A Single MAPKKK Regulates the Hog1 MAPK Pathway in the Pathogenic FungusCandida albicans

Abstract

The Hog1 mitogen-activated protein kinase (MAPK) plays a central role in stress responses in the human pathogen Candida albicans. Here, we have investigated the MAPK kinase kinase (MAPKKK)-dependent regulation of the pathway. In contrast to the Hog1 pathway in Saccharomyces cerevisiae, which is regulated by three MAPKKKs (Ssk2, Ssk22, and Ste11), our results demonstrate that Hog1 in C. albicans is regulated by a single MAPKKK Ssk2. Deletion of SSK2 results in comparable stress and morphological phenotypes exhibited by hog1Delta cells, and Ssk2 is required for the stress-induced phosphorylation and nuclear accumulation of Hog1, and for Hog1-dependent gene expression. Furthermore, phenotypes associated with deletion of SSK2 can be circumvented by expression of a phosphomimetic mutant of the MAPKK Pbs2, indicating that Ssk2 regulates Hog1 via activation of Pbs2. In S. cerevisiae, the Hog1 pathway is also regulated by the MAPKKK Ste11. However, we can find no connection between Ste11 and the regulation of Hog1 in C. albicans. Furthermore, expression of a chimeric Pbs2 protein containing the Ste11-dependent regulatory region of S. cerevisiae Pbs2, fails to stimulate Ste11-dependent stress signaling in C. albicans. Collectively, our data show that Ssk2 is the sole MAPKKK to relay stress signals to Hog1 in C. albicans and that the MAPK signaling network in C. albicans has diverged significantly from the corresponding network in S. cerevisiae.

Master and Commander in Fungal Pathogens: the Two-Component System and the HOG Signaling Pathway

“…In C. albicans, deletion of SSK2, but not STE11, generates phenotypes highly comparable to those of pbs2⌬ and hog1⌬ mutants in stress response and morphology and completely abolishes phosphorylation and nuclear accumulation of Hog1 in response to external stress (33). Notably, the ssk2⌬ mutant exhibits constitutively filamentous morphology, like the hog1⌬ mutant, with increased expression of hypha-specific genes such as HWP1 and ECE1, while the ssk1⌬ mutant is almost completely defective in hyphal formation (33). This indicates that Ssk2 is negatively regulated by either Ssk1 or other, unknown, signaling components for controlling the morphological differentiation of C. albicans.…”

“…In addition, sexual differentiation under nitrogen starva- In contrast to the model yeasts, most fungal pathogens appear to contain a single Ssk2-like MAPKKK. Two recent studies showed that a single MAPKKK is sufficient to control the Pbs2-Hog1 signaling pathway in C. albicans and C. neoformans (12,33). In C. albicans, deletion of SSK2, but not STE11, generates phenotypes highly comparable to those of pbs2⌬ and hog1⌬ mutants in stress response and morphology and completely abolishes phosphorylation and nuclear accumulation of Hog1 in response to external stress (33).…”

“…Two recent studies showed that a single MAPKKK is sufficient to control the Pbs2-Hog1 signaling pathway in C. albicans and C. neoformans (12,33). In C. albicans, deletion of SSK2, but not STE11, generates phenotypes highly comparable to those of pbs2⌬ and hog1⌬ mutants in stress response and morphology and completely abolishes phosphorylation and nuclear accumulation of Hog1 in response to external stress (33). Notably, the ssk2⌬ mutant exhibits constitutively filamentous morphology, like the hog1⌬ mutant, with increased expression of hypha-specific genes such as HWP1 and ECE1, while the ssk1⌬ mutant is almost completely defective in hyphal formation (33).…”

“…In C. neoformans, none of the Sho1-like membrane protein has been discovered in its genome (15). The findings that deletion of SSK2 completely abolishes Hog1 phosphorylation and phenotypes of the ssk2⌬ mutant are almost identical to those of the hog1⌬ mutant in both C. neoformans and C. albicans further suggest that the Ssk1-Ssk2-dependent pathway is the major signaling branch for activation of Hog1 in the human fungal pathogens studied to date (12,33).…”

Oxidative stress response pathways: Fission yeast as archetype

“…Similar to MAPKs, SAPKs are activated through a series of upstream events to induce the expression of stress-protective genes and coordinate diverse cellular processes that confer stress resistance (Kyriakis & Avruch, 2001). The SAPK modules that are present in fungal species are remarkably conserved in mammals ( Figure 2B): Win1/ Wis4-Wis1-Sty1 (S. pombe) (Millar et al, 1995;Samejima et al, 1997;Shieh et al, 1997a), SSK2/22-PBS2-HOG1 (S. cerevisiae) (Brewster et al, 1993;Posas & Saito, 1998;Posas et al, 1996), SskB-PbsB-HogA (A. nidulans) (Aguirre et al, 2005;Kawasaki et al, 2002;Vargas-Pérez et al, 2007), Ssk2-Pbs2-Hog1 (C. albicans) (Cheetham et al, 2007;San José et al, 1996) and MTK1/ASK1/TAK1/MLKs/MEKKsMKKs-p38 (a, b, g, d) (mammals) (Mielke & Herdegen, 2000;Nebreda & Porras, 2000;Paul et al, 1997;Tibbles & Woodgett, 1999). Among other TFs, the mammalian p38 SAPK pathway regulates ATF2, which is the ortholog of the fission yeast Atf1 TF (Table 1 and Figure 2B) (Dérijard et al, 1994).…”