Characterization of inhibitor-bound alpha-synuclein dimer: role of alpha-synuclein N-terminal region in dimerization and inhibitor binding.

alpha-Synuclein is a major component of filamentous inclusions that are histological hallmarks of Parkinson's disease and other alpha-synucleinopathies. Previous analyses have revealed that several polyphenols inhibit alpha-synuclein assembly with low micromolar IC(50) values, and that SDS-stable, noncytotoxic soluble alpha-synuclein oligomers are formed in their presence. Structural elucidation of inhibitor-bound alpha-synuclein oligomers is obviously required for the better understanding of the inhibitory mechanism. In order to characterize inhibitor-bound alpha-synucleins in detail, we have prepared alpha-synuclein dimers in the presence of polyphenol inhibitors, exifone, gossypetin, and dopamine, and purified the products. Peptide mapping and mass spectrometric analysis revealed that exifone-treated alpha-synuclein monomer and dimer were oxidized at all four methionine residues of alpha-synuclein. Immunoblot analysis and redox-cycling staining of endoproteinase Asp-N-digested products showed that the N-terminal region (1-60) is involved in the dimerization and exifone binding of alpha-synuclein. Ultra-high-field NMR analysis of inhibitor-bound alpha-synuclein dimers showed that the signals derived from the N-terminal region of alpha-synuclein exhibited line broadening, confirming that the N-terminal region is involved in inhibitor-induced dimerization. The C-terminal portion still predominantly exhibited the random-coil character observed in monomeric alpha-synuclein. We propose that the N-terminal region of alpha-synuclein plays a key role in the formation of alpha-synuclein assemblies.