The acidity of intracellular compartments and the extracellular environment is crucial to various cellular processes, including membrane trafficking, protein degradation, bone resorption and sperm maturation. At the heart of regulating acidity are the vacuolar (V-)ATPases--large, multisubunit complexes that function as ATP-driven proton pumps. Their activity is controlled by regulating the assembly of the V-ATPase complex or by the dynamic regulation of V-ATPase expression on membrane surfaces. The V-ATPases have been implicated in a number of diseases and, coupled with their complex isoform composition, represent attractive and potentially highly specific drug targets.

The pH of intracellular compartments in eukaryotic cells is a carefully controlled parameter that affects many cellular processes, including intracellular membrane transport, prohormone processing and transport of neurotransmitters, as well as the entry of many viruses into cells. The transporters responsible for controlling this crucial parameter in many intracellular compartments are the vacuolar (H+)-ATPases (V-ATPases). Recent advances in our understanding of the structure and regulation of the V-ATPases, together with the mapping of human genetic defects to genes that encode V-ATPase subunits, have led to tremendous excitement in this field.

The vacuolar (H+)-ATPases (or V-ATPases) function in the acidification of intracellular compartments in eukaryotic cells. The V-ATPases are multisubunit complexes composed of two functional domains. The peripheral V1 domain, a 500-kDa complex responsible for ATP hydrolysis, contains at least eight different subunits of molecular weight 70-13 (subunits A-H). The integral V0 domain, a 250-kDa complex, functions in proton translocation and contains at least five different subunits of molecular weight 100-17 (subunits a-d). Biochemical and genetic analysis has been used to identify subunits and residues involved in nucleotide binding and hydrolysis, proton translocation, and coupling of these activities. Several mechanisms have been implicated in the regulation of vacuolar acidification in vivo, including control of pump density, regulation of assembly of V1 and V0 domains, disulfide bond formation, activator or inhibitor proteins, and regulation of counterion conductance. Recent information concerning targeting and regulation of V-ATPases has also been obtained.

The vacuolar (H + )-ATPases are ATP-dependent proton pumps that function to acidify intracellular compartments and, in some cases, transport protons across the plasma membrane of eukaryotic cells. Intracellular V-ATPases play an important role in such normal physiological processes as receptor-mediated endocytosis, intracellular membrane traffic, pro-hormone processing, protein degradation and the coupled uptake of small molecules, such as neurotransmitters. They also function in the entry of various pathogenic agents, including many envelope viruses, like influenza virus, and toxins, like anthrax toxin. Plasma membrane V-ATPases function in renal pH homeostasis, bone resorption and sperm maturation, as well as in various disease processes, including renal tubular acidosis, osteopetrosis and tumor metastasis. V-ATPases are composed of a peripheral V 1 domain containing eight different subunits that is responsible for ATP hydrolysis and an integral V 0 domain containing six different subunits that translocates protons. In mammalian cells most of the V-ATPase subunits exist in multiple isoforms which are often expressed in a tissue specific manner. Isoforms of one of the V 0 subunits (subunit a) have been shown to possess information that targets the V-ATPase to distinct cellular destinations. Mutations in isoforms of subunit a lead to the human diseases osteopetrosis and renal tubular acidosis. A number of mechanisms are employed to regulate V-ATPase activity in vivo, including reversible dissociation of the V 1 and V 0 domains, control of the tightness of coupling of proton transport and ATP hydrolysis and selective targeting of V-ATPases to distinct cellular membranes. Isoforms of subunit a are involved in regulation both by control of coupling and by selective targeting. This review will begin with a brief introduction to the function, structure and mechanism of the VATPases followed by a discussion of the role of V-ATPase subunit isoforms and the mechanisms involved in regulation of V-ATPase activity. V-ATPase FunctionThe vacuolar (H + )-ATPases (V-ATPases) are a family of ATP-dependent proton pumps localized to a variety of cellular membranes of eukaryotic cells, including endosomes, lysosomes, Golgi-derived vesicles, secretory vesicles and, for some cells, the plasma membrane (1-3). V-ATPases within endosomes function to dissociate internalized ligands, such as low density lipoprotein (LDL), from their receptors, thus facilitating recycling of the receptors to the plasma membrane (4). They are also required for budding of endosomal carrier vesicles that carry internalized ligands from early to late endosomes (5) and for dissociation of lysosomal enzymes from the mannose-6-phosphate receptors that target them from the Golgi to the lysosome (6). Acidic endosomal compartments also provide the entry point for the cytotoxic portions of many envelope viruses, including influenza and Ebola virus, and toxins, such as diphtheria and anthrax toxin (7). The ability of these pathogenic agents to infect or kill ...