Cu(I) Binding to Designed Proteins Reveals a Putative Copper Binding Site of the Human Line1 Retrotransposon Protein ORF1p

-HH and -HAAAH motifs act as fishing nets for biologically relevant metal ions in metallopeptides

Histidine are one of the most common residues involved in transition metal ion binding in the active sites of metalloenzymes. In order to mimic enzymatic metal binding sites, it is crucial to understand the basic coordination modes of histidine residues, distributed at different positions in the peptide sequence. We show that: (i) the separation of two histidines has a large effect on complex stability - a sequence with adjusting histidine residues forms more stable complexes with Zn(II) than the one in which the residues are separated, while the contrary is observed for Cu(II) complexes, in which amide nitrogens participate in metal binding. No pronounced effect is observed for Ni(II) complexes, where the amides participate in binding at higher pH; (ii) non-coordinating amino acid residues (basic, acidic and aromatic ones) have a significant impact on complex stability; charged and aromatic residues may enhance Zn(II) binding, while the contrary is observed for the amide-binding Cu(II); (iii) cysteine containing sequences are much more effective Zn(II) and Ni(II) binding motifs at pH above 8, while histidine containing ligands are more suitable for effective Zn(II) and Ni(II) binding at lower pH.

Disruption of zinc (II) binding and dimeric protein structure of the XIAP-RING domain by copper (I) ions

Modulation of metalloprotein structure and function via metal ion substitution may constitute a molecular basis for metal ion toxicity and/or metal-mediated functional control. The X-linked Inhibitor of Apoptosis Protein (XIAP) is a metalloprotein that requires zinc for proper structure and function. In addition to its role as a modulator of apoptosis, XIAP has been implicated in copper homeostasis. Given the similar coordination preferences of copper and zinc, investigation of XIAP structure and function upon interaction with copper is relevant. The Really Interesting New Gene (RING) domain of XIAP is representative of a class of zinc finger proteins that utilize a bi-nuclear zinc-binding motif to maintain proper structure and ubiquitin ligase function. Herein, we report the characterization of copper (I) binding to the Zn₂-RING domain of XIAP. Electronic absorption studies that monitor copper-thiolate interactions demonstrate that the RING domain of XIAP binds 5-6 Cu(I) ions and that copper is thermodynamically preferred relative to zinc. Repetition of the experiments in the presence of the Zn(II)-specific dye Mag-Fura2 shows that Cu(I) addition results in Zn(II) ejection from the protein, even in the presence of glutathione. Loss of dimeric structure of the RING domain, which is a requirement for its ubiquitin ligase activity, upon copper substitution at the zinc-binding sites, was readily observed via size exclusion chromatography. These results provide a molecular basis for the modulation of RING function by copper and add to the growing body of literature that describe the impact of Cu(I) on zinc metalloprotein structure and function.