Ectopic Potassium Uptake in trk1 trk2 Mutants ofSaccharomyces cerevisiae Correlates with a Highly Hyperpolarized Membrane Potential

Abstract

Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K1 and Rb1. We show that this low-affinity Rb1 uptake is mediated by several independent transporters, and that trk1D cells and especially trk1D trk2D cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC6(3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K1 starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1D trk2D cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K1 transporter from Schwanniomyces occidentalis suppresses the K1-defective transport of trk1D trk2D cells but not the high hyperpolarization, and the HKT1 K1 transporter from wheat suppresses both defects, in the presence of Na1. We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1D trk2D cells and its ectopic transport of alkali cations.