Clustering of alpha-synuclein on supported lipid bilayers: role of anionic lipid, protein, and divalent ion concentration.

Alpha-synuclein is the major component of Lewy body inclusions found in the brains of patients with Parkinson's disease. Several studies indicate that alpha-synuclein binds to negatively charged phospholipid bilayers. We examined the binding of alpha-synuclein to membranes containing different amounts of negatively charged lipids using supported lipid bilayers, epifluorescence microscopy, fluorescence recovery after photobleaching, and bulk fluorescence techniques. The membranes contained phosphatidylcholine and phosphatidylglycerol. In the absence of protein, these lipids mix uniformly. Our results show that the propensity of alpha-synuclein to cluster on the membrane increases as the concentration of anionic lipid and/or protein increases. Regions on the lipid bilayer where alpha-synuclein is clustered are enriched in phosphatidylglycerol. We also observe divalent metal ions stimulate protein cluster formation, primarily by promoting lipid demixing. The importance of protein structure, lipid demixing, and divalent ions, as well as the physiological implications, will be discussed. Because membrane-bound alpha-synuclein assemblies may play a role in neurotoxicity, it is of interest to determine how membranes can be used to tune the propensity of alpha-synuclein to aggregate.