Univalent cation fluxes in yeast

Transport of H+, K+, Rb+ and Tl+ ions was studied in a wild‐type strain of Saccharomyces cerevisiae and in its mutants defective in the high‐affinity K+ transport system TRK1 and in the double mutant with an additional deletion in the TRK2 gene. In the absence of glucose K+, Rb+ and Tl+ elicited a more or less stoichiometric exchange outflow of H+, in the mutants K+ moved out of cells even in the presence of 10 mM KCl or KNO3. In the presence of glucose in the wild type, K+, Rb+ and Tl+ brought about a massive outflow of H+ while being transported inward against high concentration gradients. In the trklΔ mutant the exchange fluxes were reduced by 65‐85%, in the double mutant those of K+, Rb+ and Tl+ practically cease but outflow of H+ caused by Tl+ remained at the level of the trklΔ mutant. It appears that, in addition to the H+ export by the PMAl‐coded plasma membrane H+‐ATPase, at least three different univalent‐cation involving activities are present: the high‐affinity transport system for K+ (TRK1), another system (possibly TRK2) with different responses to K+ and Rb+, vs. Tl+, and an active system for K+ export. The first two are apparently active exchange systems for K+, Rb+, and Tl+ against H+. The source of energy for these highly active transports (acting against gradients of 1000:1 and 5000:1, respectively) is unclear.