Broken helix in vesicle and micelle-bound alpha-synuclein: Insights from site-directed spin labeling-EPR experiments and MD simulations

Human a-Synuclein (aS) is a 140 amino acid-protein strongly associated to both familial and sporadic cases of Parkinson’s disease. The physiological role of aS is still elusive, although its nerve terminal localization and its ability to interact with membranes seem to be the key factors for its physiological functions. aS is a natively unfolded, soluble protein whose primary sequence is characterized by the presence of seven imperfect 11-residue repeats potentially able to fold into an amphipathic helix. In the presence of sodium dodecyl sulfate (SDS) micelles or synthetic membranes, the first ~100 residues of aS undergo a conformational transition to a helical state. We report here on an SDSL-EPR study of the controversial interhelix region of aS, when bound to either SUV or SDS micelles, accompanied by modeling of the spin label in the two proposed protein conformations. The investigation is completed by a Molecular Dynamics (MD) simulation of the 31-52 fragment interacting with a lipid bilayer. Our data show that the 38-44 region of aS exhibits a very similar behavior in micelles and in SUV. Specifically, we find evidence for a high degree of conformational disorder rather than for the formation of a continuous helical structure. The main result of our investigation is that SUV-bound aS bears most of the features reported for it in micellar environment: an unbroken helical structure of the region around residue 40 can be ruled out. Helix breaking does not appear to be a mere consequence of the constraints imposed by the small micellar dimensions, but as an intrinsic feature of aS, when bound to amphipathic interfaces. Furthermore, we can confirm the picture of the interhelix region as characterized by conformational disorder, rather than exhibiting a single structure. This disorder might play a role in aS binding to synaptic vesicles, by allowing the protein to fit into amphipathic aggregates with different degrees of lipid packing strain.