The Clustering and Spatial Arrangement of β-Sheet Sequence, but Not Order, Govern α-Synuclein Fibrillogenesis

The intrinsically unstructured protein alpha-synuclein (as) is prone to misfold into cytotoxic beta-sheet-rich oligomers and amyloid fibrils that underlie the pathogenesis of Lewy body diseases such as Parkinson's disease. An important, recognized fibrillogenesis parameter is amino acid content, whereas the influence of amino acid sequence distribution is not as well understood. The fibril core of aS encompasses five regions of high beta-sheet propensity, termed beta 1-beta 5. Using four its variants with identical amino acid compositions but rearranged pseudorepeat motifs, we show that beta 2-beta 5 sequence clustering, but not order, is important for efficient fibrillogenesis. For molecular species progressing toward the fibrillar state, order invariably increases; i.e., the spatial arrangement of sequence elements becomes restricted. By introducing disulfide bonds in a fibril structure-based manner, we demonstrated that a successful protofibril-to-fibril conversion is dependent upon the spatial arrangement of sequence elements of high beta-sheet propensity. Moreover, a disulfide-linked aS dimer is shown to fibrillize rapidly. We propose that a conformational search underlies the emergence of a fibrillar aS nucleus that is directed by gaps in sequence between beta-sheet regions and the accessible range of spatial beta-sheet arrangements in Soluble, prefibrillar oligomers. Oil the basis of the universal cross-beta-sheet structure of amyloid fibrils, these principles are expected to apply to a wide range of amyloidogenic proteins.