Tunable metastability of condensates reconciles their dual roles in amyloid fibril formation.

Stress granules form via co-condensation of RNA-binding proteins (RBPs) containing prion-like low-complexity domains (PLCDs) with RNA molecules. Homotypic interactions among PLCDs can drive amyloid fibril formation that is enhanced by amyotrophic lateral sclerosis (ALS)-associated mutations. We report that condensation- versus fibril-driving homotypic interactions are separable for A1-LCD, the PLCD of hnRNPA1. These separable interactions lead to thermodynamically metastable condensates and globally stable fibrils. Interiors of condensates suppress fibril formation, whereas interfaces have the opposite effect. ALS-associated mutations enhance the stability of fibrils and weaken condensate metastability, thus enhancing the rate of fibril formation. We designed mutations to enhance A1-LCD condensate metastability and discovered that stress granule disassembly in cells can be restored even when the designed variants carry ALS-causing mutations. Therefore, fibril formation can be suppressed by condensate interiors that function as sinks. Condensate sink potentials are influenced by their metastability, which is tunable through separable interactions even among minority components of stress granules.