Metal interactions of alpha-synuclein probed by NMR amide-proton exchange

The aberrant aggregation of a-synuclein (aS), a disordered protein primarily expressed in neuronal cells, is strongly associated with the underlying mechanisms of Parkinson's disease. It is now established that aS has a weak affinity for metal ions and that these interactions alter its conformational properties by generally promoting self-assembly into amyloids. Here, we characterised the nature of the conformational changes associated with metal binding by aS using nuclear magnetic resonance (NMR) to measure the exchange of the backbone amide protons at a residue specific resolution. We complemented these experiments with N-15 relaxation and chemical shift perturbations to obtain a comprehensive map of the interaction between aS and divalent (Ca2+, Cu2+, Mn2+, and Zn2+) and monovalent (Cu+) metal ions. The data identified specific effects that the individual cations exert on the conformational properties of aS. In particular, binding to calcium and zinc generated a reduction of the protection factors in the C-terminal region of the protein, whereas both Cu(II) and Cu(I) did not alter the amide proton exchange along the aS sequence. Changes in the R-2/R-1 ratios from N-15 relaxation experiments were, however, detected as a result of the interaction between aS and Cu+ or Zn2+, indicating that binding to these metals induces conformational perturbations in distinctive regions of the protein. Collectively our data suggest that multiple mechanisms of enhanced aS aggregation are associated with the binding of the analysed metals.