Killing of Candida albicans by human salivary histatin 5 is modulated, but not determined, by the potassium channel TOK1

Secrets of the fungus-specific potassium channel TOK family

Several families of potassium (K⁺) channels are found in membranes of all eukaryotes, underlining the importance of K⁺ uptake and redistribution within and between cells and organs. Among them, TOK (tandem-pore outward-rectifying K⁺) channels consist of eight transmembrane domains and two pore domains per subunit organized in dimers. These channels were originally studied in yeast, but recent identifications and characterizations in filamentous fungi shed new light on this fungus-specific K⁺ channel family. Although their actual function in vivo is often puzzling, recent works indicate a role in cellular K⁺ homeostasis and even suggest a role in plant-fungus symbioses. This review aims at synthesizing the current knowledge on fungal TOK channels and discussing their potential role in yeasts and filamentous fungi.

Zinc Binding Inhibits Cellular Uptake and Antifungal Activity of Histatin-5 in Candida albicans

Histatin-5 (Hist-5) is a polycationic, histidine-rich antimicrobial peptide with potent antifungal activity against the opportunistic fungal pathogen Candida albicans. Hist-5 can bind metals in vitro, and metals have been shown to alter the fungicidal activity of the peptide. Previous reports on the effect of Zn²⁺ on Hist-5 activity have been varied and seemingly contradictory. Here, we present data elucidating the dynamic role Zn²⁺ plays as an inhibitory switch to regulate Hist-5 fungicidal activity. A novel fluorescently labeled Hist-5 peptide (Hist-5*) was developed to visualize changes in internalization and localization of the peptide as a function of metal availability in the growth medium. Hist-5* was verified for use as a model peptide and retained antifungal activity and mode of action similar to native Hist-5. Cellular growth assays showed that Zn²⁺ had a concentration-dependent inhibitory effect on Hist-5 antifungal activity. Imaging by confocal microscopy revealed that equimolar concentrations of Zn²⁺ kept the peptide localized along the cell periphery rather than internalizing, thus preventing cytotoxicity and membrane disruption. However, the Zn-induced decrease in Hist-5 activity and uptake was rescued by decreasing the Zn²⁺ availability upon addition of a metal chelator EDTA or S100A12, a Zn-binding protein involved in the innate immune response. These results lead us to suggest a model wherein commensal C. albicans may exist in harmony with Hist-5 at concentrations of Zn²⁺ that inhibit peptide internalization and antifungal activity. Activation of host immune processes that initiate Zn-sequestering mechanisms of nutritional immunity could trigger Hist-5 internalization and cell killing.

Candida albicans Potassium Transporters

Potassium is basic for life. All living organisms require high amounts of intracellular potassium, which fulfils multiple functions. To reach efficient potassium homeostasis, eukaryotic cells have developed a complex and tightly regulated system of transporters present both in the plasma membrane and in the membranes of internal organelles that allow correct intracellular potassium content and distribution. We review the information available on the pathogenic yeast Candida albicans. While some of the plasma membrane potassium transporters are relatively well known and experimental data about their nature, function or regulation have been published, in the case of most of the transporters present in intracellular membranes, their existence and even function have just been deduced because of their homology with those present in other yeasts, such as Saccharomyces cerevisiae. Finally, we analyse the possible links between pathogenicity and potassium homeostasis. We comment on the possibility of using some of these transporters as tentative targets in the search for new antifungal drugs.

TOK channels use the two gates in classical K+ channels to achieve outward rectification

TOKs are outwardly rectifying K⁺ channels in fungi with two pore-loops and eight transmembrane spans. Here, we describe the TOKs from four pathogens that cause the majority of life-threatening fungal infections in humans. These TOKs pass large currents only in the outward direction like the canonical isolate from Saccharomyces cerevisiae (ScTOK), and distinct from other K⁺ channels. ScTOK, AfTOK1 (Aspergillus fumigatus), and H99TOK (Cryptococcus neoformans grubii) are K⁺ -selective and pass current above the K⁺ reversal potential. CaTOK (Candida albicans) and CnTOK (Cryptococcus neoformans neoformans) pass both K⁺ and Na⁺ and conduct above a reversal potential reflecting the mixed permeability of their selectivity filter. Mutations in CaTOK and ScTOK at sites homologous to those that open the internal gates in classical K⁺ channels are shown to produce inward TOK currents. A favored model for outward rectification is proposed whereby the reversal potential determines ion occupancy, and thus, conductivity, of the selectivity filter gate that is coupled to an imperfectly restrictive internal gate, permitting the filter to sample ion concentrations on both sides of the membrane.

