Fig. 1. Distinct modalities for the plasma membrane V-ATPase. (A) On its own, a plasma membrane V-ATPase (V) can electrically polarise an apical domain to well over 100 mV. (B) When partnered with an apical chloride channel (typically a ClC), it is capable of bulk acidification. (C) When partnered with an apical alkali-metal/proton exchanger, it can drive active trans-epithelial transport of (for example) K⁺.