Characterization of alpha-synuclein interactions with selected aggregation-inhibiting small molecules

The 140-residue protein alpha-synuclein (aS) has been implicated in the molecular chain of events leading to Parkinson's disease, which relates to the hierarchical aggregation of aS into soluble oligomers and insoluble fibrils. A number of small organic molecules have been reported to inhibit aS aggregation. Here, the interactions of chlorazole black E, Congo red, lacmoid, PcTS-Cu (2+), and rosmarinic acid with aS are examined by NMR spectroscopy to identify aS sequence elements that are masked by these compounds. Surprisingly, similar aS interaction sites, encompassing residues 3-18 and 38-51, were obtained for all molecules at equimolar small molecule:aS ratios. At higher ratios, virtually the entire amphiphilic region of aS (residues 2-92) is affected, revealing the presence of additional, lower affinity interaction sites. Upon rearranging the high-affinity interaction sites over the aS amphiphilic region in an aS mutant form, perturbations of the entire amphiphilic region were found to have already been obtained at equimolar ratios, indicating a high specificity for the original binding sites. CD spectroscopy reveals that, in the presence of the small molecules, the aS structure is still dominated by random-coil characteristics. The strongest effects are exerted by molecules that contain sulfonate groups adjacent to aromatic systems, often present in multiple copies in a symmetrical arrangement, suggesting that these elements are useful for developing an aS-specific chemical chaperone.