Yeast Yeast 2006; 23: 581–589. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/yea.1376 Research Article The yeast potassium transporter TRK2 is able to substitute for TRK1 in its biological function under low K and low pH conditions Bertha Michel*, Carlos Lozano, Miriam Rodr´ ıguez, Roberto Coria, Jorge Ram´ ırez and Antonio Pe˜ na Depto de Gen´ etica Molecular, Instituto de Fisiolog´ ıa Celular, UNAM, Circuito Exterior s/n, Ciudad Universitaria, D.F. 04510, M´ exico *Correspondence to: Bertha Michel, Departmento de Gen´ etica Molecular, Instituto de Fisiolog´ ıa Celular, Universidad Nacional Aut´ onoma de M´ exico, Circuito Exterior s/n, Ciudad Universitaria, D.F. 04510 M´ exico, M´ exico. E-mail: bmichel@ifc.unam.mx Received: 10 January 2006 Accepted: 16 March 2006 Abstract In S. cerevisiae, K + transport relies principally on two structurally related membrane proteins, known as Trk1p and Trk2p. Direct involvement in cation movements has been demonstrated for Trk1p, which is a high-affinity K + transporter. Initially described as a low-affinity K + transporter, Trk2p seems to play a minor role in K + transport, since its activity is only apparent under very specific conditions, such as in a ∆sin3 background. Here we show that growth of a ∆trk1∆sin3 double mutant, under K + -limiting conditions or at low pH, is Trk2p-dependent, and by Northern blot analysis we demonstrate that deletion of SIN3 results in transcriptional derepression of TRK2. In addition, we show that heterologous overexpression of TRK2 with the inducible GAL1 promoter bypasses Sin3p repression in a ∆trk1∆trk2 double mutant and fully restores growth under non-permissive conditions. Furthermore, kinetic experiments in a ∆trk1∆sin3 double mutant revealed a K + transporter with an apparent high affinity and a moderate capacity. Taken together, these results indicate that TRK2 encodes a functional K + transporter that, under our experimental conditions, displays distinctive kinetic characteristics. Copyright  2006 John Wiley & Sons, Ltd. Keywords: yeast; K + transport regulation; TRK1; TRK2 Introduction Potassium (K + ) uptake in the yeast Saccharomyces cerevisiae is mediated principally by the plasma- membrane proteins Trk1p and Trk2p. The role of Trk1p in this process seems to be predominant over Trk2p, since trk1 mutants lose the high-affinity component of potassium uptake and the ability to grow on low potassium media (Gaber et al., 1988; Ko et al., 1991). TRK1 encodes a 1235 amino acid protein and contains 12 hydrophobic segments that form the four putative MPM motifs (where M corresponds to the hydrophobic segments and P to an α-helix that enters the membrane and connects the two M segments) characteristic of K + channels and transporters from many organisms (Ko et al., 1991; Rodriguez-Navarro, 2000). TRK2 encodes a shorter protein; it is 889 amino acid residues long, and is 55% identical to Trk1p in its overall sequence. Trk2p also contains four MPM segments whose identity to those of Trk1p MPM segments rises to 70–90%. The principal difference between Trk1p and Trk2p lies between the first and second MPM motifs; in Trk1p this loop is 642 amino acids long, while in Trk2p this intracellular segment is significantly shorter (326 residues) (Miranda et al., 2002). There has been controversy as to whether Trk2p is a low- or a medium-affinity potassium trans- porter, in part due to changes in its activity depend- ing on the growing conditions (Ko et al., 1990; Ramos et al., 1994). In addition, whereas there is no experimental evidence for transcription reg- ulation of TRK1, there are several reports that indicate that TRK2 undergoes a rather complex Copyright  2006 John Wiley & Sons, Ltd.