Explaining the Structural Plasticity of α-Synuclein

Given that alpha-synuclein has been implicated in the pathogenesis of several neurodegenerative disorders, deciphering the structure of this protein is of particular importance. While monomeric alpha-synuclein is disordered in solution, it can form aggregates rich in cross-beta structure, relatively long helical segments when bound to micelles or lipid vesicles, and a relatively ordered helical tetramer within the native cell environment. To understand the physical basis underlying this structural plasticity, we generated an ensemble for monomeric alpha-synuclein using a Bayesian formalism that combines data from NMR chemical shifts, RDCs, and SAXS with molecular simulations. An analysis of the resulting ensemble suggests that a non-negligible fraction of the ensemble (0.08, 95% confidence interval 0.03-0.12) places the minimal toxic aggregation-prone segment in alpha-synuclein, NAC(8-18), in a solvent exposed and extended conformation that can form cross-beta structure. Our data also suggest that a sizable fraction of structures in the ensemble (0.14, 95% confidence interval 0.04-0.23) contains long-range contacts between the N- and C-termini. Moreover, a significant fraction of structures that contain these long-range contacts also place the NAC(8-18) segment in a solvent exposed orientation, a finding in contrast to the theory that such long-range contacts help to prevent aggregation. Lastly, our data suggest that alpha-synuclein samples structures with amphipathic helices that can self-associate via hydrophobic contacts to form tetrameric structures. Overall, these observations represent a comprehensive view of the unfolded ensemble of monomeric alpha-synudein and explain how different conformations can arise from the monomeric protein.