The influence of V-type H+-ATPases on vesicle fusion processes

The proton pumping activity of the V-type H+-ATPase (V-ATPase) complex is essential to maintain the luminal acidification of defined cellular compartments. A broad range of physiological functions, e.g. the proteolytic degradation within lysosomes, intracellular protein sorting and signalling by the mTORC1 (mechanistic target of rapamycin complex 1) depend on functional V‑ATPase complexes. In addition, there is an ongoing debate about a role for the V‑ATPase V0 sector in promoting membrane fusion processes. The present work addresses the influence of the V-ATPase complex on vesicle fusion events in genetically defined mouse models, lacking either the V-ATPase V0 subunit a3 or the V-ATPase accessory subunit 2 (ATP6AP2), a chaperone for V-ATPase V0 sector assembly. In fibroblasts, a detailed characterisation of endosome and phagosome maturation revealed functional vesicle fusion upon knockout of the V-ATPase V0 subunit a3. Similarly, phagosome maturation proceeded in ATP6AP2-deficient macrophages with significantly decreased V‑ATPase V0 subunit levels. Even the pharmacological reduction of accessible, free V0 sectors or pharmacologically inhibited vesicle acidification did not disturb the acquisition of lysosomal markers to phagosomes. In summary, neither the presence nor the activity of the V‑ATPase complex were required for endocytic and phagosomal fusion processes. Therefore, a general fusion promoting influence of V‑ATPases is not supported by the data obtained in this study [...]