Phosphoric Metabolites Link Phosphate Import and Polysaccharide Biosynthesis for Candida albicans Cell Wall Maintenance

Ning Ning Liu, Maikel Acosta-Zaldívar, Wanjun Qi, Joann Diray-Arce, Louise A. Walker, Theodore J. Kottom, Rachel Kelly, Min Yuan, John M. Asara, Jessica Ann Lasky-Su, Ofer Levy, Andrew H. Limper, Neil A.R. Gow, Julia R. Köhler* (Corresponding Author)

*Corresponding author for this work

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The Candida albicans high-affinity phosphate transporter Pho84 is required for normal Target of Rapamycin (TOR) signaling, oxidative stress resistance, and virulence of this fungal pathogen. It also contributes to C. albicans' tolerance of two antifungal drug classes, polyenes and echinocandins. Echinocandins inhibit biosynthesis of a major cell wall component, beta-1,3-glucan. Cells lacking Pho84 were hypersensitive to other forms of cell wall stress beyond echinocandin exposure, while their cell wall integrity signaling response was weak. Metabolomics experiments showed that levels of phosphoric intermediates, including nucleotides like ATP and nucleotide sugars, were low in pho84 mutant compared to wild-type cells recovering from phosphate starvation. Nonphosphoric precursors like nucleobases and nucleosides were elevated. Outer cell wall phosphomannan biosynthesis requires a nucleotide sugar, GDP-mannose. The nucleotide sugar UDP-glucose is the substrate of enzymes that synthesize two major structural cell wall polysaccharides, beta-1,3- and beta-1,6-glucan. Another nucleotide sugar, UDP-N-acetylglucosamine, is the substrate of chitin synthases which produce a stabilizing component of the intercellular septum and of lateral cell walls. Lack of Pho84 activity, and phosphate starvation, potentiated pharmacological or genetic perturbation of these enzymes. We posit that low substrate concentrations of beta-o-glucan- and chitin synthases, together with pharmacologic inhibition of their activity, diminish enzymatic reaction rates as well as the yield of their cell wall stabilizing products. Phosphate import is not conserved between fungal and human cells, and humans do not synthesize beta-D-glucans or chitin. Hence, inhibiting these processes simultaneously could yield potent antifungal effects with low toxicity to humans.

IMPORTANCE Candida species cause hundreds of thousands of invasive infections with high mortality each year. Developing novel antifungal agents is challenging due to the many similarities between fungal and human cells. Maintaining phosphate balance is essential for all organisms but is achieved completely differently by fungi and humans. A protein that imports phosphate into fungal cells, Pho84, is not present in humans and is required for normal cell wall stress resistance and cell wall integrity signaling in C. albicans. Nucleotide sugars, which are phosphate-containing building block molecules for construction of the cell wall, are diminished in cells lacking Pho84. Cell wall-constructing enzymes may be slowed by lack of these building blocks, in addition to being inhibited by drugs. Combined targeting of Pho84 and cell wall-constructing enzymes may provide a strategy for antifungal therapy by which two sequential steps of cell wall maintenance are blocked for greater potency.

Original languageEnglish
Article numbere03225-19
Number of pages19
Issue number2
Early online date17 Mar 2020
Publication statusPublished - 17 Mar 2020

Bibliographical note


We declare no conflicts of interest.

We thank Jesús Pla for his kind gift of the anti-Mkc1 antibody and Kristin Moffitt and Richard Malley for generous advice in ELISA technology and use of the ELISA reader. We thank Tahmeena Chowdhury for scientific discussions leading up to this work. We thank the Candida Genome Database.

N.-N.L., M.A.-Z., W.Q., and J.R.K. were supported by R21 AI137716 and by Boston Children’s Hospital Department of Pediatrics. M.A.-Z. was partially funded by the Alfonso Martin Escudero Foundation. J.D.-A. and O.L. were funded by the Boston Children’s Hospital Department of Pediatrics and U19 AI118608-01A1. N.A.R.G. was supported by the Wellcome Trust and the Medical Research Council Centre for Medical Mycology (MR/N006364/1).


  • antifungal agents
  • Candida albicans
  • cell wall
  • chitin synthase
  • glucan synthase
  • nucleotide sugar
  • Pho84
  • phosphate metabolism
  • Antifungal agents
  • Nucleotide sugar
  • Chitin synthase
  • Cell wall
  • Glucan synthase
  • Phosphate metabolism


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