HspB8 controls molecular aging of FUS condensates by domain-specific protein interactions Part 2

Biomolecular condensates formed by phase separation offer a confined space where protein-protein interactions (PPIs) facilitate distinct biochemical reactions. As their aberrant behavior is associated with disease, it is crucial to understand the molecular organization of PPIs within condensates. Using quantitative time-resolved crosslinking mass spectrometry (XL-MS) we directly and specifically monitor PPIs and protein dynamics inside condensates formed by the protein fused in sarcoma (FUS) and identify its folded RNA recognition motif (RRM) as a key player of aberrant molecular aging. We find that the chaperone HspB8, but not a disease-associated mutant, prevents FUS aging. In XL-MS and partitioning experiments we find that the disordered region of HspB8 directs its α-crystallin domain (αCD) into FUS droplets where HspB8 prevents aging via condensate-specific αCD–RRM interactions. We propose that chaperones like HspB8 prevent aberrant phase transitions by stabilizing aggregation prone folded domains inside condensates in times of cellular stress.