The maintenance of potassium homeostasis is crucial for all types of cells, including Candida glabrata. Three types of plasma-membrane systems mediating potassium influx with different transport mechanisms have been described in yeasts: the Trk1 uniporter, the Hak cation-proton symporter and the Acu ATPase. The C. glabrata genome contains only one gene encoding putative system for potassium uptake, the Trk1 uniporter. Therefore, its importance in maintaining adequate levels of intracellular potassium appears to be critical for C. glabrata cells. In this study, we first confirmed the potassium-uptake activity of the identified gene’s product by heterologous expression in a suitable S. cerevisiae mutant, further we generated a corresponding deletion mutant in C. glabrata and analysed its phenotype in detail. The obtained results show a pleiotropic effect on the cell physiology when CgTRK1 is deleted, affecting not only the ability of trk1Δ to grow at low potassium concentrations, but also the tolerance to toxic alkali-metal cations and cationic drugs, as well as the membrane potential and intracellular pH. Taken together, our results find the sole potassium uptake system in C. glabrata cells to be a promising target in the search for its specific inhibitors and in developing new antifungal drugs.

Yeasts usually have one or two high-affinity potassium transporters. Two complete and one interrupted gene encoding three types of putative potassium uptake system exist in Candida albicans SC5314. As high intracellular potassium is essential for many yeast cell functions, the existence of three transporters with differing transport mechanisms (Trk uniporter, Hak cation-proton symporter, Acu ATPase) may help pathogenic C. albicans cells to acquire the necessary potassium in various organs and tissues of the host. When expressed in Saccharomyces cerevisiae lacking their own potassium uptake systems, all three putative transporters were able to provide cells with the ability to grow with low amounts of potassium over a broad range of external pH. Only CaTrk1 was properly recognized and secreted to the plasma membrane. Nevertheless, even the small number of CaHak1 and mainly CaAcu1 molecules which reached the plasma membrane resulted in an improved growth of cells in low potassium concentrations, suggesting a high affinity and capacity of the transporters. A single-point mutation restored the complete CaACU1 gene, and the resulting protein not only provided cells with the necessary potassium but also improved their tolerance to toxic lithium. In contrast to its known homologues, CaAcu1 did not seem to transport sodium.

A high intracellular concentration of potassium (200-300 mmol/L) is essential for many yeast cell functions, such as the regulation of cell volume and pH, maintenance of membrane potential, and enzyme activation. Thus, cells use high-affinity specific transporters and expend a lot of energy to acquire the necessary amount of potassium from their environment. In Candida genomes, genes encoding 3 types of putative potassium uptake systems were identified: Trk uniporters, Hak symporters, and Acu ATPases. Tests of the tolerance and sensitivity of C. albicans, C. dubliniensis, C. glabrata, C. krusei, C. parapsilosis, and C. tropicalis to various concentrations of potassium showed significant differences among the species, and these differences were partly dependent on external pH. The species most tolerant to potassium-limiting conditions were C. albicans and C. krusei, while C. parapsilosis tolerated the highest KCl concentrations. Also, the morphology of cells changed with the amount of potassium available, with C. krusei and C. tropicalis being the most influenced. Taken together, our results confirm potassium uptake and accumulation as important factors for Candida cell growth and suggest that the sole (and thus probably indispensable) Trk1 potassium uptake system in C. krusei and C. glabrata may serve as a target for the development of new antifungal drugs.