The mitogen-activated protein kinase homolog HOG1 gene controls glycerol accumulation in the pathogenic fungus Candida albicans

Abstract

The Candida albicans HOG1 gene (HOG1 Ca ) was cloned by functional complementation of the osmosensitive phenotype associated with Saccharomyces cerevisiae hog1⌬ mutants. HOG1 Ca codes for a 377-amino-acid protein, 78% identical to S. cerevisiae Hog1p. A C. albicans hog1 null mutant was found to be sensitive to osmotic stress and failed to accumulate glycerol on high-osmolarity media.Fungi, like other microorganisms, are able to respond to changes in extracellular osmolarity, adjusting their intracellular composition to prevent the dehydration that could impair normal cell growth (4). Recently, different signal transduction pathways involving members of the mitogen-activated protein (MAP) kinase family have been shown to regulate different aspects of cell physiology (2, 7, 10, 18), one of them being responsible for adaptation to high osmolarity in Saccharomyces cerevisiae (5). The HOG1 gene (5) (high-osmolarity glycerol response pathway) encodes a MAP kinase which plays an essential role in osmoadaptation. Candida albicans is a commensal dimorphic pathogenic yeast which under situations that diminish the host immune response is able to colonize the human body and seriously compromise human health (20). Interest in this organism as a model pathogen has increased in view of the relevance of the fungal infections that it causes, especially in industrialized countries (12). The identification of signal transduction pathways in pathogenic fungi is essential for understanding fungal mechanisms of adaptation to a complex and changing environment like the human body and also for identifying potential novel targets in antifungal therapy. In this work, we have addressed the existence of a HOG pathway in C. albicans.A C. albicans gene library (19) was used to screen S. cerevisiae JBY10 (MATa ura3 leu2 his3 trp1 lys2 ade2 hog1-⌬1:: TRP1) (obtained from M. Gustin) by electroporation. Of more than 200,000 transformant clones, 121 were initially selected for an osmoresistant phenotype on 1 M sorbitol-minimal SD medium (0.67% yeast nitrogen base without amino acids, 2% glucose) plates. Three of these positive transformants were further characterized. Sequencing of approximately 1.6 kb from the insert present in one of them, pHOG1c24.2 (Fig. 1), allowed us to identify an open reading frame which encoded a putative protein, CaHog1p, of 377 amino acids (42.3 kDa), with an overall 78% identity (86% similarity) to S. cerevisiae Hog1p, this similarity not being restricted to the 11 MAP kinase subdomains but also evident within the carboxy-terminal nonkinase domain. CaHog1p was 55.7% identical to S. cerevisiae Fus3p, 53.9% identical to Homo sapiens Erk1p, 52% identical to C. albicans Mkc1p, and 48.8% identical to S. cerevisiae Slt2p. A TGY motif, characteristic of hyperosmolarityactivated MAP kinases (6), is found in subdomain VIII in C. albicans Hog1p, similar to Xenopus laevis Mpk2 (21) and mammalian p38 (14) and CSBP kinases (17), which have been shown to complement S. cerevisiae hog1⌬ mutants. HOG1 Ca did not complement S. cerevisi...

The Pbs2 MAP kinase kinase is essential for the oxidative-stress response in the fungal pathogen Candida albicans

“…We present evidence that Pbs2 plays an important role in the response to oxidative and osmotic stress. Deletion of the PBS2 gene in C. albicans leads to the expected osmosensitive phenotype that has already been described for the hog1 mutant (San José et al, 1996), as also occurs with homologous routes in other organisms (Brewster et al, 1993;Millar et al, 1995;Shiozaki & Russell, 1995). The hierarchical relationship between Pbs2 and Hog1 regarding osmotic stress seems to be linear, as pbs2 and hog1 mutants displayed a similar osmosensitivity, independent of the osmolyte and the state of the medium (liquid or solid).…”

“…Four MAP kinases have so far been identified in this organism: Mkc1, the homologue to the Slt2/Mpk1 MAP kinase from S. cerevisiae (NavarroGarcía et al, 1995), Cek1, homologue to Kss1 (Csank et al, 1998), Cek2, homologue to Fus3 (Chen et al, 2002) and Hog1, homologue to the Hog1 MAP kinase. Hog1 has been implicated in different functions in C. albicans, such as glycerol accumulation, morphological transitions, cellwall biogenesis and virulence (Alonso-Monge et al, 1999;San José et al, 1996). It has also been shown how this MAP kinase is phosphorylated when exposed to NaCl and hydrogen peroxide (Alonso-Monge et al, 2003).…”

“…Candida albicans is a pathogenic yeast of great clinical interest (Fox, 1993;Odds, 1988 , 1999;San José et al, 1996). It has also been shown how this MAP kinase is phosphorylated when exposed to NaCl and hydrogen peroxide (Alonso-Monge et al, 2003).…”

“…sty1 2 mutants are sterile and exhibit a G 2 cell-cycle delay, indicating additional roles for Sty1 in meiosis and cell-cycle progression (Millar, 1999;Shiozaki & Russell, 1995. The range of environmental insults that activate Sty1 is similar to the stimuli that activate the mammalian SAPKs: JNKs and p38/CSBP (GalchevaGargova et al, 1994;Kyriakis & Avruch, 1996), which control a wide variety of physiological and pathological conditions.Candida albicans is a pathogenic yeast of great clinical interest (Fox, 1993;Odds, 1988 , 1999;San José et al, 1996). It has also been shown how this MAP kinase is phosphorylated when exposed to NaCl and hydrogen peroxide (Alonso-Monge et al, 2003).…”

The MAP kinase Mkc1p is activated under different stress conditions in Candida albicans

“…C. albicans HOG1 has been cloned in budding yeast and complements the lack of growth of a Saccharomyces cerevisiae hog1 mutant in a high-osmolarity medium. Likewise, the C. albicans hog1 mutant has been found to be unable to grow in the presence of high concentrations of extracellular solutes (NaCl, KCl, sorbitol) and also to accumulate internal glycerol in response to an osmotic challenge (San José et al, 1996). It was later found that Hog1p is implicated in morphogenesis, since hog1 mutants form filaments even under non-inducing conditions.…”