Cytosolic Acidification Is the First Transduction Signal of Lactoferrin-induced Regulated Cell Death Pathway

In yeast, we reported the critical role of K⁺-efflux for the progress of the regulated cell death (RCD) induced by human lactoferrin (hLf), an antimicrobial protein of the innate immune system that blocks Pma1p H⁺-ATPase. In the present study, the K⁺ channel Tok1p was identified as the K⁺ channel-mediating K⁺-efflux, as indicated by the protective effect of extracellular K⁺ (30 mM), K⁺-channel blockers, and the greater hLf-resistance of TOK1-disrupted strains. K⁺-depletion was necessary but not sufficient to induce RCD as inferred from the effects of valinomycin, NH₄Cl or nigericin which released a percentage of K⁺ similar to that released by lactoferrin without affecting cell viability. Cytosolic pH of hLf-treated cells decreased transiently (0.3 pH units) and its inhibition prevented the RCD process, indicating that cytosolic acidification was a necessary and sufficient triggering signal. The blocking effect of lactoferrin on Pma1p H⁺-ATPase caused a transitory decrease of cytosolic pH, and the subsequent membrane depolarization activated the voltage-gated K⁺ channel, Tok1p, allowing an electrogenic K⁺-efflux. These ionic events, cytosolic accumulation of H⁺ followed by K⁺-efflux, constituted the initiating signals of this mitochondria-mediated cell death. These findings suggest, for the first time, the existence of an ionic signaling pathway in RCD.

Monovalent cation transporters at the plasma membrane in yeasts

Maintenance of proper intracellular concentrations of monovalent cations, mainly sodium and potassium, is a requirement for survival of any cell. In the budding yeast Saccharomyces cerevisiae, monovalent cation homeostasis is determined by the active extrusion of protons through the Pma1 H⁺ -ATPase (reviewed in another chapter of this issue), the influx and efflux of these cations through the plasma membrane transporters (reviewed in this chapter), and the sequestration of toxic cations into the vacuoles. Here, we will describe the structure, function, and regulation of the plasma membrane transporters Trk1, Trk2, Tok1, Nha1, and Ena1, which play a key role in maintaining physiological intracellular concentrations of Na⁺ , K⁺ , and H⁺ , both under normal growth conditions and in response to stress.

Promising Antifungal Targets Against Candida albicans Based on Ion Homeostasis

In recent decades, invasive fungal infections have been increasing significantly, contributing to high incidences and mortality in immunosuppressed patients. Candida albicans (C. albicans) is the most prevalent opportunistic fungal pathogen in humans that can cause severe and often fatal bloodstream infections. Current antifungal agents have several limitations, including that only a small number of classes of antifungals are available, certain of which have severe toxicity and high cost. Moreover, the emergence of drug resistance is a new limitation to successful patient outcomes. Therefore, the development of antifungals with novel targets is an essential strategy for the efficient management of C. albicans infections. It is widely recognized that ion homeostasis is crucial for all living cells. Many studies have identified that ion-signaling and transduction networks are central to fungal survival by regulating gene expression, morphological transition, host invasion, stress response, and drug resistance. Dysregulation of ion homeostasis rapidly mediates cell death, forming the mechanistic basis of a growing number of compounds that elicit antifungal activity. Most of the potent antifungals have been widely used in the clinic, and certain of them have low toxicity, meaning that they may be expected to be used as antifungal drugs in the future. Hence, we briefly summarize the homeostasis regulation of several important ions, potential antifungal targets based on these ion-signaling networks, and antifungal compounds based on the disruption of ion homeostasis. This summary will help in designing effective drugs and identifying new targets for combating fungal diseases.

