Cytoplasmic fluidization triggers breaking spore dormancy in fission yeast.

The cytoplasm is a complex, crowded environment that influences myriad cellular processes including protein folding and metabolic reactions. Recent studies have suggested that changes in the biophysical properties of the cytoplasm play a key role in cellular homeostasis and adaptation. However, it still remains unclear how cells control their cytoplasmic properties in response to environmental cues. Here, we used fission yeast spores as a model system of dormant cells to elucidate the mechanisms underlying regulation of the cytoplasmic properties. By tracking fluorescent tracer particles, we found that particle mobility decreased in spores compared to vegetative cells, and rapidly increased at the onset of dormancy breaking upon glucose addition. This cytoplasmic fluidization depended on glucose sensing via the cAMP-PKA pathway. PKA activation led to trehalose degradation through trehalase Ntp1, thereby increasing particle mobility as the amount of trehalose decreased. In contrast, the rapid cytoplasmic fluidization did not require de novo protein synthesis, cytoskeletal dynamics, or cell volume increase. Furthermore, the measurement of diffusion coefficients with tracer particles of different sizes suggests that the spore cytoplasm impedes the movement of larger protein complexes (40-150 nm) such as ribosomes, while allowing free diffusion of smaller molecules ([~]3 nm) such as second messengers and signaling proteins. Our experiments have thus uncovered a series of signaling events that enable cells to quickly fluidize the cytoplasm at the onset of dormancy breaking. Significance statementCellular processes are influenced by the biophysical properties of the cytoplasm such as crowding and viscoelasticity. Although it has been suggested that cells tune the cytoplasmic properties in response to environmental changes, the molecular mechanisms remain unclear. Here, we used the dormant fission yeast spores and uncovered signaling pathways that facilitate cytoplasmic fluidization during dormancy breaking. Furthermore, we tracked the mobility of intracellular tracer particles, and found that the spore cytoplasm impedes the mobility of larger protein complexes, while allowing free diffusion of smaller molecules. These results suggest that small signaling proteins can diffuse relatively freely in the spore cytoplasm and have the ability to transmit dormancy breaking signals, while the motion of large complexes, such as ribosomes, is restricted.