BPS2026 – Dissection of the suppressive effects of biomolecular condensates on amyloid fibril assembly

Amyloids formed by RNA-binding proteins such as TDP-43, FUS, TIA1, or hnRNPA1 are the histopathological hallmark of a spectrum of neurodegenerative diseases including ALS, frontotemporal dementia, and multisystem proteinopathy. Stress granules are formed by co-condensation of RBPs and untranslated RNA and widely viewed as crucibles for amyloid fibril formation and neurodegenerative diseases. Given that protein condensation and fibril formation are both highly concentration-dependent and use overlapping sequence grammars, understanding their mutual effects requires careful thermodynamic and kinetic measurements over a range of concentrations. Here, we used the intrinsically disordered prion-like domain of hnRNPA1 (A1-LCD), which forms disordered condensates and ordered amyloid fibrils, to characterize how condensates influence fibril formation. Disease-causing mutations promote fibril formation and weaken condensate formation of A1-LCD, showing that the interactions underlying condensate and fibril formation are distinct and separable. Condensates have dual effects on fibril formation: condensates are metastable, off-pathway sinks for soluble protein and suppress fibril formation in their interiors, but their interfaces catalyze nucleation of fibrils. Fibrils grow in the dilute phase, and protein efflux from metastable condensates can become rate-limiting for fibril growth. These insights are generalizable to multi-component condensates formed by RGG-repeat peptides and nucleic acids, which favorably partition A1-LCD. In certain concentration regimes, multi-component condensates can be stable relative to homotypic amyloid fibrils, thermodynamically suppressing fibril growth yet enabling interface-catalyzed nucleation that is determined by the density of A1-LCD at interfaces. Our combined results suggest that multi-component stress granules have multiple features that are optimal for suppression of amyloid fibril assembly and therefore may function as potent suppressors rather than crucibles of pathological processes.