Complex coacervation reshapes the aggregation landscape of tau

Biomolecular condensates are increasingly implicated in protein aggregation, yet their contribution is often reduced to that of concentrating reactants. Whether the phase state of a protein itself changes how it aggregates remains unclear. Using complex coacervates of a tau repeat-domain construct (K12) with heparin, we show that phase separation does not simply accelerate tau aggregation but redirects it along a distinct kinetic regime. Amyloid nucleation and growth are largely associated with the condensed phase, where intact phase-separated mixtures nucleate with a markedly shortened lag phase, whereas the corresponding dilute phase contributes little to overall amyloid formation. Aggregation kinetics in this regime become largely decoupled from total protein concentration. Because phase equilibrium pins the composition of the dense phase, additional tau partitions largely into the coexisting dilute phase without substantially altering the reacting population. This weak concentration dependence provides a kinetic signature of compartmentalized aggregation, and it recurs across chemically distinct coacervates formed with heparin, RNA, and polyglutamate, pointing to a general feature of coacervate-mediated tau assembly rather than a heparin-specific effect. Aggregation within coacervates also yields fibrils with altered morphology and secondary structure, suggesting access to alternative regions of the assembly landscape, and shows reduced sensitivity to bulk pH perturbations. Together, these results show that condensation changes tau aggregation by defining the local reaction environment: phase equilibrium buffers the dense-phase composition, in turn altering aggregation kinetics and the properties of the amyloid formed.