Plant potassium nutrition in ectomycorrhizal symbiosis: properties and roles of the three fungal TOK potassium channels in Hebeloma cylindrosporum

Ectomycorrhizal fungi play an essential role in the ecology of boreal and temperate forests through the improvement of tree mineral nutrition. Potassium (K⁺ ) is an essential nutrient for plants and is needed in high amounts. We recently demonstrated that the ectomycorrhizal fungus Hebeloma cylindrosporum improves the K⁺ nutrition of Pinus pinaster under shortage conditions. Part of the transport systems involved in K⁺ uptake by the fungus has been deciphered, while the molecular players responsible for the transfer of this cation towards the plant remain totally unknown. Analysis of the genome of H. cylindrosporum revealed the presence of three putative tandem-pore outward-rectifying K⁺ (TOK) channels that could contribute to this transfer. Here, we report the functional characterization of these three channels through two-electrode voltage-clamp experiments in oocytes and yeast complementation assays. The expression pattern and physiological role of these channels were analysed in symbiotic interaction with P. pinaster. Pine seedlings colonized by fungal transformants overexpressing two of them displayed a larger accumulation of K⁺ in shoots. This study revealed that TOK channels have distinctive properties and functions in axenic and symbiotic conditions and suggested that HcTOK2.2 is implicated in the symbiotic transfer of K⁺ from the fungus towards the plant.

Antifungal Activity and Action Mechanism of Histatin 5-Halocidin Hybrid Peptides against Candida ssp

The candidacidal activity of histatin 5 is initiated through cell wall binding, followed by translocation and intracellular targeting, while the halocidin peptide exerts its activity by attacking the Candida cell membrane. To improve antimicrobial activities and to understand the killing mechanism of two peptides, six hybrid peptides were designed by conjugating histatin 5 and halocidin. A comparative approach was established to study the activity, salt tolerance, cell wall glucan binding assay, cytotoxicity, generation of ROS and killing kinetics. CD spectrometry was conducted to evaluate secondary structures of these hybrid peptides. Furthermore the cellular localization of hybrid peptides was investigated by confocal fluorescence microscopy. Of the six hybrid congeners, di-PH2, di-WP2 and HHP1 had stronger activities than other hybrid peptides against all tested Candida strains. The MIC values of these peptides were 1–2, 2–4 and 2–4 μg/ml, respectively. Moreover, none of the hybrid peptides was cytotoxic in the hemolytic assay and cell-based cytotoxicity assay. Confocal laser microscopy showed that di-PH2 and HHP1 were translocated into cytoplasm whereas di-WP2 was accumulated on surface of C. albicans to exert their candidacidal activity. All translocated peptides (Hst 5, P113, di-PH2) were capable of generating intracellular ROS except HHP1. Additionally, the KFH residues at C-terminal end of these peptides were assumed for core sequence for active translocation.

A new look at the antibiotic amphotericin B effect on Candida albicans plasma membrane permeability and cell viability functions

Amphotericin B (AmB) is an antifungal polyene for which the most accepted mode of action is formation of protein-like ion channels in the cell membrane. Patch-clamp research on Candida albicans protoplasts carried out in the outside-out configuration showed that application of 0.05 and 0.1 μM AmB caused a decrease in seal resistance. Such a phenomenon can be correlated with a decrease in membrane tightness. AmB applied at a 0.05 μM concentration also caused a decrease in the number of active TOK1 (two-pore outward rectifiers) potassium channels, but did not significantly change their open probability. The results indicate that in C. albicans protoplast AmB causes a decrease in cell membrane integrity by interaction with its lipid phase but not with ion channels. Fluorescence microscopy techniques showed that AmB treatment, in clinical concentrations, had no effect on the percentage of PI-positive protoplasts. AmB treatment in the concentrations tested did not cause a rapid reduction of the number of C. albicans protoplasts. However, there was a significant loss of replication competency and numerous morphological and physiological disorders, including cytoplasm shrinking, abnormal morphology of the nucleus and mitochondria, a sudden decrease in the MTT reduction level and oxidative stress. Our results show that the induction of yeast cell death by AmB, at therapeutic doses, is a multistage and long-term process involving multiple intracellular pathways.

