Alpha-Synuclein Promotes Structural Reorganization of Raft-Like Membranes

Abstract

α-Synuclein is an intrinsically-disordered protein in vitro that undergoes spontaneous coil-helix transitions in the presence of large membranes causing structural disruption [1-2]. One potential membrane target for this interaction is a raft-like lipid mixture that mimics the exocytotic active zone of the neuron. Using solid-state 2D 13C magic-angle spinning (MAS) NMR and electron microscopy (EM), we studied site-resolved molecular and macroscopic structural changes that occur in the presence of wild-type α-synuclein or its N-terminal (1-25) consensus sequence. The raft system is a highly-ordered, heterogeneous system composed of liquid-ordered and liquid-disordered phases. This was determined through interpretation of chemical shifts and residual magnetic dipolar couplings (RDCs). We also used a mean-torque structural model that defined average cross-sectional areas and volumetric hydrocarbon thicknesses for the lipid domains. α-Synuclein caused striking changes of the RDCs and chemical shifts of the membrane components. These perturbations suggested that α-synuclein associates interfacially with the membrane, specifically through hydrogen-bonding with EYSM and cholesterol, yielding drastic changes in the membrane hydrocarbon region. Molecular cross-sectional areas for each lipid are significantly increased, and there is a striking 6 A decrease in membrane thickness. These site-specific changes in molecular structure reflect a protein-assisted mixing of membrane components, and disruption of raft-like domains. Corroborating our NMR structural analysis, the EM images revealed a multilamellar distribution of large vesicles that is dramatically disrupted by α-synuclein. Our results define both molecular and macroscopic membrane structures, and suggest that lipid heterogeneity is an important aspect of the α-synuclein-membrane interaction that may be key in understanding the molecular pathology of Parkinson's disease [3]. [1] K. Beyer (2007) Cell Biochem. Biophys.47 285-299. [2] T. Bartels et al. (2010) Biophys. J. (in press). [3] F. Kamp et al. (2010) EMBO J. (in press).