Abstract
Candida albicans is an opportunistic human fungal pathogen and able to cause life-threatening infections in immunocompromised patients. C. albicans has a unique morphological transition between white and opaque phases. These two cell forms have different properties in virulence, mating capability, biofilm formation and host-cell interaction. White-opaque transition is regulated by several external stimuli, such as CO2, N-acetylglucosamine and oxidative stress. Our previous study revealed that deletion of SSK2, PBS2 or HOG1 gene resulted in 100% white-to-opaque switching on SC medium, and suppressed the mating response. Hog1 protein has two important phosphoacceptors, Thr-174 and Tyr-176, and will be activated when phosphorylated in response to stimuli. In this study, we first demonstrated that two conserved phosphorylation sites are not only required for stress response, but also involved in white-opaque switching, mating and pheromone-stimulated cell adhesion. Six Hog1 point-mutated strains were generated, including non-phosphorylated strains (HOG1T174A, HOG1Y176F and HOG1T174A,Y176F) and mimic phosphorylated strains (HOG1T174D, HOG1Y176D and HOG1T174D,Y176D). Point mutation on Thr-174, Tyr-176 or in combination of Hog1 protein in MTL homozygous strains of C. albicans would stimulate opaque cell formation in a frequency of 100% on SC medium. Similar to the hog1 mutant, all of the point mutation strains were sensitive to both osmotic and oxidative stresses. Western blotting showed that Hog1 proteins were expressed in these mutants, but could not be phosphorylated when treated with H2O2. Deletion of PTP2 or PTP3 gene would not impact white-to-opaque switching on Lee’s NAG medium. Furthermore, mating projections of point-mutated strains were significantly shorter and their mating efficiencies were lower than those of the wild-type. In addition, numbers of pheromone-induced cell adhesion of white cells in all point-mutated strains reduced. Taken together, our study demonstrated that mutation on either Thr-174 or Tyr-176 of Hog1 resulted in similar characters with hog1 mutants in white-opaque transition, sexual mating and pheromone-induced cell adhesion in C. albicans.
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