Molecular Role of Ca²⁺ and Hard Divalent Metal Cations on Accelerated Fibrillation and Interfibrillar Aggregation of α-Synuclein

α-Synuclein (αSyn) is an intrinsically disordered protein, the aggregation of which is highly related to the pathology of diverse α-synucleinopathies. Various hard divalent metal cations have been shown to affect αSyn aggregation. Especially, Ca²⁺ is suggested to be a crucial ion due to its physiological relevance to α-synucleinopathies. However, the molecular origin of αSyn aggregation mediated by the metal ions is not fully elucidated. In this study, we revealed that hard divalent metal ions had almost identical influences on αSyn aggregation. Based on these similarities, the molecular role of Ca²⁺ was investigated as a representative metal ion. Herein, we demonstrated that binding of multiple Ca²⁺ ions induces structural transition of αSyn monomers to extended conformations, which promotes rapid αSyn fibrillation. Additionally, we observed that Ca²⁺ induced further interfibrillar aggregation via electrostatic and hydrophobic interactions. Our results from multiple biophysical methods, including ion mobility-mass spectrometry (IM-MS), synchrotron small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), provide detailed information on the structural change of αSyn and the aggregation process mediated by Ca²⁺. Overall, our study would be valuable for understanding the influence of Ca²⁺ on the aggregation of αSyn during the pathogenesis of α-synucleinopathies.