SummaryExtracellular cues regulate cell fate, and this is mainly achieved through the engagement of specific transcriptional programs. The TORC1 and TORC2 complexes mediate the integration of nutritional cues to cellular behavior, but their interplay is poorly understood. Here, we use fission yeast to investigate how phosphatase activity participates in this interplay during the switch from proliferation to sexual differentiation. We find that loss of PP2A-B55Pab1 enhances the expression of differentiation-specific genes and leads to premature conjugation. pab1 deletion brings about a transcriptional profile similar to TORC1 inactivation, and deletion of pab1 overcomes the repression of differentiation genes in cells overexpressing TORC1. Importantly, we show that this effect is mediated by an increased TORC2-AKT (Gad8) signaling. Under nutrient-rich conditions, PP2A-B55Pab1 dephosphorylates Gad8 Ser546, repressing its activity. Conversely, TORC1 inactivation upon starvation leads to the inactivation of PP2A-B55Pab1 through the Greatwall-Endosulfin pathway. This results in the activation of Gad8 and the commitment to differentiation. Thus, PP2A-B55Pab1 enables a crosstalk between the two TOR complexes that controls cell-fate decisions in response to nutrient availability.

The mitogen-activated protein kinase (MAPK) Sty1 is essential for the regulation of transcriptional responses that promote cell survival in response to different types of environmental stimuli in Schizosaccharomyces pombe. In fission yeast, three distinct eukaryotic initiation factor 2␣ (eIF2␣) kinases, two mammalian HRI-related protein kinases (Hri1 and Hri2) and the Gcn2 ortholog, regulate protein synthesis in response to cellular stress conditions. In this study, we demonstrate that both Hri1 and Hri2 exhibited an autokinase activity, specifically phosphorylated eIF2␣, and functionally replaced the endogenous Saccharomyces cerevisiae Gcn2. We further show that Gcn2, but not Hri1 or Hri2, is activated early after exposure to hydrogen peroxide and methyl methanesulfonate (MMS). Cells lacking Gcn2 exhibit a later activation of Hri2. The activated MAPK Sty1 negatively regulates Gcn2 and Hri2 activities under oxidative stress but not in response to MMS. In contrast, Hri2 is the primary activated eIF2␣ kinase in response to heat shock. In this case, the activation of Sty1 appears to be transitory and does not contribute to the modulation of the eIF2␣ kinase stress pathway. In strains lacking Hri2, a type 2A protein phosphatase is activated soon after heat shock to reduce eIF2␣ phosphorylation. Finally, the MAPK Sty1, but not the eIF2␣ kinases, is essential for survival upon oxidative stress or heat shock, but not upon MMS treatment. These findings point to a regulatory coordination between the Sty1 MAPK and eIF2␣ kinase pathways for a particular range of stress responses.

SummaryIn fission yeast, three distinct eukaryotic initiation factor 2a (eIF2a) kinases (Hri1, Hri2 and Gcn2), regulate protein synthesis in response to various environmental stresses. Thus, Gcn2 is activated early after exposure to hydrogen peroxide (H 2 O 2 ) and methyl methanesulfonate (MMS), whereas Hri2 is the primary activated eIF2a kinase in response to heat shock. The function of Hri1 is still not completely understood. It is also known that the mitogen-activated protein kinase Sty1 negatively regulates Gcn2 and Hri2 activities under oxidative stress. In this study, we demonstrate that Hri1 is mainly activated, and its expression upregulated, during transition from exponential growth to the stationary phase in response to nutritional limitation. Accordingly, both Hri1 and Gcn2, but not Hri2, are activated upon nitrogen source deprivation. In contrast, Hri2 is stimulated early during glucose starvation. We also found that Gcn2 is implicated in nitrogen starvation-induced growth arrest in the cell cycle G1 phase as well as in the non-selective protein degradation process caused upon this particular cellular stress. Moreover, Gcn2, but not Hri1 or Hri2, is essential for survival of cells growing in minimal medium, upon oxidative stress or glucose limitation. We further show that eIF2a phosphorylation at serine 52 by the eIF2a kinases is necessary for efficient cell cycle arrest in the G1 phase, for the consequent protein degradation and for sexual differentiation, under nitrogen starvation. Therefore, the eIF2a kinase signalling pathway modulates G1 phase cell cycle arrest, cell survival and mating under nutritional stress in the fission yeast Schizosaccharomyces pombe.