Yeast Tok1p channel is a major contributor to membrane potential maintenance under chemical stress

The molecular basis of pH sensing by the human fungal pathogen Candida albicans TOK potassium channel

Two-pore domain, outwardly rectifying potassium (TOK) channels are exclusively expressed in fungi. Human fungal pathogen TOK channels are potential antifungal targets, but TOK channel modulation in general is poorly understood. Here, we discovered that Candida albicans TOK (CaTOK) is regulated by extracellular pH, in contrast to TOK channels from other fungal species tested. Low pH increased CaTOK channel outward currents (pKa = 6.0), hyperpolarized the voltage-dependence of TOK activation, and increased pore selectivity for K+ over Na+, shifting the reversal potential (E REV) toward E K. Mutating H144 in the S1-S2 extracellular linker partially diminished pH sensitivity, suggesting H144 forms part of the CaTOK pH sensor. Functional analysis of chimeras made with pH-insensitive Saccharomyces cerevisiae TOK and point mutants revealed that CaTOK V462 and S466 in the final transmembrane segment complete the pH-responsive elements. A tripartite network of residues thus endows CaTOK with the ability to respond functionally to changes in pH.

Molecular Basis of Yeasts Antimicrobial Activity-Developing Innovative Strategies for Biomedicine and Biocontrol

In the context of the growing concern regarding the appearance and spread of emerging pathogens with high resistance to chemically synthetized biocides, the development of new agents for crops and human protection has become an emergency. In this context, the yeasts present a huge potential as eco-friendly agents due to their widespread nature in various habitats and to their wide range of antagonistic mechanisms. The present review focuses on some of the major yeast antimicrobial mechanisms, their molecular basis and practical applications in biocontrol and biomedicine. The synthesis of killer toxins, encoded by dsRNA virus-like particles, dsDNA plasmids or chromosomal genes, is encountered in a wide range of yeast species from nature and industry and can affect the development of phytopathogenic fungi and other yeast strains, as well as human pathogenic bacteria. The group of the "red yeasts" is gaining more interest over the last years, not only as natural producers of carotenoids and rhodotorulic acid with active role in cell protection against the oxidative stress, but also due to their ability to inhibit the growth of pathogenic yeasts, fungi and bacteria using these compounds and the mechanism of competition for nutritive substrate. Finally, the biosurfactants produced by yeasts characterized by high stability, specificity and biodegrability have proven abilities to inhibit phytopathogenic fungi growth and mycelia formation and to act as efficient antibacterial and antibiofilm formation agents for biomedicine. In conclusion, the antimicrobial activity of yeasts represents a direction of research with numerous possibilities of bioeconomic valorization as innovative strategies to combat pathogenic microorganisms.

Yeast Trk1 Potassium Transporter Gradually Changes Its Affinity in Response to Both External and Internal Signals

Yeasts need a high intracellular concentration of potassium to grow. The main K+ uptake system in Saccharomyces cerevisiae is the Trk1 transporter, a complex protein with four MPM helical membrane motifs. Trk1 has been shown to exist in low- or high-affinity modes, which reflect the availability of potassium in the environment. However, when and how the affinity changes, and whether the potassium availability is the only signal for the affinity switch, remains unknown. Here, we characterize the Trk1 kinetic parameters under various conditions and find that Trk1’s KT and Vmax change gradually. This gliding adjustment is rapid and precisely reflects the changes in the intracellular potassium content and membrane potential. A detailed characterization of the specific mutations in the P-helices of the MPM segments reveals that the presence of proline in the P-helix of the second and third MPM domain (F820P and L949P) does not affect the function of Trk1 in general, but rather specifically prevents the transporter’s transition to a high-affinity state. The analogous mutations in the two remaining MPM domains (L81P and L1115P) result in a mislocalized and inactive protein, highlighting the importance of the first and fourth P-helices in proper Trk1 folding and activity at the plasma membrane.

Engineering membrane asymmetry to increase medium-chain fatty acid tolerance in Saccharomyces cerevisiae

Saccharomyces cerevisiae is an attractive chassis for the production of medium-chain fatty acids, but the toxic effect of these compounds often prevents further improvements in titer, yield, and productivity. To address this issue, Lem3 and Sfk1 were identified from adaptive laboratory evolution mutant strains as membrane asymmetry regulators. Co-overexpression of Lem3 and Sfk1 [Lem3(M)-Sfk1(H) strain] through promoter engineering remodeled the membrane phospholipid distribution, leading to an increased accumulation of phosphatidylethanolamine in the inner leaflet of the plasma membrane. As a result, membrane potential and integrity were increased by 131.5% and 29.2%, respectively; meanwhile, the final OD₆₀₀ in the presence of hexanoic acid, octanoic acid, and decanoic acid was improved by 79.6%, 73.4%, and 57.7%, respectively. In summary, this study shows that membrane asymmetry engineering offers an efficient strategy to enhance medium-chain fatty acids tolerance in S. cerevisiae, thus generating a robust industrial strain for producing high-value biofuels.

Plasma Membrane Potential of Candida albicans Measured by Di-4-ANEPPS Fluorescence Depends on Growth Phase and Regulatory Factors

The potential of the plasma membrane (Δѱ) regulates the electrochemical potential between the outer and inner sides of cell membranes. The opportunistic fungal pathogen, Candida albicans, regulates the membrane potential in response to environmental conditions, as well as the physiological state of the cell. Here we demonstrate a new method for detection of cell membrane depolarization/permeabilization in C. albicans using the potentiometric zwitterionic dye di-4-ANEPPS. Di-4-ANEPPS measures the changes in the cell Δѱ depending on the phases of growth and external factors regulating Δѱ, such as potassium or calcium chlorides, amiodarone or DM-11 (inhibitor of H⁺-ATPase). We also demonstrated that di-4-ANEPPS is a good tool for fast measurement of the influence of amphipathic compounds on Δѱ.

Hyper-Osmotic Stress Elicits Membrane Depolarization and Decreased Permeability in Halotolerant Marine Debaryomyces hansenii Strains and in Saccharomyces cerevisiae

The use of seawater and marine microorganisms can represent a sustainable alternative to avoid large consumption of freshwater performing industrial bioprocesses. Debaryomyces hansenii, which is a known halotolerant yeast, possess metabolic traits appealing for developing such processes. For this purpose, we studied salt stress exposure of two D. hansenii strains isolated from marine fauna. We found that the presence of sea salts during the cultivation results in a slight decrease of biomass yields. Nevertheless, higher concentration of NaCl (2 M) negatively affects other growth parameters, like growth rate and glucose consumption rate. To maintain an isosmotic condition, the cells accumulate glycerol as compatible solute. Flow cytometry analysis revealed that the osmotic adaptation causes a reduced cellular permeability to cell-permeant dye SYBR Green I. We demonstrate that this fast and reversible phenomenon is correlated to the induction of membrane depolarization, and occurred even in presence of high concentration of sorbitol. The decrease of membrane permeability induced by osmotic stress confers to D. hansenii resistance to cationic drugs like Hygromycin B. In addition, we describe that also in Saccharomyces cerevisiae the exposure to hyper-osmotic conditions induced membrane depolarization and reduced the membrane permeability. These aspects are very relevant for the optimization of industrial bioprocesses, as in the case of fermentations and bioconversions carried out by using media/buffers containing high nutrients/salts concentrations. Indeed, an efficient transport of molecules (nutrients, substrates, and products) is the prerequisite for an efficient cellular performance, and ultimately for the efficiency of the industrial process.