NMR unveils an N-terminal interaction interface on acetylated-α-synuclein monomers for recruitment to fibrils

Amyloid fibril formation of ?-synuclein (?S) is associated with multiple neurodegenerative diseases, including Parkinson?s disease (PD). Growing evidence suggests that progression of PD is linked to cell-to-cell propagation of ?S fibrils, which leads to seeding of endogenous intrinsically disordered monomer via templated elongation and secondary nucleation. A molecular understanding of the seeding mechanism and driving interactions is crucial to inhibit progression of amyloid formation. Here, using relaxation-based solution NMR experiments designed to probe large complexes, we probe weak interactions of intrinsically disordered acetylated-?S (Ac-?S) monomers with seeding-competent Ac-?S fibrils and seeding-incompetent off-pathway oligomers to identify Ac-?S monomer residues at the binding interface. Under conditions that favor fibril elongation, we determine that the first 11 N-terminal residues on the monomer form a common binding site for both fibrils and off-pathway oligomers. Additionally, the presence of off-pathway oligomers within a fibril seeding environment suppresses seeded amyloid formation, as observed through thioflavin-T fluorescence experiments. This highlights that off-pathway ?S oligomers can act as an auto-inhibitor against ?S fibril elongation. Based on these data taken together with previous results, we propose a model in which Ac-?S monomer recruitment to the fibril is driven by interactions between the intrinsically disordered monomer N terminus and the intrinsically disordered flanking regions (IDR) on the fibril surface. We suggest that this monomer recruitment may play a role in the elongation of amyloid fibrils and highlight the potential of the IDRs of the fibril as important therapeutic targets against seeded amyloid formation.

Automated summary

This study demonstrates that under conditions favoring fibril elongation, the first 11 N-terminal residues of α-synuclein monomers form a common binding site for both fibrils and off-pathway oligomers. Presence of off-pathway oligomers in the fibril seeding environment suppresses seeded amyloid formation, acting as an auto-inhibitor against fibril elongation.