Cip1 tunes cell cycle arrest duration upon calcineurin activation

Dual regulation of the levels and function of Start transcriptional repressors drives G1 arrest in response to cell wall stress

BACKGROUND

Many different stress signaling pathways converge in a common response: slowdown or arrest cell cycle in the G1 phase. The G1/S transition (called Start in budding yeast) is a key checkpoint controlled by positive and negative regulators. Among them, Whi7 and Whi5 are transcriptional repressors of the G1/S transcriptional program, yeast functional homologs of the Retinoblastoma family proteins in mammalian cells. Under standard conditions, Whi7 plays a lesser role than Whi5 in Start inhibition. However, under cell wall stress, Whi7 is induced and plays a more important role in G1/S control. In this work, we investigated the functional hallmarks of Whi7 and Whi5, which determine their strength as Start inhibitors under cell wall stress.

METHODS

The response of Saccharomyces cerevisiae to Calcofluor White was investigated to characterize the regulation and function of Whi7 and Whi5 under cell wall stress. To control their protein levels, we used dose-dependent β-estradiol-induced expression and auxin-induced degron protein fusions. We also performed Chromatin Immunoprecipitation assays to investigate Whi7 and Whi5 association with Start promoters and scored cell cycle arrest and re-entry using cell microscopy assays.

RESULTS

We found that cell wall stress promoted the specific upregulation of the Whi7 Start repressor. First, although cell wall stress increases Whi7 protein levels, this is not the only determinant behind the Whi7 function in promoting G1 arrest. Indeed, artificial induction of Whi5 at the same protein level resulted in a lower G1 block. Second, under cell wall stress, Whi7 was specifically recruited to SBF-target promoters, independent of the increase in its protein levels or cell cycle stage. Finally, we found that Whi7 protein instability further increased during cell wall stress and that Whi7 degradation triggered advanced cell cycle re-entry.

CONCLUSIONS

Here, we show that cell wall stress signaling specifically enhances Whi7 function as a Start transcriptional repressor. Importantly, we identified new Whi7-specific regulatory mechanisms that do not operate in the Whi5 repressor. Our results indicate that cells may benefit from stress-specific repressors to ensure the stress-induced G1 arrest and that Whi7 rapid degradation may be particularly important to resume cell cycle upon adaptation.

Calcineurin promotes adaptation to chronic stress through two distinct mechanisms

Adaptation to environmental stress requires coordination between stress-defense programs and cell cycle progression. The immediate response to many stressors has been well characterized, but how cells survive in challenging environments long term is unknown. Here, we investigate the role of the stress-activated phosphatase calcineurin (CN) in adaptation to chronic CaCl2 stress in Saccharomyces cerevisiae. We find that prolonged exposure to CaCl2 impairs mitochondrial function and demonstrate that cells respond to this stressor using two CN-dependent mechanisms-one that requires the downstream transcription factor Crz1 and another that is Crz1 independent. Our data indicate that CN maintains cellular fitness by promoting cell cycle progression and preventing CaCl2-induced cell death. When Crz1 is present, transient CN activation suppresses cell death and promotes adaptation despite high levels of mitochondrial loss. However, in the absence of Crz1, prolonged activation of CN prevents mitochondrial loss and further cell death by upregulating glutathione biosynthesis genes thereby mitigating damage from reactive oxygen species. These findings illustrate how cells maintain long-term fitness during chronic stress and suggest that CN promotes adaptation in challenging environments by multiple mechanisms.

Dynamic phosphorylation of Hcm1 promotes fitness in chronic stress

Cell survival depends upon the ability to adapt to changing environments. Environmental stressors trigger an acute stress response program that rewires cell physiology, downregulates proliferation genes and pauses the cell cycle until the cell adapts. Here, we show that dynamic phosphorylation of the yeast cell cycle-regulatory transcription factor Hcm1 is required to maintain fitness in chronic stress. Hcm1 is activated by cyclin dependent kinase (CDK) and inactivated by the phosphatase calcineurin (CN) in response to stressors that signal through increases in cytosolic Ca2+. Expression of a constitutively active, phosphomimetic Hcm1 mutant reduces fitness in stress, suggesting Hcm1 inactivation is required. However, a comprehensive analysis of Hcm1 phosphomutants revealed that Hcm1 activity is also important to survive stress, demonstrating that Hcm1 activity must be toggled on and off to promote gene expression and fitness. These results suggest that dynamic control of cell cycle regulators is critical for survival in stressful environments.

Calcineurin promotes adaptation to chronic stress through two distinct mechanisms

Adaptation to environmental stress requires coordination between stress-defense programs and cell cycle progression. The immediate response to many stressors has been well characterized, but how cells survive in challenging environments long-term is unknown. Here, we investigate the role of the stress-activated phosphatase calcineurin (CN) in adaptation to chronic CaCl2 stress in Saccharomyces cerevisiae. We find that prolonged exposure to CaCl2 impairs mitochondrial function and demonstrate that cells respond to this stressor using two CN-dependent mechanisms - one that requires the downstream transcription factor Crz1 and another that is Crz1-independent. Our data indicate that CN maintains cellular fitness by promoting cell cycle progression and preventing CaCl2-induced cell death. When Crz1 is present, transient CN activation suppresses cell death and promotes adaptation despite high levels of mitochondrial loss. However, in the absence of Crz1, prolonged activation of CN prevents mitochondrial loss and further cell death by upregulating glutathione (GSH) biosynthesis genes thereby mitigating damage from reactive oxygen species. These findings illustrate how cells maintain long-term fitness during chronic stress and suggest that CN promotes adaptation in challenging environments by multiple mechanisms.