Wsh3/Tea4 is an essential component of the Tea1 cell-end complex. In addition to its role in bipolar growth during the normal cell cycle, the Wsh3-Tea1 complex, together with the stress-signaling MAPK cascade, contributes to cell-polarity maintenance under stress conditions.

Members of the mitogen-activated protein kinase (MAPK) subfamily responsive to environmental stress stimuli are known as SAPKs (stress-activated protein kinases), which are conserved from yeast to humans. In the fission yeast Schizosaccharomyces pombe, Spc1/Sty1 SAPK is activated by diverse forms of stress, such as osmostress, oxidative stress and heat shock, and induces gene expression through the Atf1 transcription factor. Sin1 (SAPK interacting protein 1) was originally isolated as a protein that interacts with Spc1, and its orthologs were also found in diverse eukaryotes. Here we report that Sin1 is not required for the stress gene expression regulated by Spc1 and Atf1, and that Sin1 is an essential component of TOR (target of rapamycin) complex 2 (TORC2). TORC2 is not essential for cell viability in S. pombe but plays important roles in cellular survival of stress conditions through phosphorylation and activation of an AGC-family protein kinase, Gad8. In addition, inactivation of Gad8 results in a synthetic growth defect with cdc25-22, a temperature-sensitive mutation of the Cdc25 phosphatase that activates Cdc2 kinase at G 2 /M. Gad8 also positively regulates expression of the CDK inhibitor gene rum1 + , which is essential for cell cycle arrest in G 1 after nitrogen starvation. These results strongly suggest that the TORC2-Gad8 pathway has multiple physiological functions in cellular stress resistance and cell cycle progression at both G 1 /S and G 2 /M transitions. IntroductionStress-activated protein kinases (SAPKs) form an evolutionarily conserved subfamily of the MAP kinase (MAPK), members of which are stimulated in response to environmental stress. The prototype of SAPK was first identified in budding yeast as Hog1, which is mainly activated by high osmolarity stress. 1 On the other hand, human JNK (c-Jun N-terminal Kinase) and p38 SAPKs 2 as well as Spc1/Sty1 in the fission yeast Schizosaccharomyces pombe 3 are activated in response to diverse forms of stress, including osmostress, heat shock, oxidative stress and nutritional starvation. In S. pombe, activated Spc1 induces expression of a set of stress resistance genes through the Atf1 transcription factor. [4][5][6] Cells lacking the functional Spc1-Atf1 pathway are hypersensitive to diverse forms of stress conditions, indicating the essential role of the SAPK signaling in cellular survival of environmental changes. In addition, Spc1 positively regulates the initiation of mitosis independently of the Atf1-regulated gene expression. [7][8][9] Sin1 (SAPK interacting protein 1) was isolated by a yeast twohybrid screen as a protein that interacts with Spc1 MAPK. 10 It was reported that a sin1 mutant was hypersensitive to various stress conditions and defective in the stress gene expression regulated by the Spc1-Atf1 pathway and that by Pap1, an AP-1 family transcription factor. 10 Sin1 is widely conserved among eukaryotic species, 11,12 and its mammalian ortholog also interacts with JNK SAPK 13 and MEKK2 MAPK kinase kinase. 14 More recently, however, Si...

Intracellular thiols like L-cysteine and glutathione play a critical role in the regulation of cellular processes. Escherichia coli has multiple L-cysteine transporters, which export L-cysteine from the cytoplasm into the periplasm. However, the role of L-cysteine in the periplasm remains unknown. Here we show that an L-cysteine transporter, YdeD, is required for the tolerance of E. coli cells to hydrogen peroxide. We also present evidence that L-cystine, a product from the oxidation of L-cysteine by hydrogen peroxide, is imported back into the cytoplasm in a manner dependent on FliY, the periplasmic L-cystine-binding protein. Remarkably, this protein, which is involved in the recycling of the oxidized L-cysteine, is also found to be important for the hydrogen peroxide resistance of this organism. Furthermore, our analysis of the transcription of relevant genes revealed that the transcription of genes encoding FliY and YdeD is highly induced by hydrogen peroxide rather than by L-cysteine. These findings led us to propose that the inducible L-cysteine/Lcystine shuttle system plays an important role in oxidative stress tolerance through providing a reducing equivalent to the periplasm in E. coli.

Summary Background From yeast to human, TOR (Target Of Rapamycin) kinase plays pivotal roles in coupling extracellular stimuli to cell growth and metabolism. TOR kinase functions in two distinct protein complexes, TOR complex 1 (TORC1) and 2 (TORC2), which phosphorylate and activate different AGC-family protein kinases. TORC1 is controlled by the small GTPase Rheb, but little is known about TORC2 regulators. Results We have identified the Ryh1 GTPase, a human Rab6 ortholog, as an activator of TORC2 signaling in the fission yeast Schizosaccharomyces pombe. Mutational inactivation of Ryh1 or its guanine nucleotide exchange factor compromises the TORC2-dependent phosphorylation of the AGC-family Gad8 kinase. In addition, the effector domain of Ryh1 is important for its physical interaction with TORC2 and for stimulation of TORC2 signaling. Thus, GTP-bound Ryh1 is likely to be the active form stimulatory to TORC2–Gad8 signaling. Consistently, expression of the GTP-locked mutant Ryh1 is sufficient to promote interaction between TORC2 and Gad8 and to induce Gad8 hyper-phosphorylation. The loss of functional Ryh1, TORC2 or Gad8 brings about similar vacuolar fragmentation and stress sensitivity, further corroborating their involvement in a common cellular process. Human Rab6 can substitute Ryh1 in S. pombe and therefore, Rab6 may be a potential activator of TORC2 in mammals. Conclusions In its GTP-bound form, Ryh1, an evolutionarily conserved Rab GTPase, activates TORC2 signaling to the AGC kinase Gad8. The Ryh1 GTPase and the TORC2–Gad8 pathway are required for vacuolar integrity and cellular stress resistance in S. pombe.