Ectopic potassium uptake in trk1 trk2 mutants of Saccharomyces cerevisiae correlates with a highly hyperpolarized membrane potential

Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K+ and Rb+. We show that this low-affinity Rb+ uptake is mediated by several independent transporters, and that trk1 Delta cells and especially trk1 Delta trk2 Delta cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC(6)(3) fluorescence. On the basis of the latter analyses, it is preposed that Tsk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K+ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1 Delta trk2 Delta cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K+-transporter from Schwanniomyces occidentalis suppresses the K+-defective transport of trk1 Delta trk2 Delta cells but not the high hyperpolarization, and the HKT1 K+ transporter from wheat suppresses both defects, in the presence of Na+. 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 trk1 Delta trk2 Delta cells and its ectopic transport of alkaki cations.