A Novel Mechanism of Ion Homeostasis and Salt Tolerance in Yeast: the Hal4 and Hal5 Protein Kinases Modulate the Trk1-Trk2 Potassium Transporter

Abstract

The regulation of intracellular ion concentrations is a fundamental property of living cells. Although many ion transporters have been identified, the systems that modulate their activity remain largely unknown. We have characterized two partially redundant genes from Saccharomyces cerevisiae, HAL4/SAT4 and HAL5, that encode homologous protein kinases implicated in the regulation of cation uptake. Overexpression of these genes increases the tolerance of yeast cells to sodium and lithium, whereas gene disruptions result in greater cation sensitivity. These phenotypic effects of the mutations correlate with changes in cation uptake and are dependent on a functional Trk1-Trk2 potassium transport system. In addition, hal4 hal5 and trk1 trk2 mutants exhibit similar phenotypes: (i) they are deficient in potassium uptake; (ii) their growth is sensitive to a variety of toxic cations, including lithium, sodium, calcium, tetramethylammonium, hygromycin B, and low pH; and (iii) they exhibit increased uptake of methylammonium, an indicator of membrane potential. These results suggest that the Hal4 and Hal5 protein kinases activate the Trk1-Trk2 potassium transporter, increasing the influx of potassium and decreasing the membrane potential. The resulting loss in electrical driving force reduces the uptake of toxic cations and improves salt tolerance. Our data support a role for regulation of membrane potential in adaptation to salt stress that is mediated by the Hal4 and Hal5 kinases.

Regulation of Saccharomyces cerevisiae maltose fermentation by cold temperature and CSF1

“…3) to several toxic cations (calcium, hygromycin B, tetramethylammonium) and acidic pH, but not to high potassium concentrations. This phenotype is consistent with hyperpolarization of the plasma membrane, a phenomenon observed in strains lacking the Trk1-Trk2 potassium transporters, or lacking the kinases that activate these transporters (7). Although the mechanism by which CSF1 allows normal nutrient uptake at low temperatures is still unknown, our results indicate that the csf1∆ mutant strain displays a complex pleiotropic phenotype which includes deficiencies in ion homeostasis and salt tolerance.…”

An Osmotically Induced Cytosolic Ca2+ Transient Activates Calcineurin Signaling to Mediate Ion Homeostasis and Salt Tolerance of Saccharomyces cerevisiae

“…Calcineurin post-transcriptionally activates the Trk1-Trk2 K ϩ transport system to the high affinity state resulting in higher K ϩ /Na ϩ selective uptake and reduced Na ext ϩ influx (8,15). The Hal4p/Hal5p signal pathway also activates the Trk1-Trk2 transport system high affinity transition, although the post-translational process has not been defined nor is it known whether Ca 2ϩ is required for the function of this pathway (16). Calcineurin post-transcriptionally regulates Vcx1p, which together with Pmc1p and Pmr1p function to evacuate Ca 2ϩ from the cytosol (17,18).…”

Regulation of Monovalent Ion Homeostasis and pH by the Ser-Thr Protein Phosphatase SIT4 in Saccharomyces cerevisiae

“…Hal3p acts as an inhibitory subunit of Ppz1p regulating its function on salt tolerance (15). Deletion of two protein kinase homologs YCR101c/SAT4/ HAL4 (16) and YJL165c/HAL5 causes salt and pH sensitivity apparently as a result of deficient Trk1p and Trk2p activation (5).…”

“…When yeast cells are subjected to Na ϩ stress, its uptake is inhibited by increasing K ϩ import through Trk1p. However, this does not inhibit completely Na ϩ entry (4,5). Two transport systems pump out Na ϩ that has entered the cell as follows: a cluster of up to five P-ATPases PMR2A/ENA1-4 (6, 7) and a Na ϩ /H ϩ antiporter, encoded by NHA1 (8,9).…”