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

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.

Alkali Metal Cation Transport and Homeostasis in Yeasts

“…Therefore, the primary function of TRK2, at least in trk1 cells, was proposed to be K ϩ influx (127). This has been subsequently confirmed by other laboratories (168,228). It has been reported that strains deficient for both K ϩ transporters (trk1⌬ trk2) exhibit hypersensitivity to low extracellular pH that can be suppressed by high concentrations of K ϩ but not Na ϩ .…”

“…This additional phenotype is most probably a side effect derived from the fact that at acidic pH, the cells become depolarized and K ϩ requirements are higher. This is probably the reason why either TRK1 or TRK2 can completely suppress both the K ϩ transport defect and low-pH hypersensitivity of trk1 trk2 cells (127,168).…”

“…This idea was reinforced by the modest contribution of Trk2 to total K ϩ uptake in wild-type cells and by the low Trk2-mediated potassium uptake activity measured in trk1 TRK2 strains (127,278). However, this scenario was too simplistic, since it was shown that Trk2 may be a high/moderateaffinity K ϩ transporter (168,228) and, on the other hand, a residual potassium transport process with a K m in the mM range was present in mutants lacking both the TRK1 and TRK2 genes (148).…”

“…Several reports indicate that TRK2 undergoes a rather complex transcriptional regulation; in this respect, it has been shown that TRK2 expression is negatively regulated by the Rpd3/Sin3 histone deacetylase complex and by a URS1 element located within the promoter region (278, 279). Peña's group has also reported that Northern blot analysis showed no TRK2 transcripts in a wild-type strain or in a trk1⌬ mutant, strongly suggesting that in these strains TRK2 was not expressed and that deletion of SIN3 resulted in derepression of TRK2 transcription (168). However, as for TRK1 expression, TRK2 expression is not altered by perturbations in cation homeostasis.…”

Identification and Characterization of Vnx1p, a Novel Type of Vacuolar Monovalent Cation/H+ Antiporter of Saccharomyces cerevisiae

“…Trk1p, the high affinity K ϩ transporter, seems to be predominant over Trk2p, since trk1 mutants lose the ability to grow in low K ϩ media (1,2). Trk2p, the low affinity K ϩ transporter, is able to substitute Trk1p activity only under K ϩ limiting conditions or at low pH (3). Na ϩ and other alkali cations enter the cell through K ϩ uptake systems, and steadystate cytosolic Na ϩ concentrations are established by the balance between Na ϩ inward and Na ϩ outward fluxes among the cytosol and the external medium or the vacuole.…”