X‑ray Crystallographic Structure of Oligomers Formed by a Toxic β‑Hairpin Derived from α‑Synuclein: Trimers and Higher-Order Oligomers

Oligomeric
assemblies of the protein α-synuclein are thought
to cause neurodegeneration in Parkinson’s disease and related
synucleinopathies. Characterization of α-synuclein oligomers
at high resolution is an outstanding challenge in the field of structural
biology. The absence of high-resolution structures of oligomers formed
by α-synuclein impedes understanding the synucleinopathies at
the molecular level. This paper reports the X-ray crystallographic
structure of oligomers formed by a peptide derived from residues 36–55
of α-synuclein. The peptide 1a adopts a β-hairpin
structure, which assembles in a hierarchical fashion. Three β-hairpins
assemble to form a triangular trimer. Three copies of the triangular
trimer assemble to form a basket-shaped nonamer. Two nonamers pack
to form an octadecamer. Molecular modeling suggests that full-length
α-synuclein may also be able to assemble in this fashion. Circular
dichroism spectroscopy demonstrates that peptide 1a interacts
with anionic lipid bilayer membranes, like oligomers of full-length
α-synuclein. LDH and MTT assays demonstrate that peptide 1a is toxic toward SH-SY5Y cells. Comparison of peptide 1a to homologues suggests that this toxicity results from
nonspecific interactions with the cell membrane. The oligomers formed
by peptide 1a are fundamentally different than the proposed
models of the fibrils formed by α-synuclein and suggest that
α-Syn36–55, rather than the NAC, may nucleate
oligomer formation.