POTASSIUM STIMULATION OF CORN ROOT PLASMALEMMA ATPASE .2. H+-PUMPING IN NATIVE AND RECONSTITUTED VESICLES WITH PURIFIED ATPASE

The stimulation by K+ of the initial rate of H+-pumping by ATPase was studied in native plasmalemma (Zea mays L. var Mona) vesicles and in reconstituted vesicles with enzyme purified on a glycerol gradient. In reconstituted vesicles, a very high H+-pumping rate (200,000% quenching per minute per milligram protein) was obtained with 9-amino-6-chloro-2-methoxyacridine provided that the pump was short-circuited by K+-valinomycin. A constant ionic strength was used to prevent indirect stimulation by the electrostatic effects of K+ salts. Indirect stimulation of H+-pumping by the short-circuiting effect of internal K+, could be abolished by using the permeant anions NO3- and Br- in native, but not in reconstituted vesicles. In both materials, half-stimulation of the H+-pumping by K+ was observed at about 5 millimolar. The same stimulation was obtained when K+ was present only in the external solution or when it was present both outside and inside the vesicles. It was concluded that the stimulating effect of K+ on the H+-pumping evidenced in these experiments on both native and reconstituted vesicles was due to a direct effect of the cation on the cytoplasmic face of the ATPase. These results are discussed within the context of the hypothesis of an active K+ transport driven by the ATPase through a direct H+/K+ exchange mechanism.