Journal of Biological Chemistry
Volume 275, Issue 39, 29 September 2000, Pages 30623-30630
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PROTEIN STRUCTURE AND FOLDING
Brain S100A5 Is a Novel Calcium-, Zinc-, and Copper Ion-binding Protein of the EF-hand Superfamily*

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S100A5 is a novel member of the EF-hand superfamily of calcium-binding proteins that is poorly characterized at the protein level. Immunohistochemical analysis demonstrates that it is expressed in very restricted regions of the adult brain. Here we characterized the human recombinant S100A5, especially its interaction with Ca2+, Zn2+, and Cu2+. Flow dialysis revealed that the homodimeric S100A5 binds four Ca2+ ions with strong positive cooperativity and an affinity 20–100-fold higher than the other S100 proteins studied under identical conditions. S100A5 also binds two Zn2+ ions and four Cu2+ ions per dimer. Cu2+ binding strongly impairs the binding of Ca2+; however, none of these ions change the α-helical-rich secondary structure. After covalent labeling of an exposed thiol with 2-(4′-(iodoacetamide)anilino)-naphthalene-6-sulfonic acid, binding of Cu2+, but not of Ca2+ or Zn2+, strongly decreased its fluorescence. In light of the three-dimensional structure of S100 proteins, our data suggest that in each subunit the single Zn2+ site is located at the opposite side of the EF-hands. The two Cu2+-binding sites likely share ligands of the EF-hands. The potential role of S100A5 in copper homeostasis is discussed.

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Published, JBC Papers in Press, July 5, 2000, DOI 10.1074/jbc.M002260200

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This work was supported by Swiss National Science Foundation Grant 0031-050510.97, the EMDO Foundation, Union Bank of Switzerland (on behalf of an anonymous client), the Julius Klaus Foundation, and BIOMED 2, European Union Grant BMH4CT950319/BBW Switzerland Grant 950215-1.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.