In the fission yeast Schizosaccharomyces pombe the Wak1p/ Win1p-Wis1p-Sty1p stress-activated protein kinase (SAPK) pathway relays environmental signals to the transcriptional machinery and modulates gene expression via a cascade of protein phosphorylation. Cells of S. pombe subjected to cold shock (transfer from 28°C to 15°C) transiently activated the Sty1p mitogen-activated protein kinase (MAPK) by phosphorylation. Induction of this response was completely abolished in cells disrupted in the upstream response regulator Mcs4p. The cold-triggered Sty1p activation was partially dependent on Wak1p MAPKKK and fully dependent on Wis1p MAPKK suggesting that the signal transmission follows a branched pathway, with the redundant MAPKKK Win1p as alternative transducer to Wis1p, which subsequently activates the effector Sty1p MAPK. Also, the bZIP transcription factor Atf1p became phosphorylated in a Sty1p-dependent way during the cold shock and this phosphorylation was found responsible for the increased expression of gpd1 + , ctt1 + , tps1 + and ntp1 + genes. Strains deleted in transcription factors Atf1p or Pcr1p were unable to grow upon incubation at low temperature whereas those disrupted in any member of the SAPK pathway were able to do so. These data reveal that S. pombe responds to cold by inducing the SAPK pathway. However, such activation is dispensable for yeast growth in cold conditions, supporting that the presence of Atf1/Pcr1 heterodimers, rather than an operative SAPK pathway, is critical to ensure yeast growth at low temperature by an as yet undefined mechanism.Keywords: cold; SAPK pathway; fission yeast.Low temperature is an important environmental signal for all living organisms. Adaptive response to cold stress involves synthesis of several types of proteins. In bacteria, thermal downshifts induce cold-shock proteins (Csp) that function as RNA chaperones favouring efficient translation of mRNAs at low temperature [1]. However, in eukaryotes no proteins homologous to bacterial Csp's have been isolated and cold shock-inducible proteins range from structural components involved in ribosomal biogenesis to transcriptional regulation factors that activate gene expression in response to a drop in temperature [2,3].The mitogen-activated protein kinase (MAPK) signalling pathways are critical for the sensing and response of eukaryotic cells to changes in the external environment [4]. These MAPK cascades are highly conserved through evolution and serve to transduce signals to the nucleus, which result in new patterns of gene expression [5,6]. Each MAPK module comprises at least three protein kinases: a MAP kinase is activated through phosphorylation on specific threonine and tyrosine residues by a MAPK kinase (MAPKK or MEK) which is in turn activated by phosphorylation in one or several serine and threonine residues by a MAPKK kinase (MAPKKK or MEKK). Recently, different studies have revealed a key role for MAPK cascades in the response of metazoan cells to osmotic changes, heat shock, oxidative stress and UV radi...

In Schizosaccharomyces pombe, glucose concentrations below a certain threshold trigger the stress-activated protein kinase (SAPK) signal transduction pathway and promote increased transcription of Atf1-dependent genes coding for the general stress response. Removal of glucose specifically induces the nuclear accumulation of green fluorescent protein-labeled Pap1 (GFP-Pap1) and the expression of genes dependent on this transcription factor. In contrast, depletion of the nitrogen source triggers the SAPK pathway but does not activate Pap1-dependent gene transcription, indicating that carbon stress rather than growth arrest leads to an endogenous oxidative condition that favors nuclear accumulation of Pap1. The reductant agents glutathione or N-acetylcysteine suppress the nuclear accumulation of GFP-Pap1 induced by glucose deprivation without inhibiting the activation of the MAPK Sty1. In addition, cells expressing a mutant GFP-Pap1 unable to accumulate into the nucleus upon hydrogen peroxide-mediated oxidative stress failed to show this protein into the nucleus in the absence of glucose. These results support the concept of a concerted action between the SAPK pathway and the Pap1 transcription factor during glucose exhaustion by which glucose limitation induces activation of the SAPK pathway prior to the oxidative stress caused by glucose deprivation. The ensuing induction of Atf1-dependent genes (catalase) decreases the level of hydroperoxides allowing Pap1 nuclear accumulation and function. Congruent with this interpretation, glucose-depleted cells show higher adaptive response to exogenous oxidative stress than those maintained in the presence of glucose.

The Wis1p-Sty1p mitogen-activated protein kinase cascade is a major signalling system in the fission yeast Schizosaccharomyces pombe for a wide range of stress responses. It is known that trehalose functions as a protective metabolite to counteract deleterious effects of environmental stresses. Herein it is reported that the expression of genes related to trehalose metabolism in S. pombe, ntp1+ (neutral trehalase) and tps1 + [trehalose-6-phosphate (T6P) synthase], is partially regulated by the Sty1p kinase under salt-induced osmotic stress and conditions of slight oxidative stress and is fully dependent on this kinase under severe oxidative stress. This control is carried out through transcription factors Atf1p/Pcr1p during osmotic stress and through Pap1p during exposure to low levels of oxidative stress. However, all three transcription factors are needed for gene expression under conditions of extreme oxidative stress. In addition, a role for Sty1p in the modulation of post-transcriptional activation of trehalase mediated by Pka1p/Sck1p kinases, as well as in the activity of T6P synthase under such stressful conditions has been demonstrated. These results reveal a novel dual action of the Wis1p-Sty1p pathway in the regulation of trehalose metabolism in fission yeast.

Neutral trehalases mobilize trehalose accumulated by fungal cells as a protective and storage carbohydrate. A structural feature of these enzymes is the presence of an EF-like motif similar to that shown by many Ca2+-binding proteins. In this study we provide direct evidence for physical binding of Ca2+ to neutral trehalase (Ntp1p) of the fission yeast Schizosaccharomyces pombe, and show that aspartic residues at positions 97 and 108 in the conserved putative Ca2+-binding motif of Ntp1p appear to be responsible for this interaction. Mutations in these residues do not interfere with the ability of Ntp1p to associate in vivo with trehalose-6-phosphate synthase, but prevent activation of neutral trehalase triggered by the addition of glucose or by subjecting cells to stressing conditions. Strains expressing Ntp1p variants that are unable to bind Ca2+ partially resemble those devoid of the ntp1+ gene in terms of trehalose hyperaccumulation. Gel filtration of cell extracts from wild-type cells after EDTA treatment or from cells containing Ntp1p with mutations in aspartic acid residues within the Ca2+-binding site revealed that Ntp1p eluted mainly in an inactive conformation instead of the dimeric or trimeric active form of the enzyme. These results suggest that activation of S. pombe Ntp1p under different conditions depends upon Ca2+ binding through the Ca2+-binding motif as a prerequisite for correct enzyme oligomerization to its active form. Given the high degree of conservation of the Ca2+ accommodation site, this might be a general mechanism regulating neutral trehalase activity in other yeasts and filamentous fungi.

In the fission yeast Schizosaccharomyces pombe, a heat shock enhances transcription of the ntp1 + gene, encoding the hydrolytic enzyme neutral trehalase. As compared to wild-type cells, cells devoid of the MAP kinase Sty1p showed a strong decrease in ntp1 + expression induced by the temperature upshift, indicating that the stressactivated protein kinase (SAPK) pathway regulates the expression of this gene during heat shock. The transcription factor Atf1p, which is the main downstream target for Sty1p in the SAPK pathway, appears to be involved in such control, since ntp1