POTASSIUM-TRANSPORT INTO PLANT VACUOLES ENERGIZED DIRECTLY BY A PROTON-PUMPING INORGANIC PYROPHOSPHATASE

Potassium is accumulated in plant vacuoles against an inside-positive membrane potential. The mechanism facilitating energized K+ transport has remained obscure. However, electrogenic activity of the inorganic pyrophosphatase (H+-PPase) at the vacuolar membrane is dependent on cytoplasmic K+, raising the possibility that the enzyme translocates K+ into the vacuole. Membrane currents generated by the H+-PPase were measured (using a patch clamp technique) in intact vacuoles isolated from Beta vulgaris storage tissue. A significant orthophosphate-dependent outward current mediated by the enzyme in reverse mode is evoked only when potassium is present at the vacuolar face of the tonoplast, suggesting that potassium is a translocated ion. Furthermore, current-voltage analysis of the effects of extravacuolar potassium and pH on the reversal potential of the H+-PPase-generated current points to direct translocation of K+ and H+ by the enzyme. Thus the H+-PPase represents a distinct class of eukaryote translocase and could facilitate vacuolar K+ accumulation in vivo.