Abstracts
How condensates can suppress prion-like domain fibril formation
Tanja Mittag
St. Jude Research, Memphis, USA
Stress granules form under stress conditions via phase separation of untranslated mRNA and RNA-binding proteins. These stress granules are currently viewed as crucibles for or sights of fibril formation in a spectrum of neurodegenerative diseases including ALS and frontotemporal dementia. Biophysical investigations indeed showed that fibrils appear earlier in the presence of condensates than in their absence and are often bundled at sites with condensates. We and others recently demonstrated that interfaces of mono-component condensates can indeed promote fibril nucleation, but that their interiors suppress fibril formation. Given that stress granules are complex assemblies formed by multiple proteins and RNA, we asked whether multi-component condensates suppress or promote fibril formation and what the underlying mechanism are. Here, we employed programmable peptide-nucleic acid condensates that favorably partition the low-complexity domain of the RNA-binding protein hnRNPA1 (A1-LCD) and its disease mutant D262V. A combination of measurements assessing kinetics of fibril formation, stabilities of fibrils and condensates, material and transport properties revealed several insights. (i) Fibrils form in the dilute phase, and the condensate interiors suppress fibril formation. (ii) Condensates sequester soluble protein, thereby lowering the protein concentrations in the dilute phase that can fuel fibril growth. (iii) Multi-component condensates have lower relative concentrations of fibril-forming protein at interfaces, lowering nucleation and increasing lag times. (iv) Higher condensate viscoelasticity slows down protein efflux from the dense phase and therefore fibril growth. These principles render multi-component condensates into effective suppressors of fibril formation, suggesting that stress granules may act as protective modulators rather than crucibles for pathological fibril formation.