Probing the effect of the disordered flank regions on amyloid fibril growth and proliferation.

Amyloid fibrils typically consist of a dense core made up of β-strands, with disordered flanks on either side, and are sometimes interrupted by disordered loop regions (the fuzzy coat). α-Synuclein found in Lewy Bodies of Parkinson's Disease patients is mostly C-terminally truncated, meaning that a large fraction of the fuzzy coat of disease-related fibrils is enzymatically degraded in the amyloid state. We demonstrate that the proteolytic removal of the fuzzy coat leads to enhanced fibril-fibril interactions and flocculation, which renders the study of the role of the fuzzy coat in bulk solution very challenging. In order to overcome these challenges, here we use Quartz Crystal Microbalance with Dissipation (QCM-D), a surface based biosensing technique, to study the effects of proteolytic removal of the fuzzy coat of α-synuclein amyloids. We demonstrate that Dissipation-Frequency analysis can illuminate multiple simultaneous reactions and characterize the monomer-fibril interactions in detail. We find that removal of the fuzzy coat increases apparent fibril elongation rates permanently. Utilizing kinetic models, we demonstrate that our results cannot be rationalized by alterations of the elongation rate of fibrils alone, but indicate that proteolytic cleavage of the fuzzy coat of α-synuclein fibrils can lead to the formation of new growth-competent fibril ends. We propose that such phenomena may be highly relevant for understanding disease-related α-synuclein amyloid formation. Furthermore we suggest that the QCM-D is a particularly attractive platform for studying post-translational modifications in real-time and their effect on amyloid growth or molecular interactions.