Nanoscale profiling of evolving intermolecular interactions in ageing FUS condensates.

Protein condensates can exist in different states with distinct material properties, corresponding to specific cellular functions. These material properties, however, remain difficult to characterise, in part due to the technical challenges associated with studying condensed states. Here, to address this problem, we combine a microfluidic sample deposition technique that preserves the solution properties of condensates on surfaces with a nanometre-resolution spatial mapping method to characterise the time-dependent material properties of condensates of the fused in sarcoma (FUS) protein. This approach revealed two distinct phase transitions within FUS condensates. We first observed a spatially heterogeneous disorder-to-order transition initiating at the condensate interfaces and associated with intermolecular β-sheet formation. This process was then followed by the gelation of the condensate core, arising from an increase in the density of intermolecular interactions between intrinsically disordered regions. Overall, this study identifies molecular conformations associated with emergent phases of FUS condensates, and establishes a technology platform to understand the role of nanometre-scale phase changes in protein condensates.