Efflux of potassium induced by dio-9, a plasma membrane ATPase inhibitor in the yeast Schizosaccharomyces pombe.

The active uptake of L-leucine is 50% inhibited by 10 pg of Dio-S/ml in glycolyzing cells of the yeast Schizosaccharomyces pombe in which the respiration has been inhibited by antimycin. Similar inhibition is obsetied in the nuclear respiratory-deficient mutant RD32 which lacks mitochondrial ATPase activity. Therefore the mitochondrial ATPase is not involved in the inhibition by Dio-9 of the cellular uptake of metabolites. On the other hand, the ATPase activities of purified plasma membrane fractions isolated from the wild type or the mutant RD32 were both 50% inhibited by 10 pg of Dio-S/ml. Moreover, in the presence of glucose, Dio-9 induces an efflux of K+ simultaneous to an influx of H+ with a half-time for equilibration of 2 min. Uncouplers like carbonylcyanide m-chlorophenylhydrazone increases the velocity of the potassium and proton movements induced by Dio-9 by reducing the half-time for equilibration to about 20 s. In the absence of glucose, Dio-9 does not induce cation movements unless uncouplers are present. The addition of CaCI, or NaCl decreases the H+ influx without modifying the Dio-9induced K+ efflux, suggesting that Ca2+ and Na+ can substitute for H+ to balance the K+ efflux. For increasing concentrations of Dio-9, the apparent stoichiometry of the K+ and H+ movements varies largely from negative to positive values finally tending toward 1.0 at saturating concentrations of the inhibitor. This variation reflects a dual action of Dio-9 on proton movements which can be discriminated by the effects of 80 mM external KC1 which totally abolishes H+ influx at Dio-9 concentrations higher than 50 pg/ml but allows the expression of H+ efflux at concentrations lower than 10 pg of Dio-g/ml. We conclude that Dio-9 elicits an electrogenic exit of K+ along the concentration gradient of this cation. The high electric membrane potential so generated must be balanced by the uptake of cations. Therefore in ATP-depleted cells, the ejection of K+ induced by Dio-9 can be observed only if a proton-conducting agent is added to the medium to balance the K+ efflux by an H+ influx. In the presence of glucose and Dio-9, protons or other cations such as Ca’+ or Na+ are taken up by the cell to neutralize the electric potential generated by the exit of K+. The Dio-g-induced K+ efflux is not second-