Sequence-encoded grammars determine material properties and physical aging of protein condensates

We demonstrate that nascent versus aged condensates formed by prion-like low complexity domains (PLCDs) are distinct types of viscoelastic materials. Nascent PLCD condensates are terminally viscous Maxwell fluids whose viscoelastic moduli are governed by the strengths of aromatic stickers. Upon physical aging, PLCD condensates transition, on sequence-specific timescales, from fluids to non-fibrillar, elastic Kelvin-Voigt solids. Accordingly, fluid-like condensates of PLCDs are metastable, whereas elastic solids are the stable material states. Fluid-to-solid transitions are accelerated by mutations to spacers that weaken metastable fluids with respect to stable solids. Evolutionarily selected PLCD sequence features that enhance the metastabilities of fluid-like condensates are likely to render the barriers for conversion from fluids to solids to be insurmountable on timescales that are relevant to cellular processes.