α-Synuclein Forms Distinct Micelle-Like Assemblies at Low Ionic Strengths

Intrinsically disordered proteins (IDPs) often associate into biomolecular condensates. While micron-sized condensates are observed above a well-defined saturation concentration, the formation of nanoscale clusters also occurs at subsaturated concentrations. The diversity and physical nature of such nanoscale clusters remain underexplored with respect to the well-established condensate state. Here we show that the Parkinson's disease-associated protein α-synuclein forms highly monodisperse assemblies at low ionic strength, distinct from the larger polydisperse assemblies previously observed at higher ionic strengths. Using a range of biophysical methods, we identify monodisperse clusters of α-synuclein composed of ∼20 molecules that maintain their size over a broad concentration range. These clusters are stable over time and do not progress to amyloid fibril formation. Their response to mutational perturbations and chemical destabilization, as well as their thermal cycling support a micelle-like assembly. Increasing ionic strength shifts the system toward the formation of larger, polydisperse nanoclusters and condensates that promote amyloid fibril formation. Our results reveal a hitherto uncharacterized assembly state of α-synuclein and highlight the complexity of nanoscale assembly processes by IDPs.