Copper and Zinc Binding Modulates the Aggregation and Neurotoxic Properties of the Prion Peptide PrP106−126

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Abstract
The abnormal form of the prion protein (PrP) is believed to be responsible for the transmissible spongiform encephalopathies. A peptide encompassing residues 106−126 of human PrP (PrP106−126) is neurotoxic in vitro due its adoption of an amyloidogenic fibril structure. The Alzheimer's disease amyloid β peptide (Aβ) also undergoes fibrillogenesis to become neurotoxic. Aβ aggregation and toxicity is highly sensitive to copper, zinc, or iron ions. We show that PrP106−126 aggregation, as assessed by turbidometry, is abolished in Chelex-100-treated buffer. ICP-MS analysis showed that the Chelex-100 treatment had reduced Cu2+ and Zn2+ levels approximately 3-fold. Restoring Cu2+ and Zn2+ to their original levels restored aggregation. Circular dichroism showed that the Chelex-100 treatment reduced the aggregated β-sheet content of the peptide. Electron paramagnetic resonance spectroscopy identified a 2N1S1O coordination to the Cu2+ atom, suggesting histidine 111 and methionine 109 or 112 are involved. Nuclear magnetic resonance confirmed Cu2+ and Zn2+ binding to His-111 and weaker binding to Met-112. An N-terminally acetylated PrP106−126 peptide did not bind Cu2+, implicating the free amino group in metal binding. Mutagenesis of either His-111, Met-109, or Met-112 abolished PrP106−126 neurotoxicity and its ability to form fibrils. Therefore, Cu2+ and/or Zn2+ binding is critical for PrP106−126 aggregation and neurotoxicity.