The variable sensitivity of Candida
species to NaCl has been observed and exploited previously, mainly in connection with the original niche of the species, e.g. the high NaCl tolerance of C. parapsilosis
strains isolated from sea water or hypersaline brines [22
] or in clinical microbiological tests aiming to distinguish among Candida
species (NaCl sensitivity of C. dubliniensis
]). In this work, we performed a series of tests to characterize the tolerance of four different pathogenic Candida
species to different alkali metal cations, and addressed the question of whether the observed differences in salt tolerance could be based on the transport activity and/or specificity of Candidas'
antiporters. Our results showed clearly that, besides differences in their tolerance to sodium, the four species tested also differ in their sensitivity to highly toxic lithium and their tolerance of high external concentrations of non-toxic potassium cations (Figure ). C. glabrata
and C. dubliniensis
are the most sensitive to Li+
cations, the latter being the least alkali-metal-cation tolerant of the species tested.
To elucidate the role of Na+
antiporters in Candidas'
salt tolerance, we isolated and heterologously expressed genes encoding putative antiporters in the most and the least tolerant species, C. parapsilosis
and C. dubliniensis
. The functional characterization of these encoded proteins in an S. cerevisiae
mutant lacking its own alkali-metal-cation exporters revealed that both antiporters have broad substrate specificity. They recognize at least four different cations (K+
), but transport them with varying capacities and/or affinities. C. parapsilosis
and C. albicans
Cnh1 proteins are very efficient transporters and their capacities exceed that of S. cerevisiae
Nha1p. On the other hand, the C. dubliniensis
antiporter has a much lower transport activity and its ability to recognize and transport the smallest cation, toxic lithium, is very limited compared to the other two Candida
antiporters (Figure , Table ). The large difference between the transport activities of the evolutionary close C. albicans
and C. dubliniensis
Cnh1 antiporters is worth noting, as these two proteins shared the highest level of identity (Table ). Their core membrane section (twelve tms and connecting loops) is almost 99% identical and their N-termini, composed of 11 aa residues, are 100% identical. These two antiporters differ slightly (compared to other antiporters) in the length and composition of their hydrophilic C-termini. Whether the observed transport capacities are based on the C-terminal difference remain to be established, though previous studies showed that the C-termini of yeast Na+
antiporters are not the most important factor in this activity [7
] and that a single amino-acid exchange in one of the transmembrane domains can significantly influence both the substrate specificity and transport capacity of the antiporter [28
Though the C. parapsilosis
Cnh1 antiporter mediates a high and efficient sodium efflux from S. cerevisiae
cells (similarly as the Ca
Cnh1p, Figure and Table ), its physiological role in C. parapsilosis
remains to be established. A recent study [20
] showed that the Cnh1 antiporter is mainly important in potassium homeostasis in C. albicans
cells and its role in Na+
detoxification is rather marginal.
The differences observed in alkali-metal-cation tolerance between C. dubliniensis and C. parapsilosis/C. albicans species were also found upon testing the alkali-metal-cation tolerance of S. cerevisiae cells expressing the antiporters of these three species. Efflux measurements confirmed that the differing tolerances of S. cerevisiae cells were based on the differing transport activities of the Candida antiporters. Altogether, our results suggest that the activity of plasma-membrane Na+/H+ antiporters is one of the factors determining the tolerance to high external concentrations of alkali metal cations in pathogenic Candida species.