The yeast potassium transporter TRK2 is able to substitute for TRK1 in its biological function under low K and low pH conditions

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 Δ sin3 background. Here we show that growth of a Δ trk1 Δ 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 Δ trk1 Δ trk2 double mutant and fully restores growth under non‐permissive conditions. Furthermore, kinetic experiments in a Δ trk1 Δ 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.