Structural Insights of Fe3+ Induced α-synuclein Fibrillation in Parkinson's Disease.

Amyloid aggregation of α-synuclein (α-syn) in Lewy bodies (LBs) is the pathological hallmark of Parkinson's disease (PD). Iron, especially Fe3+, is accumulated in substantia nigra of PD patients and co-deposited with α-syn in LBs. However, how Fe3+ modulates α-syn fibrillation at molecular level remains unclear. In this study, we found that Fe3+ can promote α-syn fibrillation at low concentration while inhibit its fibrillation at high concentration. NMR titration study shows poor interaction between α-syn monomer and Fe3+. Instead, we found that Fe3+ binds to α-syn fibrils. By using cryo-electron microscopy (cryo-EM), we further determined the atomic structure of α-syn fibril in complex with Fe3+ at the resolution of 2.7 Å. Strikingly, two extra electron densities adjacent to His50 and Glu57 were observed as putative binding sites of Fe3+ and water molecules, suggesting that Fe3+ binds to the negative cleft of the fibril and stabilizes the fibril structure for promoting α-syn aggregation. Further mutagenesis study shows mutation of His50 abolishes the Fe3+-facilitated fibrillation of α-syn. Our work illuminates the structural basis of the interaction of Fe3+ and α-syn in both monomeric and fibrillar forms, and sheds light on understanding the pathological role of Fe3+ in α-syn aggregation in PD.