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The sucrose non-fermenting 1 ( SNF1 ) complex is a heterotrimeric protein kinase complex that is an ortholog of the mammalian AMPK complex and is evolutionally conserved in most eukaryotes. This complex contains a catalytic subunit (Snf1), a regulatory subunit (Snf4) and a scaffolding subunit (Sip1/Sip2/Gal73) in budding yeast. Although the function of AMPK has been well studied in Saccharomyces cerevisiae and Candida albicans , the role of AMPK in Candida tropicalis has never been investigated. In this study, we focused on SNF4 in C. tropicalis as this fungus cannot produce a snf1Δ mutant. We demonstrated that C. tropicalis SNF4 shares similar roles in glucose derepression and is necessary for cell wall integrity and virulence. The expression of both SNF1 and SNF4 was significantly induced when glucose was limited. Furthermore, snf4Δ strains exhibited high sensitivity to many surface-perturbing agents because the strains contained lower levels of glucan, chitin and mannan. Interestingly, in contrast to C. albicans sak1Δ and snf4Δ , C. tropicalis snf4Δ exhibited phenotypes for cell aggregation and pseudohypha production. These data indicate that SNF4 performs convergent and divergent roles in C. tropicalis and possibly other unknown roles in the C. tropicalis SNF1-SNF4 AMPK pathway.

Candida albicans is the most prevalent fungal pathogen in humans, particularly in immunocompromised patients. In this study, by screening a C. albicans mutant library, we first identified that the MSS2 gene, an ortholog of Saccharomyces cerevisiae MSS2 required for mitochondrial respiration, mediates chitosan resistance. Upon treatment with 0.2% chitosan, the growth of mss2Δ strains was strikingly impaired, and MSS2 expression was significantly repressed by chitosan. Furthermore, mss2Δ strains exhibited slow growth on medium supplemented with glycerol as the sole carbon source. Similar to the chitosan-treated wild-type strain, the mss2Δ strain exhibited a significantly impaired ATP production ability. These data suggest that an antifungal mechanism of chitosan against C. albicans acts by inhibiting MSS2 gene expression, leading to repression of mitochondrial function. Normal respiratory function is suggested to be required for fungal virulence. Interestingly, the mss2Δ mutant strains exhibited significantly impaired invasive ability in vitro and ex vivo but retained normal hyphal development ability in liquid medium. Furthermore, the MSS2 deletion strains could not form robust biofilms and exhibited significantly reduced virulence. Collectively, these results demonstrated that the antifungal effect of chitosan against C. albicans is mediated via inhibition of mitochondrial biogenesis. These data may provide another strategy for antifungal drug development via inhibition of fungal mitochondria.