Two new polymorphic structures of alpha-synuclein solved by cryo-electron microscopy

Intracellular inclusions containing an enrichment of alpha-synuclein protein are characteristic for several neuropathological diseases including Parkinson9s disease (PD). Recent studies using cryo-electron microscopy and helical reconstruction approaches had determined the structure of fibrillar recombinant human alpha-synuclein (polymorphs 1a and 1b). Here, we describe two new polymorphic atomic structures of alpha-synuclein fibrils (polymorph 2a at 3.1 Angstroem resolution and polymorph 2b at 3.5 Angstroem), which show a radically different arrangement in the protofilament compared to the previously investigated polymorphs. All structures are fibrils of 10 nm diameter that are composed of two protofilaments. The strong steric-zipper geometry at the protofilament interface in previously described polymorphs is absent in the polymorphs described here. Instead, they interact via intermolecular salt-bridges between amino acids K45, E57 (polymorph 2a) or E46 (polymorph 2b). The non-amyloid component (NAC) region of alpha-synuclein is fully buried by previously non-described interactions with the N-terminus. A hydrophobic cleft, the location of familial PD mutation sites, and the nature of the protofilament interface now invite to formulate hypotheses about fibril formation, growth and stability. If such structures can be identified in human brain they open new avenues for the design of tools such as small molecules or antibodies targeting the various amyloid forms of alpha-synuclein for better diagnosis and assessment of potential therapies targeting alpha-synuclein deposits.