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

The deposition of misfolded proteins in cytoplasmic protein bodies requires the concerted action of stress-inducible protein-sorting factors and molecular chaperones. Protein sequestration during acute stress is a cellular strategy that adjusts the flux of misfolded proteins to the capacities of the protein quality control system.

Membrane-less compartments, such as P bodies and stress granules, play major roles in cell biology and disease. The dynamics and physical properties of these compartments are very diverse, ranging from liquid-like to solid-like behavior. Importantly, changes in compartment dynamics have been linked to changes in compartment function or pathology. However, appropriate tools to investigate the physical properties of membrane-less compartments are still lacking. The aliphatic alcohol hexanediol has been proposed as a tool to differentiate between liquid-like and solid-like assemblies in living cells. We show that in vitro reconstituted compartments formed by the RNAbinding protein FUS are rapidly dissolved by hexanediol. In contrast, solid-like fibers of FUS, which have been linked to the disease amyotrophic lateral sclerosis (ALS), are resistant to hexanediol treatment, supporting the idea that hexanediol can differentiate between liquid-and solid-like assemblies. We further show that hexanediol can be used to examine the physical properties of membrane-less compartments in vivo. We find that hexanediol dissolves dynamic, liquid-like assemblies, such as P bodies, whereas solid-like assemblies, such as protein aggregates and cytoskeletal assemblies, are largely resistant to hexanediol. Finally, we report here that extended exposure of yeast and mammalian cells to hexanediol is cytotoxic and causes abnormal changes in cell morphology, which trigger the formation of aberrant assemblies. We therefore urge care in the use of hexanediol, especially when cells are exposed to hexanediol for extended times. In summary, we propose that hexanediol is a powerful tool to probe the physical properties of membrane-less compartments, but only if adequate controls are included and precautions are taken.

How cells adapt to varying environmental conditions is largely unknown. Here, we show that, in budding yeast, the RNA-binding and stress granule protein Pub1 has an intrinsic property to form condensates upon starvation or heat stress and that condensate formation is associated with cell-cycle arrest. Release from arrest coincides with condensate dissolution, which takes minutes (starvation) or hours (heat shock). In vitro reconstitution reveals that the different dissolution rates of starvation- and heat-induced condensates are due to their different material properties: starvation-induced Pub1 condensates form by liquid-liquid demixing and subsequently convert into reversible gel-like particles; heat-induced condensates are more solid-like and require chaperones for disaggregation. Our data suggest that different physiological stresses, as well as stress durations and intensities, induce condensates with distinct physical properties and thereby define different modes of stress adaptation and rates of recovery.