Solution structure of the Cu(I) and apo forms of the yeast metallochaperone, Atx1

The ¹H NMR solution structure of the Cu(I)-bound form of Atx1, a 73-amino acid metallochaperone protein from the yeast Saccharomyces cerevisiae, has been determined. Ninety percent of the ¹H and 95% of the ¹⁵N resonances were assigned, and 1184 meaningful NOEs and 42 ³J_HNHα and 60 ¹J_HN residual dipolar couplings provided a family of structures with rmsd values to the mean structure of 0.37 ±0.07 Å for the backbone and 0.83 ±0.08 Å for all heavy atoms. The structure is constituted by four antiparallel β strands and two α helices in a βαββαβ fold. Following EXAFS data [Pufahl, R., Singer, C. P., Peariso, K. L., Lin, S.-J., Schmidt, P. J., Fahrni, C. J., Cizewski Culotta, V., Penner-Hahn, J. E., and O'Halloran, T. V. (1997) Science 278, 853-856], a copper ion can be placed between two sulfur atoms of Cys15 and Cys18. The structure of the reduced apo form has also been determined with similar resolution using 1252 meaningful NOEs (rmsd values for the family to the mean structure are 0.67 ±0.12 Å for the backbone and 1.00 ± 0.12 Å for all heavy atoms). Comparison of the Cu(I) and apo conformations of the protein reveals that the Cu(I) binding cysteines move from a buried site in the bound metal form to a solvent-exposed conformation on the surface of the protein after copper release. Furthermore, copper release leads to a less helical character in the metal binding site. Comparison with the Hg(II)-Atxl solid-state structure [Rosenzweig, A. C., Huffman, D. L., Hou, M. Y., Wernimont, A. K., Pufahl, R. A., and O'Halloran, T. V. (1999) Structure 7, 605-617] provides insights into the copper transfer mechanism, and a pivotal role for Lys65 in the metal capture and release process is proposed.