Identification and mechanism of action of the plant defensin NaD1 as a new member of the antifungal drug arsenal against Candida albicans

In recent decades, pathogenic fungi have become a serious threat to human health, leading to major efforts aimed at characterizing new agents for improved treatments. Promising in this context are antimicrobial peptides produced by animals and plants as part of innate immune systems. Here, we describe an antifungal defensin, NaD1, with activity against the major human pathogen Candida albicans, characterize the mechanism of killing, and identify protection strategies used by the fungus to survive defensin treatment. The mechanism involves interaction between NaD1 and the fungal cell surface followed by membrane permeabilization, entry into the cytoplasm, hyperproduction of reactive oxygen species, and killing induced by oxidative damage. By screening C. albicans mutant libraries, we identified that the high-osmolarity glycerol (HOG) pathway has a unique role in protection against NaD1, while several other stress-responsive pathways are dispensable. The involvement of the HOG pathway is consistent with induction of oxidative stress by NaD1. The HOG pathway has been reported to have a major role in protection of fungi against osmotic stress, but our data indicate that osmotic stress does not contribute significantly to the adverse effects of NaD1 on C. albicans. Our data, together with previous studies with human beta-defensins and salivary histatin 5, indicate that inhibition of the HOG pathway holds promise as a broad strategy for increasing the activity of antimicrobial peptides against C. albicans.

Identification and analysis of cation channel homologues in human pathogenic fungi

Fungi are major causes of human, animal and plant disease. Human fungal infections can be fatal, but there are limited options for therapy, and resistance to commonly used anti-fungal drugs is widespread. The genomes of many fungi have recently been sequenced, allowing identification of proteins that may become targets for novel therapies. We examined the genomes of human fungal pathogens for genes encoding homologues of cation channels, which are prominent drug targets. Many of the fungal genomes examined contain genes encoding homologues of potassium (K⁺), calcium (Ca²⁺) and transient receptor potential (Trp) channels, but not sodium (Na⁺) channels or ligand-gated channels. Some fungal genomes contain multiple genes encoding homologues of K⁺ and Trp channel subunits, and genes encoding novel homologues of voltage-gated K_v channel subunits are found in Cryptococcus spp. Only a single gene encoding a homologue of a plasma membrane Ca²⁺ channel was identified in the genome of each pathogenic fungus examined. These homologues are similar to the Cch1 Ca²⁺ channel of Saccharomyces cerevisiae. The genomes of Aspergillus spp. and Cryptococcus spp., but not those of S. cerevisiae or the other pathogenic fungi examined, also encode homologues of the mitochondrial Ca²⁺ uniporter (MCU). In contrast to humans, which express many K⁺, Ca²⁺ and Trp channels, the genomes of pathogenic fungi encode only very small numbers of K⁺, Ca²⁺ and Trp channel homologues. Furthermore, the sequences of fungal K⁺, Ca²⁺, Trp and MCU channels differ from those of human channels in regions that suggest differences in regulation and susceptibility to drugs.

Salivary histatin 5 internalization by translocation, but not endocytosis, is required for fungicidal activity in Candida albicans

Salivary histatin 5 (Hst 5) is a cationic salivary protein with high fungicidal activity against Candida albicans. Binding to the cell wall followed by intracellular translocation is required for killing; however, specific binding components and critical toxic events are not understood. In this study, laminarin (β-1,3-glucan) but not sialic acid, mannan or pustulan mediated Hst 5 binding to C. albicans, and was disassociated by 100 mM NaCl. Time-lapse confocal microscopy revealed a dose-dependent rate of cytosolic uptake of Hst 5 that invariably preceded propidium iodide (PI) entry, demonstrating that translocation itself does not disrupt membrane integrity. Cell toxicity was manifest by vacuolar expansion followed by PI entrance; however, loss of endocytotic vacuolar trafficking of Hst 5 did not reduce killing. Extracellular NaCl (100 mM), but not sorbitol, prevented vacuolar expansion and PI entry in cells already containing cytosolic Hst 5, thus showing a critical role for ionic balance in Hst 5 toxicity. Hst 5 uptake, but not cell wall binding, was blocked by pretreatment with azide or carbonyl cyanide m-chlorophenylhydrazone; however, 10% of de-energized cells had membrane disruption. Thus, Hst 5 is capable of heterogeneous intracellular entry routes, but only direct cytosolic translocation causes cell death as a result of ionic efflux.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

