Glutathione utilization by Candida albicans requires a functional glutathione degradation (DUG) pathway and OPT7, an unusual member of the oligopeptide transporter family.

Candida albicans lacks the ability to survive within its mammalian host in the absence of endogenous glutathione biosynthesis. To examine the ability of this yeast to utilize exogenous glutathione, we exploited the organic sulfur auxotrophy of C. albicans met15Δ strains. We observed that glutathione is utilized efficiently by the alternative pathway of glutathione degradation (DUG pathway). The major oligopeptide transporters OPT1-OPT5 of C. albicans that were most similar to the known yeast glutathione transporters were not found to contribute to glutathione transport to any significant extent. A genomic library approach to identify the glutathione transporter of C. albicans yielded OPT7 as the primary glutathione transporter. Biochemical studies on OPT7 using radiolabeled GSH uptake revealed a K(m) of 205 µm, indicating that it was a high affinity glutathione transporter. OPT7 is unusual in several aspects. It is the most remote member to known yeast glutathione transporters, lacks the two highly conserved cysteines in the family that are known to be crucial in trafficking, and also has the ability to take up tripeptides. The transporter was regulated by sulfur sources in the medium. OPT7 orthologues were prevalent among many pathogenic yeasts and fungi and formed a distinct cluster quite remote from the Saccharomyces cerevisiae HGT1 glutathione transporter cluster. In vivo experiments using a systemic model of candidiasis failed to detect expression of OPT7 in vivo, and strains disrupted either in the degradation (dug3Δ) or transport (opt7Δ) of glutathione failed to show a defect in virulence.

Glutathione biosynthesis in the yeast pathogens Candida glabrata and Candida albicans: essential in C. glabrata, and essential for virulence in C. albicans.

Redox pathways play a key role in pathogenesis. Glutathione, a central molecule in redox homeostasis in yeasts, is an essential metabolite, but its requirements can be met either from endogenous biosynthesis or from the extracellular milieu. In this report we have examined the importance of glutathione biosynthesis in two major human opportunistic fungal pathogens, Candida albicans and Candida glabrata. As the genome sequence of C. glabrata had suggested the absence of glutathione transporters, we initially investigated exogenous glutathione utilization in C. glabrata by disruption of the MET15 gene, involved in methionine biosynthesis. We observed an organic sulphur auxotrophy in a C. glabrata met15Δ strain; however, unlike its Saccharomyces cerevisiae counterpart, the C. glabrata met15Δ strain was unable to grow on exogenous glutathione. This inability to grow on exogenous glutathione was demonstrated to be due to the lack of a functional glutathione transporter, despite the presence of a functional glutathione degradation machinery (the Dug pathway). In the absence of the ability to obtain glutathione from the extracellular medium, we examined and could demonstrate that γ-glutamyl cysteine synthase, the first enzyme of glutathione biosynthesis, was essential in C. glabrata. Further, although γ-glutamyl cysteine synthase has been reported to be non-essential in C. albicans, we report here for what is believed to be the first time that the enzyme is required for survival in human macrophages in vitro, as well as for virulence in a murine model of disseminated candidiasis. The essentiality of γ-glutamyl cysteine synthase in C. glabrata, and its essentiality for virulence in C. albicans, make the enzyme a strong candidate for antifungal development.