Regulation of gene expression by stress-activated protein kinases (SAPKs) is essential for cell adaptation to extracellular stimuli. Exposure of yeast to high osmolarity results in activation of the SAPK Hog1, which associates with transcription factors bound at target promoters and stimulates transcriptional initiation. Unexpectedly, activated Hog1 also associates with elongating Pol II and components of the elongation complex. Hog1 is selectively recruited to the entire coding region of osmotic stress genes, but not to constitutively expressed genes. Selective association of Hog1 with the transcribed region of osmoresponsive genes is determined by the 3' untranslated region (3' UTR). Lastly, Hog1 is important for the amount of the RNA polymerase II (Pol II) elongation complex and of mRNA produced from genes containing osmoresponsive coding regions. Thus, in addition to its various functions during transcriptional initiation, Hog1 behaves as a transcriptional elongation factor that is selective for genes induced upon osmotic stress.

Anaphase chromatin bridges can lead to chromosome breakage if not properly resolved before completion of cytokinesis. The NoCut checkpoint, which depends on Aurora B at the spindle midzone, delays abscission in response to chromosome segregation defects in yeast and animal cells. How chromatin bridges are detected, and whether abscission inhibition prevents their damage, remain key unresolved questions. We find that bridges induced by DNA replication stress and by condensation or decatenation defects, but not dicentric chromosomes, delay abscission in a NoCut-dependent manner. Decatenation and condensation defects lead to spindle stabilization during cytokinesis, allowing bridge detection by Aurora B. NoCut does not prevent DNA damage following condensin or topoisomerase II inactivation; however, it protects anaphase bridges and promotes cellular viability after replication stress. Therefore, the molecular origin of chromatin bridges is critical for activation of NoCut, which plays a key role in the maintenance of genome stability after replicative stress.

Sir2 histone deacetylase prevents programmed cell death caused by sustained activation of the Hog1 stress-activated protein kinaseSustained activation of Hog1 in yeast inhibits mitochondrial respiration and leads to ROS accumulation, which causes cell death. Activation of Sir2 in response to Hog1 activation relieves the Hog1- induced oxidative stress and prevents cell death.

Highly selective, cell-permeable and fast-acting inhibitors of individual kinases are sought-after as tools for studying the cellular function of kinases in real time. A combination of small molecule synthesis and protein mutagenesis, identified a highly potent inhibitor (1-Isopropyl-3-(phenylethynyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine) of a rationally engineered Hog1 serine/threonine kinase (Hog1T100G). This inhibitor has been successfully used to study various aspects of Hog1 signaling, including a transient cell cycle arrest and gene expression changes mediated by Hog1 in response to stress. This study also underscores that the general applicability of this approach depends, in part, on the selectivity of the designed the inhibitor with respect to activity versus the engineered and wild type kinases. To explore this specificity in detail, we used a validated chemogenetic assay to assess the effect of this inhibitor on all gene products in yeast in parallel. The results from this screen emphasize the need for caution and for case-by-case assessment when using the Analog-Sensitive Kinase Allele technology to assess the physiological roles of kinases.

Although the benefit of sirtuin activation in age-related diseases is well-characterized, the benefit of sirtuin activation in acute diseases has been elusive. Here we discuss that, at least in yeast, Sir2 activation prevents programmed cell death induced by the sustained activation of the stress activated protein kinase (SAPK) Hog1, the yeast homologue of the p38 SAPK. Sir2 prevents ROS formation and maximize cell survival upon SAPK activation. The conserved function of Sir2 in age-related diseases and the conserved role of SAPKs open the possibility of a novel role for sirtuins in cell fate determination in eukaryotic cells.

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