Rewiring yeast osmostress signalling through the MAPK network reveals essential and non-essential roles of Hog1 in osmoadaptation

Abstract

Mitogen-activated protein kinases (MAPKs) have a number of targets which they regulate at transcriptional and post-translational levels to mediate specific responses. The yeast Hog1 MAPK is essential for cell survival under hyperosmotic conditions and it plays multiple roles in gene expression, metabolic regulation, signal fidelity and cell cycle regulation. Here we describe essential and non-essential roles of Hog1 using engineered yeast cells in which osmoadaptation was reconstituted in a Hog1-independent manner. We rewired Hog1-dependent osmotic stress-induced gene expression under the control of Fus3/Kss1 MAPKs, which are activated upon osmostress via crosstalk in hog1Δ cells. This approach revealed that osmotic up-regulation of only two Hog1-dependent glycerol biosynthesis genes, GPD1 and GPP2, is sufficient for successful osmoadaptation. Moreover, some of the previously described Hog1-dependent mechanisms appeared to be dispensable for osmoadaptation in the engineered cells. These results suggest that the number of essential MAPK functions may be significantly smaller than anticipated and that knockout approaches may lead to over-interpretation of phenotypic data.

Osmostress enhances activating phosphorylation of Hog1 MAP kinase by mono‐phosphorylated Pbs2 MAP 2K

“…In WT cells, the three modes occurred at 0.2-0.8 M NaCl, 1.0-1.2 M NaCl, and > 1.4 M NaCl ( Fig 9D). The WT profile is consistent with the numerous earlier reports (Van Wuytswinkel et al, 2000;Muzzey et al, 2009;Miermont et al, 2013;Babazadeh et al, 2014;English et al, 2015).…”

An integrated view on a eukaryotic osmoregulation system

“…This observation, together with the properties of the re-routed osmoregulation system (Babazadeh et al 2014) suggest that glycerol accumulation and volume recovery are critical in osmoadaptation and if those are possible they may overcome lack of many other Hog1-dependent effects, probably also because volume recovery diminishes the stimulus and hence signalling through the HOG system, which through its routing to Fus3 and Kss1 may cause deleterious effects. In the presence of Hog1, volume recovery allows deactivation of Hog1 and relieve of the cell cycle block mediated by active Hog1.…”

“…A hog1Δ strain (Fig. 3) that expresses under the Fus3-Kss1 controlled FUS1 promoter an unphosphorylatable hyperactive version of Gpd1 as well as Gpp2 displayed glycerol accumulation and volume recovery profiles essentially as wild type, and could also grow under high osmolarity conditions (Babazadeh et al 2014). Quite remarkably it appears that in this system Fps1 is closed even in the absence of Hog1 since expression of an N-terminal truncation of Fps1 caused osmo-sensitivity and deletion of FPS1 did not affect osmotolerance of the strain.…”

Tunable bet hedging in yeast responses to osmotic stress

“…The high osmolarity glycerol (HOG) signaling pathway is central to an elaborate stress response that reduces cellular damage and death in unpredictably changing osmotic environments where the balance between external solutes and free water pressure in the cell can change suddenly (Hohmann, 2002). A main function of the HOG pathway is the production and accumulation of intracellular glycerol, which restores water balance and, as demonstrated by a large body of work from many labs, is essential for survival, adaptation and proliferation in hyperosmotic stress (Babazadeh et al, 2014;Clotet and Posas, 2007;Hohmann, 2002;Hohmann et al, 2007;Nadal et al, 2002;Saito and Posas, 2012). In the wild, yeast and other microorganisms must balance immediate survival against evolutionary fitness.…”