Selectivity of Protein Interactions along the Aggregation Pathway of a-Synuclein

Abstract

The aggregation of -SYN follows a cascade of oligomeric, prefibrillar and fibrillar forms, culminating in the formation of Lewy Bodies (LB), the pathological hallmarks of Parkinsons Disease in neurons. Whilst -synuclein is a major contributor to LB, these dense accumulations of protein aggregates and tangles of fibrils contain over 70 different proteins. However, the potential for interactions between these proteins and the different aggregated species of -SYN is largely unknown. We hypothesized that the proteins present in the Lewy Bodies are trapped or pulled into the aggregates in a hierarchical manner, by binding at specific stages of the aggregation of -SYN. In this study we uncover a map of interactions of a total of 65 proteins, against different species formed by -SYN. We measured binding to monomeric -SYN using AlphaScreen, a sensitive nano-bead assay for detection of protein-protein interactions. To access different oligomeric species, we made use of the pathological mutants of -SYN (A30P, G51D and A53T), which form oligomeric species with distinct properties. Finally, we used bacterially expressed recombinant -SYN to generate amyloid fibrils and measure interactions with a pool of GFP-tagged potential partners. Binding to oligomers and fibrils was measured by two-color coincidence detection (TCCD) on a single molecule spectroscopy setup. Overall, we demonstrate that LB components are selectively recruited to specific steps in the formation of the LB, explaining their presence in the inclusions. Only a few proteins were found to interact with -SYN monomers at detectable levels, and only a subset recognizes the oligomeric -SYN including autophagosomal proteins. We therefore propose a new model for the formation of Lewy Bodies, where selectivity of protein partners at different steps drives the arrangement of these structures, uncovering new ways to modulate aggregation. Significance StatementThe molecular complexity of the Lewy Bodies has been a major hindrance to a bottom-up reconstruction of these inclusions, protein by protein. This work presents an extensive dataset of protein-protein interactions, showing that despite its small size and absence of structure, -SYN binds to specific partners in the LB, and that there is a clear selectivity of interactions between the different -SYN species along the self-assembly pathway. We use single-molecule methods to deconvolute number and size of the co-aggregates, to gain detailed information about the mechanisms of interaction. These observations constitute the basis for the elaboration of a global interactome of -SYN.