Breaking spore dormancy in budding yeast transforms the cytoplasm and the solubility of the proteome

Abstract

The biophysical properties of the cytoplasm are major determinants of key cellular processes and adaptation. Many yeasts produce dormant spores that can withstand extreme conditions. We show that spores of Saccharomyces cerevisiae exhibit extraordinary biophysical properties, including a highly viscous and acidic cytosol. These conditions alter the solubility of more than 100 proteins such as metabolic enzymes that become more soluble as spores transit to active cell proliferation upon nutrient repletion. A key regulator of this transition is the heat shock protein, Hsp42, which shows transient solubilization and phosphorylation, and is essential for the transformation of the cytoplasm during germination. Germinating spores therefore return to growth through the dissolution of protein assemblies, orchestrated in part by Hsp42 activity. The modulation of spores’ molecular properties are likely key adaptive features of their exceptional survival capacities.

mRNA condensation fluidizes the cytoplasm

“…Despite this difference, we have found evolutionarily conserved roles for the dynamic processes of polysome collapse and mRNA condensation in mediating stress-induced increased mesoscale macromolecular diffusivity within the cytoplasm (Figure 6). Several studies have demonstrated that decreases in ATP cause yeast or bacterial cells to transition to a glassy/solid state in the cytoplasm (Munder et al, 2016;Parry et al, 2014;Plante et al, 2023). In our study, we also found that the reduction of ATP and cytoplasmic pH results in decreased mesoscale diffusivity.…”

“…We found that the initial fluidization of the cytoplasm was important for the formation of Q-bodies. It will be interesting to investigate if other mesoscale assemblies that are known to assemble during ATP depletion (Marini et al, 2020;Munder et al, 2016;Plante et al, 2023) also require cytoplasmic fluidization for their formation.…”

“…Microrheology is the inference of the physical properties of a material from the observation of the motion of passive probes embedded within the environment (Mason and Weitz, 1995). Passive rheological probes have proven to be effective tools for inferring the biophysical properties of different cellular compartments (Chambers et al, 2022;Delarue et al, 2018;Munder et al, 2016;Parry et al, 2014;Plante et al, 2023;Shu et al, 2022.;Xiang et al, 2021).…”

Fungal spore swelling and germination are restricted by the macrophage phagolysosome

“…Previous studies suggest this may not be the case, since the provision of nutrients can promote the early onset polarized growth of poorly swollen spores whilst nutrient depletion can prevent germination of fully swollen spores ( Ekundayo and Carlile, 1964 ; Feofilova et al., 2012 ). More recently, the breaking of dormancy of spores of ascomycete yeasts has been shown to require reduction of the very high internal viscosity or “glassy state” of the spore cytoplasm due to the high internal concentration of compatible solutes such as trehalose ( Plante et al., 2023 ). The transition from a highly viscous to a more fluid cytoplasm more capable of metabolic activity is regulated by chaperones that enable the solubilisation and refolding of the spore proteome ( Plante et al., 2023 ).…”

The breaking of fungal spore dormancy: A coordinated transition

“…How do these dormant spores with limited metabolic activity transition from this nearly inactive state to a lively germling? A recent study published in PLOS Biology by Plante and colleagues shows that the proper transition is at least in part orchestrated by chaperones, specifically Hsp42 [6]. The authors show that this heat shock protein is activated by phosphorylation during very early stages of germination and ensures proper resolubilization of proteins during breaking of dormancy.…”