Accumulation of filamentous aggregates of α-synuclein is a pathological hallmark of several neurodegenerative diseases, including Parkinson's disease (PD). The interaction between α-synuclein and phospholipids has been shown to play a critical role in the aggregation of α-synuclein. Most structural studies have, however, been focused on α-synuclein filaments formed in the absence of lipids. Here, we report the structural investigation of α-synuclein filaments assembled under the quiescent condition in the presence of anionic lipid vesicles using electron microscopy (EM), including cryogenic electron microscopy (cryo-EM). Our transmission electron microscopy (TEM) analyses reveal that α-synuclein forms curly protofilaments at an early stage of aggregation. The flexible protofilaments were then converted to long filaments after a longer incubation of 30 days. More detailed structural analyses using cryo-EM reveal that the long filaments adopt untwisted structures with different diameters, which have not been observed in previous α-synuclein fibrils formed in vitro. The untwisted filaments are rather similar to straight filaments with no observable twist that are extracted from patients with dementia with Lewy bodies. Our structural studies highlight the conformational diversity of α-synuclein filaments, requiring additional structural investigation of not only more ex vivo α-synuclein filaments but also in vitro α-synuclein filaments formed in the presence of diverse cofactors to better understand the molecular basis of diverse molecular conformations of α-synuclein filaments.
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