Abstract
Biomolecular condensates (BMCs) are assemblies of hundreds to many thousands of macromolecules within cells that are organized without physical barriers. Condensate function is dictated not only by its molecular composition, but also by substructural organization and molecular mobility. One hypothesis for the onset of multiple protein aggregation diseases is that the increased densities of specific proteins within BMCs promotes the formation of solid inclusions. However, deciphering the internal structural and functional properties of BMCs at the nanoscale and identifying the initiating events of inclusion formation requires tools with high spatiotemporal precision. Here we show using single molecule and other microscopy approaches that single component Fused in Sarcoma (FUS) condensates exhibit confinement and contain clusters with higher FUS density even at early timepoints. Upon aging, condensates displayed altered physical properties and reduced monomer mobility, and yet most FUS monomers diffused throughout the condensate within seconds. While an increase in connectivity over time explains reduced mobility, the large fraction of molecules retaining high mobility suggests a sponge-like structure rather than a system-spanning network. These findings indicate that a pseudo-equilibrium between distinct structural connectivities can exist within simple condensates, suggesting the potential for substantial structural and functional complexity of BMCs at the nanoscale.