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 Deltasin3 background. Here we show that growth of a Deltatrk1Deltasin3 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 Deltatrk1Deltatrk2 double mutant and fully restores growth under non-permissive conditions. Furthermore, kinetic experiments in a Deltatrk1Deltasin3 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.
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