A role for Ppz1p-mediated regulation of the Trk1p high affinity potassium transporter in cell cycle progression.
Stephanie Merchan, Ramon Serrano, Lynne Yenush
IBMCP, Uni. Politecnica de Valencia, Camino de vera, Valencia, 46022, Spain
Intracellular pH and K + concentrations must be tightly controlled because they affect many cellular activities including cell growth and death. The mechanisms of homeostasis of H + and K + are only partially understood. In the yeast, Saccharomyces cerevisiae, proton efflux is mediated by the Pma1p H + -ATPase. As this pump is electrogenic, the activity of the Trk1,2 K + uptake system is crucial for sustained Pma1p operation. Thus, the coordinated activity of these two systems determines cell volume, turgor, membrane potential, nutrient uptake, and pH. Genetic evidence indicates that Trk1p is activated by the Hal4,5p kinases and inhibited by the Ppz1,2p phosphatases. Here, we show that Trk1p, present in plasma membrane 'rafts', interacts with Ppz1p, that Trk1p is phosphorylated in vivo and its level of phosphorylation increases in ppz1,2 mutants. In addition, we show that ppz1,2 mutants are sensitive to the DNA-damaging agents, hydroxyurea and UV irradiation, and that they present both morphological and arrest/progression defects during alpha-factor treatments. These cell cycle phenotypes are no longer observed upon further disruption of the TRK1 and TRK2 genes in the ppz1,2 mutant. Taken together, these data indicate that the Ppz1,2 phosphatases establish a connection between the regulation of high affinity potassium uptake and cell cycle progression in Saccharomyces cerevisiae.