a short unstructured polypeptide chain at the C-terminus (DIII). The wild-type (WT) protein shows an extended X-ray absorption fine structure (EXAFS) spectrum characteristic of cluster formation, but it is not known how such a cluster is formed. Cys to Ser mutagenesis was used to investigate the Cu binding in more detail. Single Cys to Ser mutations, as represented by C22S and C244S, did little to affect the metal binding ratios of hCCS. Both mutants still showed approximately 2 Cu(I) ions and 1 Zn ion per protein. The double mutants C22/24S and C244/246S, on the other hand, showed Cu binding stoichiometries close to 1:1. The Zn-EXAFS of WT CCS showed a 3-4 histidine ligand environment that is consistent with Zn binding in the SOD-like domain II of CCS. The Zn environment remained unchanged between wild type and all of the mutant CCS proteins. Single Cys to Ser mutations displayed lower activity than WT protein, although close to full activity could be rescued by increasing the CCS:SOD ratios to 8:1 in the assay mixture. The structure of the Cu centers of the single mutants as revealed by EXAFS was also similar to that of WT protein, with clear indications of a Cu cluster. On the other hand, the double mutants showed a greater degree of perturbation. The DI C22/25S mutant was 70% active and formed a cluster with a more intense Cu-Cu interaction. The DIII C244/246S mutant retained only a fraction (16%) of activity and did not form a cluster. The results suggest the formation of a DIII-DIII cluster within a dimeric or tetrameric protein and further suggest that this cluster may be an important element of the copper transfer machinery.
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