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 α-Synuclein (αS) is thought to involve membrane binding. Therefore, the interaction of αS with membranes composed of zwitterionic (POPC) and anionic (POPG) lipids was investigated through the mobility of spin labels attached to the protein. Differently labelled variants of 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 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 partially bound form of αS. Thus, the interaction of αS with membranes of low charge densities might be initiated at helix 1. The local binding information thus obtained gives a more differentiated picture of the affinity of αS to membranes. These findings contribute to our understanding of the details and structural consequences of αS-membrane interactions