Uptake of Selenite by Saccharomyces cerevisiae Involves the High and Low Affinity Orthophosphate Transporters

Abstract

Although the general cytotoxicity of selenite is well established, the mechanism by which this compound crosses cellular membranes is still unknown. Here, we show that in Saccharomyces cerevisiae, the transport system used opportunistically by selenite depends on the phosphate concentration in the growth medium. Both the high and low affinity phosphate transporters are involved in selenite uptake. When cells are grown at low P i concentrations, the high affinity phosphate transporter Pho84p is the major contributor to selenite uptake. When phosphate is abundant, selenite is internalized through the low affinity P i transporters (Pho87p, Pho90p, and Pho91p). Accordingly, inactivation of the high affinity phosphate transporter Pho84p results in increased resistance to selenite and reduced uptake in low P i medium, whereas deletion of SPL2, a negative regulator of low affinity phosphate uptake, results in exacerbated sensitivity to selenite. Measurements of the kinetic parameters for selenite and phosphate uptake demonstrate that there is a competition between phosphate and selenite ions for both P i transport systems. In addition, our results indicate that Pho84p is very selective for phosphate as compared with selenite, whereas the low affinity transporters discriminate less efficiently between the two ions. The properties of phosphate and selenite transport enable us to propose an explanation to the paradoxical increase of selenite toxicity when phosphate concentration in the growth medium is raised above 1 mM.Although toxic at high concentrations, selenium is required in many cells, because it is translationally incorporated as selenocysteine into selenoproteins that perform specific and essential functions (1). Cells must ensure selenium uptake to sustain this metabolism. However, little is known about selenium transport. Because selenium and sulfur are chalcogen elements that have many chemical properties in common, selenium shares metabolic pathways with sulfur. Accordingly, selenate was shown to be taken up by the yeast Saccharomyces cerevisiae sulfate permeases (2). Similarly, in plants, selenate is taken up by roots via the high affinity sulfate transporters (3).On the other hand, specific selenite transporters have never been identified so far. The sulfate ABC transporter of Escherichia coli mediates selenite uptake in addition to that of selenate (4). However, repression of the expression of that transporter does not quench selenite uptake completely, indicating that an alternative entry pathway exists for selenite (5). In S. cerevisiae, which does not possess selenoproteins, an energy-dependent uptake of selenite, distinct from that of selenate, was reported. Characterization of the kinetics of selenite uptake suggested the existence of two transport systems: a high affinity system at low selenite concentration and a low affinity system at higher concentration (6). Recently, a study of selenite uptake by wheat (Triticum aestivum) roots showed it to be an active process competitively inhibited by phosph...

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“…This can be formed from other Se forms such as selenite [20], the predominant environmental form. In the presence of glucose, selenite enters the yeast cell through the high affinity phosphate transporter Pho84 in low phosphate conditions and through both Pho84 and the low affinity transporters Pho87/Pho90/Pho91 in high phosphate conditions [21]. Once inside the cell, selenite causes double-strand breaks, high mutagenicity rate, cell cycle arrest and protein hypercarbonylation which is indicative of extensive protein oxidative damage [22]–[24].…”

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“…Studies show that in E. coli selenate/selenite uptake is via the sulfate ABC transporter complex encoded by the cysAWTP operon (Sirko et al 1990;Turner et al 1998). Another study in eukaryotic system, Saccharomyces cerevisae, demonstrates the uptake of selenite ions by orthophosphate transporters due to the similar size and charge of phosphate and selenite oxyions (Lazard et al 2010). However, in our study there was down-regulation of inorganic phosphate transporter (BC1453).…”

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“…The high affinity transporter strongly selects 270 phosphate over selenite, whereas the low affinity transporter does not discriminate efficiently 271 between the two anions (Lazard et al 2010). Thus the low affinity transporter can potentially 272 have a higher conductance to selenite than the high affinity transporter (Lazard et al 2010 310 velutipes and other 11 species of phylogenetically and ecologically varied mushrooms were 311 tested, and 10 of them showed the reducing ability (Fig. S3).…”

“…In S. cerevisiae, which is phylogenetically close to F. velutipes, the selenite uptake 268 involves the high and low affinity phosphate transporters at low and high (1 mM) phosphate 269 concentrations, respectively (Lazard et al 2010). The high affinity transporter strongly selects 270 phosphate over selenite, whereas the low affinity transporter does not discriminate efficiently 271 between the two anions (Lazard et al 2010).…”

“…In S. cerevisiae, which is phylogenetically close to F. velutipes, the selenite uptake 268 involves the high and low affinity phosphate transporters at low and high (1 mM) phosphate 269 concentrations, respectively (Lazard et al 2010). The high affinity transporter strongly selects 270 phosphate over selenite, whereas the low affinity transporter does not discriminate efficiently 271 between the two anions (Lazard et al 2010). Thus the low affinity transporter can potentially 272 have a higher conductance to selenite than the high affinity transporter (Lazard et al 2010 310 velutipes and other 11 species of phylogenetically and ecologically varied mushrooms were 311 tested, and 10 of them showed the reducing ability (Fig.…”