The role of released ATP in killing Candida albicans and other extracellular microbial pathogens by cationic peptides

A unifying theme common to the action of many cationic peptides that display lethal activities against microbial pathogens is their specific action at microbial membranes that results in selective loss of ions and small nucleotides chiefly ATP. One model cationic peptide that induces non-lytic release of ATP from the fungal pathogen Candida albicans is salivary histatin 5 (Hst 5). The major characteristic of Hst 5-induced ATP release is that it occurs rapidly while cells are still metabolically active and have polarized membranes, thus precluding cell lysis as the means of release of ATP. Other cationic peptides that induce selective release of ATP from target microbes are lactoferricin, human neutrophil defensins, bactenecin, and cathelicidin peptides. The role of released extracellular ATP induced by cationic peptides is not known, but localized increases in extracellular ATP concentration may serve to potentiate cell killing, facilitate further peptide uptake, or function as an additional signal to activate the host innate immune system at the site of infection.

Synthetic histidine-rich peptides inhibit Candida species and other fungi in vitro: role of endocytosis and treatment implications

A family of histidine-rich peptides, histatins, is secreted by the parotid gland in mammals and exhibits marked inhibitory activity against a number of Candida species. We were particularly interested in the mechanism by which histidine-rich peptides inhibit fungal growth, because our laboratory has synthesized a variety of such peptides for drug and nucleic acid delivery. In contrast to naturally occurring peptides that are linear, peptides made on synthesizers can be varied with respect to their degrees of branching. Using this technology, we explored whether histidine-lysine (HK) polymers of different complexities and degrees of branching affect the growth of several species of Candida. Polymers with higher degrees of branching were progressively more effective against Candida albicans, with the four-branched polymer, H2K4b, most effective. Furthermore, H2K4b accumulated efficiently in C. albicans, which may indicate its ability to transport other antifungal agents intracellularly. Although H2K4b had greater antifungal activity than histatin 5, their mechanisms were similar. Toxicity in C. albicans induced by histatin 5 or branched HK peptides was markedly reduced by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate, an inhibitor of anion channels. We also determined that bafilomycin A1, an inhibitor of endosomal acidification, significantly decreased the antifungal activity of H2K4b. This suggests that the pH-buffering and subsequent endosomal-disrupting properties of histidine-rich peptides have a role in their antifungal activity. Moreover, the ability of the histidine component of these peptides to disrupt endosomes, which allows their escape from the lysosomal pathway, may explain why these peptides are both effective antifungal agents and nucleic acid delivery carriers.

Candida albicans cell wall ssa proteins bind and facilitate import of salivary histatin 5 required for toxicity

Fungicidal activity of Hst 5 is initiated by binding to cell surface proteins on Candida albicans, followed by intracellular transport to cytoplasmic effectors leading to cell death. As we identified heat shock 70 proteins (Ssa1p and/or Ssa2p) from C. albicans lysates that bind Hst 5, direct interactions between purified recombinant Ssa proteins and Hst 5 were tested by pull-down and yeast two-hybrid assays. Pulldown of both native complexes and those stabilized by cross-linking demonstrated higher affinity of Hst 5 for Ssa2p than for Ssa1p, in agreement with higher levels of interactions between Ssa2p and Hst 5 measured by yeast two-hybrid analyses. C. albicans ssa1Delta and ssa2Delta mutants were constructed to examine Hst 5 binding, translocation, and candidacidal activities. Both ssa1Delta and ssa2Delta mutants were indistinguishable from wild-type cells in growth and hyphal formation. However, C. albicans ssa2Delta mutants were highly resistant to the candidacidal activity of Hst 5, although the ssa1Delta mutant did not have any significant reduction in killing by Hst 5. Total cellular binding of 125I-Hst 5 in the ssa2Delta mutant was reduced to one-third that of wild-type cells, in contrast to the ssa1Delta mutant whose total cellular binding of Hst 5 was similar to the wild-type strain. Intracellular transport of Hst 5 was significantly impaired in the ssa2Delta mutant strain, but only mildly so in the ssa1Delta mutant. Thus, C. albicans Ssa2p facilitates fungicidal activity of Hst 5 in binding and intracellular translocation, whereas Ssa1p appears to have a lesser functional role in Hst 5 toxicity.

