Vacuolar H+-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H+-ATPases fulfill special tasks in the kidney. Vacuolar H+-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H+-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H+-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H+-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H+-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

The recruitment of the small GTPase Arf6 and ARNO from cytosol to endosomal membranes is driven by V-ATPase-dependent intra-endosomal acidification. The molecular mechanism that mediates this pH-sensitive recruitment and its role are unknown. Here, we demonstrate that Arf6 interacts with the c-subunit, and ARNO with the a2-isoform of V-ATPase. The a2-isoform is targeted to early endosomes, interacts with ARNO in an intra-endosomal acidification-dependent manner, and disruption of this interaction results in reversible inhibition of endocytosis. Inhibition of endosomal acidification abrogates protein trafficking between early and late endosomal compartments. These data demonstrate the crucial role of early endosomal acidification and V-ATPase/ARNO/Arf6 interactions in the regulation of the endocytic degradative pathway. They also indicate that V-ATPase could modulate membrane trafficking by recruiting and interacting with ARNO and Arf6; characteristics that are consistent with the role of V-ATPase as an essential component of the endosomal pH-sensing machinery.

Vacuolar-type H+-ATPase (V-ATPase)-driven proton pumping and organellar acidification is essential for vesicular trafficking along both the exocytotic and endocytotic pathways of eukaryotic cells. Deficient function of V-ATPase and defects of vesicular acidification have been recently recognized as important mechanisms in a variety of human diseases and are emerging as potential therapeutic targets. In the past few years, significant progress has been made in our understanding of function, regulation, and the cell biological role of V-ATPase. Here, we will review these studies with emphasis on novel direct roles of V-ATPase in the regulation of vesicular trafficking events.