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Description
The fungal pathogen Candida albicans affects over 1 billion individuals annually by secreting toxin, candidalysin. In 2016, candidalysin was the first identified in human fungal pathogen Candida albicans. Candidalysin is generated from parental protein Ece1p (271 amino acids), encoded by the ECE1 gene. Ece1p is processed by Kex proteinases, yielding 31 amino acids candidalysin peptide and seven non-candidalysin Ece1 peptides. The cytolytic peptide candidalysin is amphipathic, hydrophobic and cationic in nature (+3), properties that are hypothesised to be key factors during pore formation, membrane destabilisation and toxicity. Recently, orthologs of C. albicans candidalysin in C. dubliniensis and C. tropicalis were identified. The candidalysin family of cytolysins have different amino acid sequences, are amphipathic, and adopt a predominantly α-helical secondary structure in solution.
Here, the molecular mechanisms of membrane permeabilisation by candidalysin toxins have been characterised using Orbit e16, a multi-channel recording technique. This technology enables to quantify the real-time binding of candidalysin peptides to reconstituted planar lipid bilayers on a chip. While applying constant membrane potentials to the bilayers, candidalysin wild-type and modified conjugates are added to the bilayer and membrane binding and disruption are detected by electrical current changes.
These are two major findings:
(1) Candidalysin family candidalysins bind to the bilayers and induce membrane permeabilisation like antimicrobial peptides (e.g. Melittin). The membrane permeabilisation properties can be quantified by measuring dwell times, which are defined as the time that elapsed between candidalysin addition and bilayer permeabilization. Shorter dwell times indicate faster membrane permeabilisation. At 2 µM of candidalysin, the candidalysins of C. dubliniensis and C. tropicalis permeabilise the membrane 4 to 7 times faster than C. albicans candidalysin at 2 µM, indicating different potencies of candidalysin family toxins.
(2) The membrane permeabilising potential of candidalysin can be prevented by the presence of albumin. In electrophysiological current measurements, candidalysin preincubated with murine albumin shows slowed membrane permeabilisation function in a dose-dependent manner, demonstrating that albumin neutralizes candidalysin cytotoxicity. In addition, the direct binding of murine albumin to candidalysin is confirmed by isothermal calorimetry technique, indicating hydrophobic interactions.
These observations identify important steps for C. albicans membrane transport at a molecular level.
References
- Richardson et al. mBio. 2022 Jan-Feb; 13(1): e03510-21.
- Austermeier et al. mBio. 2021 Jun 29;12(3):e0053121.
| Keywords | Infectious disease, Membrane biophysics, Toxin, Candida, Oral infection |
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