HspB8 prevents aberrant phase transitions of FUS by chaperoning its folded RNA-binding domain.

Edgar E Boczek,Simon Alberti,Peter Heid,Jie Wang,Anthony A Hyman,Kai-Michael Kammer,Ina Poser,Daniel Mateju,Marcus Jahnel,Louise Jawerth,Lars Hubatsch,Julius Fürsch,Marie Laura Niedermeier,Laura Mediani,Andrej Pozniakovski,Serena Carra,Xiao Yan,Martine Ruer-Gruß,Florian Stengel

doi:10.7554/elife.69377

Abstract

Aberrant liquid-to-solid phase transitions of biomolecular condensates have been linked to various neurodegenerative diseases. However, the underlying molecular interactions that drive aging remain enigmatic. Here, we develop quantitative time-resolved crosslinking mass spectrometry to monitor protein interactions and dynamics inside condensates formed by the protein fused in sarcoma (FUS). We identify misfolding of the RNA recognition motif of FUS as a key driver of condensate aging. We demonstrate that the small heat shock protein HspB8 partitions into FUS condensates via its intrinsically disordered domain and prevents condensate hardening via condensate-specific interactions that are mediated by its α-crystallin domain (αCD). These αCD-mediated interactions are altered in a disease-associated mutant of HspB8, which abrogates the ability of HspB8 to prevent condensate hardening. We propose that stabilizing aggregation-prone folded RNA-binding domains inside condensates by molecular chaperones may be a general mechanism to prevent aberrant phase transitions.

Highlights

Condensate formation by liquid-liquid phase separation (LLPS) leads to a local density change of proteins (1, 2)
The general trend is for interactions between RNA recognition motif (RRM) domains and within RRM domains to decrease as condensates age. 186 The small heat shock protein HspB8 partitions into fused in sarcoma (FUS) condensates and interacts with the RRM domain To test whether these changes in links are driven by aging, we looked for ways to slow down the aging process. small heat shock proteins (sHSPs) are ATP-independent chaperones and are structurally divided into intrinsically disordered regions (IDRs) and a folded chaperone domain called α-crystallin domain (38)
We find the majority of inter-links within the droplets were formed between the α-crystallin domain (αCD) of HspB8 and the RRM of FUSm, and to a lesser degree the FUS-nuclear localization sequence (NLS)

Summary

Introduction

Condensate formation by liquid-liquid phase separation (LLPS) leads to a local density change of proteins (1, 2). The molecular aging process is accelerated by ALS-linked mutations in FUS and other RBPs (4, 5) This suggests that molecular aging of liquid condensates such as stress granules can be a disease process. Aberrant stress granules have been linked to disease and many sHSPs are associated with neurodegenerative disorders (18) This suggests that chaperones such as sHSPs may regulate the properties of stress granules and presumably the molecular aging process of stress granule proteins such as FUS. 110 To determine the molecular changes during the aging of stress granule proteins such as FUS, it is critical to monitor protein-protein interactions (PPIs) and conformational dynamics within condensates. These condensate[124] specific interactions are altered in a disease-associated HspB8 mutant, resulting in 125 its inability to prevent FUS aging

Methods

Results

Discussion

Conclusion