Salt-Induced Hydrophobic C-Terminal Region of α-Synuclein Triggers Its Fibrillation under the Mimic Physiologic Condition

α-Synuclein (αS) causes Parkinson's disease due to the structural alteration into amyloid fibrils that form after the interaction with synaptic membranes in neurons. To understand the alternation mechanism, the effect of salt (NaCl) on the interaction of αS with synaptic mimic membrane was characterized at the molecular level because salt triggered the amyloid fibril formation. The membrane-bound conformation (or the initial conformation before fibrillation) showed that NaCl decreased the number of helical structures and Tyr residues interacting with the membrane surface compared to when NaCl was absent, implying an increase in solvent-exposed regions. The N-terminal region of αS interacted with the membrane, forming the helical structures regardless of NaCl, while the C-terminal region formed a random structure with weak membrane interaction, but NaCl inhibited the interaction of its hydrophobic area, suggesting that salt promoted amyloid fibril formations by exposing the hydrophobic C-terminal region, which can intermolecularly interact with free αS.