Helical α-synuclein forms highly conductive ion channels

alpha-Synuclein (alpha S) is a cytosolic protein involved in the etiology of Parkinson's disease (PD). Disordered in an aqueous environment, alpha S develops a highly helical conformation when bound to membranes having a negatively charged surface and a large curvature. It exhibits a membrane-permeabilizing activity that has been attributed to oligomeric protofibrillar forms. In this study, monomeric wild-type alpha S and two mutants associated with familial PD, E46K and A53T, formed ion channels with well-defined conductance states in membranes containing 25-50% anionic lipid and 50% phosphatidylethanolamine (PE) in the presence of a trans-negative potential. Another familial mutant, A30P, known to have a lower membrane affinity, did not form ion channels. Ca2+ prevented channel formation when added to membranes before alpha S and decreased channel conductance when added to preformed channels. In contrast to the monomer, membrane permeabilization by oligomeric alpha S was not characterized by formation of discrete channels, a requirement for PE lipid, or a membrane potential. Channel activity, alpha-helical contents thermal stability of membrane-bound alpha S determined by far-UV CD, and lateral mobility of alpha S bound to planar membranes measured by fluorescence correlation spectroscopy were correlated. It was inferred that discrete ion channels with well-defined conductance states were formed in the presence of a membrane potential by one or several molecules of monomeric alpha S in an alpha-helical conformation and that such channels may have a role in the normal function and/or pathophysiology of the protein.