Human antimicrobial peptides: defensins, cathelicidins and histatins

Antimicrobial peptides, which have been isolated from many bacteria, fungi, plants, invertebrates and vertebrates, are an important component of the natural defenses of most living organisms. The isolated peptides are very heterogeneous in length, sequence and structure, but most of them are small, cationic and amphipathic. These peptides exhibit broad-spectrum activity against Gram-positive and Gram-negative bacteria, yeasts, fungi and enveloped viruses. A wide variety of human proteins and peptides also have antimicrobial activity and play important roles in innate immunity. In this review we discuss three important groups of human antimicrobial peptides. The defensins are cationic non-glycosylated peptides containing six cysteine residues that form three intramolecular disulfide bridges, resulting in a triple-stranded beta-sheet structure. In humans, two classes of defensins can be found: alpha-defensins and beta-defensins. The defensin-related HE2 isoforms will also be discussed. The second group is the family of histatins, which are small, cationic, histidine-rich peptides present in human saliva. Histatins adopt a random coil conformation in aqueous solvents and form alpha-helices in non-aqueous solvents. The third group comprises only one antimicrobial peptide, the cathelicidin LL-37. This peptide is derived proteolytically from the C-terminal end of the human CAP18 protein. Just like the histatins, it adopts a largely random coil conformation in a hydrophilic environment, and forms an alpha-helical structure in a hydrophobic environment.

In the yeast potassium channel, Tok1p, the external ring of aspartate residues modulates both gating and conductance

The yeast plasma-membrane potassium channel, Tok1p, is a voltage-dependent outward rectifier, the gating and steady-state conductance of which are conspicuously modulated by extracellular [K(+)] ([K(+)](o)). Activation is slow at high [K(+)](o), showing time constants (tau(a)) of approximately 90 ms when [K(+)](o) is 150 mM (depolarizing step to +100 mV), and inactivation is weak (<30%) during sustained depolarization. Lowering [K(+)](o) accelerates activation, increases peak current, and enhances inactivation, so that at 15 mM [K(+)](o) tau(a) is less than 50 ms and inactivation suppresses approximately 60% of peak current. Two negative residues, Asp292 and Asp426, near the mouth of the assembled channel, modulate both kinetics and conductance of the channel. Charge neutralization in the mutant Asp292Asn allows fast activation (tau(a) approximately 20 ms) at high [K(+)](o), peak currents diminishing with decreasing [K(+)](o), and fast, nearly complete, inactivation. The voltage dependence of tau(a) persists in the mutant, but the [K(+)](o) dependence almost disappears. Similar but smaller changes are seen in the Asp426Asn mutant, implying that pore geometry in the functional channel has twofold, not fourfold, symmetry.

The TRK1 potassium transporter is the critical effector for killing of Candida albicans by the cationic protein, Histatin 5

The principal feature of killing of Candida albicans and other pathogenic fungi by the catonic protein Histatin 5 (Hst 5) is loss of cytoplasmic small molecules and ions, including ATP and K(+), which can be blocked by the anion channel inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. We constructed C. albicans strains expressing one, two, or three copies of the TRK1 gene in order to investigate possible roles of Trk1p (the organism's principal K(+) transporter) in the actions of Hst 5. All measured parameters (Hst 5 killing, Hst 5-stimulated ATP efflux, normal Trk1p-mediated K(+) ((86)Rb(+)) influx, and Trk1p-mediated chloride conductance) were similarly reduced (5-7-fold) by removal of a single copy of the TRK1 gene from this diploid organism and were fully restored by complementation of the missing allele. A TRK1 overexpression strain of C. albicans, constructed by integrating an additional TRK1 gene into wild-type cells, demonstrated cytoplasmic sequestration of Trk1 protein, along with somewhat diminished toxicity of Hst 5. These results could be produced either by depletion of intracellular free Hst 5 due to sequestered binding, or to cooperativity in Hst 5-protein interactions at the plasma membrane. Furthermore, Trk1p-mediated chloride conductance was blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in all of the tested strains, strongly suggesting that the TRK1 protein provides the essential pathway for ATP loss and is the critical effector for Hst 5 toxicity in C. albicans.