The Activity of Yeast Hog1 MAPK Is Required during Endoplasmic Reticulum Stress Induced by Tunicamycin Exposure

Abstract

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the so-called unfolded protein response (UPR), a conserved signaling pathway that drives the transcription of genes such as chaperones and folding enzymes. Nevertheless, the activity of the UPR accounts only for a part of the gene expression program activated upon ER stress. Moreover, the mechanism(s) for how cells adapt and survive to this stress are largely unknown. Here, we show that the yeast high osmolarity glycerol (HOG) pathway plays a role in ER stress resistance. Strains lacking the MAPK Hog1p displayed sensitivity to tunicamycin or ␤-mercaptoethanol, whereas hyperactivation of the pathway enhanced their resistance. However, these effects were not due to Hog1p-mediated regulation of the UPR. Northern blot analysis demonstrated that Hog1p controls the tunicamycininduced transcriptional change of GPD1 and that wild-type cells exposed to the drug accumulated glycerol in a Hog1p-dependent manner. Consistent with this, deletion of genes involved in glycerol synthesis caused increased sensitivity to tunicamycin, whereas overexpression of GPD1 provided higher tolerance to both wildtype and hog1⌬ mutant cells. Quite remarkably, these effects were mediated by the basal activity of the MAPK because tunicamycin exposure does not trigger the phosphorylation of Hog1p or its nuclear import. Hence, our results describe new aspects of the yeast response to ER stress and identify additional functions of glycerol and the Hog1p MAPK to provide stress resistance.

Characterization of the S. cerevisiae inp51 mutant links phosphatidylinositol 4,5-bisphosphate levels with lipid content, membrane fluidity and cold growth

“…KCl, 1 mM MgSO 4 ), and protein extracts were prepared and processed for galactosidase activity as previously described [106]. One unit is defined as the amount of enzyme that is able to convert 1 nmol of the substrate o-NPG per min under the assay conditions.…”

ER stress response mechanisms in the pathogenic yeastCandida glabrataand their roles in virulence

“…Tunicamycin also induces cell wall stress, which may provide an explanation for why certain genes implicated in cell wall biogenesis were found to be upregulated during the UPR in tunicamycin-treated S. cerevisiae and C. albicans cells 11 , 58 , 60 . In contrast, the loss of Ire1 alone in C. glabrata did not confer a cell wall-defective phenotype, and the upregulation of some cell wall-related genes upon tunicamycin exposure was mainly dependent on calcineurin, but not Ire1 12 .…”

Inhibition of MAPK Hog1 Results in Increased Hsp104 Aggregate Formation Probably through Elevated Arsenite Influx into the Cells, an Approach with Numerous Potential Applications

“…Another likely explanation would be that inhibition of the kinase activity of the Hog1 for a longer time leads to protein misfolding and thereby induction of heat shock protein responses. Hog1 kinase activity has been shown required to provide a necessary function to cope with unfolded protein accumulation due to ER stress [49]. Considering the fact that Hog1 becomes activated upon a wide variety of environmental cues and is involved in transcription, translation, transport, as well as cell cycle adaptations in response to different conditions [23] [24] [29], these are all plausible scenarios.…”