The C-Terminal Domain of α-Synuclein Confers Steric Stabilization on Synaptic Vesicle-Like Surfaces

While alpha-synuclein, an intrinsically disordered protein linked to Parkinson's disease, has been shown to associate with membrane organelles, its overall cellular function remains nebulous. alpha-Synuclein binds to membranes through its amino-terminal domain (first approximate to 100 residues), but there is no consensus on the biophysical function of the carboxyl-terminal domain (last approximate to 40 residues) due, in part, to its lack of strong interaction partners and persisting intrinsic disorder even when membrane bound. Here, by directly applying force on alpha-synuclein bound to spherical nanoparticle-supported lipid bilayers (SSLBs) and tracking higher-order structural changes through small-angle X-ray scattering, strong evidence is presented that alpha-synuclein sterically stabilizes membrane surfaces through its carboxyl-terminal domain. Full-length alpha-synuclein dramatically increases the critical osmotic pressure at which SSLBs cluster (P-C approximate to 1.3 x 10(5) Pa) compared to alpha-synuclein without the carboxyl-terminal domain (P-C approximate to 1.9 x 10(4) Pa) at physiological salt and temperature conditions. This clustering of alpha-synuclein-bound SSLBs is shown to be reversible and sensitive to monovalent/divalent salt, both features of grafted polyelectrolyte-mediated steric stabilization. In elucidating the biophysical function of alpha-synuclein in the framework of polymer science, it is demonstrated that the carboxyl-terminal domain can potentially utilize its persisting intrinsic disorder to functionalize membrane surfaces.