Co-existence of Two Different α-Synuclein Oligomers with Different Core Structures Determined by Hydrogen/Deuterium Exchange Mass Spectrometry

Neurodegenerative disorders are characterized by the formation of protein oligomers and amyloid fibrils, which in the case of Parkinson's disease involves the protein alpha-synuclein (alpha SN). Cytotoxicity is mainly associated with the oligomeric species, but we still know little about their assembly and structure. Hydrogen/deuterium exchange (HDX) monitored by mass spectrometry is used to analyze oligomers formed by wild-type (wt) alpha SN and also three familial alpha SN mutants (A30P, E46K, and A53T). All four variants show coexistence of two different oligomers. The backbone amides of oligomer type I are protected from exchange with D2O until they dissociate into monomeric alpha SN by EX1 exchange kinetics. Fewer residues are protected against exchange in oligomer type II, but this type does not revert to alpha SN monomers. Both oligomers are protected in the core sequence Y39-A89. Based on incubation studies, oligomer type I appears to form straight fibrils, while oligomer type II forms amorphous clusters that do not directly contribute to the fibrillation process.