Alkali-metal-cation influx and efflux systems in nonconventional yeast species (original) (raw)
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Yeast as a model organism to study transport and homeostasis of alkali metal cations
Physiological research, 2004
To maintain an optimum cytoplasmic K(+)/Na+ ratio, cells employ three distinct strategies: 1) strict discrimination among alkali metal cations at the level of influx, 2) efficient efflux of toxic cations from cells, and 3) selective sequestration of cations in organelles. Cation efflux and influx are mediated in cells by systems with different substrate specificities and diverse mechanisms, e.g. ATPases, symporters, antiporters, and channels. Simple eukaryotic yeast Saccharomyces cerevisiae cells proved to be an excellent model for studying the transport properties and physiological function of alkali-metal-cation transporters, and the existence of mutant strains lacking their own transport systems provided an efficient tool for a molecular study of alkali-metal-cation transporters from higher eukaryotes upon their expression in yeast cells.
Dual System for Potassium Transport In Saccharomyces Cerevisiae
Journal of bacteriology, 1984
Like all other organisms, Saccharomyces cerevisiae accu-mulates K+ from the external medium to fulfill cellular requirements. The transport of K+ in yeasts has been the subject of very extensive studies (2) that have clarified many kinetic aspects of the process. All of the alkali cations ...
Molecular Microbiology, 2003
Saccharomyces cerevisiae cells express three defined potassium-specific transport systems en-coded by TRK1 , TRK2 and TOK1 . To gain a more complete understanding of the physiological function of these transport proteins, we have constructed a set of isogenic yeast strains carrying all combinations of trk1 Δ, trk2 Δ and tok1 Δ null mutations. The in vivo K + transport characteristics of each strain have been documented using growth-based assays, and the in vitro biochemical and electrophysiological properties associated with K + transport have been determined. As has been reported previously, Trk1p and Trk2p facilitate high-affinity potassium uptake and appear to be functionally redundant under a wide range of environmental conditions. In the absence of TRK1 and TRK2 , strains lack the ability specifically to take up K + , and trk1 Δ trk2 Δ double mutant cells depend upon poorly understood non-specific cation uptake mechanisms for growth. Under conditions that impair the activity of the non-specific uptake system, termed NSC1, we have found that the presence of functional Tok1p renders cells sensitive to Cs + . Based on this finding, we have established a growth-based assay that monitors the in vivo activity of Tok1p.
FEMS Yeast Research, 2006
There are three different sodium transport systems (Ena1-4p, Nha1p, Nhx1p) in Saccharomyces cerevisiae. The effect of their absence on the tolerance to alkalimetal cations and on the membrane potential was studied. All three sodium transporters were found to participate in the maintenance of Na 1 , Li 1 , K 1 and Cs 1 homeostasis. Measurements of the distribution of a fluorescent potentiometric probe (diS-C 3 (3) assay) in cell suspensions showed that the lack of all three transporters depolarizes the plasma membrane. The overexpression of the Na 1 ,K 1 /H 1 antiporter Nha1 resulted in the hyperpolarization of the plasma membrane and consequently increased the sensitivity to Cs 1 , Tl 1 and hygromycin B. This is the first evidence that the activity of a Na 1 ,K 1 /H 1 antiporter could play a role in the homeostatic regulation of the plasma membrane potential in yeast cells.
Journal of Fungi, 2021
The three families of yeast plasma membrane potassium influx transporters are represented in Candida albicans: Trk, Acu, and Hak proteins. Hak transporters work as K+-H+ symporters, and the genes coding for Hak proteins are transcriptionally activated under potassium limitation. This work shows that C. albicans mutant cells lacking CaHAK1 display a severe growth impairment at limiting potassium concentrations under acidic conditions. This is the consequence of a defective capacity to transport K+, as indicated by potassium absorption experiments and by the kinetics parameters of Rb+ (K+) transport. Moreover, hak1− cells are more sensitive to the toxic cation lithium. All these phenotypes became much less robust or even disappeared at alkaline growth conditions. Finally, transcriptional studies demonstrate that the hak1− mutant, in comparison with HAK1+ cells, activates the expression of the K+/Na+ ATPase coded by CaACU1 in the presence of Na+ or in the absence of K+.
FEMS Yeast Research, 2012
Saccharomyces cerevisiae extrudes K + cations even when potassium is only present in scarce amounts in the environment. Lost potassium is taken up by the Trk1 and Trk2 uptake systems. If the Trk transporters are absent or nonfunctional, the efflux of potassium is significantly diminished. A series of experiments with strains lacking various combinations of potassium efflux and uptake systems revealed that all three potassium-exporting systems the Nha1 antiporter, Ena ATPase and Tok1 channel contribute to potassium homeostasis and are active upon potassium limitation in wild-type cells. In trk1D trk2D mutants, the potassium efflux via potassium exporters Nha1 and Ena1 is diminished and can be restored either by the expression of TRK1 or deletion of TOK1. In both cases, the relative hyperpolarization of trk1D trk2D cells is decreased. Thus, it is the plasma-membrane potential which serves as the common mechanism regulating the activity of K + exporting systems. There is a continuous uptake and efflux of potassium in yeast cells to regulate their membrane potential and thereby other physiological parameters, and the cells are able to quickly and efficiently compensate for a malfunction of potassium transport in one direction by diminishing the transport in the other direction.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2002
Previous descriptions by whole-cell patch clamping of the calcium-inhibited non-selective cation channel (NSC1) in the plasma membrane of Saccharomyces cerevisiae (H. . 181 (1999)) suggested that this inwardly rectifying pathway could relieve the growth inhibition normally imposed on yeast by disruption of its potassium transporters, Trk1p and Trk2p. Now, demonstration of multiple parallel effects produced by various agonists and antagonists on both NSC1 currents and growth (of trk1vtrk2v strains), has identified this non-selective cation pathway as the primary low-affinity uptake route for potassium ions in yeast. Factors which suppress NSC1-mediated inward currents and inhibit growth of trk1vtrk2v cells include (i) elevating extracellular calcium over the range of 10 WM^10 mM, (ii) lowering extracellular pH over the range 7.5^4, (iii) blockade of NSC1 by hygromycin B, and (iv) to a lesser extent by TEA . Growth of trk1vtrk2v cells is also inhibited by lithium and ammonium; however, these ions do not inhibit NSC1, but instead enter yeast cells via NSC1. Growth inhibition by lithium ions is probably a toxic effect, whereas growth inhibition by ammonium ions probably results from competitive inhibition, i.e. displacement of intracellular potassium by entering ammonium. ß