Spin-Label EPR on α-Synuclein Reveals Differences in the Membrane Binding Affinity of the Two Antiparallel Helices

The putative function of the Parkinson's disease-related protein alpha-Synuclein (alpha S) is thought to involve membrane binding. Therefore, the interaction of alpha S with membranes composed of zwitter ionic (POPC) and anionic (POPG) lipids was investigated through the mobility of spin labels attached to the protein. Differently labelled variants of alpha S were produced, containing a spin label at positions 9, 18 (both helix 1), 69, 90 (both helix 2), and 140 (C terminus). Protein binding to POPC/POPG vesicles for all but alpha S140 resulted in two mobility components with correlation times of 0.5 and 3 ns, for POPG mole fractions >0.4. Monitoring these components as a function of the POPG mole fraction revealed that at low negative charge densities helix 1 is more tightly bound than helix 2, this indicates a partly bound form of alpha S. Thus, the interaction of alpha S with membranes of low charge densities might be initiated at helix). The local binding information thus obtained gives a more differentiated picture of the affinity of alpha S to membranes. These findings contribute to our understanding of the details and structural consequences of alpha S-membrane interactions.