Hown in Figure 1A. In ste11Dssk1Dssk22D mutant, the phosphorylation of Hog1p peaked within 10 min and disappeared within 20 min under 1.0M sorbitol. The duration of the phosphorylated state of Hog1p in ste11Dssk1Dssk22D mutant was also shorter than wild type (Figure 1B). However, the response to the stress in the ste11Dssk1Dssk22D mutant was quick. The activation of Ssk22p, on the other hand, was totally dependent on Ssk1p. In ste11Dssk1Dssk2D mutant, we could not detect any phosphorylation of Hog1p under hyperosmotic stress (Figure 2B). Our results suggest that there may be an unidentified factor that activates Ssk2p under osmotic stress in addition to Ssk1p. Here we name the unidentified factor “X factor” temporarily. The growth of ste11Dssk1Dssk22D mutant was faster than that of the ste11Dssk1Dssk2D mutant (Figure 2E). It has been reported that Ssk2p is specialized to promote actin cytoskeleton reassembly after osmotic shock [31,32]. This function requires the kinase activity of Ssk2p [26,31]. Osmotic stress induces a rapid disassembly of the actin cytoskeleton [31,33]. Actin cytoskeleton disassembly induces Ssk2p to translocate from the cytosol to the septin cytoskeleton of the bud neck [26,31,32]. Therefore, we asked whether actin disassembly would activate the Ssk2p to activate the HOG pathway. Lat B was used to induce rapid and complete disassembly of the actin cytoskeleton in strains BY4741 and ste11Dssk1D [34]. Within 20 min of Lat B treatment, neither strain displayed activation of Hog1p (Figure 2C). After 20 min incubation of both cells in 200 uM lat B, samples were fixed for Rd-phalloidin staining of actin structures. No actin structures were observed in the cells (Figure 2D). The results were in accordance with previous observation that activity of Hog1p activity is affected neither by actin-destabilizing drug latrunculin A, nor by actin-stabilizing drug jasplakinolide [21]. These results indicate that X factor may not be the actin disassembly.A Receiver Domain (Amino Acids 177,240) Near the Nterminus of SSK2 is Needed for the Activation of SSK2 Independent of SSKAs observed above, Ssk2p can be GM6001 biological activity activated without Ssk1p under osmotic stress, whereas the Ssk22p cannot. We carried out a sequence alignment analysis of the two proteins Ssk2p and Ssk22p. As shown in Figure 3, the sequence comparison shows that Ssk2p and Ssk22p are quite similar. The similarity of the kinase domains of these two MAPKKKs is higher than that of the N-terminal noncatalytical domains. Ssk2p is larger than Ssk22p, mainly due to an extra N-terminal segment (1,176). There isSsk2p can be Activated Independent of Ssk1p under Severe Osmotic StressAs described above, the HOG pathway was activated in the ssk1Dste11D mutant under osmotic stress but not in the get GKT137831 ste11Dssk2Dssk22D mutant, which indicated Ssk2p and Ssk22p may be activated independent of Ssk1p under osmotic stress. It hasAlternative Activation of Ssk2p in Osmotic StressFigure 1. Hog1p phosphorylation level and growth phenotypes for the wild type (WT) and mutant yeast 18334597 cells under various osmotic and salt stress conditions. A. Hog1p MAPK phosphorylation (P-Hog1p) was detected in the ssk1Dste11D mutant under hyperosmotic stress. Cells were exposed to different level of osmotic stress induced by sorbitol (concentration shown) in YPD medium for the time indicated. B. Same experiment as in A but for the wild type strain which shows higher sensitivity and a longer duration of the response. C. Hog1p phosph.Hown in Figure 1A. In ste11Dssk1Dssk22D mutant, the phosphorylation of Hog1p peaked within 10 min and disappeared within 20 min under 1.0M sorbitol. The duration of the phosphorylated state of Hog1p in ste11Dssk1Dssk22D mutant was also shorter than wild type (Figure 1B). However, the response to the stress in the ste11Dssk1Dssk22D mutant was quick. The activation of Ssk22p, on the other hand, was totally dependent on Ssk1p. In ste11Dssk1Dssk2D mutant, we could not detect any phosphorylation of Hog1p under hyperosmotic stress (Figure 2B). Our results suggest that there may be an unidentified factor that activates Ssk2p under osmotic stress in addition to Ssk1p. Here we name the unidentified factor “X factor” temporarily. The growth of ste11Dssk1Dssk22D mutant was faster than that of the ste11Dssk1Dssk2D mutant (Figure 2E). It has been reported that Ssk2p is specialized to promote actin cytoskeleton reassembly after osmotic shock [31,32]. This function requires the kinase activity of Ssk2p [26,31]. Osmotic stress induces a rapid disassembly of the actin cytoskeleton [31,33]. Actin cytoskeleton disassembly induces Ssk2p to translocate from the cytosol to the septin cytoskeleton of the bud neck [26,31,32]. Therefore, we asked whether actin disassembly would activate the Ssk2p to activate the HOG pathway. Lat B was used to induce rapid and complete disassembly of the actin cytoskeleton in strains BY4741 and ste11Dssk1D [34]. Within 20 min of Lat B treatment, neither strain displayed activation of Hog1p (Figure 2C). After 20 min incubation of both cells in 200 uM lat B, samples were fixed for Rd-phalloidin staining of actin structures. No actin structures were observed in the cells (Figure 2D). The results were in accordance with previous observation that activity of Hog1p activity is affected neither by actin-destabilizing drug latrunculin A, nor by actin-stabilizing drug jasplakinolide [21]. These results indicate that X factor may not be the actin disassembly.A Receiver Domain (Amino Acids 177,240) Near the Nterminus of SSK2 is Needed for the Activation of SSK2 Independent of SSKAs observed above, Ssk2p can be activated without Ssk1p under osmotic stress, whereas the Ssk22p cannot. We carried out a sequence alignment analysis of the two proteins Ssk2p and Ssk22p. As shown in Figure 3, the sequence comparison shows that Ssk2p and Ssk22p are quite similar. The similarity of the kinase domains of these two MAPKKKs is higher than that of the N-terminal noncatalytical domains. Ssk2p is larger than Ssk22p, mainly due to an extra N-terminal segment (1,176). There isSsk2p can be Activated Independent of Ssk1p under Severe Osmotic StressAs described above, the HOG pathway was activated in the ssk1Dste11D mutant under osmotic stress but not in the ste11Dssk2Dssk22D mutant, which indicated Ssk2p and Ssk22p may be activated independent of Ssk1p under osmotic stress. It hasAlternative Activation of Ssk2p in Osmotic StressFigure 1. Hog1p phosphorylation level and growth phenotypes for the wild type (WT) and mutant yeast 18334597 cells under various osmotic and salt stress conditions. A. Hog1p MAPK phosphorylation (P-Hog1p) was detected in the ssk1Dste11D mutant under hyperosmotic stress. Cells were exposed to different level of osmotic stress induced by sorbitol (concentration shown) in YPD medium for the time indicated. B. Same experiment as in A but for the wild type strain which shows higher sensitivity and a longer duration of the response. C. Hog1p phosph.
