Structural reorganization underlying stress-induced cytoplasmicsolidification in yeast

Cells employ diverse strategies to rapidly adapt to sudden environmental changes. In yeast, cytoprotective solidification in response to starvation and energy depletion (ED) has been reported and associated with extensive mesoscale macromolecular assembly. Yet, the structural and molecular basis underlying such whole-cell level liquid-to-solid phase transitions remain unknown. Here, we use cryo-electron tomography to characterize the subcellular organization of intact yeast cells exposed to ED and other stressors, and to untangle the effects of molecular crowding versus cytoplasmic acidification previously suggested to underpin solidification. We visualize self-assembly of macromolecules and complexes into ordered assemblies and condensates under ED, and quantify ribosome and polysomes concentrations to probe changes in cytoplasmic crowding. Combined with live-cell microscopy, we pinpoint supramolecular assembly induced by acidification, rather than a uniform increase in intracellular crowding, as the structural basis of cytoplasmic solidification that supports yeast cells adaptation in response to environmental stresses.