Yeast Yeast 2004; 21: 569–582. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/yea.1126 Research Article The trehalose pathway and intracellular glucose phosphates as modulators of potassium transport and general cation homeostasis in yeast Jose M. Mulet, Santiago Alejandro, Carlos Romero # and Ram ´ on Serrano* Instituto de Biolog´ ıa Molecular y Celular de Plantas, Universidad Polit´ ecnica de Valencia-CSIC, Camino de Vera, 46022 Valencia, Spain *Correspondence to: Ram´ on Serrano, Instituto de Biolog´ ıa Molecular y Celular de Plantas, Universidad Polit´ ecnica de Valencia-CSIC, Camino de Vera, 46022 Valencia, Spain. E-mail: rserrano@ibmcp.upv.es # Present address: Instituto Valenciano de Investigaciones Agrarias (IVIA), 46113 Moncada, Spain. Received: 14 October 2003 Accepted: 25 February 2004 Abstract Trk, encoded by the partially redundant genes TRK1 and TRK2, is the major potassium transporter of Saccharomyces cerevisiae. This system is specific for potassium and rubidium but, by reducing the electrical membrane potential of the plasma membrane, Trk decreases the uptake of toxic cations such as lithium, calcium, aminoglycosides and polyamines, which are transported by other systems. Gain- and loss-of-function studies indicate that TPS1, a gene encoding trehalose-6-phosphate synthase and known to modulate glucose metabolism, activates Trk and reduces the sensitivity of yeast cells to many toxic cations. This effect is independent of known regulators of Trk, such as the Hal4 and Hal5 protein kinases and the protein phosphatase calcineurin. Mutants defective in isoform 2 of phosphoglucomutase (pgm2) and mutants defective in isoform 2 of hexokinase (hxk2 ) exhibit similar phenotypes of reduced Trk activity and increased sensitivity to toxic cations compared with tps1 mutants. In all cases Trk activity was positively correlated with levels of glucose phosphates (glc-1-P and glc-6-P). These results indicate that Tps1, like Pgm2 and Hxk2, increases the levels of glucose phosphates and suggest that these metabolites, directly or indirectly, activate Trk. Copyright  2004 John Wiley & Sons, Ltd. Keywords: Saccharomyces cerevisiae ; trehalose; glucose phosphates; salt tolerance; potassium transport Introduction Under normal growth conditions, living cells must ensure an internal neutral pH, a content of potas- sium of about 10 −1 M and the exclusion of sodium and other toxic cations from the intracellular medium. In order to achieve cation homeostasis, fungal and plant cells contain proton pumps, potas- sium transporters, systems mediating the extrusion or vacuolar accumulation of toxic cations and reg- ulatory pathways for all these systems (Serrano and Rodr´ ıguez-Navarro, 2001). In the yeast Saccharomyces cerevisiae, the plasma membrane H + -ATPase is encoded by the PMA1 gene (Serrano, 1991) and the major K + transporter is the high-affinity Trk system, encoded by the TRK1 and TRK2 genes (Rodr´ ıguez-Navarro, 2000). These two major electrogenic systems modulate the electrical membrane potential of the yeast plasma membrane (enhanced by Pma1 and decreased by Trk) and therefore the uptake of toxic cations, such as sodium, lithium, calcium, polyamines and hygromycin B, mediated by other transporters (Mulet et al., 1999; Goosens et al., 2000). Other important biophysical parameters, such as cell turgor and intracellular pH, also depend on Pma1 and Trk activities (Serrano, 1991). The important role of Pma1 and Trk in cellular physiology explains that these transporters are subjected to multiple regulations, with intracellu- lar K + ,H + and Na + concentrations and sugar metabolism as the best-known regulatory fac- tors (Portillo, 2000). The protein kinases Hal4p Copyright  2004 John Wiley & Sons, Ltd.