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

While α‐synuclein, an intrinsically disordered protein linked to Parkinson's disease, has been shown to associate with membrane organelles, its overall cellular function remains nebulous. α‐Synuclein binds to membranes through its amino‐terminal domain (first ≈100 residues), but there is no consensus on the biophysical function of the carboxyl‐terminal domain (last ≈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 α‐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 α‐synuclein sterically stabilizes membrane surfaces through its carboxyl‐terminal domain. Full‐length α‐synuclein dramatically increases the critical osmotic pressure at which SSLBs cluster (PC ≈ 1.3 × 105 Pa) compared to α‐synuclein without the carboxyl‐terminal domain (PC ≈ 1.9 × 104 Pa) at physiological salt and temperature conditions. This clustering of α‐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 α‐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. The specific function of the intrinsically disordered protein α‐synuclein remains unknown despite its unequivocal link to Parkinson's disease. Herein, small‐angle X‐ray scattering of synaptic vesicle‐mimics is used to show that α‐synuclein sterically stabilizes these mimics even under applied depletion attraction. The polymeric C‐terminal domain of α‐synuclein is the predominant contributor of this effect, strikingly similar to polyelectrolyte‐mediated stabilization of colloids.