On the transient interactions of α-synuclein in different dimensions

0.-Synuclein (Syn) is a neuronal protein predominantly found in the brain, whose native function seems to be associated with vesicle trafficking. While intrinsically disordered in solution, the first ca. 100 residues adopt an amphipathic -helical structure when the protein adsorbs onto membranes. Additionally, the aggregation of Syn into highly ordered {beta}-sheet rich amyloid fibrils is associated with Parkinso[n]s disease. The different regions of Syn and the interactions between them have been reported to play a key role in the behaviour of the protein in solution, its membrane binding, and its aggregation into fibrils. This study employs photo-induced cross-linking of unmodified proteins (PICUP) to capture and identify the transient contacts of Syn in different conformational states: free in solution, adsorbed to membranes, and aggregated into fibrils. By using tyrosine-to-phenylalanine mutations to block the reactivity of specific amino acid residues, we establish key cross-links in each state. In solution, we identify internal contacts between the N and C termini of monomers, as well as inter-monomer contacts between C termini in oligomers. When Syn is adsorbed to membranes, the internal cross-linking is blocked, while cross-linking between C-terminal regions persists. In fibrils, cross-linking is significantly reduced, primarily occurring between C-terminal residues of adjacent monomers. This work highlights the utility of PICUP for reporting on the transient contacts that occur on the pathways of self- and co-assembly of Syn.