Rosenzweig A C, Huffman D L, Hou M Y, Wernimont A K, Pufahl R A, O'Halloran T V
Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, IL 60208, USA.
Structure. 1999 Jun 15;7(6):605-17. doi: 10.1016/s0969-2126(99)80082-3.
Metallochaperone proteins function in the trafficking and delivery of essential, yet potentially toxic, metal ions to distinct locations and particular proteins in eukaryotic cells. The Atx1 protein shuttles copper to the transport ATPase Ccc2 in yeast cells. Molecular mechanisms for copper delivery by Atx1 and similar human chaperones have been proposed, but detailed structural characterization is necessary to elucidate how Atx1 binds metal ions and how it might interact with Ccc2 to facilitate metal ion transfer.
The 1.02 A resolution X-ray structure of the Hg(II) form of Atx1 (HgAtx1) reveals the overall secondary structure, the location of the metal-binding site, the detailed coordination geometry for Hg(II), and specific amino acid residues that may be important in interactions with Ccc2. Metal ion transfer experiments establish that HgAtx1 is a functional model for the Cu(I) form of Atx1 (CuAtx1). The metal-binding loop is flexible, changing conformation to form a disulfide bond in the oxidized apo form, the structure of which has been solved to 1.20 A resolution.
The Atx1 structure represents the first structure of a metallochaperone protein, and is one of the largest unknown structures solved by direct methods. The structural features of the metal-binding site support the proposed Atx1 mechanism in which facile metal ion transfer occurs between metal-binding sites of the diffusible copper-donor and membrane-tethered copper-acceptor proteins. The Atx1 structural motif represents a prototypical metal ion trafficking unit that is likely to be employed in a variety of organisms for different metal ions.
金属伴侣蛋白在真核细胞中负责将必需但可能有毒的金属离子运输并递送至不同位置和特定蛋白质。在酵母细胞中，Atx1蛋白将铜转运至运输ATP酶Ccc2。虽然已经提出了Atx1及类似人类伴侣蛋白进行铜递送的分子机制，但仍需要详细的结构表征来阐明Atx1如何结合金属离子以及它可能如何与Ccc2相互作用以促进金属离子转移。
Atx1的Hg(II)形式（HgAtx1）的X射线结构分辨率为1.02 Å，揭示了整体二级结构、金属结合位点的位置、Hg(II)的详细配位几何结构以及可能在与Ccc2相互作用中起重要作用的特定氨基酸残基。金属离子转移实验表明HgAtx1是Atx1的Cu(I)形式（CuAtx1）的功能模型。金属结合环是灵活的，在氧化的脱辅基形式中会改变构象以形成二硫键，其结构已解析至1.20 Å分辨率。
Atx1结构代表了金属伴侣蛋白的首个结构，并且是通过直接方法解析的最大未知结构之一。金属结合位点的结构特征支持了所提出的Atx1机制，即在可扩散的铜供体和膜结合的铜受体蛋白的金属结合位点之间发生便捷的金属离子转移。Atx1结构基序代表了一种典型的金属离子运输单元，可能在多种生物体中用于不同的金属离子。
