Effect of ATP on Phase Separation in FUS Condensates*

The fused-in-sarcoma (FUS) protein is known to phase separate from the cellular medium into liquid-like condensate droplets. Aberrant FUS phase separation is seen in diseases such as ALS and frontotemporal dementia. Adenosine triphosphate (ATP), the common cellular energy carrier, was recently observed to promote FUS condensation at low ATP concentrations and dissolve FUS condensates at high ATP concentrations. However, the mechanism by which this occurs has not yet been determined. Here, we used all-atom simulations to elucidate, at the microscopic level, how ATP affects FUS phase separation. In our simulations, two FUS proteins were placed in electrolyte solution and simulated, with and without ATP, in the presence of harmonic restraints that probed local and global inter-FUS forces. Analysis of the simulation trajectories reveals changes in the effective force between FUS molecules, number of inter-FUS contacts, and FUS radius of gyration mediated by the formation of ATP-FUS contacts and ATP bridges between FUS molecules, charting a microscopic mechanism by which ATP may affect phase separation.