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An Amyloidogenic Fragment of the Spike Protein from SARS-CoV-2 Virus Stimulates the Aggregation and Toxicity of Parkinson’s Disease Protein Alpha-Synuclein Emerging evidence suggests that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may have long-term deleterious effects on the central nervous system and even contribute to post-COVID neurological syndromes. Interestingly, inflammation-induced proteolytic processing of the Spike protein of SARS-CoV-2 leads to the generation of peptides capable of aggregating into amyloid fibrils in vitro. Herein, we investigate the in vitro effect of a fibrillogenic fragment of the Spike protein [Spike 194–203 (S194)] on the aggregation and toxicity of the Parkinson’s disease (PD) protein α-synuclein (αSyn). Our results indicate that S194 fibrils stimulate in a concentration-dependent manner the fibrillation of αSyn monomer, resulting in aggregates with increased capacity of inducing lipid vesicle leakage and toxicity to neuroblastoma cells, in comparison with either αSyn or S194 alone. Bidimensional NMR (1H–15N-HSQC) suggests that S194 fibrils cause a higher perturbation in both the N-terminal region (sequence: 19–68) and the hydrophobic central domain of the αSyn monomer (sequence: 71–95), which is corroborated by protein–peptide docking and molecular dynamics simulations. In contrast with fibrils from wild-type αSyn, aggregates from the PD variant A30P exhibited a remarkable accelerative effect on S194 fibrillation. Similarly, fibrils from amyloid-β peptides, which are linked to Alzheimer’s disease, exhibited a pro-aggregating effect on the S194 monomer. Taken together, these findings might contribute to a broader understanding of the potential connections between SARS-CoV-2 infection and amyloid-related neurodegenerative disorders, highlighting areas that may warrant further investigation. https://pubs.acs.org/doi/10.1021/acschemneuro.5c00478