Fig. 4. Utility of the V-type H⁺ ATPase expressed in plasma membranes. (A) Coupling of proton secretion to K⁺ secretion via electrogenic 2H⁺/K⁺ antiport in apical membranes of goblet cells in insect midgut (Azuma et al., 1995). (B) Secretion of strong acid across the ruffled border membrane (apical membrane) into the lacunar space of osteoclasts, serving the acid digestion of bone (Chatterjee et al., 1992; Cleiren et al., 2001). (C) Coupling of proton secretion to oligopeptide absorption in the apical brush border membrane of the renal proximal tubule (Lee and Kim, 2004; Wagner et al., 2004).