Buchholz, H. E. | Search bioRxiv

bioRxiv2025

Stress granules and protein aggregates reveal intracellular resource competition

Stress granules are biomolecular condensates that form in response to environmental stress and disassemble once normal conditions are restored. However, when disassembly fails, stress granules can persist and solidify. While stress granule solidification has been well documented, the cellular mechanisms underlying the transition from reversible to persistent stress granules remain unclear. Persistent stress granules can seed the formation of pathological aggregates, such as TDP-43 in amyotrophic lateral sclerosis1, 2. Although amyloid and tau aggregates are hallmarks of Alzheimers disease, a subset of patients also develop TDP-43 deposits, suggesting a possible role for stress granule solidification in Alzheimers disease progression3-5. Despite theoretical models explaining why persistence and ensuing solidification occurs, strong in vivo evidence is lacking6. Here we show that competition for limited chaperone resources drive stress granule persistence. In the presence of TDP-43 aggregates or yeast amyloid proteins called prions, stress granule disassembly is slowed or halted disassembly. Using yeast prions as a model, we show that the addition of chaperones, specifically the AAA+ ATPase molecular chaperone, Hsp104, resulted in resumption of stress granule disassembly. Our results demonstrate that the competition for shared resources, such as molecular chaperones, can limit stress granule disassembly. We suspect that the presence of pathological aggregates results in resource competition within the aging brain, contributing to the persistence of stress granules and their subsequent solidification and aggregation.

cell biology↗

bioRxiv2025

Hsp70 chaperones, Ssa1 and Ssa2, limit poly(A) binding protein aggregation

Molecular chaperones play a central role in maintaining protein homeostasis. The highly conserved Hsp70 family of chaperones have major functions in folding of nascent peptides, protein refolding, and protein aggregate disassembly. In yeast, loss of two Hsp70 proteins, Ssa1 and Ssa2, is associated with decreased cellular growth and shortened lifespan. While heterologous or mutant temperature sensitive proteins form anomalous large cytoplasmic inclusions in ssa1{Delta}ssa2{Delta} strains, it is unclear how endogenous wildtype proteins behave and are regulated in the presence of limiting Hsp70s. Using the wildtype yeast Poly A binding protein (Pab1), which is involved in mRNA binding and forms stress granules (SGs) upon heat shock, Pab1 forms large inclusions in approximately half of ssa1{Delta}ssa2{Delta} cells in the absence of stress. Overexpression of Ssa1, Hsp104, and Sis1 almost completely limits the formation of these large inclusions in ssa1{Delta}ssa2{Delta}, suggesting that excess Ssa1, Hsp104 and Sis1 can each compensate for the lower levels of Ssa proteins. Upon heat shock, SGs also form in cells whether large Pab1 inclusions are present or not. Surprisingly, cells containing only SGs disassemble faster than wildtype, whereas cells with both large inclusions disassemble slower albeit completely. We suspect that disassembly of these large inclusions is linked to the elevated heat shock response and elevated Hsp104 and Sis1 levels in ssa1{Delta}ssa2{Delta} strains. We also observed that wildtype cultures grown to saturation also form large Pab1-GFP inclusions. These inclusions can be partially rescued by overexpression of Ssa1. Taken together, our data suggests that Hsp70 not only plays a role in limiting unwanted protein aggregation in normal cells, but as cells age, the depletion of active Hsp70 possibly underlies the age-related aggregation of endogenous proteins.

cell biology↗