The vacuolar H + -ATPase (V-ATPase) is a multimeric enzyme complex that acidi®es organelles of the vacuolar system in eukaryotic cells. Proteins that interact with the V-ATPase may play an important role in controlling the intracellular localization and activity of the proton pump. The neuroendocrine-enriched V-ATPase accessory subunit Ac45 may represent such a protein as it has been shown to interactwith themembrane sector of theV-ATPase inonlya subset of organelles.