The Parkinson's disease substantia nigra is characterized by the loss of dopaminergic neurons and the presence of cytoplasmic fibrillar Lewy bodies in surviving neurons. The major fibrillar protein of Lewy bodies is alpha-synuclein. Two point mutations in the alpha-synuclein gene are associated with autosomal-dominant Parkinson's disease (FPD). Studies of the in vitro fibrillization behavior of the mutant proteins suggest that fibril precursors, or alpha-synuclein protofibrils, rather than the fibrils, may be pathogenic. Atomic force microscopy (AFM) revealed two distinct forms of protofibrillar alpha-synuclein: rapidly formed spherical protofibrils and annular protofibrils, which were produced on prolonged incubation of spheres. The spherical protofibrils bound to brain-derived membrane fractions much more tightly than did monomeric or fibrillar alpha-synuclein, and membrane-associated annular protofibrils were observed. The structural features of alpha-synuclein annular protofibrils are reminiscent of bacterial pore-forming toxins and are consistent with their porelike activity in vitro. Thus, abnormal membrane permeabilization may be a pathogenic mechanism in PD.