Protein misfolding diseases are a group of devastating disorders characterized by structural conversion of a soluble protein into an amyloid-like aggregate. Typically, the structural conversion occurs by misfolding of a single disease-associated protein, such as alpha-synuclein (alpha S) in Parkinson's disease, amyloid-beta in Alzheimer's disease, and prion protein (PrP) in transmissible spongiform encephalopathies (TSEs). However, accumulating evidence has implicated that cross-interactions between heterologous amyloidogenic proteins dramatically impact on amyloidogenesis and disease pathology. Here we show alpha S in a monomeric state can suppress amyloidogenesis of PrP in vitro. Thioflavin-T assays and transmission electron miscopy revealed that monomeric alpha S inhibits the nucleation step of amyloidogenesis without inhibiting the growing step. Surface plasmon resonance and co-sedimentation assays neither detected interaction between alpha S and monomeric PrP nor fibrillar PrP. These results suggested that alpha S suppress amyloidogenesis of PrP by binding to a transiently accumulated intermediate, such as a partially unfolded state. Moreover, we found that oligomeric alpha S, which was recently suggested to interact with PrP, also did not interact with PrP. Taken together, our study revealed a chaperon-like activity of alpha S against PrP amyloidogenesis, suggesting a possible involvement of alpha S in the pathology of TSEs. (C) 2019 Elsevier Inc. All rights reserved.