Misfolding of the
cellular prion protein (PrPC) is associated with the development of fatal neurodegenerative diseases
called transmissible spongiform encephalopathies (TSEs). Metal ions
appear to play a crucial role in PrPCmisfolding. PrPCis a combined Cu(II) and Zn(II) metal-binding protein, where
the main metal-binding site is located in the octarepeat (OR) region.
Thus, the biological function of PrPCmay involve the transport
of divalent metal ions across membranes or buffering concentrations
of divalent metal ions in the synaptic cleft. Recent studies have
shown that an excess of Cu(II) ions can result in PrPCinstability,
oligomerization, and/or neuroinflammation. Here, we have used biophysical
methods to characterize Cu(II) and Zn(II) binding to the isolated
OR region of PrPC. Circular dichroism (CD) spectroscopy
data suggest that the OR domain binds up to four Cu(II) ions or two
Zn(II) ions. Binding of the first metal ion results in a structural
transition from the polyproline II helix to the β-turn structure,
while the binding of additional metal ions induces the formation of
β-sheet structures. Fluorescence spectroscopy data indicate
that the OR region can bind both Cu(II) and Zn(II) ions at neutral
pH, but under acidic conditions, it binds only Cu(II) ions. Molecular
dynamics simulations suggest that binding of either metal ion to the
OR region results in the formation of β-hairpin structures.
As the formation of β-sheet structures can be a first step toward
amyloid formation, we propose that high concentrations of either Cu(II)
or Zn(II) ions may have a pro-amyloid effect in TSE diseases.