Candidaalbicansmutants with mutations in mitogen-activated protein (MAP) kinaseHOG1displayed an increased sensitivity to agents producing reactive oxygen species, such as oxidants (menadione, hydrogen peroxide, or potassium superoxide), and UV light. Consistent with this finding,C.albicansHog1 was activated not only in response to an increase in external osmolarity, as happens with itsSaccharomycescerevisiaehomologue, but also in response to hydrogen peroxide. The Hog1-mediated response to oxidative stress was different from that of transcription factor Cap1, the homologue ofS.cerevisiaeYap1, as shown by the different sensitivities to oxidants and the kinetics of cell death ofcap1Δ,hog1, andhog1cap1Δ mutants. Deletion ofCAP1did not influence the level of Hog1 phosphorylation, and deletion ofHOG1did not affect Cap1 nuclear localization. Moreover, we show that theHOG1gene plays a role in chlamydospore formation, another oxygen-related morphogenetic event, as demonstrated by the fact thathog1cells were unable to generate these thick-walled structures in several media through a mechanism different from that of theEFG1regulator. This is the first demonstration of the role of the Hog1-mediated MAP kinase pathway in resistance to oxidative stress in pathogenic fungi, and it allows us to propose a molecular model for the oxidative stress response inC.albicans.

MAP (mitogen-activated protein) kinase-mediated pathways are key elements in sensing and transmitting the response of cells to environmental conditions by the sequential action of phosphorylation events. In the fungal pathogenCandida albicans, different routes have been identified by genetic analysis, and especially by the phenotypic characterization of mutants altered in the Mkc1, Cek1/2 and Hog1 MAP kinases. The cell integrity (orMKC1-mediated) pathway is primarily involved in the biogenesis of the cell wall. The HOG pathway participates in the response to osmotic stress while the Cek1 pathway mediates mating and filamentation. Their actual functions are, however, much broader and Mkc1 senses several types of stress, while Hog1 is also responsive to other stress conditions and participates in two morphogenetic programmes: filamentation and chlamydospore formation. Furthermore, it has been recently shown that Cek1 participates in a putative pathway involved in the construction of the cell wall and which seems to be operative under basal conditions. As these stimuli are frequently encountered in the human host, they provide a reasonable explanation for the significant reduction in pathogenicity that several signal transduction mutants show in certain animal models of virulence. MAPK pathways therefore represent an attractive multienzymic system for which novel antifungal therapy could be designed.

The Hog1 mitogen-activated protein (MAP) kinase mediates an adaptive response to both osmotic and oxidative stress in the fungal pathogen Candida albicans . This protein also participates in two distinct morphogenetic processes, namely the yeast-to-hypha transition (as a repressor) and chlamydospore formation (as an inducer). We show here that repression of filamentous growth occurs both under serum limitation and under other partially inducing conditions, such as low temperature, low pH, or nitrogen starvation. To understand the relationship of the HOG pathway to other MAP kinase cascades that also play a role in morphological transitions, we have constructed and characterized a set of double mutants in which we deleted both the HOG1 gene and other signaling elements (the CST20 , CLA4 , and HST7 kinases, the CPH1 and EFG1 transcription factors, and the CPP1 protein phosphatase). We also show that Hog1 prevents the yeast-to-hypha switch independent of all the elements analyzed and that the inability of the hog1 mutants to form chlamydospores is suppressed when additional elements of the CEK1 pathway ( CST20 or HST7 ) are altered. Finally, we report that Hog1 represses the activation of the Cek1 MAP kinase under basal conditions and that Cek1 activation correlates with resistance to certain cell wall inhibitors (such as Congo red), demonstrating a role for this pathway in cell wall biogenesis.

The Candida albicans HOG1 gene (HOG1CA) was cloned by functional complementation of the osmosensitive phenotype associated with Saccharomyces cerevisiae hog1 delta mutants. HOG1CA 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.

TheCandida albicans MKC1gene encodes a mitogen-activated protein (MAP) kinase, which has been cloned by complementation of the lytic phenotype associated withSaccharomyces cerevisiae slt2(mpk1) mutants. In this work, the physiological role of this MAP kinase in the pathogenic fungusC. albicanswas characterized and a role forMKC1in the biogenesis of the cell wall suggested based on the following criteria. First,C. albicans mkc1Δ/mkc1Δ strains displayed alterations in their cell surfaces under specific conditions as evidenced by scanning electron microscopy. Second, an increase in specific cell wall epitopes (O-glycosylated mannoprotein) was shown by confocal microscopy inmkc1Δ/mkc1Δ mutants. Third, the sensitivity to antifungals which inhibit (1,3)-β-glucan and chitin synthesis was increased in these mutants. In addition, evidence for a role for theMKC1gene in morphological transitions inC. albicansis presented based on the impairment of pseudohyphal formation ofmkc1Δ/mkc1Δ strains on Spider medium and on the effect of its overexpression onSacch. cerevisiaecolony morphology on SLADH medium. Using the two-hybrid system, it was also demonstrated thatMKC1is able to interact specifically withSacch. cerevisiaeMkk1p and Mkk2p, the MAP-kinase kinases of thePKC1-mediated route ofSacch. cerevisiae, and to activate transcription inSacch. cerevisiaewhen bound to a DNA-binding element. These results suggest a role for this MAP kinase in the construction of the cell wall ofC. albicansand indicate its potential relevance for the development of novel antifungals.