Stressed cells shut down translation, release mRNA molecules from polysomes, and form stress granules (SGs) via a network of interactions that involve G3BP. Here we focus on the mechanistic underpinnings of SG assembly. We show that, under non-stress conditions, G3BP adopts a compact auto-inhibited state stabilized by electrostatic intramolecular interactions between the intrinsically disordered acidic tracts and the positively charged arginine-rich region. Upon release from polysomes, unfolded mRNAs outcompete G3BP auto-inhibitory interactions, engendering a conformational transition that facilitates clustering of G3BP through protein-RNA interactions. Subsequent physical crosslinking of G3BP clusters drives RNA molecules into networked RNA/protein condensates. We show that G3BP condensates impede RNA entanglement and recruit additional client proteins that promote SG maturation or induce a liquid-to-solid transition that may underlie disease. We propose that condensation coupled to conformational rearrangements and heterotypic multivalent interactions may be a general principle underlying RNP granule assembly.

RNA-protein (RNP) granules have been proposed to assemble by forming solid RNA/protein aggregates or through phase separation into a liquid RNA/protein phase. Which model describes RNP granules in living cells is still unclear. In this study, we analyze P bodies in budding yeast and find that they have liquid-like properties. Surprisingly, yeast stress granules adopt a different material state, which is reminiscent of solid protein aggregates and controlled by protein disaggregases. By using an assay to ectopically nucleate RNP granules, we further establish that RNP granule formation does not depend on amyloid-like aggregation but rather involves many promiscuous interactions. Finally, we show that stress granules have different properties in mammalian cells, where they show liquid-like behavior. Thus, we propose that the material state of RNP granules is flexible and that the solid state of yeast stress granules is an adaptation to extreme environments, made possible by the presence of a powerful disaggregation machine.DOI: http://dx.doi.org/10.7554/eLife.06807.001

Stress granules (SG) are membrane‐less compartments involved in regulating mRNAs during stress. Aberrant forms of SGs have been implicated in age‐related diseases, such as amyotrophic lateral sclerosis (ALS), but the molecular events triggering their formation are still unknown. Here, we find that misfolded proteins, such as ALS‐linked variants of SOD1, specifically accumulate and aggregate within SGs in human cells. This decreases the dynamics of SGs, changes SG composition, and triggers an aberrant liquid‐to‐solid transition of in vitro reconstituted compartments. We show that chaperone recruitment prevents the formation of aberrant SGs and promotes SG disassembly when the stress subsides. Moreover, we identify a backup system for SG clearance, which involves transport of aberrant SGs to the aggresome and their degradation by autophagy. Thus, cells employ a system of SG quality control to prevent accumulation of misfolded proteins and maintain the dynamic state of SGs, which may have relevance for ALS and related diseases.

Stress granules (SGs) are ribonucleoprotein complexes induced by stress. They sequester mRNAs and disassemble when the stress subsides, allowing translation restoration. In amyotrophic lateral sclerosis (ALS), aberrant SGs cannot disassemble and therefore accumulate and are degraded by autophagy. However, the molecular events causing aberrant SG formation and the molecular players regulating this transition are largely unknown. We report that defective ribosomal products (DRiPs) accumulate in SGs and promote a transition into an aberrant state that renders SGs resistant to RNase. We show that only a minor fraction of aberrant SGs is targeted by autophagy, whereas the majority disassembles in a process that requires assistance by the HSPB8-BAG3-HSP70 chaperone complex. We further demonstrate that HSPB8-BAG3-HSP70 ensures the functionality of SGs and restores proteostasis by targeting DRiPs for degradation. We propose a system of chaperone-mediated SG surveillance, or granulostasis, which regulates SG composition and dynamics and thus may play an important role in ALS.

Cellular stress leads to reprogramming of mRNA translation and formation of stress granules (SGs), membraneless organelles consisting of mRNA and RNA-binding proteins. Although the function of SGs remains largely unknown, it is widely assumed they contain exclusively nontranslating mRNA. Here we re-examine this hypothesis using single-molecule imaging of mRNA translation in living cells. While our data confirms that non-translating mRNAs are preferentially recruited to SGs, we find unequivocal evidence for translation of mRNA localized to SGs. Our data indicate that SG-associated translation is not rare and that the entire translation cycle (initiation, elongation and termination) can occur for these transcripts. Furthermore, translating mRNAs can be observed transitioning between the cytosol and SGs without changing their translational status. Together, these results argue against a direct role for SGs in inhibition of mRNA translation.