Megadalton-sized dityrosine aggregates of α-synuclein retain high degrees of structural disorder and internal dynamics.

Heterogeneous aggregates of the human protein -synuclein (Syn) are abundantly found in Lewy body inclusions of Parkinsons disease patients. While structural information on classical Syn amyloid fibrils is available, little is known about the conformational properties of disease-relevant, non-canonical aggregates. Here, we analyze the structural and dynamic properties of megadalton-sized dityrosine adducts of Syn that form in the presence of reactive oxygen species and cytochrome c, a proapoptotic peroxidase that is released from mitochondria during sustained oxidative stress. In contrast to canonical cross-{beta} amyloids, these aggregates retain high degrees of internal dynamics, which enables their characterization by solution-state NMR spectroscopy. We find that intermolecular dityrosine crosslinks restrict Syn motions only locally whereas large segments of concatenated molecules remain flexible and disordered. Indistinguishable aggregates form in crowded in vitro solutions and in complex environments of mammalian cell lysates, where relative amounts of free reactive oxygen species rather than cytochrome c are rate limiting. We further establish that dityrosine adducts inhibit classical amyloid formation by maintaining Syn in its monomeric form and that they are non-cytotoxic despite retaining basic membrane-binding properties. Our results suggest that oxidative Syn aggregation scavenges cytochrome cs activity into the formation of amorphous, high molecular-weight structures that may contribute to aggregate diversity in Lewy body deposits.