Heterotypic FG-Nup98-tau condensates form nested assemblies through stoichiometry-dependent phase transitions
The dysfunction of nucleocytoplasmic transport (NCT) and tau aggregation are emerging as interconnected hallmarks of neurodegenerative tauopathies. However, the molecular basis by which components of the nuclear pore complex and tau interact remains unclear. Here, we combine experimental and computational approaches to elucidate the mechanism of heterotypic phase separation between the FG-repeat domain of nucleoporin Nup98 (FG-Nup98) and tau. In vitro assays revealed that FG-Nup98 and tau undergo coacervation, forming dynamic condensates whose morphology and dynamics depend on stoichiometry, macromolecular crowding, and ionic strength. FRAP indicated reduced tau mobility within FG-Nup98-rich condensates, supporting a scaffold-client model. Complementary computational analyses revealed hierarchical binding energetics: FG-Nup98 self-association is strongest, followed by FG-Nup98-tau and tau-tau interactions. While FG-Nup98 forms stable homotypic networks, tau-tau contacts are transient but energetically favorable, which suggests that elevated tau concentrations may trigger a transition from droplets to tau aggregates. Together, these results establish that multivalent FG-Nup98-tau interactions drive condensate formation that could potentially perturb the permeability barrier of the nuclear pore. This study elucidates the coordinated behaviors of FG-Nup98-tau condensates and provides a framework for understanding NCT defects in tauopathies.